Infrared Thermal Imaging as a Rapid Tool for Identifying Water‐Stress Tolerant Maize Genotypes of Different Phenology

The main task of this research was to evaluate canopy temperature and Crop Water Stress Index (CWSI) by assessing genotype variability of maize performance for different water regimes. To that end, three hundred tropical and subtropical maize hybrids with different phenology in terms of date of anthesis were evaluated. The influence of phenology on the change in canopy temperatures and CWSI was not equal during the three dates of measurement. At the end of vegetative growth (82 days after sowing, DAS) and at the blister stage (DAS 97), a high significant difference in temperatures and CWSI (P < 0.001) were obtained between the early‐ and late‐maturity genotypes. During anthesis (DAS 89), phenology had a significant effect (P < 0.01) only for the well‐watered genotypes, while under water‐stress conditions, no differences were found between early and late genotypes in terms of canopy temperature and CWSI. High significant differences (P < 0.001) in stomatal conductance (g_s) between early and late genotypes for different treatments were observed. A relationship (R² = 0.62) between g_s and canopy temperature was obtained. Under a water‐stress canopy, temperature was measured at anthesis, which was negatively correlated with grain yield of the early (r = ?0.55)‐ and late (r = ?0.46)‐maturity genotypes in the water‐stressed condition.     

Effects of Timing of Drought Stress on Phenology, Yield and Yield Components of Short-duration Pigeonpea

Nine short-duration pigeonpea genotypes were given adequate soil moisture throughout growth or subjected to water stress during the late vegetative and flowering (stress 1), flowering and early pod development (stress 2), or podfill (stress 3) growth stages under field conditions. The stress 1 treatment had no significant effect on the time to flowering. No stress treatment affected maturity or inter-plant flowering synchronization. The interval from a newly opened flower to a mature pod was about 30 days for all genotypes, and was unchanged in plants that were recovenng from stress 1 or undergoing stress 2. Seed yield was reduced to the greatest extent by stress 2 (by 37 %) and not significantly affected by stress 3 for all genotypes. No consistent differences were found between determinate and indeterminate genotypes in the ability to maintain seed yield under both stress 1 and stress 2. The harvest index was significantly reduced (22 %) by stress 2 but not by stress 1. However, under each soil moisture treatment, genotypic differences for seed yield were associated largely with differences in total dry matter production (TDM). For all genotypes, the number of pods m^-2 was the only yield component significantly affected by the water stress treatments. The stability of other yield components should be fully exploited to improve the stability of seed yield under drought conditions (drought resistance). Possible characteristics which may improve the drought resistance of short-duration pigeonpea include the ability to maintain TDM, low flowering synchronization, small pod size with few seeds pod^-1, and large 100-seed mass.     

Relationship between Biomass Production and Nitrogen Fixation under Drought-Stress Conditions in Peanut Genotypes with Different Levels of Drought Resistance

The relationship between biomass production and N₂ fixation under drought‐stress conditions in peanut genotypes with different levels of drought resistance is not well understood. The objective of this study was to determine the effect of drought on biomass production and N₂ fixation by evaluating the relative values of these two traits under well watered and water‐stress conditions. Twelve peanut genotypes were tested under field conditions in the dry seasons of 2003/2004 and 2004/2005 in north‐east Thailand. A split‐plot design with four replications was used. Main‐plot treatments were three water regimes [field capacity (FC), 2/3 available soil water (AW) and 1/3 AW], and sub‐plot treatments were 12 peanut lines. Data were recorded on biomass production and N₂ fixation under well watered and water‐stress conditions. Genotypic variations in biomass production and N₂ fixation were found at all water regimes. Biomass production and N₂ fixation decreased with increasing levels of drought stress. Genotypes did not significantly differ in reductions for biomass production, but did differ for reductions in N₂ fixation. High biomass production under both mild and severe drought‐stress conditions was due largely to high potential biomass production under well‐watered conditions and, to a lesser extent, the ability to maintain high biomass production under drought‐stress conditions. High N₂ fixation under drought stress also was due largely to high N₂ fixation under well‐watered conditions with significant but lower contributions from the ability to maintain high nitrogen fixation under drought stress. N₂ fixation at FC was not correlated with the reduction in N₂ fixation at 2/3 AW and 1/3 AW. Positive relationships between N₂ fixed and biomass production of the tested peanut genotypes were found at both levels of drought stress, and the relationship was stronger the more severe the drought stress. These results suggested that the ability to maintain high N₂ fixation under drought stress could aid peanut genotypes in maintaining high yield under water‐limited conditions.     

