Movements of sunflower and shade avoidance (Viewpoint)
One of the best-known plant movements is the phototropic solar tracking in sunflower (Helianthus annuus). Kutschera and Briggs (pp. 1–8) examine reports from the literature together with observations from 10-week-old sunflower plants made on rainy, cloudy and clear days in California and find that movements occur only under open-sky conditions, and they are confined to the upper third of the stem and cease at anthesis, leaving the sunflower heads facing east. Based on photon-fluence and photosynthesis measurements on the upper leaves, and published data, they conclude that, in populations of competing sunflower plants, the shade avoidance response, which may optimize CO2-assimilation, appears to be the ‘driving force’ behind these conspicuous plant movements.
From genes to networks: genetic control of root growth (Invited Review)
Roots are able to coordinate their growth with changing environments by adjusting root development. Slovak et al. (pp. 9–24) describe the root system and review key studies that elucidated basic principles and pathways underlying the regulation of its growth and development. They further illustrate how systems biology has helped to address the complexity of these processes. Finally, they argue for pursuing systems genetics approaches which merge genetics, genomics, systems biology and phenomics to move beyond the limitations of past and current approaches.
Long-tongued hawkmoths form a pollination niche
Unrelated organisms that share similar niches often exhibit patterns of convergent evolution in functional traits. Johnson and Raguso (pp. 25–36) study long-tongued hawkmoths and flowers with very long (>8 cm) tubes for both native and invasive species in Africa and determine that the hawkmoth Agrius convolvuli, and to a much lesser extent the related species Coelonia fulvinotata, comprise a distinct long-tongued pollinator niche that is occupied by a large guild of plant species. From the patterns of convergent evolution among guild members it seems likely that white floral colouration and copious amounts of nectar are important for establishing mutualisms with long-tongued hawkmoths. The study highlights the value of a niche perspective for understanding the geographical context and functional significance of floral traits.
Phylogeography and reproduction modes in Limonium spp.
The genus Limonium Mill. (Plumbaginaceae) has long been recognized to have sexual and apomictic (asexual seed formation) modes of reproduction. Rois et al. (pp. 37–50) examine correlations between phylogeographical patterns and modes of reproduction in diploid and tetraploid Limonium spp. They study haplotype sharing within and among species with distinct ploidy levels using cpDNA haplotype distribution, ploidy groups, species groups and geographic origin. Reproductive biology analyses showed that diploid species mainly form sexual embryo sacs while tetraploid species have only apomictic embryo sacs. Altogether, the findings provide evidence for a pattern of ‘geographical parthenogenesis’ of coastal diploid and tetraploid Limonium spp.
Tiller production and survival in wheat
Tillering, or branching, is a key determinant of plant architecture and grain yield in wheat. Xie et al. (pp. 51–66) identify the genetic basis of tiller production and survival through a detailed quantitative trait locus (QTL) analysis in a bread wheat × spelt mapping population. In addition to genetic control, a low red:far red ratio (R:FR) is associated with early tillering cessation, few total shoots, high infertile shoot number and shoot abortion. They also determine the trade-offs between fertile shoot number and other yield components. These findings provide a deeper insight into the genetic and environmental determination of tillering dynamics and, in turn, yield potential in wheat.
Disturbance, density-dependent processes and genetic relatedness
The impact of biotic interactions on the spatial distribution of genotypes within populations is not well known. Fajardo et al. (pp. 67–77) examined the role of competition and facilitation on the degree of conspecific genetic relatedness of nearby individuals of tree populations of Nothofagus pumilio following large-scale fires in Patagonia (Chile). They found that trees located in the interior of second-growth forests had significantly lower relatedness suggesting a fading of the recolonization structure by competition, whereas trees located in the edge of the same second-growth forests showed a positive and highly significant relatedness among trees, resulting from facilitation.
