Physiological parameters of salt tolerance during germination and seedling growth of Sorghum bicolor cultivars of the same subtropical origin

Abstract

Salt-tolerant ecotypes (or cultivars, varieties, etc.) of different plant species have been long known to evolve in nature. In the past few years, plant breeders have made significant achievements regarding salt tolerance in a number of potential crops using artificial selection. The aim of this work was to evaluate and screening of the natural sea water (Red sea) tolerance of 7 Saudi local (Baish, Jazan; 17.388086, 42.524070) cultivars of sorghum (Sorghumbicolor L., Moench; Poaceae) with respect to the performance of some physiological parameters such as germination, shoot and root development which could be recommended to local farmers and plant breeders. The shoot growth of the studied sorghum cultivars were significantly affected by the exposure to sea water. Root growth was different among cultivars even when treated with normal water. The cultivar C3 (mix white and red seeds) was observed as more salt tolerant and cultivar C4 (whitish seeds) was more salt sensitive on the basis of the germination-ability and shoot development. Cultivar C3 was also observed to produce better seeds compared with the other cultivars. Results of this experiment can be useful to the local sorghum growing farmers or as a genetic resource for the development of sorghum cultivars with improved germination under salt stress.

1. Introduction

Management measures to overcome the salinity related problems are expensive and until now, did not provide permanent solutions. In contrast, biotic approaches for overcoming salinity related problems have been gaining attention within the past few decades. Salt-tolerant ecotypes (or cultivars, varieties etc.) of different plant species have been long known to evolve in nature (Ashraf and Wu, 1994). In the past few years, plant breeders have made significant achievements regarding salt tolerance in a number of potential crops using artificial selection and conventional breeding approaches (Ashraf, 2004). For example, screening of seven salinity tolerant and ten salinity sensitive sorghum genotypes was reported (Krishnamurthy et al., 2007). In general, sorghum is known as moderately tolerant to salinity (Igartua et al., 1995). Approach like combination of molecular, physiological, biochemical and metabolic aspects of salt tolerance is essential to overcome the effects of salinity and develop salt-tolerant plant varieties (Roychoudhury and Chakraborty, 2013). Salt tolerance is usually assessed as the percent biomass production in saline versus control conditions over a prolonged period of time or in terms of survival (Munns, 2002). However, parameters related with plant growth (such as germination, leaf and root growth) at the seedling stage may provide a less time consuming, easy and inexpensive natural way to select salt tolerant cultivars to be used for cultivation or breeding programs. Farmers themselves may conduct such experiments without the requirements of expensive apparatus. Screening of such locally grown cultivars for salt tolerance can strengthen the breeding programs by identifying genotypes with high salt tolerance and yield potential. The tolerant genotypes could be recommended for cultivation in moderately salt affected areas (Roy et al., 2014). Previously, experiments on the selection of salt tolerance cultivars of sorghum were conducted using artificial salt medium (Azhar and McNeilly, 1987; El-Naim et al., 2012; Rani et al., 2012). Recent studies from Saudi Arabia on the selection of sorghum genotypes under saline water irrigation did not include any of the Saudi local cultivars (Hefny et al., 2013; Metwali, 2013). Hence, the aim of this work was evaluation and screening of the relative natural sea water (Red sea) tolerance of 7 Saudi local cultivars of sorghum with respect to the performance of some physiological parameters such as germination, shoot and root development which could be recommended to local farmers and plant breeders.

2. Materials and methods

2.1. Plant material

The specimens (panicles of different cultivars of Sorghum bicolor; Fig. 1) were collected from the sorghum growing area in Saudi Arabia (Baish, Jazan; 17°23′17.1′′N, 42°31′26.6′′E).

Figure 1.

Panicles of the S. bicolor (commonly known as Sorghum/Milo/Dohra Rafi’ah/Jowari) cultivars (C1-C7) of Baish, Jizan, Saudi Arabia.

2.2. Plant growth and salinity

Natural sea-water was used in this study for testing salt tolerance of the sorghum cultivars. Red sea water was collected from Jeddah, Saudi Arabia. Germination percentage, fresh shoot, root length and weight of the studied cultivars were investigated in this study.

