Research advances in major cereal crops for adaptation to abiotic stresses

Abstract

With devastating increase in population there is a great necessity to increase crop productivity of staple crops but the productivity is greatly affected by various abiotic stress factors such as drought, salinity. An attempt has been made a brief account on abiotic stress resistance of major cereal crops viz. In spite of good successes obtained on physiological and use molecular biology, the benefits of this high cost technology are beyond the reach of developing countries. This review discusses several morphological, anatomical, physiological, biochemical and molecular mechanisms of major cereal crops related to the adaptation of these crop to abiotic stress factors. It discusses the effect of abiotic stresses on physiological processes such as flowering, grain filling and maturation and plant metabolisms viz. photosynthesis, enzyme activity, mineral nutrition, and respiration. Though significant progress has been attained on the physiological, biochemical basis of resistance to abiotic stress factors, very little progress has been achieved to increase productivity under sustainable agriculture. Therefore, there is a great necessity of inter-disciplinary research to address this issue and to evolve efficient technology and its transfer to the farmers’ fields.

Abbreviations

ABA

abscisic acid

APRI

alternate partial root zone irrigation

AM

arbuscular mycorrhiza

ASI

anthesis-silking interval

CAT

catalase

CGR

crop growth rate

DRI

drought resistance index

GA

gibberelic acid

GPX

glutathione peroxidase

GR

glutathione reductase

GST

glutathione-S transferase

HSP

heat shock protein

LWP

leaf water potential

MAS

marker assisted selection

MDA

malonaldehyde

MnSOD

manganese superoxide dismutase

MT

more tillage

NAR

net assimilation rate

NDVI

normalized difference vegetation index

NT

no tillage

OA

osmotic adjustment

PEG

poly-ethylene glycol

POX

peroxidase

QTL

quantitative trait loci

ROS

reactive oxygen species

RUE

radiation use efficiency

SA

salicylic acid

SPAW

soil plant air water

TE

transpiration efficiency

TTC

triphenyltetrazolium chloride

VDAC

voltage dependent anion channel

WSI

water stress index

WUE

water use efficiency

Introduction

Rapidly changing global climate is affecting the crop productivity thus food availability in the world. Over 8,00,000 people in West Africa suffer from malnutrition and severe food shortage due to prolonged drought conditions. The United Nations World Food Program (WFP) reported that about 10 million people are affected in Eastern Sahel region of West Africa due to poor crop production. Over 70,000 livestock have died in Tanzania due to dry spell during 2009–10. The symptom is not limited to the African countries alone. Eastern India is facing rainfall deficit in rainy seasons, which will severely affect rice production. Similar drought has hit Vietnam reducing water level in Mekong River to 20-y low. The South western regions of China, which receives ample water for cultivation, did not receive sufficient rain in 2010. Thus, there is a growing concern on the changing pattern of crop cultivation and shift in agricultural operations. Besides, indiscriminate uses of fertilizer, pesticides and increasing salinity to three-fourth of arable lands are affecting crop productivity of the world converting lands unfertile and barren. Concerted research activities have been directed throughout the world to identify/breed crop cultivars resistant to these stresses. They are successful in bringing out resistant cultivars but with poor yields.

Agriculture is considered the biggest consumer of quality water, particularly in the non-Industrial countries. All the operations and activities related to agriculture thus will be affected by water shortage. Usable water is going to be the most precious commodity in near future. Reuse of groundwater and waste water, reclamation of degraded lands, identification of drought tolerant genotypes, sustainable use of cultivation inputs, dependence on micro irrigation and identification of alternate natural resources that can reduce water use in agriculture are major options to increase sustainability.

Therefore, in the context of above situation, global warming, heat stress, drought, salinity etc., affect crop productivity drastically and threaten global food security. Under this situation, there is a great necessity of multidisciplinary research to select/breed high yielding crop cultivars for resistance to these abiotic and biotic stresses which can sustain/enhance crop productivity in the farmers’ fields. To fulfil these objectives there is an immediate need for innovation of very simple techniques in screening crop cultivars. We developed very simple technique for screening and selecting crop cultivars for tolerance to salinity, drought, heat stress, flooding, pollen viability to high temperature etc. For this, we developed very simple techniques to screen all pipe line hybrids and parents (with high yield potential) and selecting genotypes for resistance to different stress factors. Using these simple techniques, cereal crop cultivars with high yielding background can be evaluated for stress tolerance with satisfactory results.^118,122

Cereal crops are the major source of food material and nutritional components for human health and feed for livestock throughout the world. With increasing population, there is an increasing demand for food and other commodities but the productivity of these crops is endangered with ever increasing global warming, heat stress, drought, salinity and other abiotic stresses, thereby threatening food security and causing starvation death especially in African countries. Under this situation, it needs to be mentioned that 1/5th of arable land of the world is arid and semiarid and 2/3 of it is saline thereby affecting crop productivity severely. Under this situation, the selection of drought and salt tolerant crop cultivars is considered as a feasible alternative to sustain the productivity of the crops in these regions.

Though enormous researches have been undertaken, the results of which are published in reputed journals on various aspects with special reference to the mechanism of resistance to these stresses and their management, but insignificant progress has been attained for their practical utility in the farmer’s fields. It has been argued by the eminent plant breeders namely TJ Flowers that salinity and drought resistances are complex traits, very difficult to manage for genetic improvement. However, good successes have been not achieved to use molecular biology such as marker assisted selection, transgenic tools, but the practical use of these techniques are beyond the reach of developing countries for this high cost technology.

Successful establishment, survival and productivity of a crop mainly depend on adaptation of the crop to environmental conditions (abiotic) in which the crop grows. Several morphological, anatomical, physiological, biochemical and molecular mechanisms play an important role in adaption of the crop to abiotic stress factors.

Abiotic stress play important role in cultivated crops. It mainly affects the various growth stages which ultimately resulted loss in the yield. Physiological process such as flowering, grain filling and maturation was highly effected abiotic stresses. Plant metabolisms which include photosynthesis, enzyme activity, mineral nutrition, and respiration are affected by several abiotic stress factors such as drought, salinity, heat, chilling, and UV radiation stress. In the context of abiotic stress few studies have been given under various stress factors in major cereal crops.

Sorghum (Sorghum bicolor)

Salinity tolerance

Salt stress affects sorghum growth and causes several physical and biochemical changes. Genotypic variability in sorghum hybrids and their parents for NaCl-salinity tolerance was observed at the seedling stage and salinity tolerant hybrids and parents have been selected for possible incorporation in breeding program.¹ Seed priming increased germination and reduced the delay in germination time. Salinity causes several physiological, biochemical changes in sorghum.²

Biochemical changes

Among the enzymes related to nitrogen metabolism, nitrate reductase (NR) is more sensitive than nitrite reductase (NIR) to NaCl stress in vivo as well as in vitro, and anionic salinity is more toxic to NR activity than is cationic salinity.³ Salt-induced accumulation of betaine and betaine aldehyde dehydrogenase (BADH) mRNA coincides with the presence of ABA, suggesting involvement of these enzymes in stress tolerance.⁴ Several biochemical mechanisms occur in glossy sorghum lines for resistance to salinity. Seventeen glossy sorghum lines showed highly significant differences among genotypes for different seedling traits as well as contents of total chlorophyll, hydrocyanic acids and epicuticular wax.⁵ Proline accumulation appears to be a reaction to salt stress damage and not a plant response associated with salt tolerance.⁶

Adaptive mechanism

NaCl acts as a trigger for adaptation to a whole range of environmental perturbations. Adaptation to salinity is not a pre-programed response of the plants, and may be related to learning processes occurring in animals.⁷ High capacity of sorghum seedlings to recover after salt stress relief appears to be related to an adequate partition of carbon between shoots and roots and to changes in absorption, transport and re-translocation of salts.⁸ Over-expression of reactive oxygen species-scavenging enzymes e.g. glutathione-S-transferases and L-ascorbate peroxidase was in agreement in the context of stress responses in other plants. However, with respect to the physiological functions of other identified proteins such as putative sialin, putative inorganic pyrophosphatase, RNA binding protein, and serine/threonine-protein kinase.⁹

Drought

Drought is a major problem for all the cultivated crops. But the impact of the drought is less in case of semi-arid crops like sorghum, pearlmillet, and peanut, etc. Even though, severe drought causes considerable yield loss in sorghum, flowering and grain filling are highly affected by drought compared to vegetative stage. Drought affects adversely various physiological functions, leaf growth, and inflorescence development. Several mechanisms operate for drought resistance. The crop shows different mechanisms for drought resistance viz. 1) drought escape, 2) drought tolerance, and 3) drought avoidance. Drought affects the growth and productivity of sorghum.

