Sensitivity of Two Quinoa (ChenopodiumquinoaWilld.) Varieties to Progressive Drought Stress

Quinoa (ChenopodiumquinoaWilld.) is a highly nutritious Andean seed crop which shows great potential to grow under a range of hostile environments. The objective of this study was to investigate the differences of drought tolerance of a Bolivian (Achachino) and a Danish (Titicaca) variety, and especially drought‐related adaption strategies. Soil water status was expressed as the fraction of transpirable soil water (FTSW). Relative stomatal conductance (RSC), relative transpiration (RT) and relative leaf water potential (RLW) were calculated by determining stomatal conductance, transpiration rate and leaf water potential of the drought‐treated plants relative to those of fully irrigated plants. The responses of RSC, RT and RLW to decreasing FTSW were described by a linear‐plateau model. The critical value of FTSW was the threshold of FTSW where the parameters studied decreased. The thresholds increased C_S for stomatal conductance, C_T for transpiration and C_Lfor leaf water potential. Achachino showed significantly lower C_T and C_L when compared with Titicaca, implying that transpiration and leaf water potential were less affected under mild drought conditions in the Bolivian variety. C_S in Achachino was significantly higher than C_L and C_T, which indicated that stomatal conductance declined before transpiration and leaf water potential were reduced. Such difference was found in Titicaca where reduction of leaf area had more effect on transpiration than stomatal closure. Slower growth rate and smaller leaf area in combination with a lower stomatal conductance was found to contribute to drought resistance in Achachino. ABA concentration in the xylem sap tended to increase in both varieties after 2 days onset of drought, prior to decline in leaf water potential. Titicaca showed significantly (P < 0.05) higher ABA concentration when compared with Achachino under both fully irrigated and drought conditions. Titicaca had higher xylem nutrient concentration in comparison with Achachino in both fully‐watered and drought plants at day 2 after onset of soil drying. It was concluded that Titicaca was more sensitive to progressive drought than Achachino which avoided water loss by means of lower growth rate and smaller leaf area.     

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.     

外源ABA对干旱胁迫下玉米叶片光合能力及气孔开度的影响

为探究旱胁迫下外源ABA对玉米品种叶片光合作用以及气孔开度的影响情况,以玉米品种‘郑单988’为研究对象,采用PEG、ABA、ABA+PEG不同处理对不同时间节点下玉米苗期叶片光合作用、气孔张开程度和叶绿素含量变化进行研究。发现不同处理下3 个时间节点光合速率、蒸腾速率、气孔导度和叶绿素含量均低于对照,而气孔闭合数目均高于对照;ABA+PEG处理的4 个指标均处于PEG和ABA处理之间。说明干旱胁迫下玉米为了维持自身的生长发育,通过关闭气孔减少水分蒸腾;外施ABA可增加干旱胁迫下气孔的关闭数目和叶绿素含量,减少水分流失的同时提高旱胁迫下植株的光合速率,最终提高植株的抗旱能力。     

Maize Hybrid Variability for Transpiration Decrease with Progressive Soil Drying

Drought is ubiquitous in rainfed cropping systems and often limits maize yields. The sensitivity of transpiration response early in progressive soil drying is a trait with potential to improve crop drought resistance. Simulation studies demonstrated that increased sensitivity to drying soil leading to restricted transpiration rates results in conservation of soil water during vegetative stages for possible use during grain filling. In contrast to other crops, there have been no studies characterizing genotypic variability for this trait in maize. Experiments in controlled environments were conducted to characterize the fraction of transpirable soil water (FTSW) threshold on drying soil for 36 hybrids selected for variation in the field for drought resistance, regions of adaptation and stay green. While FTSW thresholds varied among hybrids from 0.60 to 0.33, these thresholds were not uniformly associated with level of drought resistance in the field. Nevertheless, this study demonstrated a high FTSW threshold corresponded with drought resistance observed in some modern maize germplasm (hybrids #7, 17, 24, 27 and 32). This knowledge can enable breeding work seeking to exploit this adaptive trait to improved drought tolerance in low threshold FTSW germplasm.     