In brief, Petri-plates were lined with 1 sterilized 90 mm of filter paper (Hermann Paulsen, Germany) and moistened with 2.0 ml of different dilutions of Red sea water (1.6%, 3.1%, 6.3%, 12.5%, 25.0%, 50.0% and 100%). Control sets were moistened with an equal amount of autoclaved milliQ water (0% of sea-water). Seeds of S. bicolor were immersed in 1% of HgCl₂ solution for about 60 s in a 50 ml tube (Corn bring, Japan) then repeatedly washed and dried overnight at room temperature. Successively, 5 seeds of each cultivar were placed on the filter paper in the petri plate and moistened with 2.0 ml of sea-water. The petri plates were then sealed with parafilm (Pechiney Plastic Packaging Company, USA) to avoid losing moisture. The Petri plates were arranged on the laboratory table at room temperature (±25 °C; ±12 h of normal light). To screen the comparative sea water tolerant cultivars, seedling were collected after 12 days of moistening the petri plates. The sampling was done for all 5 seeds/cultivars for all sea-water concentrations. Data were recorded on the fresh length and weight of shoot and root of each cultivar. Data (shoot and root growth of different cultivars for all the sea water treatments) were analyzed using a one way ANOVA and post hoc comparison (Tukey’s test) was conducted for each cultivar.

3. Results and discussion

Simple classification of sorghum (S. bicolor, L., Moench) on the basis of their mature heads and spikelets include 5 basic races (bicolor, guinea, caudatum, kafir and durra). Sorghum-hybrids were further classified into 15 races. However subraces, commonly used for cultivation (feterita, kaura, sorgo, sudangrass etc.) do not require a formal Latin name (Harlan and De-Wet, 1972). Sorghum can be grouped on genetic relatedness and phenotype-based racial classification. Genetic groups might differ from racial groups in several ways (Brown et al., 2011). Phenotypic variations among panicles of studied sorghum cultivars (C1-C7) are illustrated in Fig. 1. The studied sorghum cultivars had panicles consisting of deep to light red and white seeds. Cultivars C1, C2 and C3 had deep to light red seeds. Cultivars C4 and C6 had more whitish with a few reddish seeds. Cultivars C5 and C7 had combinations of white and reddish seeds. Previous study (Bafeel, 2014) with the same 7 cultivars of sorghum showed two major groups on the basis of RAPD (Random Amplified Polymorphic DNA) fingerprinting pattern. Five cultivars (C2, C3, C4, C5 and C6) belong to the same group, although their panicles varied in size, shape and color. Two cultivars (C1 and C7) has more genetic similarities compared with the other 5 cultivars. Previously 571 cultivars of sorghum were recognized, however they were grouped into a single species. Plant breeders can take the advantage of this diversity and can manipulate the genetic make-up of this group to acquire the best crop (Ramph and Reynolds, 2005).

Individual weight of the sorghum seed cultivars used in this study varied significantly (F, 16.4; P, 0.000). High rate and short duration of sorghum grain development were generally associated with larger seed size (Tuinstra et al., 1997). Cultivars C3 and C6 produced better seeds (34 ± 2 mg) compared with the other 5 cultivars (29 ± 2 mg) used in this study (Fig. 2).

Figure 2.

Individual seed weight of the studied sorghum cultivars; F, 16.4; P, 0.000). Different letters represent significantly different (Tukey test; P ⩽ 0.01).

Germination was observed after 3 days for all treatments (0–50% of sea water). All the seeds of the 7 cultivars germinated when treated with normal water (Fig. 3). In contrast, none of the seeds of the 7 cultivars germinated when treated with 100% of sea water (data are not shown). All the seeds of the cultivars C1, C3 and C5 germinated when treated with sea water. However, some seeds of the cultivars C2, C4, C6 and C7 failed to germinate (77–97% germination) when treated with sea water. Cultivar C4 was more affected (76.7% germination) by sea water compared with the others (Fig. 3).

Figure 3.

Germination (%) of different sorghum cultivars treated with normal and sea water. C1-C7, cultivars; NW, normal water; SW, sea water.

