Reflectance and biochemical responses of maize plants to drought and re‐watering cycles

The ability to recover from drought stress after re‐watering is an important feature that will enable plants to cope with the predicted increase in episodic drought. The effects of pre‐drought and re‐watering conditions on leaf spectral properties and their relationships with the biochemical processes that underlie the recovery from pre‐drought conditions should be better understood. The reflectance spectra, 10 spectral reflectance indices (SRIs) and biochemical characteristics of maize (Zea mays) leaves were monitored 7, 14, 21 and 28 days after the initiation of soil drought stress during two successive cycles of drought and re‐watering periods. The leaf reflectance of the two inbred maize lines increased under the drought stress, especially in the visible spectral range. In addition, an obvious recovery of the leaf reflectance was only observed in the first re‐watering period, and its value remained higher than that of the control plants during the second recovery period. A recovery lag in the pigment contents was also observed during the second cycle. The recovery variations in the pattern and magnitude of the SRIs and the total contents of C, N and P that were measured in response to the re‐watering during both cycles were diverse and complex; both full and partial recoveries were observed. The SRIs representing different physiological attributes of plant growth, including the water index, red edge position, photochemical reflectance index and near‐infrared reflectance at 800 nm, showed strong linear relationships (P < 0.01 or 0.05) with the growth and biochemical traits across the successive drought and re‐watering cycles. The results suggest that maize plants can adjust their leaf reflectance properties and employ growth and biochemical strategies to adapt to cyclic drought stress and recover from drought stress after re‐watering.     

Historical comparisons show evolutionary changes in drought responses in European plant species after two decades of climate change

Plants must continuously respond to environmental changes, and a timely question is whether and how populations respond to ongoing global warming and increased drought frequencies and intensities. Plants can either respond through migration or through phenotypic plasticity or their populations can adapt evolutionarily, which encompasses the evolution of trait means and of trait plasticity. One way to detect such evolutionary changes within plant populations is through historical comparisons where plants grown from seeds collected in the past (“ancestors”) are compared to freshly collected seeds from the same populations (“descendants”) in common garden experiments. We used 21- to 26-year-old seeds stored in seed banks for two multi-species experiments that investigated changes in phenotypic traits and their plasticity conferring drought tolerance in early life stages of European plant species. In the first experiment, we used seedlings of four Mediterranean species, ceased watering and recorded their day of mortality. In the second experiment, we studied phenotypic responses to drought in juvenile plants of nine species originating from temperate regions in Europe. In one of four species in the first experiment, descendants survived significantly longer without watering and were smaller than their ancestors. In the second experiment, descendant plants were generally taller under well-watered conditions but smaller under drought than their ancestors, thus showing stronger plasticity. Our historical comparisons suggest that some populations have likely evolved through changes in trait means and plasticity in ways consistent with adaptation to increased drought. Using seed bank material for historical comparisons has several weaknesses, such as unknown sampling protocols or invisible fractions. However, we show how accurately sampled and stored seed bank collections can be used similar to the resurrection approach for investigating rapid evolutionary processes in early life stages of plants under climate change.     

Leaf damage decreases fitness and constrains phenotypic plasticity to drought of a perennial herb

Mediterranean-type ecosystems are increasingly prone to drought stress. Herbivory might limit plant functional responses to water shortage. This may occur as a result of plant resource depletion or due to the fact that leaf damage and drought may elicit opposite phenotypic responses. We evaluated the impact of herbivory on plant fitness in the field, and the effects of leaf damage on phenotypic plasticity to reduced soil moisture in a greenhouse. The study species was Convolvulus demissus, a perennial herb endemic to central Chile, which has a Mediterranean-type climate. Controlled herbivory by chrysomelid beetles (natural herbivores) in the field had a negative impact on plant fitness, estimated as number of fruits. Whereas reduced soil moisture alone did not affect seedling survival, damaged seedlings (simulated herbivory) had greater mortality when growing under water shortage. The hypothesis that herbivory would constrain phenotypic plasticity was supported by significant statistical interactions between leaf damage and soil moisture, followed by inspections of reaction norms. This was verified both overall (all phenotypic traits taken together, MANOVA) and in four of the six traits evaluated (ANOVAs). When plants were damaged, the reaction norms in response to low soil moisture of water use efficiency, root:shoot ratio and xylem water potential showed reduced slopes. While undamaged plants increased root biomass in response to low moisture, the opposite trend was found for damaged plants. The simultaneous occurrence of herbivory and drought events might curtail recruitment in plant populations of central Chile and other Mediterranean-type ecosystems due to the inability of damaged seedlings to show functional responses to low soil moisture. This finding is of ecological significance in view of current and projected trends of increased aridity in these ecosystems.     

