Photosynthetic Properties and Response to Drought in Cucumber Seedlings Acclimatized to Different Vapor-pressure-deficit Levels

Abstract

Hardening with a high vapor-pressure-deficit (VPD) can decrease conductance (g_s) 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 g_s 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 (P_n) and g_s 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 (C_i) was used to quantify the effects of VPD acclimatization on carboxylation efficiency. P_n did not differ significantly between acclimatization VPDs, but g_s was much lower (×0.36) in high-VPD leaves at an ambient CO₂ concentration of 400 μmol·mol⁻¹. Thus, the intrinsic WUE (= P_n/g_s) of the high-VPD leaves was much higher (×2.85). The C_i of high-VPD seedlings was lower than that of low-VPD seedlings, but, this did not cause any significant reduction in P_n in the high-VPD treatments because the decrease in C_i occurred within a range in which photosynthesis was not limited by ribulose-1,5-bisphosphate carboxylation in this experiment. The P_n–C_i 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 g_s 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 g_s 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 g_s effectively.