Dissecting the combined effects of air temperature and relative humidity on water-use efficiency of barley under drought stress

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 degrees C) and relative humidity (RH: 60 and 75%) levels, respectively, resulting in four VPD levels, viz. VPD1 (0.59 kPa, 20 degrees C + 75% RH), VPD2 (0.94 kPa, 20 degrees C + 60% RH), VPD3 (1.06 kPa, 30 degrees C + 75% RH) and VPD4 (1.70 kPa, 30 degrees 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 (DMshoot) 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), DMshoot 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, DMshoot, 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.