Variation in tolerance to drought among Scandinavian populations of <Emphasis Type="Italic">Arabidopsis lyrata</Emphasis>

The ability to cope with water limitation influences plant distributions, and several plant traits have been interpreted as adaptations to drought stress. In Scandinavia, the perennial herb Arabidopsis lyrata occurs in open habitats that differ widely in climate and water availability in summer, suggesting differential selection on drought-related traits. We conducted two greenhouse experiments to examine differentiation in drought response traits among six Scandinavian populations, and to determine whether leaf trichomes confer protection against drought. We quantified tolerance to drought as fitness (survival and biomass of survivors) when exposed to drought relative to fitness under non-drought conditions. Two Swedish populations from shores along the Bothnian Bay had higher tolerance to drought than four riverbed populations from Norway. Under conditions of drought, the shore populations experienced less leaf damage compared to the riverbed populations, and their survival and biomass were less reduced relative to non-drought conditions. Across populations, tolerance to drought was positively related to leaf mass per area and negatively related to flowering propensity and proportion roots, but not related to plant size at the initiation of the drought treatment. In populations polymorphic for trichome production, trichome-producing plants were more tolerant to drought than glabrous plants. The results suggest that both leaf morphology and life-history traits contribute to differential drought response in natural populations of A. lyrata, and that this system offers excellent opportunities for examining the adaptive value and genetic basis of drought-related traits.     

Maize leaf temperature responses to drought: Thermal imaging and quantitative trait loci (QTL) mapping

Leaf temperature has been shown to vary when plants are subjected to water stress conditions. Recent advances in infrared thermography have increased the probability of recording drought tolerant responses more accurately. The aims of this study were to identify the effects of drought on leaf temperature using infrared thermography. Furthermore, the genomic regions responsible for the expression of leaf temperature variation in maize seedlings (Zea mays L.) were explored. The maize inbred lines Zong3 and 87-1 were evaluated using infrared thermography and exhibited notable differences in leaf temperature response to water stress. Correlation analysis indicated that leaf temperature response to water stress played an integral role in maize biomass accumulation. Additionally, a mapping population of 187 recombinant inbred lines (RILs) derived from a cross between Zong3 and 87-1 was constructed to identify quantitative trait loci (QTL) responsible for physiological traits associated with seedling water stress. Leaf temperature differences (LTD) and the drought tolerance index (DTI) of shoot fresh weight (SFW) and shoot dry weight (SDW) were the traits evaluated for QTL analysis in maize seedlings. A total of nine QTL were detected by composite interval mapping (CIM) for the three traits (LTD, RSFW and RSDW). Two co-locations responsible for both RSFW and RSDW were detected on chromosomes 1 and 2, respectively, which showed common signs with their trait correlations. Another co-location was detected on chromosome 9 between LTD and shoot biomass, which provided genetic evidence that leaf temperature affects biomass accumulation. Additionally, the utility of a thermography system for drought tolerance breeding in maize was discussed.     

Different combinations of morpho‐physiological traits are responsible for tolerance to drought in wild tomatoes Solanum chilense and Solanum peruvianum