Physiological responses of six bread wheat and durum wheat genotypes to water stress

Summary The responses of six wheat genotypes to water stress were analysed. Soil moisture (H), leaf water potential (_w), photosynthesis (P_N), stomatal resistance (r_s) and transpiration (T) were measured during a water stress. The genotypes investigated differed in their stress avoidance (_w-H relationship) and their stress tolerance (P_N-_w and r_s-_w relationships). The most important differences observed concern the mechanisms of tolerance at low leaf water potential: two varieties, Haurani 27 and Baalback, can then maintain a high photosynthetic activity. These observations are in agreement with the drought resistance characteristics already known for these genotypes. Possible applications to wheat breeding are considered     

Yield and Osmotic Adjustment Capacity of Barley Under Terminal Water-Stress Conditions

This paper reports the influence of the osmotic adjustment capacity and turgor potential (TP) on tolerance to drought in 12 cultivated genotypes, six breeding lines and six cultivars of barley (Hordeum vulgare L.) under terminal water‐stress conditions. When the crops reached the flag leaf stage, half of the experimental plots in which they were grown were submitted to water stress treatment and the remainder maintained under optimal irrigation conditions. Differences were seen in the osmotic adjustment, relative water content (RWC) and water potential (WP) of the different genotypes. Two of the breeding lines showed the greatest osmotic adjustment capacities, maintaining their TPs better than the other genotypes studied. A positive, significant correlation was found between yield and osmotic adjustment capacity, RWC and WP under water‐stress conditions. Osmotic adjustment capacity and TP were linearly related, indicating that as water stress increases osmotic adjustment favours the maintenance of higher TPs. Significant correlations were found between osmotic adjustment capacity and grain filling rate and grain yield. Under conditions of terminal water stress, yield was negatively correlated with the number of days to ear emergence; no correlation was found between osmotic adjustment capacity and earliness. Early ear emergence, a good osmotic adjustment capacity and high RWC values all contributed to yield increases under terminal water‐stress conditions.     

Physiological and Molecular Characterization of Faba bean (Vicia faba L.) Genotypes for Adaptation to Drought Stress

Faba bean (Vicia faba L.) is one of the most important and drought sensitive grain legumes. Drought stress is thus one of major constraints in global faba bean production. In this study, twenty local and exotic faba bean genotypes were characterized on physiological and molecular basis. Seeds of faba bean genotypes (six per pot) were sown in poly venyl chloride pots. After seedling emergence, soil moisture was maintained at 100%, 50% and 25% of field capacity designated as well watered, moderate drought and severe drought, respectively. Drought stress significantly influenced the leaf area, leaf temperature, stomatal conductance, relative leaf water contents, grain yield and water‐use efficiency. Faba bean genotypes also differed for the leaf area, leaf temperature, relative leaf water contents, grain yield and water‐use efficiency. Faba bean genotypes Kamline and L.4 were better equipped to curtail water loss, maintain tissue water status, produce stable grain yield and had better water‐use efficiency under mild and severe drought stress, and may be used in breeding programmes. Amplified fragment length polymorphism markers showed high potential in detecting polymorphism and estimating genetic diversity among faba bean genotypes. Unweighted pair group method with arithmetic mean cluster analysis of the genotypes illustrated considerable association between molecular diversity, genetic background and geographic origin. In crux, high polymorphic rate and polymorphism information content values, together with the low genetic similarity observed among tested genotypes suggests a high level of heterogeneity, which may be used in breeding programmes to assemble different drought tolerance mechanisms in one genotype.     