Seedling stage

Various research inputs have been undertaken by Maiti and his team which have confirmed that glossy lines were more resistant to drought at the seedling stage compared to those in non-glossy ones^10,11; variability of glossy sorghum strains for epicuticular wax, chlorophyll and hydrocyanic acid content at the seedling stage¹² and few morpho-physiological characters such intensity glossiness and density of no-glandular trichomes in relation to shootfly tolerance (Atherigona socata Ronf) damaging shoot meristem thereby reducing productivity¹³ and to shootfly tolerance and drought resistance at the seedling stage.¹³ Variability in physiological and biochemical characteristics of glossy/non-glossy sorghums developed under different abiotic stresses at the seedling stage.¹⁴

Glossy lines showed better growth and higher water use efficiency compared to the non-glossy lines,¹² although there was variability among genotypes at the seedling stage.¹⁰ Distinct protein profile has been observed in some glossy and nonglossy sorghum lines.¹² In addition, forage productivity of glossy lines was higher.¹² Biochemical changes such as in glossy and non-glossy lines in sorghum were also noticed.¹⁵ The glossy lines have greater capacity in the uptake of phosphate than that of non-glossy sorghum lines.¹⁶ Differential protein production was observed by resistant and susceptible sorghum genotypes under different stress conditions.¹⁴

Physiological and biochemical characteristics of glossy/non-glossy sorghums developed under 3 stress factors, drought, high temperature and salinity have been undertaken by De la Rosa-Ibarra et al.¹⁴ Greater mineral acquisition by glossy than in non-glossy sorghum has been observed.¹⁷ While reviewing research advances in sorghum for resistance to drought, high and low temperature and salinity and the mechanisms of resistance,¹⁸ the role of epicuticular wax load has been reported on water-use efficiency in bloomless and sparse-bloom mutants of S. bicolor L.¹⁹ Water use efficiency (WUE) of sweet sorghum is higher than other C4 crops.²⁰

Physiological and biochemical changes in sorghum

Grain sorghum cultivated in semi-arid regions where heat and drought stress are prevalent. Leaf-rolling may alter the leaf surface microclimate so that stomata may remain open and growth continues without associated high rates of water loss.²¹ The decrease in biomass in response to water deficit was associated more with a reduction in radiation-use efficiency. Mobilization of pre-anthesis assimilates to grain occurred in sorghum and millet but not in maize.²²

Osmotic adjustment in pollen grains may be used as a measure in adaption of sorghum genotypes to drought. The change in size and shape of the pollen grains under osmotic stress was considered as a measure of osmotic adjustment (OA).²³ Osmoregulation enabled photosynthesis to continue and that a significant amount of the resultant assimilates may have been diverted to root growth.²⁴ The resistant lines showed slower shoot and root growth rates, slower soil water-extraction rates, but higher root:shoot ratios than the susceptible lines, which may be due to their higher leaf water-potentials in the stress treatment.²¹ The varieties differed in the stress thresholds at which stomatal and metabolic limitations to photosynthesis occur. At lower PEG concentrations, there was a decrease in the levels of intercellular CO₂ concentration in all varieties that could be attributed to stomatal closure.²⁵ Great diversity for physiological and yield traits such as chlorophyll content, leaf temperature, grain numbers and grain weight per panicle, harvest index and yield.²⁶ Large stem reserve storage at the onset of grain filling ascribe stable grain filling under any stress which depresses the photosynthetic source during grain filling.²⁷

In short, drought stress in C4 crop plants can be ameliorated at elevated CO₂ owing to lower stomatal conductance and sustaining intercellular CO₂. No tillage (NT) had slightly greater water use, gave consistently greater grain yields compared with more tillage (MT). Larger fraction of evapotranspiration was diverted to transpiration under NT compared with MT.²⁸

Heat stress

When extreme stress conditions developed, the hybrid's performance depended on its genetic background more than on heterosis.²⁹ Two major heat shock proteins (HSPs) of apparent mol. wt. 65 kD and 62 kD were observed in all the genotypes of sorghum tested when the incubation temperature of the 40 h seedlings was raised from 35°C to 45°C for 2 hrs.³⁰

Chilling stress

Priming of sorghum seeds in 300 g·L⁻¹ PEG for 2 d at 25°C could be used to enhance sorghum germination at low temperature, while the inclusion of plant hormones per se into priming media could be more effective than double hormone combinations.³¹

Synthesis

In the context of the literature, it may be stated emphatically that in semi-arid world, various abiotic stress factors occur together to affect crop productivity drastically. Among the various abiotic stress factors drought, salinity, heat stress and chilling reduces sorghum productivity. Significant research inputs have been directed to understand the gravity of each stress factors, its effect on crop growth, and the mechanism of tolerance with reasonable success. Techniques have been developed to screen and select the genotypes for tolerance to each stress factor, although not highly satisfactory. Genotypes with tolerance to these factors could produce sustainable yield under these stress prone areas. Finally the utilization of biotechnological tools could be helpful to develop tolerant sorghum genotypes with promising yields

Pearlmillet (Pennisetum glaucum)

Pearlmillet is well known for adaptation in the arid and semiarid regions where various abiotic stresses such as drought, salinity, high temperature etc., affect the yield potential of the crop. Environments play an important role on pearlmillet growth and adaptation. Researcheshave beenundertaken to understand the gravity of the abiotic stress related problems and their management.

Salinity stress

Pearlmillet is susceptible to salinity with drasticgermination reduction. Salt stress remarkably elevated the activities of catalase (CAT) and glutathione peroxidase (GPX) antioxidant enzymes at vegetative and reproductive stages. Salinity treatment decreased potassium uptake but application of potassium increased potassium content in leaves.³² Desai et al.³³ studied structural and functional analysis of a salt stress inducible gene encoding voltage dependent anion channel (VDAC) from pearlmillet that were differentially up-regulated following salt stress.

Differences in salinity tolerance were found among cultivars and landraces.^34,35 Genotypic variability of salinity tolerance was observed and hybrids were selected for salinity tolerance.³⁶ A novel technique was developed Maiti et al.³⁶ for screening pearlmillet genotypes for tolerance to salinity at the seedling and adult stage. Some genotypes are selected for salinity tolerance at 0.25 m NaCl (E.C. =22 dS/m).

Alleviation of salinity stress

Chilling alleviated the adverse effect of salt stress in pearlmillet in terms of fresh and dry weights of shoots and roots. Chilling also reduced Cl⁻ accumulation and, to a lesser extent, that of Na⁺, and enhanced K⁺ and Ca²⁺ accumulation in the shoots and roots under both saline and non-saline substrates. Salicylic acid (SA) could be used as a potential growth regulator to improve salt tolerance in plants.³⁷ Ashraf et al.³⁸ suggested priming seeds before planting by treating them with inorganic or organic chemicals and/or with high or low temperatures and exogenous application of organic chemicals, such as glycine betaine, proline, or plant growth regulators, or inorganic chemicals to plants under salinity stress and the selection of salt tolerant lines and its application in breeding.

Drought

Though pearlmillet is well known for adaptation to arid and semi-arid conditions but severe drought stress affects the growth and grain yield of pearlmillet at different stages in arid and semi-arid regions of the world.

Reproductive development

Drought stress during early phase can delay or completely inhibit flowering, both through an inhibition of floral induction and development. Stress during early grain development cut off the kernel sink potential by reducing the number of endosperm cells and amyloplasts. A water deficit during any stage of grain development causes the premature cessation of grain filling.

Yield components

The major factor determining grain yield of a genotype in both stress treatments was its time to flowering. Grain yield and its components were severely reduced by the terminal stress, but were little affected by the midseason drought owing to compensation by later tillers for yield lost on the earlier shoots. Maintenance of panicle number did not seem to be important for maintenance of yield under drought stress.³⁹ Selection under optimum conditions for yield components representing a resource allocation pattern favoring high yield under severe drought stress, combined with a capability to increase grain yield if assimilates are available.⁴⁰ Root shoot dry mass, survival percentage and leaf elongation rate can be considered as selection criteria for identification of drought and heat tolerant hybrids.⁴¹

Osmotic adjustment and total root length (TRL) were dependent on the degree of water deficit, and these traits were the most important factors regarding turgor maintenance and plant growth under drought conditions during the seedling stage.⁴²

Pearlmillet landraces are more tolerant to drought than the high yielding cultivars. It is necessary to study the rooting patter and root depth of landraces which may contribute to better adaptation under drought situations for deep roots.⁴³ Hybridization of landrace populations with elite composites can produce germplasm that combines drought tolerance of traditional material with high production potential of elite genetic material.

Water relations

Rapid control of leaf area by senescence is the predominant mechanism in late season drought, inducing long-term avoidance of dehydration of the upper leaves on eared shoots.⁴⁴

Drought stress induces biochemical changes in pearlmillet

The control of reactive oxygen species (ROS) and the stability of photosynthetic pigments under stress conditions are considered to contribute to drought tolerance. Drought increased activities of the antioxidant enzymes like SOD, GR and GST in pearlmillet at the ear head emergence stage.⁴⁵ Aputative drought tolerance QTL on LG 2 was identified, which may enhance drought tolerance by favoring a particular phenotype with adaptation to terminal stress.⁴⁶

Rao and Saxton⁴⁷ made analysis of soil water and water stress for pearlmillet in an Indian arid region using the SPAW Model. The relationship between WSI and pearlmillet grain yields of the Jodhpur district was Y = −45.38 WSI + 526.18 (r = −0.94).Drought tolerance isprimarily expressed in traits relating to the ability to maintain grain numbers under stress. Considerable progress in yield under stress should be possible by selection for earlier flowering and improved yield potential.⁴⁸

Heat stress

Genetic variation in seedling emergence and survival is largely due to tolerance of high temperatures rather than tolerance of soil moisture deficit, although some interaction occurred. Athermotolerance index was showed to be highly heritable.⁴⁹ Significant correlation was found between the ability of membrane thermostability to acclimate and seedling survival under heat stress in the field.⁵⁰ High temperature caused a significant increase in uptake of N, P, and K⁺ in pearlmillet, but the uptake of Ca²⁺, Mg²⁺, Na⁺ and S remained unaffected.⁵¹ High temperature imposed after imbibition the germination was reduced from 50°C to 45°C, and there was a small reduction in the rate of germination but not in G_m. Optimum time of sowing in the tropics when maximum daytime soil temperature at the depth of sowing is in the range of 45–50°C.⁵²

Synthesis

Though pearlmillet is known to be well adapted to semi-arid situations, several abiotic stresses such as drought, heat stress and salinity affects its productivity. Good research progress has been attained in understanding the gravity of the problem, its effect on physiological, biochemical and biotechnology of some of these stresses. Few techniques such as priming and other methods have been suggested for the alleviations of few of these stresses. Marker assisted selection (MAS) could facilitate in genetic improvement of the crop for stress resistance. Drought affects both vegetative and reproductive development such as the reduction of leaf area development, growth rate, delay in floral initiation, rapid grain filling leading to small seed size. Drought causes some biochemical changes in the plants affecting enzyme activities and accumulation of proline. DNA markers such as QTL have been identified related to drought resistance.

A novel strategy needs to be developed for evaluating pearlmilletcultivars for adaptation to these abiotic stresses with reasonable yield potentials. The evaluation and selection of pipe line hybrids adapted to multi-location trials could offer great potential for increasing crop productivity under abiotic stresses in the semi-arid tropic of the world.