Thresholds for leaf expansion and transpiration response to soil water deficit in a range of sunflower genotypes

The sunflower (Helianthus annuus L.) crop in southern Europe suffers from intense and frequent periods of water deficit. Minimisation of water loss in response to water deficit is a major aspect of drought tolerance and can be achieved through the lowering of either leaf area expansion rate or transpiration per unit leaf area (stomatal conductance). During three greenhouse pot experiments, leaf expansion (LE) and transpiration (TR) rates were monitored as the soil dried progressively for about 15 days. This study aimed to quantify the response of these two physiological processes to water deficit, expressed as the fraction of transpirable soil water (FTSW): response thresholds (onset of decline) were estimated on 25 sunflower genotypes from different generations of selection history. From these relationships, the thresholds below which LE and TR started to decrease from the control were calculated: little change was observed until FTSW was close to 0.6 for leaf expansion and 0.4 for daily transpiration. Variability in the response of all genotypes for expansion and transpiration control was better described using specific thresholds for each genotype rather than generic thresholds for sunflower. The ranking of genotypes was found to be unaffected for transpiration rate control, but this was not the case for leaf expansion. Identified response thresholds were not mutually correlated, suggesting that sunflower controls leaf expansion and transpiration rate independently. Neither was correlated with the release date of the genotype, suggesting that these traits were not subject to selection within evaluation environments.     

Water Relations and Transpiration of Quinoa (Chenopodium quinoa Willd.) Under Salinity and Soil Drying

Drought and salinity are the two major factors limiting crop growth and production in arid and semi‐arid regions. The separate and combined effects of salinity and progressive drought in quinoa (Chenopodium quinoa Willd.) were studied in a greenhouse experiment. Stomatal conductance (g_s), leaf water potential (Ψ_l), shoot and root abscisic acid concentration ([ABA]) and transpiration rate were measured in full irrigation (FI; around 95 % of water holding capacity (WHC)) and progressive drought (PD) treatments using the irrigation water with five salinity levels (0, 10, 20, 30 and 40 dS m^?1); the treatments are referred to as FI₀, FI₁₀, FI₂₀, FI₃₀, FI₄₀; PD₀, PD₁₀, PD₂₀, PD₃₀, PD₄₀, respectively. The measurements were carried out over 9 days of continuous drought. The results showed that increasing salinity levels decreased the total soil water potential (Ψ_T) and consequently decreased g_s and Ψ_l values in both FI and PD. During the drought period, the xylem [ABA] extracted from the shoots increased faster than that extracted from the roots. A reduction in Ψ_T, caused by salinity and soil drying, reduced transpiration and increased apparent root resistance (R) to water uptake, especially in PD₀ and PD₄₀ during the last days of the drought period. The reasons for the increase in apparent root resistance are discussed. At the end of the drought period, the minimum value of relative available soil water (RAW) was reached in PD₀. Under non‐saline conditions, Ψ_l decreased sharply when RAW reached 0.42 or lower, but under the saline conditions of PD₁₀ and PD₂₀, the threshold values of RAW were 0.67 and 0.96, respectively. In conclusion, due to the additive effect of osmotic and matric potential during soil drying on soil water availability, quinoa should be re‐irrigated at higher RAW in salt‐affected soils, i.e. before the soil water content reaches the critical threshold level causing the drop in Ψ_l resulting in stomatal closure.     

Atmospheric and soil water deficit induced changes in chemical and hydraulic signals in wheat (Triticum aestivum L.)

Plant responses to soil drying and the metabolic basis of drought-induced limitations in stomatal opening are still being discussed. In this study, we investigate the roles of root-born chemical and hydraulic signals on stomatal regulation in wheat genotypes as affected by soil drought and vapour pressure deficit. Twelve consecutive pot experiments were carried out in a glasshouse. Two bread wheat cultivars (Gönen and Basribey) were subjected to drought under high and low vapour pressure deficit (VPD) in a growth chamber. Total dry matter, specific leaf area, xylem ABA content, xylem osmotic potential, xylem pH, root water potential (RWP), stomatal conductance, leaf ABA content and photosynthetic activity were determined daily during 6 days after the onset of treatments (DAT). In the first phase of drought stress, soil drying induced an increase in the xylem ABA with a peak 3 DAT while RWP drastically decreased during the same period. Then the osmotic potential of leaves decreased and leaf ABA content increased 4 DAT. A similar peak was observed for stomatal conductance during the early stress phase, and it became stable and significantly higher than in well-watered conditions especially in high vapour deficit conditions (H-VPD). Furthermore, xylem pH and xylem osmotic potential appeared to be mostly associated with atmospheric moisture content than soil water availability. The results are discussed regarding possible drought adaptation of wheat under different atmospheric humidity.     