Trait identification and QTL validation for reproductive stage drought resistance in rice using selective genotyping of near flowering RILs

Drought resistance is becoming an indispensable character for rice improvement due to the dwindling global water resources. Genetic improvement for drought resistance is achieved through physiological dissection and genetic analysis of independent component traits associated with crop productivity under stress. A subset mapping population of 93 near flowering recombinant inbred lines with uniform phenology was constituted for genetic analysis of reproductive stage drought resistance. The population was phenotyped for 22 physio-morphological traits under two contrasting water regimes imposed at reproductive stage. Broad sense heritabilities of morphological traits were lower under stress than irrigated. Predominant association of plant height, panicle exsertion and harvest index with grain yield were observed under stress. The sustenance of panicle exsertion through maintaining growth during moisture stress was found as a significant trait associated with the grain yield through minimizing spikelet sterility. Selective genotyping was carried out with 23 polymorphic microsatellite markers of the established target genomic regions for drought resistance. The study validated the association of a QTL region on the long arm of chromosome 1 with plant height, panicle length, panicle exsertion, biological yield and stomatal conductance under stress. This region, flanked by markers RM246 and RM315, was known to possess the semi-dwarf gene, sd-1. Role of another major interval lying between RM256 and RM149 on chromosome 8 in defining the drought resistance could be established through identification of QTLs associated with leaf rolling, panicle exsertion, plant height, panicle length, senescence and biological yield under moisture stress condition. Few other QTLs were also identified.     

Genotypic variability in plant water status of French bean under drought stress

Seven genotypes of French bean (Phaseolus vulgaris L.) were evaluated under semi-controlled conditions at the Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh to analyze genotypic variability in leaf water status under water stress. The plants were grown under two moisture regimes, viz. 80% field capacity (FC) and 50% FC throughout the growing season. The genotypes showed significant variation in water relation traits. Genotypes BB24 and BB43 maintained higher relative water content (RWC), but lower turgid weight/dry weight ratio (TW/DW) and water uptake capacity (WUC). When drought susceptibility index (DSI) among the genotypes was considered, BB24 was found the most tolerant to drought and BB04 was the most susceptible one. A close positive relationship between leaf TW/DW and DSI under drought was recorded (R ² = 0.627). Leaf TW/DW was decreased considerably due to water stress by 10% in genotype BB24 followed by BB43 (13%), and both BARI bushbean-2 and BB04 (19%). Stomatal aperture and whole plant transpiration rate were found minimal in the BB24 and BB43 compared to that of BB04 and BARI bushbean-2. Considering these water relation traits, genotypes BB24 and BB43 may be considered as relatively tolerant to tissue dehydration. The study also revealed that the TW/DW, WUC, stomatal aperture, and whole plant transpiration rate was negatively and significantly associated with yield; however, the RWC was positively correlated with yield under water stress conditions.     

Understanding physiological and morphological traits contributing to drought tolerance in barley

Drought stress is a major limiting factor for crop production in the arid and semi‐arid regions. Here, we screened eighty barley (Hordeum vulgare L.) genotypes collected from different geographical locations contrasting in drought stress tolerance and quantified a range of physiological and agronomical indices in glasshouse trails. The experiment was conducted in large soil tanks subjected to drought treatment of eighty barley genotypes at three‐leaf stage and gradually brought to severe drought by withholding irrigation for 30 days under glasshouse conditions. Also, root length of the same genotypes was measured from stress‐affected plants growing hydroponically. Drought tolerance was scored 30 days after the drought stress commenced based on the degree of the leaf wilting, fresh and dry biomass and relative water content. These characteristics were related to stomatal conductance, stomatal density, residual transpiration and leaf sap Na, K, Cl contents measured in control (irrigated) plants. Responses to drought stress differed significantly among the genotypes. The overall drought tolerance was significantly correlated with relative water content, stomatal conductance and leaf Na⁺ and K⁺ contents. No significant correlations between drought tolerance and root length of 6‐day‐old seedling, stomatal density, residual transpiration and leaf sap Cl^? content were found. Taking together, these results suggest that drought‐tolerant genotypes have lower stomatal conductance, and lower water content, Na⁺, K⁺ and Cl^? contents in their tissue under control conditions than the drought‐sensitive ones. These traits make them more resilient to the forthcoming drought stress.     