Rice (Oryza sativa)

Rice being a very important staple crop is grown under high input conditions, but several abiotic stresses such as salinity, drought, high and cold temperature etc., affect its productivity depending on the agroclimatic and management situations.

Salinity stress

The salt-induced inhibition of plant growth is caused not only by osmotic effects on water uptake but also by variable effects on plant cell metabolism. The excess of a specific ion can cause toxicity and can induce nutritional disorders. Several studies have been under taken to understand the effect of salinity on physiological processes and tolerance mechanisms which are discussed below.

Salinity affects vegetative growth

A review has been undertaken by Maiti et al.⁵³ on research advances on seed physiology, salinity and some other factors affecting crop growth in rice. Maiti et al.¹ studied genotypic variability in salinity tolerance of rice hybrids and their parents and thereby giving opportunity to the breeders for genetic improvement for salinity tolerance. Maiti et al.⁵³ in another study undertook a comparative study on the levels of tolerance to NaCl-salinity of some crop cultivars (sorghum, pearlmillet, rice, maize, cotton and sunflower) at early emergence and germination stage. Using a novel technique Maiti et al.⁵⁴ investigated salt tolerance of 9 rice hybrids and their parents at the seedling stage.The chlorophyll a, chlorophyll b, total chlorophyll and total carotenoids contents in the stressed seedlings considerably decreased under both acidic and alkaline stresses, especially in the salt-sensitive genotype.⁵⁵ Decrease in photosynthesis in the salinized plants depended not only on a reduction of available CO₂ by stomatal closure, but also on the increasing effects of leaf water and osmotic potential, stomatal conductance, transpiration rate, relative leaf water content, and biochemical constituents such as soluble carbohydrates, photosynthetic pigments, and protein.⁵⁶

Salinity affects reproductive growth

In salt sensitive Basmati cultivars viability of pollens reduce greatly under salinity–sodicity stress, with a reduction in starch synthase activity in pollen.⁵⁷ Prebreeding efforts for salinity have resulted in adequate knowledge on mechanisms, genetics and reliable screening techniques. The net photosynthetic rate considerably decreased in all day under saline sodic conditions compared with that under non-saline sodic conditions.⁵⁸

Salinity affects some physiological functions

Effect on senescence

Besides, an acceleration of deteriorative processes affected all leaves in salt-sensitive cultivars which was more marked in oldest than in youngest leaves of salt-resistant genotypes. NaCl-induced senescence also involved specific modifications, such as an increase in basal non-variable chlorophyll fluorescence (F₀) was observed in all cultivars or a transient increase in soluble protein concentration recorded in salt-resistant genotypes only.⁵⁹

Damage by activated oxygen species

Free radical-mediated damage of membrane may play an important role in the cellular toxicity of NaCl in rice seedlings and that salt-tolerant varieties exhibit protection mechanism against increased radical production by maintaining the specific activity of antioxidant enzymes.⁶⁰

Mechanism of salinity tolerance

Salinity tolerance comes from genes that limit the rate of salt uptake from the soil and the transport of salt throughout the plant, adjust the ionic and osmotic balance of cells in roots and shoots, and regulate leaf development and the onset of senescence. Compartmentation of Na⁺ in vacuole and accumulation of high salt in root system are important parameters for salinity tolerance. Long-term experiments show changes in aquaporin expression.⁶¹

Sodium accumulation

Varieties of rice differ in their resistance to sodium chloride salinity and also there is very high variability in sodium uptake and in survival under saline conditions by the individual plants. This is in contrast with the relative uniformity in (for example) potassium uptake, dry weight and transpiration rate. In addition, a negative correlation exists between sodium (and chloride) accumulation by individual plants and their survival in saline conditions.⁶²

Brassinosteroids reduce the impact of salt stress

A difference in the response of the protection and maintenance of the structure and function of the roots of the species to a high Na concentration is observed.⁶³ Brassinosteroids also reduce the impact of salt stress on growth and considerably restores the pigment levels and also lead to an increased activity of nitrate reductase.⁶⁴

Genetic basis of salinity tolerance

Major genes and quantitative trait loci (QTLs) were identified for tolerance for salinity, submergence, P deficiency, and Al and Fe toxicitieswere mapped. Marker-assisted selection (MAS) techniques are being developed for elongation ability and tolerance of salinity, submergence, Al toxicity, P deficiency and Zn deficiency.⁶⁵

Responses to salinity in rice can be significantly enhanced through proper nutrient management, by increasing the concentrations of nutrient elements that have favorable effects such as Ca²⁺ and Mg²⁺. Calcium is particularly more successful than both magnesium and potassium, and can be applied at relatively larger quantities in salt affected soils.⁶⁶

Proteomic comparisons of salt stress-tolerant and stress-sensitive genotypes revealed numerous constitutive and stress-induced differences in root proteins. The abundance of ascorbate peroxidase was much higher in salt-tolerant cultivar than in salt-sensitive cultivar in the absence of stress.⁶⁷ Overexpression with the bacterial genes for trehalose synthesis enhanced salt tolerance in rice, there was fourfold greater dry weight after 4 weeks in 100 mM NaCl in transformed than in untransformed plants.⁶⁸

Heat stress

High heat stress affects the growth, physiology, phonological events and productivity of rice. It reduces yield by affecting several physiological processes which include photosynthesis and other metabolisms.

Heat stress affects photosynthesis

Above 35°C, there was a reduction in the CO₂ assimilation rate, and this decrease was greater in the light than in the dark. The de-epoxidation status of the xanthophyll cycle improved considerably with increasing temperature in the light. Xanthophyll cycle plays an important role in protecting PS-II against heat-induced photoinhibition by an increase in the ascorbate pool in the chloroplast.⁶⁹

Heat stress affects phenological development and yield components

The heat stress considerably reduced anther dehiscence and pollen fertility rate in sensitive genotypes, whereas, its effects were much smaller in tolerant genotypes. Number of spikelets per panicle, seed-setting rate, 1000-grain weight, and grain yield considerably decreased in both varieties, but the yield reduction in sensitive was greater than that in tolerant. Moreover, major narrowness of grain width and major enlargement of grain length/width ratio were observed in sensitive but such effect was much smaller in tolerant ones.⁷⁰

Rice genotypes exposed to high temperature, water stress and combined high temperature and water stress during flowering to quantify their response through spikelet fertility reveal that there was a relationship between spikelet fertility and the number of germinated pollen on stigmas. All 3 stress treatments resulted in spikelet sterility, high-temperature stress caused the highest sterility in all genotypes.⁷¹

Adaptation to heat stress - leaf mesophyll

Heat stress affects mesophyll structure in flag leaves. The resistant line showed tightly arranged mesophyll cells in flag leaves, fully developed vascular bundles and some closed stomata. The mesophyll cells in flag leaves of the sensitive line were severely damaged by the high temperature stress. In contrast, the mesophyll cells in flag leaves of the resistant line maintained an intact ultrastructure below the high temperature stress.⁷²

Brassinoid alleviates heat stress effects

The Brassinoid (BR) plays a vital role in protection of rice seedlings from heat stress by enhancing the activities or expression level of protective enzymes in the leaves. The materials with different heat-tolerances might differ in the mechanism of response to heat stress with BR application.⁷⁰

Cold stress

The accumulation of ice in the intercellular spaces can potentially result in the physical disruption of cells and tissues caused in part by the formation of adhesions between the intercellular ice and the cell walls and membranes. Most of the injuries result from the severe cellular dehydration that occurs with freezing.⁷¹ Freeze-induced dehydration could have a number of effects that result in cellular damage, such as the denaturation of proteins and precipitation of various molecules. However, the finest documented injury occurs at the membrane level.⁷² Detailed analyses have confirmed that freeze-induced dehydration can cause multiple forms of membrane lesions.⁷³

Low temperatures (5–10°C) induced ethanolic fermentation in the roots and shoots of the seedlings. Production of ethanol by ethanolic fermentation may lead to low-temperature adaptation in rice plants by changing the physical properties of membrane lipids.⁷⁴

Effect of cold temperature on crop growth and reproductive functions

Grain yield was most strictly reduced by low temperatures (Tw) (below 20°C) during the reproductive period, as a result of low spikelet fertility. Decreased crop growth rate (CGR) after heading was linked with reduced radiation use efficiency (RUE), although leaf area was also reduced by low Tw.⁷⁵

Effect of low temperature on microsporic development and pollen viability

There was a significant negative relationship between spikelet sterility and both the number of engorged pollen grains per anther and anther area only after imposing cold air and cold water treatment, hence, it was concluded that these flowering traits were facultative in nature.⁷⁶

Low temperatures (5–10°C) induced ethanolic fermentation in the roots and shoots of the seedlings. Grain yield was most strictly reduced by low Tw (below 20°C) during the reproductive period, as a result of low spikelet fertility. Rice plants are sensitive to low temperature during the young microspore stage.

Physiological and biochemical effects

In leaves of rice seedlings, cold stress stimulated the phosphorylation of a 60 kDa protein in the cold-sensitive rice variety. In the cold-tolerant rice variety, this protein had already been phosphorylated.⁷⁷ Rice seedlings treated with cold tolerant seed-coating agents under chilling stress maintained considerably higher root vigour, POD, CT and SOD activities, and chlorophyll content, had low MDA content and electrolyte leakage, and accumulated more soluble sugar and free proline. The cold tolerant seed coating agent improved the ability of rice seedlings in resisting to chilling stress.⁷⁸

Freezing effect on cell permittivity

The major freezing injury was believed to be due to harm to the plasma membrane. The effects of changes in the concentration of dimethylsulfoxide used as a cryoprotectant and in the cooling rate showed similar trends for the intact cell ratio and 2, 3, 5-triphenyltetrazolium chloride (TTC) viability, although there was a quantitative difference between them.⁷⁹

A gene OsRLK1, which encodes a putative leucine rich repeat type receptorlike protein kinase, was induced by cold and salt stresses.⁸⁰ Gene expression at the mRNA level of some selected proteins revealed that transcription levels are not always concomitant to the translational level. Root proteome expression and identification of some novel proteins could be useful in better understanding the molecular basis of chilling stress responses in plants.⁸¹

Drought

Drought stress is a major constraint to rice production and yield stability in many rainfed regions of Asia, Africa, and South America. Drought affects growth of the plant and causes some morphological, biochemical, physiological and molecular mechanism in rice.