春玉米苗期光合速率、蒸腾速率及气孔导度对土壤干旱的反应

为深入探索东北地区土壤水分含量变化对春玉米光合作用、蒸腾速率和气孔导度的影响,揭示玉米苗期干旱减产的生理机制,2010年春季在东北地区中部开展分期播种与土壤水分处理试验,进行土壤湿度、玉米苗情、净光合速率(NPn)、蒸腾速率(Tr)、气孔导度(Gs)等观测,分析它们之间的关系。结果表明,春玉米苗期叶片NPn、Tr和Gs与土壤水分变化之间分别呈二次函数关系,0~20 cm深土壤湿度在19.5%以上时,玉米叶片气孔导度大,光合作用和蒸腾作用旺盛;土壤湿度在19.0%以下,随着土壤湿度下降,NPn和Tr近于线性下降,土壤湿度每降低1个百分点,NPn和Tr分别下降1.6 μmol/(m2.s)和0.5 mol/(m2?s)。玉米叶片Tr和Gs与NPn的关系为线性函数,Gs和Tr每降低1 mol/(m2?s)和1 mmol/(m2?s),NPn分别下降0.89和3.09 μmol/(m2?s)。玉米苗在干旱胁迫下气孔关闭,蒸腾作用减弱,使光合速率快速下降,进而抑制玉米营养生长,最终导致减产。     

Genotypic Variability in Drought Performance and Recovery in Cowpea under Controlled Environment

Mid‐season drought is a factor frequently limiting crop production in the moist to dry savannah zones of the tropical and subtropical regions of the world. Ten cowpea genotypes were subjected to a cycle of drought at flowering followed by re‐watering to study variation in drought performance and recovery. Drought caused a reduction in leaf assimilation rate, transpiration rate and stomatal conductance with genotypic variances of 75.4, 57.9, and 83.3 %, respectively. Only genotypic variance in stomatal conductance increased appreciably under drought. Reductions in leaf water potential as a consequence of drought positively correlated with a decline in assimilation rate, which was associated with stomatal closure. One week after re‐watering, the three gas exchange parameters of stressed plants recovered fully and attained values 10–30 % higher than the well‐watered plants with increased genotypic variability. Reductions in the total dry matter during the drought interval varied from 11 to 50 % among genotypes, but were of minor importance for the total dry matter at maturity. After stress, the gain in dry matter varied considerably among the stressed genotypes, with stressed plants showing higher gain than the unstressed plants during this interval. This was associated with increased availability of assimilates due to enhanced green leaf area duration after stress release. Variability in drought recovery among genotypes was found, and appears to be more important for final yield than responses during drought.     

Relationships between leaf morpho‐anatomy,water status and cell membrane stability in leaves of wheat seedlings subjected to severe soil drought

The impact of the genotype‐specific leaf morphological and anatomical characteristics on the ability of wheat plants to preserve leaf water balance and cell membranes stability under drought stress was investigated. Seedlings of six modern semi‐dwarf (carriers of Rht, Reduced height genes) and six old tall bread wheat varieties were subjected to soil drought by withholding watering for 6 days. Morpho‐anatomical traits (leaf area, perimeter, thickness, stomata and trichome density) of daily watered (control) plants were characterized by light microscopy, scanning and image analyses. The leaf water status in both control and stressed plants was determined by measuring the relative water content (RWC). The leaf cell membranes stability in stressed plants was estimated by conductometric determination of the membranes injury index. On average, the modern semi‐dwarf varieties had less leaf area and leaf perimeter, and less dissection index, a parameter characterizing the leaf shape. Under drought stress, the modern genotypes maintained better water balance evidenced by significantly higher leaf RWC and better‐preserved the cell membranes stability supported by significantly lower Injury index. The correlations between morpho‐anatomical traits in control plants and drought tolerance‐related traits showed that the higher the leaf dissection index (i.e. more oblong leaves), the greater the water loss and the leaf membrane damages after desiccation were. The effect of shape of the evaporating surface on the water loss was modelled using wet filter paper. Similar to plant leaves, the evaporation and, respectively, water loss from paper pieces of more oblong shape (i.e. higher dissection index) was more intensive. The elucidation of the impact of the leaf shape on transpiration might contribute to better understanding of the mechanisms used by plants to maintain water reserves during drought stress and could be a basis for developing of simple and fast screening methods aiding the selection of drought tolerant genotypes.     

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.     