Thermal phenotyping of stomatal sensitivity in spring barley

Thermometry and thermography are alternative methods used for measuring stomatal conductivity via transpirative cooling. However, the influence of mixed soil–plant information contained in thermal images compared to thermometric spot measurements on the measurement quality and relationships to agronomic traits remains unclear. To evaluate their respective influence, canopy temperature was measured simultaneously by two infrared thermometers (thermometry), which were oriented oblique to the plant canopy and mounted on a tractor, and a hand‐held, nadir oriented thermal camera (thermography) in irrigated and drought‐stressed spring barley cultivar trials in 2011. Canopy temperatures were separated from soil temperatures and extracted from the thermal images by matching thermal and RGB images. Thermometric measurements conducted at the beginning of shooting during a stable period of high radiation were more closely related to total plant biomass and straw yield at harvest than thermography under both irrigated and drought‐stressed conditions. Taking into account the results of this evaluation, thermometry was used for assessing the agronomic importance of stomatal sensitivity, the earliness of stomatal closure, of spring barley cultivars subjected to different water supply in 2013. In this year, 16 spring barley cultivars were grown under mild drought stress and rainfed conditions. A stomatal sensitivity index was derived relating canopy temperatures of the cultivars grown under rainfed and drought‐stressed conditions to each other. Under rainfed conditions, stomatal sensitivity was negatively related to grain protein yield with a coefficient of determination of R² = .43. Under increasing terminal drought stress, positive regression slopes of stomatal sensitivity to grain yield, biomass yield and culms/m² were observed with coefficients of determination amounting to R² = .22, .31 and .36, respectively. Stomatal sensitivity negatively impacts agricultural production under well‐watered conditions, but maintains productivity under conditions of terminal drought.     

Suitability of Canopy Temperature Depression in a Temperate Climate with Drought‐Stressed Winter Rye,Determined with Three Infrared Measurement Devices

Canopy temperature has been recognised as an indicator of crop water status and may thus be a useful secondary trait in selecting for yield under dry conditions. The aim of this study was to test the suitability of canopy temperature depression (CTD = T_air ? T_canopy) in a temperate climate with winter rye, by means of three infrared (IR) temperature measuring devices. In the years 2011 and 2012, 16 winter rye genotypes were examined under drought stress conditions in a rainout shelter and under well‐watered conditions. In each year, the CTD was determined several times during the growth period using two IR thermometers and an IR camera. By means of CTD, it was possible to detect drought stress and to differentiate between water regimes. The three measurement devices showed comparable results, despite greatly different costs. Under drought‐stress conditions, a significant positive correlation between grain yield and CTD was found on most measurement dates in 2011 and on some dates in 2012. When the CTD was pooled across water regimes, a significant positive correlation between grain yield and CTD was obtained on every measurement date. However, as genotypic differences for CTD were non‐existent, the correlations are less meaningful. The missing genotypic differences for CTD were rather caused by the limited genetic variability of the genotypes used in this study, than by climatic conditions. Due to this limitation, we were not able to make a concluding statement about the CTD in a temperature climate, although the results are quite promising and indicate that the CTD can potentially be used in a temperate climate.     