Drought causes physiological and morphological changes

The cultivars differed in their natural rooting pattern, with total root length but showed no variation in capability to alter root growth during stress. Selecting for deeper roots and large root length density would assist in developing cultivars which extract more soil water and therefore are more drought resistant in upland conditions.⁸²

During the drought period, there was a decline in leaf expansion rate and leaf growth stopped completely with root-zone soil water pressure potential. Severe drought in the reproductive phase resulted in large yield reductions, mostly caused by an increase in the percentage of unfilled grains and also in grain weight.⁸³ Cultivars tolerant of mild water stress had a high relative transpiration, low initial leaf area, high carbon isotope discrimination in the leaf, and low specific leaf weight. Mild water deficit increased water use effectiveness in stressed plants, caused more degradation of starch than sugar in the leaf blade, and resulted in more accumulation of these carbohydrates in the leaf sheath.⁸⁴

Phenological development, crop growth and grain yield

Severe water deficit pending apical development until rewatering occurred, while mild water deficit reduced the rate of apical development. A small growth rate during panicle development reduced grain number and potential grain size, while cultivars which recovered quickly after water deficit had a relatively larger grain yield.⁸²

Biochemical mechanism

Manganese superoxide dismutase (MnSOD), an important antioxidant enzyme, may play in the drought tolerance of rice. Transgenic plants overexpressing MnSOD exhibited less injury, measured by net photosynthetic rate.⁸⁵ SOD is a critical component of the ROS scavenging system in plant chloroplasts.

Work on genetic engineering of osmoprotectants, such as proline and glycine betaine, into the rice plant for drought tolerance improvement is in progress.⁸⁶ Changes of H₂O₂, MDA, GSH, and ABA contents and antioxidative enzyme activities correlated considerably to drought resistance of rice hybrids.⁸⁷ Drought-tolerant NILs showed a considerably higher assimilation rate at later stages both under stress and non-stress conditions which may be due todeeper root length that provides dehydration avoidance and adaptation to drought stress.⁸⁸

Some selection criteria are suggested for drought resistance in rice

Cultivars appropriate for rainfed conditions have high yield potential resulting from high harvest index under favorable growing conditions, suitable flowering time to escape severe water stress, and capability to maintain growth during drought.⁸⁹ Comparing the QTLs associated to drought resistance, QTLs for coleoptile length (CL) and drought resistance index (DRI) were situated in the same or adjacent marker interval as those related to root traits, such as number, dry weight, depth, and length of root.⁹⁰ The broad heritability of leaf water potential (LWP) was low for direct selection in the field, but might be effective through marker-assisted selection.⁹¹

Alleviation of drought

Utilization of arbuscular mycorrhiza (AM) is an effective strategy to alleviate drought stress. The AM symbiosis improved the plant photosynthetic efficiency under, induced the accumulation of the antioxidants and reduced oxidative damage. Combined effects improved the plant performance after a drought stress period.⁹² Seed priming also accelerate the process of glucose metabolism, improved the activities of antioxidants. Indica rice showed better response to seed priming than japonica rice.⁹³

Mechanisms of tolerance

Several drought-resistance mechanisms, and assumed traits which contribute to them, have been recognized for rice; important among these being drought escape via suitable phenology, root characteristics, specific dehydration escaping and tolerance mechanisms, and drought recovery. A deep root system, with high root length density at depth is useful in extracting water thoroughly in upland conditions. There is genotypic variation in expression of green leaf retention which appears to be a useful character for prolonged droughts.⁹⁴ The variation in rice root response to drought from a physiological perspective in terms of morphology and function with respect to the different growth environments (upland and lowland) commonly used by farmers.⁹⁵

Molecular breeding for drought tolerance

Genetic improvement of grain yield under drough depends on many factors including the complexity of the target production system,the genetic resources available, selection strategies and capacity to implement cost effective breeding strategy.⁹⁶ The detection and genetic mapping of major QTLs for performance under drought stress across environments are currently given a major focus, therefore, accurate phenotyping and properly integrated in marker-assisted breeding programs will provide the development of drought-resistant genotypes.⁹⁷ Improving drought tolerance of rice by selecting any single secondary traits is not expected to be effective and the identified QTLs for grain yield and related morph-physiological traits should be carefully confirmed before to be used by marker assisted selection.⁹⁸ From phenotypic and genotypic analysis, drought escape could be indirect as the main mechanism for drought tolerance. QTL detection with low inputs and could also create useful materials for breeding with wide genetic diversity for drought tolerance.⁹⁹

Excess water stress

Rice can tolerate prolonged soil flooding or complete submergence thanks to an array of adaptive mechanisms. These include an ability to elongate submerged shoot organs at faster than normal rates and to develop aerenchyma, allowing the efficient internal transport of oxygen from the re-emerged elongated shoot to submerged parts. Anoxic rice germination is mediated through coleoptile rather than root emergence. Although there is still much to learn about the biochemical and molecular basis of anaerobic rice germination, the ability of rice to maintain an active fermentative metabolism is certainly crucial.

Flooding causes physiological and biochemical changes in crops

Greater elongation growth during submergence has pronounced adverse effects on rice plant survival during submergence, and that higher initial carbohydrate allow rice plants to sustain submergence stress and regenerate quickly after desubmergence.¹⁰⁰ Aldehyde production rate was negatively correlated with plant survival, suggesting that plants utilizing excess energy reserve during submergence stress suffered the most, resulting in poor survival. The low root-to-shoot ratio and poor root growth in the surface layer in aerobic culture are attributable to the significant reduction in adventitious root number. Vigorous root growth helps in soil water uptake and in maintaining transpiration in aerobic rice.¹⁰¹

The accumulation of starch was considerably lower in plants under submergence. Photosynthetic rate was most affected below submergence in varying days of post-flowering and was also correlated to the down regulation of Ribulose bisphosphate carboxylase activity. Loss of yield under submergence is attributed both by lower sink size or sink capacity as well as subdued carbohydrate metabolism in plants and its subsequent partitioning into the grains.¹⁰²

Flooding affects crop yield

Water depth after transplanting and initial vigour of plants (height and dry weight) appeared to be the major factors in determining successful crop establishment and yield.¹⁰³ Transplanting injury, indicated by the rate of leaf emergence at 0–4 d after transplanting, was observed when the plant age of a transplanted seedling exceeded 2.6 in leaf number.¹⁰⁴

Adaptive mechanisms

Deep water rice is adapted to survive conditions of severe flooding over extended periods of time. Cyclin expression was induced at all nodes with a similar time course, suggesting that ethylene acts systemically and that root primordia respond to ethylene at an early developmental stage.¹⁰⁵ Dry matter accumulation in panicles of deepwater riceismore in the late stage of ripening.¹⁰⁶

Enzyme mechanism in flooding tolerant and sensitive line

The levels of pyruvate decarboxylase (PDC2) subunit appeared higher in shoots of tolerant relative to sensitive during stress and recovery. Alcohol dehydrogenase (ADH1) gene transcript levels were relatively higher in tolerant rice at stress and recovery stages. Transcriptional and post-transcriptional controls play a part in the response of contrasting rice types to O₂ deprivation stress.¹⁰⁷

Role of ethylene on submergence response

Ethylene coordinates the balance of GA and ABA contents, which facilitates GA-promoted elongation of shoots during submergence. Besides cell elongation, the interaction of ethylene with GA and ABA also regulates adventitious root formation, but these developmental processes are modulated by separate regulatory networks of the 3 hormones.¹⁰⁸

Synthesis

The productivity of rice is affected by several abiotic stress factors such as drought, salinity, flooding, heat stress, cold, heavy metal, and UV radiation stresses etc.

Salinity affects vegetative, and reproductive growth, pollen viability, and several physiological functions. Some physiological, biochemical and molecular mechanisms operate for tolerance to salinity. Seedling emergence percentage and root elongation with increasing salinity are considered as selection criteria for tolerance to salinity. With respect to salinity significant progress has been achieved with respect to its effect on physiological. Sarkar et al.¹⁰⁹ developed a semi-hydroponic technique in evaluating rice cultivars for tolerance to salinity. This simple technique is being utilized in mass scale screening of rice cvs for salinity tolerance.

Drought causes several morphological, physiological, growth and molecular changes in rice showing adaptive mechanism. In conclusion, drought affects rice productivity in upland rice. Several drought resistant mechanisms operate in rice being drought escape via appropriate phenology, root characteristics, specific dehydration avoidance and tolerance mechanisms, and drought recovery. A deep root system, with high root length density at depth is useful in extracting water thoroughly in upland conditions, but does not appear to offer much scope for improving drought resistance in rainfed lowland rice. Selecting for deeper roots and large root length density could assist in developing cultivars which extract more soil water and therefore are more drought resistant in upland conditions. In addition, flooding affect rice growth and production and causes several physiological and biochemical-carbohydrate metabolisms in rice. Few biochemical mechanisms operate in rice under submergence among which the emission of methane, ethylene and other biochemical traits are important.

Heat stress affects drastically the photosynthetic activities, it is suggested that the xanthophyll cycle plays an important role in protecting PSII against heat-induced photoinhibition by an increase in the ascorbate pool in the chloroplast. Heat stress affects phenological development and yield components. The heat stress considerably reduced anther dehiscence and pollen fertility rate in sensitive, whereas, its effects were much smaller in tolerant. Number of spikelet per panicle, seed-setting rate, 1000-grain weight, and grain yield considerably decreased. Brassinoid (BR) plays a vital role in protection of rice seedlings from heat stress by enhancing the activities or expression level of protective enzymes in the leaves. The resistant line showed tightly arranged mesophyll cells in flag leaves, fully developed vascular bundles and some closed stomata while the sensitive ones the disintegrated mitochondrial membranes and mesophyll cells are highly disorganized, with accumulation of osmophilic granules. Besides, these abiotic stresses several other stresses affect rice growth and productivity such as heavy metal stresses which include Arsenic, Cadmium, cupper etc.