Herbaceous species can modify leaf structure during the growing season in response to drought stress and water loss. Evolution can select combinations of traits in plants for efficient water use in restricted environments. We investigated plant traits that mediate adaptation and acclimation to water stress in two herbaceous drought‐tolerant species. Anatomical, morphological and physiological traits related to stems and leaves were examined under optimal watering (OW) and a long period of restricted watering (RW) in 11 accessions from three Solanaceae species (Solanum chilense, S. peruvianum and S. lycopersicum). The relationships between these traits were tested using linear regression and PCA. There were significant differences in anatomical traits between the species under both OW and RW, where leaf area correlated with stem diameter. Proline and total carbohydrates accumulated highly in S. chilense and S. peruvianum, respectively, and these osmolytes were strongly correlated with increased osmotic potential. Stomatal density varied between species but not between acclimation treatments, while stomatal rate was significantly higher in wild tomatoes. There was a strong positive relationship between stem growth rate and a group of traits together expressed as total stomatal number. Total stomata is described by integration of leaf area, stomatal density, height and internode length. It is proposed that constitutive adaptations and modifications through acclimation that mediate RW play an important role in tolerance to drought stress in herbaceous plants. The capacity for growth under drought stress was not associated with any single combination of traits in wild tomatoes, since the two species differed in relative levels of expression of various phenotypic traits.     

Counteractive biomass allocation responses to drought and damage in the perennial herb Convolvulus demissus

Herbivory and water shortage are key ecological factors affecting plant performance. While plant compensatory responses to herbivory include reallocation of biomass from below‐ground to above‐ground structures, plant responses to reduced soil moisture involve increased biomass allocation to roots and a reduction in the number and size of leaves. In a greenhouse study we evaluated the effects of experimental drought and leaf damage on biomass allocation in Convolvulus demissus (Convolvulaceae), a perennial herb distributed in central Chile, where it experiences summer drought typical of Mediterranean ecosystems and defoliation by leaf beetles and livestock. The number of leaves and internode length were unaffected by the experimental treatments. The rest of plant traits showed interaction of effects. We detected that drought counteracted some plant responses to damage. Thus, only in the control watering environment was it observed that damaged plants produced more stems, even after correcting for main stem length (index of architecture). In the cases of shoot : root ratio, relative shoot biomass and relative root biomass we found that the damage treatment counteracted plant responses to drought. Thus, while undamaged plants under water shortage showed a significant increase in root relative biomass and a significant reduction in both shoot : root ratio and relative shoot biomass, none of these responses to drought was observed in damaged plants. Total plant biomass increased in response to simulated herbivory, apparently due to greater shoot size, and in response to drought, presumably due to greater root size. However, damaged plants under experimental drought had the same total biomass as control plants. Overall, our results showed counteractive biomass allocation responses to drought and damage in C. demissus. Further research must address the fitness consequences under field conditions of the patterns found. This would be of particular importance because both current and expected climatic trends for central Chile indicate increased aridity.     

Environmental Heterogeneity and Population Differentiation in Plasticity to Drought in <Emphasis Type="Italic">Convolvulus Chilensis</Emphasis> (Convolvulaceae)

Plant populations may show differentiation in phenotypic plasticity, and theory predicts that greater levels of environmental heterogeneity should select for higher magnitudes of phenotypic plasticity. We evaluated phenotypic responses to reduced soil moisture in plants of Convolvulus chilensis grown in a greenhouse from seeds collected in three natural populations that differ in environmental heterogeneity (precipitation regime). Among several morphological and ecophysiological traits evaluated, only four traits showed differentiation among populations in plasticity to soil moisture: leaf area, leaf shape, leaf area ratio (LAR), and foliar trichome density. In all of these traits plasticity to drought was greatest in plants from the population with the highest interannual variation in precipitation. We further tested the adaptive nature of these plastic responses by evaluating the relationship between phenotypic traits and total biomass, as a proxy for plant fitness, in the low water environment. Foliar trichome density appears to be the only trait that shows adaptive patterns of plasticity to drought. Plants from populations showing plasticity had higher trichome density when growing in soils with reduced moisture, and foliar trichome density was positively associated with total biomass. Co-ordinating editor: F. Stuefer     

Intraspecific variation in seedling growth responses of a relict tree species Euptelea pleiospermum to precipitation manipulation along an elevation gradient