不同冬小麦品种的离体叶片失水速率差异及对供水的反应

较低的离体叶片失水速率(RWL)与抗旱性有关已经得到很多研究者的认同.选取8个冬小麦品种为试材,设置干旱(0水)、节水(2水)和足水(4水)的水分条件,研究不同冬小麦品种对供水条件的反应和生育时期的RWL变化特征.结果表明:不同品种间的RWL差异不显著.RWL随着灌水次数的增加而增高.0水处理的衡7228 RWL最低,冀5579和石麦16也较低.2水处理的石麦16最低,石麦14和冀5579也较低.4水处理下,石新618 的RWL最低,石麦14和衡7228较低,综合3种灌水结果,石麦16、冀5579和石麦14抗旱节水性较好.RWL随着生育进程而变化,拔节期较低,逐渐增加,至抽穗期达到峰值,此后明显下降,直至成熟.前期控水对降低小麦离体叶片失水速率,提高抗旱节水性有利.     

Impact of water supply on photosynthesis,water use and carbon isotope discrimination of sugar beet genotypes

Improvements in drought tolerance of crop plants require research focused on physiological processes. In 2002 and 2003 pot experiments with sugar beet were conducted in a greenhouse. Two (2002) or three (2003) different genotypes were subjected to three watering regimes (100, 50 and 20% of water holding capacity). Gas exchange, chlorophyll fluorescence and water-use efficiency (WUE) as parameters of possible relevance for drought stress tolerance in sugar beet were investigated. It was studied whether ¹³C discrimination (Δ) is suitable as an indirect measure for WUE of sugar beet.DM yield, photosynthesis rate, transpiration rate and stomatal conductance decreased with increasing severity of drought stress. In contrast, internal CO₂ partial pressure remained relatively stable and effective quantum yield of photosynthesis was reduced only under severe drought, which points at non-stomatal inhibition of photosynthesis. Different sugar beet genotypes showed significant differences in DM yield, but interactions between genotype and water supply did not occur, indicating that genotypic differences in drought tolerance did not exist. In accordance with that, drought-sensitivity of gas exchange and chlorophyll fluorescence was the same in different genotypes. Δ was higher in the leaves than in the taproot. Reductions in Δ in drought-stressed plants corresponded to about 24% higher WUE. Differentiating between plant organs, only leaf Δ was negatively correlated with WUE_L whereas taproot Δ and WUE_T were unrelated. Δ was therefore proven to be a sensitive indicator for water availability during the growing period. However, similar as other parameters relevant for drought stress tolerance it requires investigations in broader genetic material of sugar beet to detect genotypic differences.     

Effects of Mild and Severe Drought Stress on Photosynthetic Efficiency in Tolerant and Susceptible Barley (Hordeum vulgare L.) Genotypes

Drought stress is one of the most important abiotic factors which adversely affect growth, metabolism and yield of crops worldwide. The objective of this study was to determine the effects of drought stress on photosynthesis in barley and examine the differential responses of photosynthetic apparatus in relatively tolerant (Yousof) and susceptible (Morocco) barley genotypes. Plants were subjected to different levels of soil water availability including control, mild and severe drought stress. In both genotypes, drought stress led to decrease in chlorophylls, β‐carotene and stomatal conductance accompanied by decrease in CO₂ assimilation rate. Significant increase in α‐tochoperol content was only observed in Yousof cultivar under drought stress. Initial slope and plateau phase of CO₂ response curve of drought‐stressed plants as well as polyphasic chlorophyll a fluorescence transient curve (OJIP test) and fast fluorescence induction kinetics were influenced by drought stress. These parameters were more affected in Morocco cultivar by drought stress compare with Yousof. Drought stress also resulted in reduction of D1 protein content in both genotypes and accelerated photoinhibition process. Based on our results, stomatal conductance is the main factor limiting photosynthesis in Yousof genotype under mild drought stress. However, in Morocco, in addition to stomatal limitation, damage to photosystem II (PSII), reduced electron transport and carboxylation efficiencies were important parts of limitation in photosynthesis. Severe drought stress resulted in structural and biochemical impairment of light‐dependent reactions as well as carboxylation process of photosynthesis in both genotypes. Alpha‐tocopherol showed an important protective role against drought stress in Yousof cultivar as a relatively drought‐tolerant cultivar.     