Intact Plant, Callus and Field Grain Yield Responses of Tepary Bean to Drought

In a field trial involving four tepary lines (Phaseolus acutifolius A. Gray), NE#8A and NE#19 produced higher grain yield than NE#5 and NE#7 under both well watered and drought conditions. However, NE#8A is considered more resistant than NE#19 in terms of drought sensitivity index. Greenhouse investigations on intact plants indicated no differences among the four lines in leaf and stem dry mass, and leaf area. Root depth did not strictly differentiate lower‐yielding from higher‐yielding lines. In contrast to lower‐yielding lines, however, plants of higher‐yielding ones allocated greater dry matter (DM) in roots in response to imposed water stress. Distinctly, NE#19 had the greatest root : shoot (R : S) while NE#8A characterized by high net photosynthesis. Both NE#8A and NE#19 showed reduced leaf area : root dry mass ratio, stomata conductance and transpiration rate. Consequently, these two lines showed no significant changes in leaf relative water content while photosynthetic water‐use‐efficiency increased in response to water stress. Calli derived from leaf and root tissues of higher‐yielding lines exhibited low initial osmotic potential (ψ_s). These calli did not show alterations in ψ_s, DM% and relative growth rate (RGR) when subjected to water stress. Although leaf‐ and root‐derived calli of lower‐yielding lines exhibited osmotic adjustment, they suffered water stress in terms of elevated DM and reduced RGR. Overall, results suggest that dehydration‐avoidance mechanisms conditioned by increased root mass and stomata resistance accompanied with low initial cellular ψ_s sustained high grain yield of tepary under limited water supply.     

Biochar amendment of fluvio‐glacial temperate sandy subsoil: Effects on maize water uptake,growth and physiology

Coarse sandy soils have poor water retention capacity, which may constrain crop growth during drought. We investigated the effect of biochar amendment to subsoil on crop physiological processes and maize yield, comparing irrigated and drought conditions. A two‐year greenhouse experiment was conducted with one‐time application of straw biochar at concentrations of 0%, 1%, 2% and 3% (B0, B1, B2 and B3). Maize was planted twice in the same large pots one week and again 12 months after biochar application. Plants were fully irrigated until flowering; thereafter, half of them were subjected to drought. Our results indicate B2 and B3 increased soil water content at field capacity. Leaf water potential, stomatal conductance, photosynthesis and transpiration were maintained in B2 and B3 during the drying cycle in year one and in all biochar levels in year two. In the first year, B3 induced negative root geotropism and significantly reduced vegetative biomass under both irrigation schemes. Cob biomass was significantly reduced by B1 under full irrigation. In year two, B3 significantly increased cob biomass under drought. Nitrogen uptake was significantly reduced by B2 in year one, but increased significantly in B3 in year two. In both years, P uptake was significantly increased by B2 and B3. Furthermore, K uptake was significantly increased in B2 in year one and in all biochar treatments in year two. Overall, biochar improved water content of coarse sandy soil due to decreased bulk density and increased porosity after biochar amendment, consequently, improving crop physiological processes including transpiration and photosynthesis. Significant effects on yields tended to be more negative in the first year, and neutral to positive in the second year suggesting the enhancement of biochar effects with ageing. The positive effect in the second year shows biochar's potential for improving agriculture productivity in drought‐prone regions.     

Effects of Soil Moisture on Gas Exchange, Partitioning of Fed 14CO2 and Stable Carbon Isotope Composition (δ13C) of Leymus chinensis Under Two Different Diurnal Temperature Variations

In a pot experiment under controlled condition, Leymus chinensis was grown at five soil water contents and two diurnal temperature variation levels. The leaf relative water content of L. chinensis decreased under soil drought conditions, and increased at high diurnal temperature variation (30/20 °C). Severe and very severe soil drought remarkably reduced photosynthetic rate, stomatal conductance, transpiration rate and water use efficiency (WUE), especially at the low temperature variation. Severe drought mainly increased the specific radioactivity of ¹⁴C of sheaths, roots and rhizomes, but significantly decreased that of fed leaves and the not fully expanded leaves. Root percentage of total radioactivity remaining in the whole plant increased from 15 % at 30/25 °C to 28 % at 30/20 °C. Leaf carbon stable isotope composition (δ¹³C) increased with soil drought, ranged from ?26 ‰ of the well‐watered to ?24 ‰ of severe drought treatments. High diurnal temperature variation improved leaf water status, and increased partitioning of carbon to root and δ¹³C values, especially under severe soil drought conditions. In conclusion, higher diurnal temperature variation enhanced the resistance of the plant to drought.     