Improving the accuracy of predictions regarding how plants respond to climate change is crucial to protecting biodiversity. However, little is known about the effects of seed source and elevation on the response of mountain plant species to reductions in precipitation. Here, we collected seeds of a tree species (Euptelea pleiospermum) from three seed sources and carried out a two-growing-season reciprocal transplant experiment with precipitation manipulation at three sites along an elevation gradient in the Shennongjia Mountains, central China. Variations in whole-plant traits, leaf traits, and root traits were investigated. We found that most plant traits of E. pleiospermum seedlings were affected by reductions in precipitation, and responses varied among different elevations and seed sources. Whole-plant traits, root biomass, and leaf traits related to photosynthesis capacity decreased under reduced precipitation treatments at mid and high elevation sites. Thus, climate change induced drought will likely have a negative influence on seedling growth at mid and high elevation regions. In addition, a home-site advantage in whole-plant traits and root traits was observed. However, the responses of leaf traits in most cases were not affected by seed source because of higher phenotypic plasticity. Our results suggested that both local adaptation and phenotypic plasticity were important in seedling growth responses to reduced precipitation. We also highlight the importance of taking intraspecific variation into account when studying the response of plants to changes in climate.

     

Drought affects the coordination of belowground and aboveground resource‐related traits in Solidago canadensis in China

Quantifying patterns of variation and coordination of plant functional traits can help to understand the mechanisms underlying both invasiveness and adaptation of plants. Little is known about the coordinated variations of performance and functional traits of different organs in invasive plants, especially in response to their adaptation to environmental stressors. To identify the responses of the invasive species Solidago canadensis to drought, 180 individuals were randomly collected from 15 populations and 212 ramets were replanted in a greenhouse to investigate both the response and coordination between root and leaf functional traits. Drought significantly decreased plant growth and most of the root and leaf functional traits, that is, root length, surface area, volume and leaf size, number, and mass fraction, except for the root length ratio and root mass fraction. Phenotypic plasticity was higher in root traits than in leaf traits in response to drought, and populations did not differ significantly. The plasticity of most root functional traits, that is, root length (RL), root surface area (RSA), root volume (RV), and root mass fraction (RMF), were significantly positively correlated with biomass between control and drought. However, the opposite was found for leaf functional traits, that is, specific leaf area (SLA), leaf area ratio (LAR), and leaf mass fraction (LMF). Drought enhanced the relationship between root and leaf, that is, 26 pairwise root–leaf traits were significantly correlated under drought, while only 15 pairwise root–leaf traits were significantly correlated under control conditions. Significant correlations were found between biomass and all measured functional traits except for leaf size. RV, root length ratio, RMF, total area of leaves, and LMF responded differently to water availability. These responses enable S. canadensis to cope with drought conditions and may help to explain the reason of the vast ecological amplitude of this species.     

Dealing with the genotype x environment interaction via a modelling approach: a comparison of QTLs of maize leaf length or width with QTLs of model parameters

Quantitative genetics of adaptive traits is made difficult by the genotypexenvironment interaction. A classical assumption is that QTLs identified in both stressed and control conditions correspond to constitutive traits whereas those identified only in stressed treatments are stress-specific and correspond to adaptive traits. This hypothesis was tested by comparing, in the same set of experiments, two ways of analysing the genetic variability of the responses of maize leaf growth to water deficit. One QTL detection was based on raw phenotypic traits (length and width of leaf 6) of 100 recombinant inbred lines (RILs) in four experiments with either well-watered or stressing conditions in the field or in the greenhouse. Another detection followed a method proposed recently which consists of analysing intrinsic responses of the same RILs to environmental conditions, determined jointly over several experiments. QTLs of three responses were considered: (i) leaf elongation rate per unit thermal time in the absence of stress, (ii) its response to evaporative demand in well-watered plants, and (iii) its response to soil water status in the absence of evaporative demand. The QTL of leaf length differed between experiments, but colocalized in seven cases out of 13 with QTLs of the intrinsic leaf elongation rate, even in experiments with stressing conditions. No colocalization was found between QTLs of leaf length under water deficit and QTLs of responses to air or soil water status. By contrast, QTLs of leaf width colocalized in all experiments, regardless of environmental conditions. The classical method of identifying the QTL of constitutive versus adaptive traits therefore did not apply to the experiments presented here. It is suggested that identification of the QTL of parameters of response curves provides a promising alternative for dealing with the genetic variability of adaptive traits.     