花铃期不同钾效率基因型棉花对干旱和低钾胁迫的响应差异

以钾高效和钾低效基因型棉花(HEG和LEG)为试验材料,研究了最优水肥(W1K1),干旱胁迫(W2K1),低钾胁迫(W1K2),干旱、低钾胁迫(W2K2)对其光合作用及水分、钾素利用率的影响。结果表明:干旱显著降低花铃期2种基因型棉花叶片净光合速率(Pn)、气孔导度(Gs)、蒸腾速率(Tr),导致蕾铃脱落,钾素利用率(KUE)降低,造成棉花减产。施钾促进叶片Pn及水分利用率(WUE)的提高,但基因型间存在差异,LEG叶片WUE与施钾量成正比,HEG相反。低钾胁迫下HEG的Pn、Tr下降,但Pn下降幅度小于Tr,WUE增大。较高的WUE有利于提高棉花Pn,降低蕾铃脱落率,促进钾素的高效吸收和运输。     

Physiological and Biochemical Responses of Cowpea Genotypes to Water Stress Under Glasshouse and Field Conditions

Five cowpea genotypes, Gorom local (Go), KVX61‐1 (KV), Mouride (Mo), Bambey 21 (B21) and TN88‐63 (TN), differing in their susceptibility to water stress, were studied under glasshouse and field conditions, to determine their physiological, biochemical and agronomic responses to water deficit at flowering stage. Effect of water deficit on leaf water potential (ψ_l), canopy temperature, gaseous exchange, leaf proline content, total protein and starch contents, maximal quantum yield (ϕp₀) and yield components was examined. Water deficit significantly increased the canopy temperature and the proline content of the five genotypes while ψ_l, gaseous exchanges, ϕp₀ and starch content decreased significantly. Yield components, with the exception of seed number per pod, of the five genotypes, were also significantly affected. Under glasshouse and field conditions, the results showed that stomatal closure is the common strategy used by the five genotypes to avoid dehydration. Go, Mo and TN tolerated water stress better than B21 and KV. Furthermore, Go and Mo recovered more rapidly after rewatering than B21 and KV. These latter genotypes are revealed to be sensitive with low recovery capacity. The results suggest that the maintenance of net photosynthesis and solute accumulation seem to be traits conferring water stress tolerance in Go, Mo and TN. These traits and recovery capacity could be valuable selection criteria for higher yields under water deficit conditions.     

Contrasting rice backcross inbred lines (BILs) for grain yield and heading under lowland reproductive stage moisture stress

Identification and understanding the role of physio-morphological drought responsive mechanisms leading to grain yield enhancement under water stress is a critical insight for designing appropriate strategies to breed drought-tolerant cultivars for any drought prone ecology. In this study, three pairs of contrasting BILs with varied maturity were characterized for several agronomical, physiological and morphological traits across a wide range of moisture stress environments at reproductive stage during 2012–2014. Within each group, BILs differ significantly for grain yield, heading, biomass and harvest index under drought stress, but showed similar yield potential, phenology and other traits under control condition. The most tolerant BIL, S-15 out yielded all BILs and standard checks under both conditions. Apart from superior agronomic performance, drought tolerant BILs maintained significantly higher assimilation rate, transpiration rate and transpiration efficiency compared to susceptible BILs under stress in all three groups. In addition, most tolerant BIL (S-15) showed significantly higher stomatal conductance than susceptible BIL (S-55) in early group. Among root traits, significant differences under stress was observed for root dry weight between contrasting BILs in each group, even though tolerant BILs had higher root length and root volume compared to susceptible BILs, which is non-significant. Hence, consideration of root traits an important strategy for drought avoidance in case of rice may not always contributes to significant yield improvement under moisture stress condition. Further, tolerant BILs also recorded significantly higher shoot dry weight and drought recovery score at seedling stage under stress. Our findings suggest that genotypes with higher photosynthetic efficiency and better plant water status are able to produce higher grain yield under drought stress environments.     