Maize (Zea mays)

The productivity of maizeis affected greatly by several abiotic stresses such as drought, excessive moisture (waterlogging), low temperature, and nitrogen stress. Therefore, there is a great necessity to screen and select maize for tolerance to these stress factors. Sufficient research activities are directed in these directions with reasonable success.¹¹⁰ In general, tolerance to abiotic stress is associated with morphological and physiological traits, which include root morphology and depth, plant architecture and variation in cuticle thickness, osmotic adjustment and antioxidant capacity, stomatal regulation, hormonal regulation, desiccation tolerance (membrane and protein stability), maintenance of photosynthesis, and the timing of events during reproduction.^86,111-114

Salinity stress

Salinity affects seedling growth

During early seedling growth, salinity and soil texture affected the development of the seedlings that showed symptoms of water stress in the form of lower leaf water potential, stomatal conductance, and evapotranspiration. The higher the salinity, the lower the leaf area and the dry matter production is observed. Leaf, stem, and root showed an almost similar growth reduction due to salinity.¹¹⁵ Various studies have been undertaken on genotypic variability in salinity and other stress factors.^116-118 A review has been made on cold, drought and salinity tolerance and its mechanisms of resistance in maize.¹¹⁷ Rodriguez and Maiti undertook a review on perspectives of salinity tolerance of some crops including maize.¹¹⁹ High emergence (%), profuse root system, high seedling vigour was considered for salinity tolerance in maize. Radial reflection coefficient of root played a very significant role in regulating the assimilation of NaCl in maize seedlings under salt stress.¹²⁰ Highly significant differences were observed among genotypes for different variables both for salinity and drought stress. Higher root growth under saline stress of some genotypes might be due a mechanism of resistance in maintaining osmo-regulation.¹²¹ Subsequently several research inputs have been documented by Maiti and his associates in various crops for tolerance to salinity showing genotypic variabilty.^122-125

Salinity causes several biochemical changes

Maize when exposed to salinity cause several changes occur in plants, such as ionic imbalance, disruption of cellular homeostasis, inhibition of several metabolic enzyme such as phosynthetic enzymes etc (enzyme toxicity). In response to these several defense mechanisms activate which include increase in the activity of antioxidative enzymes and lipid peroxidation. The increase in enzyme activities was more distinct in the salt-tolerant than in the salt-sensitive genotype.In roots of the salt-sensitive genotype, salinity reduced the activity of APX, GPX and SOD enzymes. CAT and GPX enzymes showhigh H₂O₂ scavenger activity in both leaves and roots.¹²⁶ O₂ generating activity negatively affected by NaCl was compatible with that of plasma membrane NADPH oxidase.¹²⁷ The production of profuse lateral roots near soil surface in response to salinity play an important role in osmotic adjustment under salt stress.³⁶ Salt- and mannitol-induced osmotic stresses in terms of xylem pressure change was seen when the transpiration rate of the plant was not significantly changed.¹²⁰

Alleviation of salt stress

Pre-treatment with 1 μM H₂O₂ to the hydroponic solution for 2 d induced an increase in salt tolerance during following exposure to salt stress. Differences in the antioxidative enzyme activities may, at least in part, explain the amplified tolerance of acclimated plants to salt stress, and that H₂O₂ metabolism is involved as signal in the processes of maize salt acclimation.¹²⁸

Molecular mechanism

Maize inbred line with high Na⁺ exclusion at the root surface and the level of xylem parenchyma.¹²⁹ The adding of calcium postponed the depolarization, and decreased the degree of depolarization caused by NaCl. High NaCl concentration leads to depolarization of maize root cell membrane, which can partly be counteracted by calcium.¹³⁰

Drought

Average loss due to drought alone can reach up to 60% in severe drought affected regions.¹³¹ In case of severe stress the stress adaptive genes present in the germplasm was expressed and helped in saving productivity of germplasm up to some extent.¹³²

In the past, tremendous progress has been made in the area of genetic enhancement for improved stable maize productivity under water limited environments, particularly through trait-based selection and improvement.^133,134 More recently new tools of biotechnology are coming forward to further accelerate the grain in selection and improvement for improving tolerance to water deficit conditions.^135,136

Effects of drought stress

Yield reduction through loss of leaf surface area during early vegetative stage

Dry climate during vegetative (4 weeks after planting) period will reduce plant and leaf size. Impact on yield will be based on the reduction in leaf area available for photosynthesis. Several maize lines described as drought resistant or drought tolerant operated with higher leaf transpiration efficiency (TE) than less drought-adapted lines. Significant genotypic variation in leaf TE exists in maize, and that TE could be improved without reducing photosynthesis.¹³⁷ In all maize cultivars, chlorophyll and carotenoid contents were considerably reduced under drought, but a recovery was observed following rewatering.

Yield loss through disruption of grain filling

Drought after silking stage up to maturity affects kernel weight. Severe drought can reduce corn yields during this period by 20 to 30%. Drought immediately following silking has impact, and can reduce yield significantly. Drought later in this period is less damaging. Drought during grain filling stage because of reduced photosynthesis accelerated leaf senescence.¹³⁸

Mineral distribution

Drought stress influences the mineral distribution in plant parts. Fresh weight content reduced by drought and salinity in spite of leaf number and caused a similar reduction. Water utilization may be reduced by extending the watering intervals, the alternate watering, or drying. Total nutrient uptake, the K and N, and shoot biomass production are improved by alternate drying and rewatering in the vertical soil profile.¹³⁹ Reduction in leaf growth under drought and saline conditions may be due to other causes rather than the limitation of nutrients in a short-term period of drought and salt stresses.¹⁴⁰

Biochemical changes

Alternate partial root zone irrigation (APRI) had small effect on the leaf relative water content, chlorophyll, proline and MDA contents and SOD and POX activities from jointing to tasselling stages. Anthocyanin and proline contents improved in all cultivars.¹⁴¹ APRI can make plants use water and nutrients more efficiently with better drought tolerance.¹⁴² Somedrought responsive proteins constantly increase during water deficit, while others stabilize after a first increase. Quantitative variation of some proteins may be attributed to ABA accumulation.¹⁴³ Under water stress, the actions of CAT, SOD and POX in leaves and roots improved sharply but declined toward the physiological maturity.^144,145 In roots and leaves, the content of soluble proteins decreased with increasing drought stress.

Strategies for crop improvement under drought stress

Cultural practices such as time and method of sowing, plant density, and reduced tillage has been found to be beneficial in minimizing drought stress. Date of sowing is crucial for maize production in water deficit areas. Early planting reduces risk of terminal drought at grain filling stage. Cultivars having robust root system, faster rate of root growth and deeper root penetration is high efficient in utilizing soil water. Several management practices have been exploited for directing the precipitation toward utilization by crop, such as tillage, water harvesting and mulching etc. Adoption of cropping practices to promote AM may increase drought stress tolerance.¹⁴⁶

Genetic enhancement for drought tolerance

During water stress ABA accumulation enhances survival but reduces productivity,¹⁴⁷ ability to protect cellular membranes and enzymes from stress, capture water from dry soils through deeper root exploitation or osmotic adjustment.¹⁴⁸ Statistical methods such as G x E interaction, genetic correlation among environments, and stability analysis etc, can use improve the process of establishing the reliability of prediction on the performance of genotypes in real target environment.¹⁴⁹ Selection on the basis of yield through multilocation testing under optimal input conditions will perform better under low input environment by virtue of their high yield potential. Byrne et al.¹⁵⁰ concluded that evaluating the genotypes under managed drought stress rather than those which occur randomly during multilocation testing is relatively much effective and efficient in selecting maize germplasm for water deficit tolerance. Use of secondary traits, in addition to grain yield, as selection criteria for tolerance to different abiotic stresses have been suggested.¹⁵¹ Edmeades et al.,¹⁵² suggested that an ideal secondary trait should be genetically associated with grain yield, highly heritable, stable within the measurement period, cheap and fast to measure, and a reliable estimator of yield potential before final harvest. Past experiences indicate that the important traits under drought are anthesis-silking interval (ASI), increased ears per plant under stress, stay green, and to a lesser extent of leaf rolling under drought.¹³⁷ Reduced tassel size in tropical maize is directly associated with the increase in ear growth at flowering and in harvest index.¹⁵³ Yield improvements under drought were paralled by increase in ear number and harvest index, while ASI declines.¹⁵⁴ The open pollinated verities having local adaptation showed 35% superiority over commercial hybrids under moderate to severe drought.¹³⁷

Molecular approach

During the past 2 decades, molecular tools have advanced the identification, mapping and isolation of genes in a wide range of crop species. DNA markers have enabled the identification of genes in the genomic regions associated with the expression of several qualitative and quantitative traits. The characterization of the gene(s) corresponding to identified QTL can be achieved and the candidate gene approach appears to be an attractive option. Drought tolerance QTLs identified thus far are likely to have limited utility for applied breeding because of their dependency on genetic background or their sensitivity to the environment.¹⁵⁵ However, a number of drought-induced genes were isolated, implicatingthat broad spectrum of biological pathways, mainly in signaling and regulatory pathways are linked to stress tolerance.¹⁵⁶

Screening for drought tolerance

Technique has been utilized for large scale screening for drought resistance in maize. Normalized difference vegetation index (NDVI), other selection criteria including leaf senescence, chlorophyll content, anthesis-silking interval, root capacitance, final grain yield, and grain yield components. The most stable trait under drought stress was kernel weight. Temperate lines with a wide adaptability can be used in drought resistance breeding for both temperate and tropical environments.¹⁵⁷

Breeding for drought tolerance

CIMMYT initiated a product-oriented breeding program targeted at improving maize for the drought-prone mid-altitudes of southern Africa. Maize varieties in Zimbabwe were selected using simultaneous selection in 3 types of environments; (i) recommended agronomic management/high rainfall conditions, (ii) low N stress, and (iii) managed drought.