干旱对夏玉米苗期叶片权衡生长的影响

叶片是植物对干旱响应最敏感的器官之一,叶片性状变化及其权衡关系能够反映植物对资源的利用策略以及对干旱的适应对策。基于2014年6个初始土壤水分梯度的夏玉米持续干旱模拟试验研究表明,随着干旱的发展,夏玉米各叶片性状均会受到影响,但不同干旱程度的影响不一致。基于水分胁迫系数及干旱持续时间提出了干旱程度的定量表达,随着干旱的发生发展,干旱程度在0—1之间变化。当干旱程度小于0.21时,夏玉米叶片性状不会受到显著影响;0.21—0.76时,叶片性状大小受到影响,但变化趋势不会发生改变;0.76—0.91时,新叶形成补偿不了老叶脱落,有效叶片数、叶干重、绿叶面积和叶含水量等性状提前出现下降趋势;大于0.91时,叶片生长几乎停滞。夏玉米叶片性状在干旱条件下的适应性生长本质上体现了其在快速生长与维持生存之间的权衡,但不同干旱程度下,夏玉米叶片性状生长的权衡策略不同:未发生干旱时,夏玉米倾向于维持较高的代谢活性,一旦干旱程度大于0,夏玉米就会降低叶片代谢活性;当干旱程度小于0.48时,夏玉米倾向于通过迅速增加叶面积来吸收较多的能量,以获得较大的生长速率,为生殖器官的生长及产量形成储备能量;当干旱程度大于0.48时,夏玉米会减小单叶面积以减少水分散失,倾向于资源贮存以提高其生存能力。     

Soil microbes alter plant fitness under competition and drought

Plants exist across varying biotic and abiotic environments, including variation in the composition of soil microbial communities. The ecological effects of soil microbes on plant communities are well known, whereas less is known about their importance for plant evolutionary processes. In particular, the net effects of soil microbes on plant fitness may vary across environmental contexts and among plant genotypes, setting the stage for microbially mediated plant evolution. Here, we assess the effects of soil microbes on plant fitness and natural selection on flowering time in different environments. We performed two experiments in which we grew Arabidopsis thaliana genotypes replicated in either live or sterilized soil microbial treatments, and across varying levels of either competition (isolation, intraspecific competition or interspecific competition) or watering (well‐watered or drought). We found large effects of competition and watering on plant fitness as well as the expression and natural selection of flowering time. Soil microbes increased average plant fitness under interspecific competition and drought and shaped the response of individual plant genotypes to drought. Finally, plant tolerance to either competition or drought was uncorrelated between soil microbial treatments suggesting that the plant traits favoured under environmental stress may depend on the presence of soil microbes. In summary, our experiments demonstrate that soil microbes can have large effects on plant fitness, which depend on both the environment and individual plant genotype. Future work in natural systems is needed for a complete understanding of the evolutionary importance of interactions between plants and soil microorganisms.     

夏玉米叶片水分变化与光合作用和土壤水分的关系

叶片是光合作用的重要器官,其含水量的变化必将影响光合作用,但关于叶片水分变化对光合作用的影响报道较少。以华北夏玉米为研究对象,利用三叶期不同水分梯度的持续干旱模拟试验资料,分析夏玉米叶片水分变化及其与叶片净光合速率和土壤水分的关系。结果表明:夏玉米叶片净光合速率对叶片水分变化的响应显著且呈二次曲线关系,叶片含水量约为70.30%时,叶片净光合速率为零;叶片含水量与土壤相对湿度呈非直角双曲线关系,叶片最大含水量约为85.14%。研究结果可为准确描述叶片水分变化对光合作用的影响及客观辨识夏玉米干旱的发生发展及监测预警提供参考。     

Do brassinosteroids mediate the water stress response?

Brassinosteroids (BRs) have been suggested to increase the resistance of plants to a variety of stresses, including water stress. This is based on application studies, where exogenously applied bioactive BRs have been shown to improve various aspects of plant growth under water stress conditions. However, it is not known whether changes in endogenous BR levels are normally involved in mediating the plant's response to stress. We have utilized BR mutants in pea ( Pisum sativum L.) to determine whether changes in endogenous BR levels are part of the plant's response to water stress and whether low endogenous BR levels alter the plant's ability to cope with water stress. In wild-type (WT) plants, we show that while water stress causes a significant increase in ABA levels, it does not result in altered BR levels in either apical, internode or leaf tissue. Furthermore, the plant's ability to increase ABA levels in response to water stress is not affected by BR deficiency, as there was no significant difference in ABA levels between WT, lkb (a BR-deficient mutant) and lka (a BR-perception mutant) plants before or 14 days after the cessation of watering. In addition, the effect of water stress on traits such as height, leaf size and water potential in lkb and lka was similar to that observed in WT plants. Therefore, it appears that, at least in pea, changes in endogenous BR levels are not normally part of the plant's response to water stress.     