Overall shoot-length for all the cultivars did not vary when treated with normal water. Sorghum cultivars were treated with different dilutions of natural Red sea water (1.6–50%). Impact of overall sea water on individual cultivars was measured. In contrast with the normal water treatment, shoot-length varied for different cultivars (F, 2.7; P, 0.02) when treated with sea-water (Fig. 4). The highest shoot growth was observed for the cultivar C3 (8.4 ± 3.4 cm) and the lowest for C4 (5.0 ± 4.7 cm) when treated with sea-water. Average shoot-length was 6.9 ± 3.7 cm for the other 5 cultivars. Shoot growth for cultivars did not differ significantly when treated with comparatively lower concentrations of sea-water (1.6%, 3.1%, 6.3% and 12.5%). However cultivars responded differently and developed different lengths of shoot when treated with 25% (F, 3.7; P, 0.008) and 50% (F, 2.9; P, 0.02) of sea-water. In general, the sorghum plant tolerance to salinity is related with reduced shoot Na⁺ concentration (Krishnamurthy et al., 2007). Average shoot length for all the cultivars under normal irrigated water was 9.0 ± 2.2 cm. However, average shoot length for all the sea water treated cultivars was observed as 6.8 ± 3.8 cm (24.4% reduction). Similar to the shoot-length, cultivar’s fresh shoot-weight did not vary (F, 2.0; P, 0.101) for the normal water. Shoot-weight for different cultivars varied (F, 2.8; P, 0.01) when treated with different dilutions of sea water (1.6–50%) (Fig. 5). Significant effect of sea water on different cultivars was not observed until 12.5% of sea water. However it varied significantly when treated with 25% (F, 3.1; P, 0.02) and 50% (F, 3.3; P, 0.01) of sea-water. Shoot-weight was observed as the highest for the cultivar C3 (52.3 ± 23.6 mg) and the lowest for the cultivar C4 (30.0 ± 29.0 mg) while the other 5 cultivar’s average shoot weight was observed as 38.1 ± 20.8 mg.

Figure 4.

Shoot-length of different sorghum cultivars treated with normal and sea water. C1–C7, cultivars. Bar, standard error of the mean (SE). Normal water F, 0.80; P, 0.58. Sea water F, 2.7; P, 0.02.

Figure 5.

Shoot-weight of different local sorghum cultivars treated with normal and sea water. C1–C7, cultivars. Bar, SE. Normal water F, 2.0; P, 0.1. Sea water F, 2.8; P, 0.01.

Unlike shoot-length, root-length for different cultivars varied (F, 2.7; P, 0.03) even when normal water was used (Fig. 6). Average root-length was 14.0 ± 3.3 cm when treated with normal water. The highest root-length was noted for the cultivar C6 (18.2 ± 2.7 cm) and the lowest for the cultivar C4 (11.3 ± 3.5 cm). Root length was also different for different cultivars when treated with sea-water (F, 3.8; P, 0.001). Average root length was (9.6 ± 5.5 cm) for all the cultivars when treated with sea water. Cultivar C4 (5.6 ± 6.0 cm) was affected significantly by sea water compared with the rest of the 6 cultivars. The highest root growth was observed for C5 and C6 (11.2 ± 5.7 cm). Unlike shoot length and weight, root length of different cultivars was different at lower levels of sea water (1.6%) (F, 3.6; P, 0.01). However, it is interesting that no variation in root length (F, 2.0; P, 0.10) was observed when treated with 25% of sea water. Average fresh root-weight for all the cultivars was 22.8 ± 4.6 mg. Similar with the root length, root-weight for different cultivars varied (F, 11.3; P, 0.00) even when normal water was used (Fig. 7). Overall impact of sea water was significant on the root weight of different cultivars (F, 4.7; P, 0.00). Root weight of different cultivars was more or less similar except C4 (6.1 ± 6.6 mg) (Fig. 6). Cultivars root weight did not vary at sea water of 3.1% (F, 2.0; P, 0.10), 12.5% (F, 1.6; P, 0.20) and 25% (F, 1.9; P, 0.12). However, it varied when treated with 1.6% (F, 4.6; P, 0.002), 6.3% (F, 5.0; P, 0.00) and 50% (F, 5.1; P, 0.00) of sea water.

Figure 6.

Root-length of different local sorghum cultivars treated with normal and sea water. C1–C7, cultivars. Bar, SE. Normal water F, 2.7; P, 0.03. Sea water F, 3.78; P, 0.001.

Figure 7.

Root-weight of different local sorghum cultivars treated with normal and sea water. C1–C7, cultivars. Bar, SE. Normal water F, 11.29; P, 0.00. Sea water F, 4.71; P, 0.00.