Drought stress leads to reduction of leaf area, stomatal closure, and reduction in assimilate supply, reduced silk growth, reduction in growth. Similar to other cereal crops the following mechanisms operate for drought resistance in maize.

Drought escape: Drought tolerant lines give early flowering and grain filling to avoid drought escape.

Drought tolerance: Drought tolerant lines give yield under drought owing to some biochemical mechanisms.

Dehydration avoidance: Dehydration avoidance is the ability of the plant retains a relatively higher level of hydration under drought stress. Apart from physiological, biochemical mechanisms this is achieved by reducing the stomatal number and size.

Anatomical screening techniques

Efficient techniques have been developed for screening maize genotypes for tolerance to excessive soil moisture. Seedling survival, seedling vigour and more number of lateral roots and production of aerenchyma for supply of oxygen are considered as selection criteria for tolerance to excessive moisture.¹¹⁰ Low partial pressure stimulates biosynthesis of ethylene by increasing ACC (1-aminocyclopropane-1- carboxylic acid) synthase activity, thus further enriching the stem with ethylene.¹⁵⁸

Excessive moisture stress

Morphological adaptations include nodal root development and changes in root geotropism. Nodal roots were initiated from both below and above ground nodes whith in 1–2 d of exposer to excessive moisture stress.¹⁵⁹ The brace roots originated from above ground surface do not suffer anoxia due to they enter surface soilthat contain air spaces. Excess soil moisture stress severely affected various growth and biochemical parameters, impaired anthesis and silking, and eventually resulted in poor kernel development and yield. However, remarkable variability was found among the genotypes studied.¹⁶⁰

During excessive moisture stressroots including newly developed adventitious roots develop aerenchyma as a response to stress. Aerenchyma provides a diffusion path of low resistance for the transport of O₂ from aerial parts to newly developed brace roots present under severe anoxic condition¹⁶¹ and provides a path for diffusion of volatile compounds such as ethylene, methane, CO₂, ethanol, and acetaldehyde.¹⁶² Excess moisture and flooding affects seedling emergence and growth and productivity of maize. Technique has been described by Zaidi et al.¹¹⁰ for screening maize cvs for tolerance to flooding. Oxidative stress may play an important role in waterlogging-stressed maize plants and that the greater protection of waterlogging-tolerant leaves and roots from waterlogging-induced oxidative damage results.¹⁶³ Waterlogging tolerance coefficient of shoot dry weight can be used for practical screening as a suitable index.¹⁶⁴

Cold stress

Maize can survive when exposed to adverse temperatures as low as 0°C and as high as 44°C; but growth starts slowing down when the temperature dips below 4.9°C. Optimum temperature for growth varies between day and night as well as entire growing season. Prolonged exposure to low temperature during the vegetative phase results in reduction in plant height, yellowing of the leaf, chlorosis, and leaf tip firing due to the death of the leaf tissue. Cold stress occurring at reproduction phase severely affects flowering and results in reduced tassel size/branches, delayed anthesis, pollen grain death, reduced silk size, and in certain cases no seed setting.

The degree of injury depends on the temperature and the duration of exposure. Wilting and discolouration of the leaves and the reduced growth are the most common symptoms. At the temperature in the range of 5–15°C and particularly around 10°C, imbibed and germinating seeds are killed by soil fungi.¹⁶⁵ Prolonged exposure to temperature of 10°C and lower, steadily decreased the rate of leaf expansion, damaged leaves, and killed shoot apical meristem.¹⁶⁶

Management of cold stress

In general single cross hybrids were tolerant to stress conditions. Among the 3 types of maturity groups, early maturing hybrids were affected due to cold for height as their vegetative phase was completed during prolonged cold situation and medium maturity single cross hybrids escaped prolonged cold stress. Under cold condition, full season cultivars are suggested for general cultivation. Field emergence of maize under early planting was improved by recurrent selection at the rate of 84%.¹⁶⁷

Chilling affect various physiological processes in maize

Genotypic variability existed for most growth parameters and was greatest for net assimilation rate (NAR). Inbred lines under field conditions in spring were influenced mainly by temperature. NAR was correlated best with temperature.¹⁶⁸ The movement of ¹⁴C -assimilates from the source cell to the phloem loading zone was inhibited more strongly in the dent-type line than in the flint-type inbred line, when the temperature was decreased from 24 to 14°C. It is assumed that the contribution of the symplastic route of phloem loading, being particularly susceptible to low temperature.¹⁶⁹ Increase in antioxidant enzyme activities (SOD; APX; GR) under chilling stress was observed in maize genotypes.¹⁷⁰ Early stages of shoot morphogenesis control the duration of the vegetative phase in cool regions, since the delay in growth at a low temperature cannot be compensated for during later growth at a higher temperature.¹⁷¹ In the inbreds the activity of SOD did not much change with the increasing length of chilling period but decreased in the hybrids, the GR activity increased higher in the inbred lines.

Mechanism of tolerance

Experiments revealed that chilling tolerant maize genotypes ABA faster and in higher amounts than chilling sensitive genotypes when exposed to chilling stress. Chilling tolerance in maize is related to the ability to accumulate ABA as a protective agent against chilling injury.¹⁷²

Maize yield is restricted by its cold-sensitivity, the molecular mechanisms of which are poorly understood. Few photosynthesis-related genes were reticent by chilling; photosynthetic apparatus is a major target of cold. Induction of chloroplast-related genes not directly engaged in photosynthesis was evident at later stages of the response.¹⁷³ ABA pre-treatment improved chilling tolerance plants, mainly by decreasing leaf conductance and by increasing root water flow. At the leaf level, they found a relationship between ABA content and chilling tolerance in both maize genotypes.¹⁷⁰

Synthesis

Water stress affect maize growth such as leaf area index, soil-water extraction and biomass accumulation during vegetative growth, partitioning of dry matter, pollen availability to the silk, grain development, and root growth. It also affects productivity which includes grain filling, gain maturity and final yield. Water stress affects several physiological-biochemicals and molecular changes in maize such as reduction in photosynthesis; dry matter accumulation; increase in proline accumulation in shoot. ABA in roots, change in mineral distribution; affect enzyme activities; molecular changes involves drought induced genes, DNA library. Increase in ABA in roots and proline under drought contribute to drought resistance in maize.

Genotypic variability for drought has been observed in number of studies. Yield stability over multi-locations is considered as criterion for drought resistance. Deep root under drought is adaptation to drought. The application of arbuscular mycorrhizae (AVM) contributes to drought alleviation. Progeny selection within the multi environment testing of conventional breeding programs can profoundly affect allele frequency in breeding populations and the stress tolerance of elite commercial products. Selection for reduced anthesis-silking interval under carefully managed moisture stress imposed at flowering provides an effective and rapid route to higher and more stable grain yield in lowland tropical maize

In view of the literature, it may be concluded that concerted research activities are concentrated on various aspects, strategy for management and excellent progress in molecular biology, identification of markers, QTL, genes for drought resistance. The breeders and the growers should clear concepts the problems of drought at different growth stages and its effects on crop productivity. Besides, efficient screening techniques need to be developed for drought resistance and its utilization in breeding program. Rooting pattern and depth contribute to adaptation to drought and salinity.

Salinity affects seedling and vegetative growth. Genotypic variability in salinity tolerance exists. Efficient technique has been developed by the author in mass screening of maize cultivars for drought resistance. Salinity causes some biochemical changes such as production of antioxidant enzymes, ascorbate peroxidase, other oxidases, induction of reactive oxygen species (ROS), increase in enzyme activity in salt tolerant genotypes caused changes in molecular level. Application of H₂O₂ alleviates salt stress.

High temperature promoted vegetative growth, biomass production. Reduced photosynthesis culm, leaf weight reduced, grain yield. Reduced cob expansion, pollen viability, pollen availability, affect floret differentiation, reduced the number of florets, flowering period extended, reduced kernel number, increased kernel abortion, finally reduced grain yield.

Cold stress affects maize growth, reduced vegetative growth, yellowing of leaf chlorosis, leaf tip firing, reduces leaf expansion, kill shoot meristem and chilling affect photosynthesis. Lower leaf area, specific leaf area, Leaf area, reduces phloem loading, decreases enzyme activity, severely affect flowering, and reduces pollen viability. Accumulation of ABA, photosynthetic related genes contribute to chilling tolerance in maize.