玉米根、茎、叶中脱落酸含量和产量形成对土壤干旱的响应

对不同程度土壤干旱胁迫下玉米根、茎、叶中脱落酸含量与产量形成进行了试验研究。充足底墒播种后采用5个土壤水分处理（分别占土壤田间持水量>80%、70%～80%、60%～70%、50%～60%、40%～50%,代号为WT1～WT5）,并遮去自然降水。试验结果表明,拔节期从中等干旱胁迫开始,玉米根、茎、叶中ABA含量大幅度增加,干旱加剧会使根     

Divergent strategies displayed by potato (Solanum tuberosum L.) cultivars to cope with soil drought

The physiological responses of potato (Solanum tuberosum L) cultivars to soil drought at the tuberization phase and their impact on agronomically important traits were investigated in potted plants under semi‐controlled conditions. Genotype‐dependent responses of potato to water deficiency were evaluated on two pairs (tolerant/sensitive) of Polish cultivars. Each pair of cultivars had a similar genetic background but was extremely different in terms of drought tolerance evaluated on the basis of loss of tuber yield under field conditions. The results clearly indicate different mechanisms of tolerance to water deficiency and the ability to recover from soil drought in two tolerant but genetically unrelated cultivars. When subjected to soil drought, the cultivar Gwiazda had low rates of transpiration and photosynthesis and low levels of stomatal conductance due to hypersensitivity to ABA, but its maximal photochemistry efficiency and PSII performance index were unchanged. Another strategy was displayed by the dehydration‐avoidant cultivar Tajfun, which kept the stomata partially open. Thus, the plants were able to retain a relatively high rate of photosynthesis over transpiration. The parameters measured for cultivar Tajfun for photosynthesis and transpiration were the same after plant rewatering, similar to the control plants. This was not the case for the cultivar Gwiazda. The ability of plants to regenerate after soil drought relief appears to be a good indicator of potato susceptibility to soil drought and allows the yield of potato tubers to be predicted. The physiological traits identified in closely related potato cultivars but differed in their drought tolerance seem to be useful for genetic engineering and breeding programmes.     

Growth and development of bambara groundnut (Vigna subterranea) in response to soil moisture: 2. Resource capture and conversion

A study was conducted in controlled environment glasshouses to investigate the effects of soil moisture on resource capture and conversion of three landraces (DipC, S19-3 and UN from Botswana, Namibia and Swaziland, respectively) of bambara groundnut (Vigna subterranea (L.) Verdc.). The study was conducted under two soil moisture treatments: an irrigated control and a drought treatment where irrigation was withheld from approximately flowering to final harvest. Drought reduced the mean fractional intercepted radiation (f) from 0.8 to less than 0.7 across landraces. The mean light extinction coefficient (=0.46) was not affected either by landrace or watering regime, while cumulative intercepted radiation (S_ci) reduced under drought because of the reduction in f. Drought reduced total transpiration (E_c) only in DipC while it had no effect on the other two landraces. Crops under irrigation extracted most of the water from the top 50 cm of the profile while those under drought extracted water down to 90 cm. The conversion coefficient for intercepted radiation (_s; g MJ⁻¹) was reduced by 32%, from 1.51 to 1.02 g MJ⁻¹. Similarly, drought reduced the dry matter/transpired water ratio (ε_w; g kg⁻¹) by 20% from 2.05 to 1.65 g kg⁻¹.     

Transpiration Dynamics in Cadmium-Treated Soybean (Glycine max L.) Plants

Soybean ( Glycine max L.) plants, cv Richland, were grown during 30 days in a nutrient solution. After this period the plants were treated with a 50 μM Cd(NO₃)₂ solution. Sap flow rate and stomatal conductance were monitored during 4 consecutive days; at the end of this period relative water content and stomatal width and length were determined on fully expanded leaves. On the second day from the Cd treatment sap, flow rate and stomatal conductance in the treated plants were reduced to 60% of the control plants. Stomatal conductance kept on decreasing up to the fourth day. Cadmium decreased the leaf relative water content and the reduction in the stomatal closure was supported by an increase in the L/W ratio of the stomata. Root water uptake should be the primary mechanism reduced by Cd stress in soybean, and this reduction is consistent with the decrease in transpiration rate and with the stomatal closure.