Plant heterotrimeric G protein function: insights from Arabidopsis and rice mutants

Heterotrimeric G proteins have been implicated in a wide range of plant processes. These include responses to hormones, drought, and pathogens, and developmental events such as lateral root formation, hypocotyl elongation, hook opening, leaf expansion, and silique development. Results and concepts emerging from recent phenotypic analyses of G-protein component mutants in Arabidopsis and rice are adding to our understanding of G-protein mechanisms and functions in higher plants.     

Plastic responses in the metabolome and functional traits of maize plants to temperature variations

Environmentally inducible phenotypic plasticity is a major player in plant responses to climate change. However, metabolic responses and their role in determining the phenotypic plasticity of plants that are subjected to temperature variations remain poorly understood. The metabolomic profiles and metabolite levels in the leaves of three maize inbred lines grown in different temperature conditions were examined with a nuclear magnetic resonance metabolomic technique. The relationship of functional traits to metabolome profiles and the metabolic mechanism underlying temperature variations were then explored. A comparative analysis showed that during heat and cold stress, maize plants shared common plastic responses in biomass accumulation, carbon, nitrogen, sugars, some amino acids and compatible solutes. We also found that the plastic response of maize plants to heat stress was different from that under cold stress, mainly involving biomass allocation, shikimate and its aromatic amino acid derivatives, and other non‐polar metabolites. The plastic responsiveness of functional traits of maize lines to temperature variations was low, while the metabolic responsiveness in plasticity was high, indicating that functional and metabolic plasticity may play different roles in maize plant adaptation to temperature variations. A linear regression analysis revealed that the maize lines could adapt to growth temperature variations through the interrelation of plastic responses in the metabolomes and functional traits, such as biomass allocation and the status of carbon and nitrogen. We provide valuable insight into the plastic response strategy of maize plants to temperature variations that will permit the optimisation of crop cultivation in an increasingly variable environment.     

Identification of QTL for drought responses in maize and their use in testing causal relationships between traits

A difficulty in identifying traits that help crop plants maintaintheir yield under droughted conditions is distinguishing betweenthose traits that contribute to yield stability under droughtand traits that do not affect yield. With the development ofmolecular markers for many crops it is now possible to identifymajor quantitative trait loci (QTL) regulating specific droughtresponses. By comparing the coincidence of such QTL for specifictraits it is possible to test much more precisely than beforewhether a particular constitutive or adaptive response to droughtstress is likely to be of significance in improving droughtresistance. We have used this approach to identify QTL for ABAcontent and other traits likely to be important in determiningdrought response in maize. Eighty-four RFLP markers were mapped in an F2 population of81 plants from a cross between parents, Polj17 (drought resistant)and F-2 (drought sensitive), that differ markedly in many constitutiveand adaptive responses to drought stress. In a soil glasshouseexperiment, from which water was withheld for 3 weeks afteranthesis, flowering time, stomatal conductance, tissue ABA contents,leaf water relations parameters and fluorescence characteristics,root pulling force, and nodal root number were measured. Theminimum number and location of genes having major effects onthe traits were determined and possible causal relationshipsamongst them tested. Comparing the coincidence of QTL for ABAcontent and stomatal conductance showed that xylem ABA contentwas more likely to have had a regulatory effect on the stomatalconductance of those plants than the whole leaf ABA content.However, both xylem and leaf ABA contents were significantlyassociated with root characteristics, suggesting that the rootingbehaviour (either constitutive or adaptive) was important inregulating stress responses, particularly in determining xylemABA contents. We also found that Fm (a measure of the activityof photosynthetic reaction centres) was positively associatedwith chlorophyll concentration per unit area. Different methodsfor comparing QTL are presented and discussed. Key words: Quantitative trait loci (QTL), ABA content, rooting behaviour, fluorescence characteristics, drought responses, maize