Hardening with a high vapor-pressure-deficit (VPD) can decrease conductance (gs) and thereby enhance drought tolerance by reducing transpiration, which is particularly useful during transplant establishment. However, high-VPD hardening may decrease photosynthetic performance due to stomatal (diffusion) and non-stomatal (mesophyll) limitation. If gs can be lowered without significantly reducing photosynthesis, water-use efficiency (WUE = CO2 assimilation/transpiration), which is an important criterion for transplant quality, would improve. We investigated the photosynthetic properties of cucumber (Cucumis sativus L.) leaves acclimatized to different VPD levels (0.4 and 3.2 kPa at 28°C), and determined whether photosynthesis was limited by stomatal or non-stomatal factors at high VPD. The net photosynthetic rate (Pn) and gs were measured at a VPD of 0.8 kPa and a leaf temperature of 28°C under saturating light. The photosynthetic response to the intercellular CO2 concentration (Ci) was used to quantify the effects of VPD acclimatization on carboxylation efficiency. Pn did not differ significantly between acclimatization VPDs, but gs was much lower (×0.36) in high-VPD leaves at an ambient CO2 concentration of 400 μmol·mol−1. Thus, the intrinsic WUE (= Pn/gs) of the high-VPD leaves was much higher (×2.85). The Ci of high-VPD seedlings was lower than that of low-VPD seedlings, but, this did not cause any significant reduction in Pn in the high-VPD treatments because the decrease in Ci occurred within a range in which photosynthesis was not limited by ribulose-1,5-bisphosphate carboxylation in this experiment. The Pn–Ci curve did not differ between the VPD treatments, indicating that carboxylation efficiency was not affected. When VPD-acclimatized seedlings experienced a limited water supply, the low-VPD leaves showed lower leaf water potential and more severe wilting than the high-VPD leaves 30 min after water limitation began. The gs of the high-VPD leaves was significantly lower (approximately ×0.5 in average) than that of the low-VPD leaves throughout the water-supply-limitation treatment. The lower gs maintained the water status of the high-VPD leaves at less-wilting levels by decreasing transpiration. These results indicate that high-VPD hardening could enhance the tolerance to short-term drought without stomatal or non-stomatal limitation of photosynthesis when controlling gs effectively.
2017 The Japanese Society for Horticultural Science (JSHS), All rights reserved.