Attack and defence in the Brassica–Sclerotinia pathosystem
Host resistance is urgently needed to control stem rot of oilseed brassicas caused by the fungus Sclerotinia sclerotiorum. Uloth et al. (pp. 79–95) show that S. sclerotiorum invades the vascular system of highly susceptible Brassica genotypes, which ensures lesions extend rapidly along the stem. In contrast, resistant Brassica genotypes restrict growth of the fungus by a combination of strategies that impede pathogen progress towards stem vascular tissues, including the hypersensitive reaction, more cell layers in the cortex, and rapid lignification within the cortex, endodermis and surrounding lesions. These complex pathogen–host interactions explain variable expressions of resistance observed in the field.
Sporogenesis and offspring ploidy levels in hybrid Dryopteris ferns
Reproduction of ferns with potentially mixed (apomictic and sexual) system remains largely unknown. Ekrt and Koutecký (pp. 97–106) use flow cytometry to assess genome sizes in offspring of pentaploid Dryopteris × critica, a hybrid of triploid apomictic D. borreri and tetraploid sexual D. filix-mas. The hybrid under study is partly fertile and shows unstable sporogenesis. Formation of aborted spores, well-developed reduced spores and unreduced diplospores was recorded in a single sporangium. The well-developed spores germinated in viable pentaploids (from unreduced spores) and half of that (∼ 2.5x; from reduced spores). They conclude that both sexual reduced and apomictic unreduced spores can be produced by a single sporangium and formed viable F2 generation.
Geographic distribution of the two cytotypes of Alnus glutinosa in Europe
In many plant groups, two or more cytotypes represent separate biological entities with distinct distribution, history and ecology. Mandák et al. (pp. 107–120) analysed ploidy level variation in populations of Alnus glutinosa over the whole Europe. In addition to diploid populations they for the first time report the occurrence of tetraploid populations of A. glutinosa. The distribution of tetraploids is far from random, forming two, geographically very well delimited populations. The first one is situated in the Iberian Peninsula extending to North Africa. The second one occupies the area of the Dinaric Alps extending to south-western Greece.
Multiple origins of dwarf shrubs on the African high mountains
Dwarf shrubs often dominate alpine and arctic environments, including those above the treeline on tropical high mountains. Gehrke et al. (pp. 121–131) show that the dwarf shrub life form evolved at least twice under alpine-like conditions in African Alchemilla (lady’s mantles). The independent evolution of dwarf shrubs in Alchemilla, that they date to as recent as the Pleistocene, highlights a selective advantage. However, despite ecological success and alpine speciation it did not trigger increased rates of diversification. They also infer reversals from (partial) woodiness back to entirely herbaceous forms, a transition rarely reported in angiosperms.
Water stress acclimation in xeric but not riparian Eucalyptus
Drought induces a decline in photosynthetic capacity but plants can acclimate to low soil moisture content, and could differ from short-term experimental results. Zhou et al. (pp. 133–144) compared Eucalyptus taxa and tested whether plants respond by modifying the functional relationships between photosynthetic traits and water stress, and whether species of contrasting habitat differ in their degree of acclimation. Under prolonged partial drought, E. occidentalis from a xeric habitat modifies the relationships and it is able to continue active transpiration and photosynthesis down to a much lower soil water potential (−3.9 MPa) relative to riverside Eucalyptus taxa.
Differential pollen placement increases pollination efficiency
Sympatric plant species that share pollinators potentially face interspecific pollen transfer and reduced reproductive success. Stewart and Dudash (pp. 145–152) examined pollen transfer by a common paleotropical nectar bat, Eonycteris spelaea, using different combinations of conspecific and heterospecific flowers in a flight cage. The four bat-pollinated flower species placed pollen on different areas of the bat’s body, and bats transferred significantly more pollen between conspecific than heterospecific flowers. Thus, in the Old World tropics, differential pollen placement is a mechanism that reduces competition for pollination among night-blooming plant species sharing a common pollinator.