Wheat (Triticum aestivum)

Salinity stress

Salinity affects morphological-physiological, biochemical and molecular changes in wheat plant.A extensive review has been undertaken by Maiti et al.¹⁷⁴ on salinity tolerance in wheat. On the other hand, genotypic variability was found among wheat varieties for salinity and osmotic stress.¹⁷⁴

Growth

Soil salinity stress affects seed vigour. Increased level of NaCl reduced the germination percentage, the growth parameters (fresh and dry weight), potassium, calcium, phosphorous and insoluble sugar contents in both the shoots and roots of 15 day old seedlings and resulted in development of burning symptoms.¹⁷⁵ After 120 d of seed sowing, plant biomass production was decreased by 49 and 65%, in response to 5 and 10 dSm⁻¹ salinity levels. The adverse effect of salinity was seen on plant weight.¹⁷⁶

The root and shoot dry weights reduce under saline conditions.^177-179 Salinity also reduces the nutrient and water uptake as well as result in reduced plant growth under salinity stress.^180,181

Fraction of seed reserve decreased with increasing drought and salinity. Appropriate efforts such as plant breeding programmes should be focused on improvement of seed reserve mobilization in order to obtained increased seedling growth under drought and salinity stress.¹⁸¹

Physiological and biochemical changes

Salt osmotic stress affects shoot growth and water status in wheat.¹⁸² Whenwheat plants are grown in aerated solutions, the rates of leaf production exceeded rates of leaf senescence at all salt concentrations. Induced accumulation of free and bound-ABA and amino acid content in harvested grains are observed.¹⁸³ Salinity increases the diffusive resistance but decreases the transpiration rate. Accumulation of Mg²⁺, Ca²⁺ and Na⁺ intensified, while that of K⁺ decreased in the salt treated plants.¹⁸⁴

Photosynthesis

Salinity induces only a small decrease in the actual PSII efficiency at midday steady-state photosynthesis, indicating that the photosynthetic electron transport is little affected by salinity. The P-N rate might allow a good discrimination between tolerant and non-tolerant cultivars.^185,186 The concentrated Na⁺ within a leaf under salinity treatments may decrease the stability of photosystem II functions and lead to photochemical inactivation.^187-189 The net photosynthetic rate (P-N), stomatal conductance^190,191 g(s) and transpiration rate (E) were reduced with the addition of NaCl.¹⁹¹ Net photosynthetic rate, stomatal conductance, pigment contents, ions contents, leaf area index, leaf area duration, leaf relative water content and dry matter accumulation of spikes decreased in both cultivars with increasing saline concentrations. Higher salinity tolerance cultivars of winter wheat could relieve senescence at the reproductive stage.¹⁹²

Biochemical changes

Wheat shoot elongation is sensitive to any diminution of water potential. Though the alleviation of toxicity by Ca²⁺ is weak solute concentrations <250 mhos, it alleviated Na⁺ and K⁺ toxicity to roots by at least 3 separate mechanisms. K⁺ was found to be more toxic to roots than Na⁺, Na⁺ was more toxic to shoots.¹⁹³ A gradual depression of photosynthesis occurs due to stomatal closure under salinity stress.¹⁸⁸ Salinity tolerant plants could produce more ATP than salt-sensitive plants.¹⁹⁴

The initial biochemical reaction to salinity at cellular level in wheat is an unspecific response and not a specific adaptation to salinity.¹⁹⁵ Salinity stress decreased relative water content, chlorophyll , carotenoids, membrane stability index, biomass and grain yield, and increased H₂O₂, thiobarbituric acid reactive substances, proline, glycine-betaine, soluble sugars, SOD, CAT and GR activities.¹⁹⁶ The most sensitive indicator of salinity stress is stomatal conductance followed by CO₂ assimilation.¹⁹⁷ The increase in salinity also causeshigh reduction in nitrate reductase activity.¹⁸³ Ascending salinity level reduced K⁺and Ca²⁺ accumulation in shoots, while the concentration of Na⁺ was increased.¹⁹⁸ The low Na⁺ genotypes show much longer chlorophyll retention than the high Na⁺ genotypes, the start of leaf senescence being prolonged by a weak or more in the low Na⁺ genotypes. The low Na⁺ genotype showed a greater yield due to enhanced grain number and grain weight in the tiller ears.¹⁹⁹ Higher salinity decreased Zn concentrations in the wheat shoots.²⁰⁰ Increase in concentrations oftoxic ions was greater at high pH than at low pH. High pH depressed the uptake of potassium and enhanced sodium, magnesium and chloride uptake particularly at 100 mol m⁻³ NaCl salinity.²⁰¹ Two varieties of durum wheat (Triticum turgidum L. subsp. durum) are known to differ in salt tolerance and Na⁺ accumulation. Genetic studies indicate that these genotypes differed at 2 major loci controlling leaf blade Na⁺ accumulation.²⁰² High salinity and sodicity, rather than B, exert the major effects on water extraction of wheat from the deep subsoil, thus negating the effect of crop B-tolerance were multiplied constraints exist.²⁰³

High presence of Mg²⁺ in saline seawater affects the plant nutritional requirements considerably. The uptake of Ca⁺² is restricted due to the competition for the enhanced uptake of ions like P and Mn.²⁰⁴ Of the 2 ions, Na⁺ and Cl⁻, the latter appears to be more damaging and results in plant dieback and reduced grain yields.²⁰⁵ The effect of sea water stress on mitochondrial ATP synthesis varies in relation to the substratrate oxidised and stress increase under severe stress, which was more pronounced for Cl⁻.²⁰⁶ NaCl increased the activities of SOD and POX.^207,208 At cellular level the wheat varieties impart salt tolerance by manipulating nutrient accumulation in leaves along with overproduction in leaves along with overproduction of antioxidative enzyme activities (SOD and POX).²⁰⁷ High accumulation of Glycine betaine (GB) mainly contributes to osmotic adjustment, which is one of the factors for improving photosynthetic capacity under salt stress.²⁰⁹

Yield

The increasing salinity of irrigation water caused significant adverse effects on yield attributes and yield of wheat.^210,211 Domestic line of wheat could be grown at a salinity concentration between 4 and 8 g/l with least reduction in biological and grain yield.²¹²

Mechanisms for salinity tolerance

In some cultivars, salinity did not reduce the grain K/Na ratio and ion selectivity, but its filling period and harvest index were reduced.²¹³ Some important proteins for salt tolerance were found to be upregulated in tolerant genotype under salt stress, such as H⁺-ATPases, glutathione S-transferase, ferritin and triosephosphate isomerise.²¹⁴ Mannitol was found to be an effective stress inducing agent.²¹⁵ Interaction between Na⁺ and Cl⁻ ions also contribute to salinity tolerance. Salt stress at seedling stage induces a decrease in relative growth rates, K concentration and leaf osmotic potential values, as well as increase in Na, proline valuesand soluble sugar content.²¹⁶ The salt-tolerant cultivar accumulated more K⁺ in both shoots and roots compared with the higher Na⁺ accumulation typical for the salt-sensitive cultivar. Optimal K⁺/Na⁺ ratio of the nutrient solution should be 16:100 for both the salt-tolerant and the salt-sensitive cultivar.¹⁹² Nemoto and Sasakum²¹⁷ investigated differential stress responses of early salt-stress responding genes respond to different inducers. Two genes, WESR1 and WESR2 were induced by both osmotic stress and exogenous ABA treatment. WESR3 responded to exogenous ABA, but not to osmotic stress. Another gene WESR4 did not show significant response to either osmotic stress or exogenous ABA treatment. They proposed that wheat has both ABA-dependent and ABA-independent salt stress signal transduction pathways.

Alleviation

Ardekani et al.²¹⁸ reported alleviation of salinity stress on wheat yield, yield components, and nutrient uptake using arbuscular mycorrhizal (AM) fungi under field conditions. AM species significantly increased the growth and nutrient uptake of the mutated line compared with the other genotypes.

Drought

Drought is a world-spread serious problem affecting grain production and quality of wheat. Water stress has affected significantly root traits and piliferous layer cell size and this impact depends on its intensity. Severe water stress markedly reduced root traits.²¹⁹

Drought affects the growth and the crop productivity of wheat

Decrease in photosynthesis soon after drought stress was imposed resulted mainly from reduced stomatal conductance and reduced mesophyll photosynthetic activity. The drought-resistance of a cultivar are lower sensitivity of CO₂ exchange rate, net CO₂ uptake to water loss ratio, stomatal resistance, relative water content and greater osmotic adjustment under stress.²²⁰ The grain yield under drought showed highly significant cultivar differences, which appeared consistent between years. As a group, tall bread wheat would out yield dwarf wheat only under very severe drought.²²¹

Biochemical changes

Drought causes several physiological, and biochemical changes in wheat crop which include photosynthesis, starch and protein. Different wheat genotypes have different physiological mechanisms to adapt themselves to changing drought stress, as observed from gene expression profiles.²²² Grains in proximal position in the spikelets produce about 80% of the spike grain yield, the proportion being slightly higher in stressed plants.²²³ Photosystem II level showed a significant decrease under severe water stress in susceptible plants.²²⁴ Under water stress amylose content, lipid content and pasting temperature reduces. The effect of water stress on accumulation of different dimethyl formamide-soluble and insoluble proteins was significant and variety dependent.²²⁵ Xyloglucans in the side chains of rhamno-galacturonan I and II of drought-tolerant cultivar significantly increased during water stress, suggesting role of thesemolecules during water stress response.²²⁶

Mechanism of resistance to drought

Water use efficiency

The rainfed treatment showed the lowest seasonal evapotranspiration (ET), biomass, and grain yield, harvest index and WUE as a result of moderate to severe water stress from jointing to grain filling. Deficit irrigation between jointing and anthesis significantly increased wheat yield and WUE through increasing both current photosynthesis and the remobilization of pre-anthesis carbon reserves.²²⁷

Relation between leaf water use efficiency and physico-chemical traits

High leaf WUE in wheat under the rained could be obtained by selecting breeding materials with high photosynthesis rate, low transpiration rate and stomatal conductance.²²⁸ Osmotic adjustment regulation is the main component for physiological machinery of wheat drought resistance which are associated with greater soluble sugar content, higher K⁺ content and greater proline content.²²⁹

Proline

Water stress induces proline accumulation in wheat. Proline content had close relationship with soil water stress threshold and wheat anti-drought.²³⁰ Stress-induced synthesis of proline conferred tolerance to water deficit in transgenic wheat. The tolerance to water deficit observed in transgenic plants was mainly due to protection mechanisms against oxidative stress and not caused by osmotic adjustment.²³¹

Alleviation of photoinhibition of drought

Glycinebetaine (GB) may protect the PSII complex from damage through accelerating D1 protein turnover and maintaining anti-oxidative enzyme activities at higher level to alleviate photodamage. Diethyldithiocarbamate as well as streptomycin treatment can impair the protective effect of GB on PSII.²³²

Abiotic stress gene

On the basis of the functions of methyltransferase and the SAM-binding motif, the SAM-binding motif of gene W89 was assumed to be connected with other proteins or transcription factors to transduce stress signals and finally regulate the expression of stress-responsive genes on the early stage of drought stress.²³³

