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

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 nonstomatal (mesophyll) limitation. If g s can be lowered without significantly reducing photosynthesis, water-use efficiency (WUE = CO 2 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 28C), 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 28C under saturating light. The photosynthetic response to the intercellular CO 2 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 CO 2 concentration of 400 molmol -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-1,5bisphosphate 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.