These findings corroborated with the previous report that salt stress significantly decreases root, shoot length and weight of sorghum cultivars. Shoot growth of sorghum cultivar is more adversely affected compared to the root growth by salt stress. Sorghum shows a substantial intra-specific variation in salinity tolerance (Rani et al., 2012). Until now genetic make-up that distinguishes some plants or varieties from others with respect to stress tolerance/resistance is not clearly understood. Salt tolerance plant taxa is described over numerous genera (Greenway and Munns, 1980). Plants are traditionally classified as halophytes or glycophytes based on their capacity to grow in a high salt medium. In general, mechanisms for salt tolerance are of two main types – (i) those minimizing the entry of salt into the plant and (ii) those minimizing the concentration of salt in the cytoplasm (Roychoudhury and Chakraborty, 2013). Plants survive in saline conditions due to osmotic adjustment which involves intracellular compartmentation and partitioning of toxic ions from the cytoplasm through energy-dependent transport into the vacuole (Hasegawa et al., 2000). Shoot growth rate regulates varietal difference in the sense that a lower shoot growth is related with proportionately higher concentration of NaCl in leaves. Other possible causes of varietal differences include apoplastic ion transport properties across root, water-use efficiency, cellular compartmentation and tissue tolerance. Entry of Na⁺ into the shoot is controlled by Ca⁺ by altering the apoplastic transpirational bypass flow. This mechanism actually operates in both salt-sensitive and salt-tolerant cultivars, but with different levels of efficiencies (Roychoudhury and Chakraborty, 2013). The major differences in Na⁺ transport between the genotypes of durum wheat (salt tolerant vs. salt sensitive Triticum turgidum subsp. durum) are (i) the rate of transfer from the root to the shoot (xylem loading), which is much lower in the salt-tolerant genotype and (ii) the capacity of the leaf sheath to extract and set apart Na⁺ as it enters the leaf. The genotypes of durum wheat were not found to differ significantly in unidirectional root uptake of Na⁺ (Davenport et al., 2005). Varietal difference in many plant species may also exist in the rate of accumulation of Na⁺ and Cl⁻ in the roots and leaves. In general, the maintenance of low cytosolic Na⁺ concentration and K⁺/Na⁺ homeostasis is so far known as an important aspect of salinity tolerance and the salt-tolerant lines were reported to have higher K⁺/Na⁺ levels (Chattopadhyay et al., 2002). The leaves and roots of the salt-tolerant genotype usually have lower internal Na⁺ and Cl⁻ concentration compared with the salt-sensitive genotype (Roychoudhury et al., 2008). Biochemical basis of varietal difference that is related with salt tolerance includes compatible solutes, polyamines, antioxidants and phytohormones (for example abscisic acid; Roychoudhury and Chakraborty, 2013). Molecular basis of varietal difference that is related with salt tolerance is supposed to be due to the constitutive overexpression of many stress-inducible genes that function in stress tolerance (Taji et al., 2004). Nonetheless, Molecular data showed that salt stress is more or less related to the ability to keep apart Na⁺ in sub-cellular compartments and/or maintain K⁺ homeostasis (Ligaba and Katsuhara, 2010). Differences in the expression of the transcript for the HKT (High-affinity K⁺ Transporter) gene (homologous to the wheat K⁺/Na⁺ symporter HKT1) in varieties was suggested as one of the factors that may distinguish salt stress-sensitive and stress tolerant lines and is correlated with a component of ion homeostasis (Golldack et al., 2002). HKT gene sequences demonstrated distinct lineages for monocots and eudicots. However, this gene was observed to be variable among rice varieties indicating environment specific adaptation (Bafeel, 2013). Salinity tolerance is unlikely to be determined by a single gene or gene product (Cheeseman, 1988). Molecular basis of salinity tolerance is more complex and probably results in the expression of many salt tolerance genes depending on their interactions and the external salt concentration (Shannon, 1984). The plant response to salinity includes increasing salt concentration around the roots to a threshold level, the rate of shoot growth or the rate of expansion of growing leaves declines dramatically as observed in this study. The growth of roots is enhanced at the cost of shoot length and the roots probe deeper into the soil for search of more water, since water deficit accompanies ionic toxicity. Growth retardation occurs as the photosynthetic capacity fails to provide carbohydrate resources for the newly emerging leaves (Roychoudhury and Chakraborty, 2013).

Nonetheless, plant breeders have successfully improved salinity tolerance of some crops through conventional selection and breeding techniques. There is a consensus among researchers that selection is more convenient and practicable if the plant species under test possesses distinctive indicators of salt tolerance (Roychoudhury and Chakraborty, 2013). Previously three sorghum cultivars out of thirteen were classified as salt tolerant (Rani et al., 2012). This study showed, cultivar C3 as the most salt tolerant and C4 as the most salt sensitive.

4. Conclusions

The shoot growth of the studied sorghum cultivars were significantly affected by exposure to sea water. Root growth was different among cultivars even when treated with normal water. The cultivar C3 (mix white and red seeds) was observed as more salt tolerant and cultivar C4 (whitish seeds) was more salt sensitive on the basis of shoot development. Cultivars C3 were also observed to produce better seeds compared with the other studied local cultivars. Results of this experiment can be useful to the local farmers or as a genetic resource for the development of sorghum cultivars with improved germination under salt stress.