Waterlogging

Waterlogging is not a major problem of wheat cultivation since the winter season receives less rainfall, but in some regions where winter rains are heavy, waterlogging may cause serious damage to plant. Waterlogging before anthesis was found to effectively enhance tolerance to waterlogging event after anthesis, by increasing net photosynthetsis, stomatal conductance and transpiration and enhancing use-efficiency of absorbed light energy. It also increased activities of SOD, APX and CT and enhanced dry matter accumulation after anthesis. Hardening by waterlogging applied before anthesis can effectively improve the tolerance of wheat to waterlogging events occurring during the generative growth stage.²³⁴

Heat stress

Heat stress affects growth and productivity of wheat, particularly at grain developmental phases. The impact of a 3-day heat shock on biomass yield was less than the effects of the pre- and post-treatment growing temperature.²³⁵ High resistance to photo-oxidative damage of the flag leaves may be the physiological basis for its high yield. Parents can be selected for improved biochemical and physiological traits and crossed to high yielding agronomically elite materials.²³⁶ HSPs synthesized to cope upwith the heat stress in different organisms are known to provide protection and repair the cellular damage caused by heat. Incorporation of terminal heat tolerance into high-yielding cultivars would require additional carbon assimilates and N inputs.²³⁷

High temperature affects anthesis and grain development

Grain yield was negatively related to thermal time accumulated above a base temperature of 31°C. Thus, fertilization and grain set was most sensitive to the maximum temperature at mid-anthesis.²³⁸ Heat stress induced ethylene production in developing wheat grains leading to kernel abortion and increased maturation in a susceptible cultivar.²³⁹ Assessments of all physiological traits showed that such genotypes can be high yielding even if under heat stressed conditions, and can be used as a gene pool in breeding programs for tolerance to heat stress.²⁴⁰

Alleviation

Salicylic acid pre-treatment could significantly alleviate damages of heat and high light stress on D1 protein and PSII of wheat leaves, and accelerate restoration of photosynthetic function.²⁴¹ Wang et al.²⁴² reported plants with pre-anthesis high-temperature acclimation showed much higher photosynthetic rates than those without pre-anthesis high-temperature acclimation.

Cold stress

Zhang and Wang²⁴³ discussed research status and future of low temperature stress on wheat. Wheat genotypes with slightly low canopy temperature are superior to conventional wheat materials in some important biological characters and particularly prominently in metabolic function and cellular structure, they still retain their superiority in some of their important biological characters and therefore have a wide range of ecological adaptability; slightly low canopy temperatures of these genotypes are closely correlated with low temperatures of their second heat sources and their vigorous plants. Thus, low temperature wheat genotypes are of great research importance and have great prospects.

Cold temperature on male sterility

Cold temperature affects yield potentials of wheat. Phenotypic characters associated with yield adaptation of wheat to a range of temperature conditions. The improvement of grain filling rate and grain weight per ear would appear to lead to increased grain yield of lines in all classes of adaptation, and wide adaptation to a range of temperature conditions.²⁴⁴

Cold stresses during the reproductive development of spring wheat cause grain-set failure in the high altitudes (>1500 m) of the world. The period from around heading until anthesis is critically sensitive to cold temperature. Mean temperatures during the period between headings to anthesis were more critical than minimum temperature as a cause of grain-set failure in cold-susceptible cultivars.²⁴⁵ Low temperature-induced accumulation of protein is sustained both in root meristems and in callus in Winter wheat but not in spring wheat.²⁴⁶ Cold temperatures and boron deficiency caused grain set failure in spring wheat. Genotypic variability was found for boron and cold temperature stress.²⁴⁷ Frosts killed spikelets, restricted internode extension (stem growth) and reduced yield. Frosts in April and May, after growth stage 33, appeared responsible for the damage symptoms observed in the crop.²⁴⁸

Mechanism for cold tolerance

Few studies have been undertaken on the mechanisms of cold tolerance in wheat.The accumulation of ice in the intercellular spaces can potentially result in the physical disruption of cells and tissues caused in part by the formation of adhesions between the intercellular ice and the cell walls and membranes (Levitt, 1980). Freeze-induced dehydration could have a number of effects that result in cellular damage, such as the denaturation of proteins and precipitation of various molecules. However, the best documented injury occurs at the membrane level.⁷²

Mechanisms that could potentially contribute to freezing tolerance would include helping to prevent or reverse freeze induced denaturation of proteins, preventing molecules from precipitating, and lessening direct physical damage caused by the accumulation of intercellular ice. Cold acclimation involves the stabilization of membranes against freeze-induced damage.⁷² Increase in membrane-freezing tolerance that occurs with cold acclimation involves changes in membrane lipid composition.⁷³

Synthesis

Although wheat being one of the most important staple crop in the world, special care is taken by the farmers for conducive growth of the crop, but several abiotic stress factors affect the growth and productivity of the cop. Among the abiotic stress factors are drought, salinity, heat and cold stress. Concerted research activities have been directed to understand the gravity of each stress and its effects on morphological, physiological and biochemical changes of the crop. Few techniques on molecular biology have been adopted which are very costly and beyond the reach of underdeveloped and developing countries. Several morpho-physiological and biochemical mechanism operate in wheat for drought resistance among which root depth, osmo-regulation, water use efficiency, proline acuumulation are important. Foliar application of glycine betaine contributes to the alleviation of drought.

Heat stress affects greatly the photosynthesis, anthesis, grain development leading to grain abortion. Heat stress induces the generation of heat shock protein. Application of salicylic acid alleviates heat stress besides pre-anthesis high-temperature acclimation alleviates damage to the flag leaf caused by post-anthesis heat stress in wheat.

Cold temperature affects greatly the growth and productivity of wheat. Great emphasis has been given on various aspects of cold tolerance in wheat. Cold temperature affects photosynthesis, pollen viability and anthesis; causing male sterility andreducing grain development and yield potential of wheat. Few studies have been undertaken on physiological, biochemical and molecular mechanisms of cold tolerance.

Increased salinity reduces seedling vigour, growth rate, shoot and root dry weight and finally yield. Salinity stress affect several physiological-biochemical activities such as reduction of transpiration, photosynthesis, increase in accumulation of Na⁺, Cl⁻ ion, depressed uptake of potassium, higher accumulation of proline etc.Several biochemical and molecular mechanisms operate for resistance to salinity such as exclusion of Na⁺ and Cl⁻ ions, accumulation of ATP, proline, ABA etc.

Perspectives of adaptation of cereal crops to abiotic stresses

Cereal crops play vital role as staple crops of the world, but the productivity of these crops are affected and reduced severely by several abiotic stress factors under low input situations. In this connection, it needs to be mentioned that 1/5th of worlds’ arable lands are affected by arid and semiarid situations and 2/3rd of it is by salinity. There is a great necessity to increase productivity in these problem areas.

In the light of world literature, the synthesis of each stress of individual crop mentioned above, it is imperative to mention that significant research inputs have been directed to understand the gravity of the problems and their solution; the effect of each stress factors on growth and the productivity of each cereal crops. Each stress factor such as drought and salinity causes several morpo-physiological and biochemical-molecular changes for adaptation to these stresses. Good progress has been attained in understanding the mechanisms of resistance to some of these stresses, but insignificant progress has been achieved to adopt strategy for improving the productivity under stress-prone areas.

Conclusions

Plant depends on its environment (abiotic factors) for water, CO₂, O₂ and minerals for several metabolic processes which ultimately influence the growth and productivity of a crop. With increasing population, there is an increasing demand for food and other commodities but the productivity of these crops is endangered with ever increasing global warming, heat stress, drought, salinity and other abiotic stresses, thereby threatening food security. Under this situation, the selection of stress resistance crop cultivars is considered as a feasible alternative to sustain the productivity of the crops in these regions.

Sufficient research inputs have been undertaken to understand the gravity of each stress factors and the effects of several abiotic stresses such as drought and salinity on several morpho-physiological traits are reviewed for abiotic stress factors. Morpho-physiological, biochemical and molecular mechanism are also discussed for each abiotic stress factor. Significant research inputs have been directed on its effect on crop growth, and the mechanism of tolerance with reasonable success. Techniques have been developed to screen and select the genotypes for tolerance to each stress factor, although not highly satisfactory. Genotypes with tolerance to these factors could produce sustainable yield under these stress prone areas.

The evaluation and selection of pipe line hybrids adapted to multi-location trials could offer great potential for increasing crop productivity under abiotic stresses in the semi-arid tropic of the world. Early vegetative stages are highly susceptible to flooding stress so proper drainage system should be maintained in the field. Several techniques have been suggested for the alleviation of few abiotic stresses such as application of brassinoids (BR), salicylic acid (SA) and application of H₂O₂ for salt or drought stress or heat stress. Few techniques have been suggested for alleviation of salinity such as inoculation of AMF, application of growth regulator, potassium nitrate etc. Techniques such as priming and other methods have also been suggested for the alleviations of some of these stresses. Mass scale screening techniques need to adopt for quick and reliable selection of tolerant lines. Marker assisted selection could facilitate in genetic improvement of the crop for stress resistance. DNA markers linked to QTLs related to drought resistance have been identified and used for MAS. Finally, the utilization of biotechnological tools could be helpful to develop tolerant sorghum genotypes with promising yields.

Green revolution could achieve the goal of increased productivity. It introduced the use of hybrid seeds, mechanical implements, synthetic fertilizers, pesticides and insecticides under irrigated conditions to achieve this. It revolutionized the cultivation practices through construction of irrigation projects and dams, digging of bore wells, thermal power projects, industries and seed companies. This was accepted as National development and employment opportunities for the youth. However, the damages caused due to the developmental activities are being experienced through pollution of air, water and soil, greenhouse gases and global warming with a cascading impact on the bioresources, biodiversity challenging the survival of the mankind. The abiotic stresses have become more complex, and new strategies are required to combat the stress factors for increased food production.

Summary statement

Understanding the physiological basis of crop production under sustainable agriculture holds key to the future success of developing abiotic stress tolerant crop plants. A good knowledge of botany in detecting crop responses and adaptation under stress prone areas will benefit the cereal abiotic stress research community.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.