As soil water deficit limits crop growth and yield, yet the combined effects of soil drying and elevated vapour pressure deficit (VPD) of the air on crop performance have not been fully understood. The objective of this study was to investigate the interactive effects of soil drying and elevated VPD on growth and physiology of barley seedlings grown under controlled climates. The plants were grown in four separate climate chambers with two air temperature (T, 20 and 30°C) and relative humidity (RH: 60 and 75%) levels, respectively, resulting in four VPD levels, viz. VPD1 (0.59 kPa, 20°C + 75% RH), VPD2 (0.94 kPa, 20°C + 60% RH), VPD3 (1.06 kPa, 30°C + 75% RH) and VPD4 (1.70 kPa, 30°C + 60% RH). Since the 4^th leaf stage, the plants were subjected to two irrigation treatments, namely well-watered and drought-stressed. The results showed that soil water deficits significantly limited leaf gas exchange rates and reduced shoot dry biomass (DM_shoot) and water consumption (WU), whereas increased leaf ABA concentration ([ABA]_leaf) and plant water-use efficiency (WUE) at each VPD level. Plants grown under VPD1 and VPD2 had greater net photosynthetic rate (A_n) and stomatal conductance (g_s) while lower transpiration rate (T_r) than those grown under VPD3 and VPD4 at both irrigation treatments. Besides, elevated VPD resulted in an increased daily transpiration per leaf area (DT) and greater soil water threshold at which DT starts to decrease. Interactions between VPD and drought were significant where the effects of drought on A_n, relative water content (RWC), DM_shoot and WUE were more severe at VPD3 and VPD4 than that at VPD1 and VPD2. When disentangling the effects of T and RH, the results showed that it was T rather than RH influenced the A_n, stomatal density and RWC, whereas the combined effect of T and RH, that is VPD, was significant in affecting g_s, T_r, [ABA]_leaf, leaf area, DM_shoot, WU and WUE, where WUE was negatively correlated with VPD. Therefore, it is essential to dissect the effects of T and RH when analysing their combined effects with soil water deficits on crop performance in a future warmer and drier climate.

中文翻译：

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干旱胁迫下气温和相对湿度对大麦水分利用效率的综合影响

由于土壤水分亏缺限制了作物的生长和产量，但尚未完全了解土壤干燥和空气蒸汽压差 (VPD) 升高对作物性能的综合影响。本研究的目的是调查土壤干燥和 VPD 升高对在受控气候下生长的大麦幼苗生长和生理的相互作用影响。植物在四个独立的气候室中生长，分别具有两个气温（T、20 和 30°C）和相对湿度（RH：60 和 75%）水平，从而产生四个 VPD 水平，即。VPD1 (0.59 kPa, 20°C + 75% RH), VPD2 (0.94 kPa, 20°C + 60% RH), VPD3 (1.06 kPa, 30°C + 75% RH) 和 VPD4 (1.70 kPa, 30°C + 60% 相对湿度）。4^日以来在叶期，植物受到两种灌溉处理，即充分浇水和干旱胁迫。结果表明，土壤水分亏缺显着限制了叶片气体交换率并降低了地上部干生物量 (DM _shot ) 和耗水量 (WU)，而增加了叶片 ABA 浓度 ([ABA]_叶) 和植物水分利用效率 (WUE)。每个 VPD 级别。在 VPD1 和 VPD2 条件下生长的植物具有较高的净光合速率 (A _n ) 和气孔导度 (g _s ) 而较低的蒸腾速率 (T _r) 比在两种灌溉处理下生长在 VPD3 和 VPD4 下的那些。此外，升高的 VPD 导致每叶面积 (DT) 的每日蒸腾量增加，并导致 DT 开始降低的土壤水分阈值增加。VPD 和干旱之间的相互作用是显着的，其中干旱对 A _n、相对含水量 (RWC)、DM_枝条和 WUE 的影响在 VPD3 和 VPD4 比在 VPD1 和 VPD2 更严重。分解T和RH的影响，结果表明是T而不是RH影响了A _n、气孔密度和RWC，而T和RH的联合作用即VPD对g _s、T的影响显着。_r , [ABA]_叶, 叶面积, DM_茎、WU 和 WUE，其中 WUE 与 VPD 呈负相关。因此，在分析 T 和 RH 的影响与土壤水分亏缺对未来更暖和更干燥气候下作物性能的综合影响时，有必要剖析 T 和 RH 的影响。