Embryonic moss sporophytes can be hardened to desiccation tolerance
Embryos of the moss Aloina bifrons are inducibly desiccation tolerant (DT), requiring a slow-dry period to confer tolerance to air-dryness. Hardening to DT describes a condition of temporary tolerance to a rapid-dry event (<30 min from full turgor to air-dryness) that is conferred by a prior slow-dry event. Brinda et al. (pp. 153–163) found for the first time that moss embryos can indeed be hardened to DT, by first slowly drying embryos and equilibrating them at 50 % relative humidity, then rehydrating the embryos and exposing them to a rapid dry event at intervals of 24 h. The degree of hardening to DT is dependent upon the prior rate of slow-drying. Finally, a new metric for assessing DT is presented as the minimum rate of drying in order for shoots or embryos to recover undamaged upon rehydration.
Ecophysiology of recalcitrant seed persistence
Several dominant tree species of temperate forests produce desiccation-sensitive seeds, a functional trait that is likely to influence regeneration under climate change. Joët et al. (pp. 165–176) use the Mediterranean Holm oak (Quercus ilex) as a model system and monitor seed water status and viability during the unfavourable winter season in two years with contrasting rainfall. They find that in situ desiccation is the main abiotic cause of mortality in winter, and seed dehydration rates can be satisfactorily estimated using integrative climate proxies including vapour pressure deficit and potential evapotranspiration. Structural equation modelling of microhabitat factors highlights the major influence on seed desiccation of canopy and hence incident radiation on the ground.
Elevated CO2 reduces nitrate inhibition of N2 fixation in Pisum
Additional carbohydrate supply resulting from enhanced photosynthesis under predicted future elevated CO2 is likely to increase symbiotic nitrogen fixation in legumes. Butterly et al. (pp. 177–185) use free-air CO2 enrichment (FACE) to study field pea, Pisum sativum, growing under different levels of CO2 and N supply in a semi-arid cropping system and find that increasing N reduces nodulation at ambient CO2 but this inhibitory effect is less under elevated CO2. The results indicate that field pea may perform well in semi-arid agricultural systems under future CO2 concentrations irrespective of soil N status and subsequent gains in N input via enhanced N2 fixation will be important for maintaining the N fertility of cropping systems.
Demographic consequences of genetic variation for a plant invasion
Genetic variation may play an important role in plant population fitness and invasion success. Li et al. (pp. 187–194) combine genetic analysis and demographic data to investigate the consequences of genetic variation for the population fitness of a perennial, invasive herb (Lupinus polyphyllus). They find that genetic variation correlates positively with the average seedling establishment in the field, but not with the fitness components of later life stages, or the long-term population growth rate (λ). The study suggests that genetic variation may facilitate plant invasions by increasing seedling establishment, but it may not necessarily affect invasion spreading after establishment.
Aridity-related stomatal differences in a South African shrub
Plants in arid environments often have predictably different phenotypes than those in moist environments. Within species, such trait–environment associations could indicate local adaptation, yet supporting evidence is rare. Carlson et al. (pp. 195–207) show both broad-scale stomatal variation and its evolutionary mechanism by combining data from a common garden with that from two wild populations. In the garden, stomatal density was higher in plants sourced from hotter, drier sites. In the wild, plants with denser stomata had higher fecundity, cooler leaves, and increased photosynthesis, but only in the drier of two sites. These findings show how stomatal density influences physiology differently among sites and uniquely demonstrates local adaptation.
Relationships between two methods to assess leaf mechanical resistance (Technical Article)
The influence of leaf mechanical properties on local ecosystem processes has resulted in a growing interest in including leaf mechanical resistance in large-scale databases of plant functional traits. Enrico et al. (pp. 209–214) explore the relationships between two techniques widely used to measure ‘force to tear’ and ‘specific work to shear’, two properties that reflect leaf mechanical resistance. They find a positive correlation between variables and, in order to predict the value of one variable on the basis of the other, they present different equations depending on the inclusion or exclusion of the midrib, and the type of leaf or leaf venation pattern.