Is Discrimination of 13C in Potato Leaflets and Tubers an Appropriate Trait to Describe Genotype Responses to Restrictive and Well‐Watered Conditions?

Selection for drought tolerance entails prioritizing plant traits that integrate critical physiological processes occurring during crop growth. Discrimination against ¹³C (?) in leaflets (?_leaflet) and tubers (?_tuber) was compared under two water regimes in two potato‐improved varieties selected to maintain yield under drought conditions (Unica and Sarnav) and one drought susceptible European cultivar (Désirée). In the control treatment, soil water content was kept at field capacity over the whole growth cycle, while in the drought treatment water supply was restricted after tuber initiation (50 % of field capacity). Gas exchange and N content per unit leaf area (N_area) as well as ? were assessed at different stages. Sarnav showed the highest tuber yield in both water conditions, suggesting that yield in the water restriction treatment was largely driven by yield potential in this genotype. Higher stomatal conductance (g_s) and N_area and lower ?_leaflet in well‐watered Sarnav suggested higher photosynthetic capacity. Under water restriction, Sarnav maintained higher g_s indicating that carbon diffusion was a key factor for biomass accumulation under water restriction. Our results suggest the use of ? determined after tuber initiation as an indirect selection indicator for tuber yield under both well‐watered and restricted soil water availability conditions.     

Elevated CO2 modulates the effects of drought and heat stress on plant water relations and grain yield in wheat

To investigate the interactive effects of drought, heat and elevated atmospheric CO₂ concentration ([CO₂]) on plant water relations and grain yield in wheat, two wheat cultivars with different drought tolerance (Gladius and Paragon) were grown under ambient and elevated [CO₂], and were exposed to post‐anthesis drought and heat stress. The stomatal conductance, plant water relation parameters, abscisic acid concentration in leaf and spike, and grain yield components were examined. Both stress treatments and elevated [CO₂] reduced the stomatal conductance, which resulted in lower leaf relative water content and leaf water potential. Drought induced a significant increase in leaf and spike abscisic acid concentrations, while elevated [CO₂] showed no effect. At maturity, post‐anthesis drought and heat stress significantly decreased the grain yield by 21.3%–65.2%, while elevated [CO₂] increased the grain yield by 20.8% in wheat, which was due to the changes of grain number per spike and thousand grain weight. This study suggested that the responses of plant water status and grain yield to extreme climatic events (heat and drought) can be influenced by the atmospheric CO₂ concentration.     

Old and New Cultivars of Soya Bean (Glycine max L.) Subjected to Soil Drying Differ in Abscisic Acid Accumulation,Water Relations Characteristics and Yield

Two old (Huangsedadou and Longxixiaohuangpi (LX)) and two new (Jindou 19 (JD) and Zhonghuang 30 (ZH)) soya bean (Glycine max (L.) Merr.) cultivars were used to investigate the influence of soil drying on the abscisic acid (ABA) accumulation in leaves, stomatal conductance (g_s), leaf water relations, osmotic adjustment (OA), leaf desiccation tolerance, yield and yield components. The greater ABA accumulation was induced by soil drying, which also inducing g_s decreased at higher soil water contents (SWC) and leaf relative water content (RWC) significantly decreased at lower SWC in the new soya bean cultivars than in the old soya bean cultivars. The soil water threshold between the value at which stomata began to close and the RWC began to decrease was significantly broader in the new cultivars than in the old cultivars. The new cultivars had significantly higher OA and lower lethal leaf water potential than old cultivars when the soil dried. The old cultivars had greater biomass, but lower grain yield than the new cultivars in well‐watered, moderate stress and severe stress conditions. Thus with soil drying, the new soya bean cultivars demonstrated greater adaptation to drought by inducing greater ABA accumulation, stomatal closure at higher SWC, enhanced OA and better water relations, associated with increased leaf desiccation tolerance, greater water use efficiency and higher yield.