Interactive effects of far-red light and vapor-pressure deficit on photosynthetic performance and water-use efficiency of cucumber seedlings

Exploring the effects of interactions between light quality and other environmental factors on photosynthetic performance is crucial for improving post-transplant performance of transplants grown under artificial lighting. We investigated the effects of far-red (FR) light and vapor-pressure deficit (VPD) on the photosynthetic performance and water-use efficiency of cucumber (Cucumis sativus) seedlings. Seedlings were grown in controlled-environment chambers under four different combinations of FR light and VPD: with or without FR light (FR+ or FR–), under low or high VPD (0.8 or 2.3 kPa). We found a synergistic interaction in which FR– combined with increased VPD enhanced photosynthetic performance. Analysis of the response curves of net photosynthetic rate (P_n) to intercellular CO₂ concentration indicated that this enhancement was mainly due to non-stomatal factors. Leaf mass per unit area, which was positively correlated with P_n, was higher under these conditions, implying increased mesophyll capacity. Intrinsic water-use efficiency (WUE_i) was higher in leaves that had been acclimatized at FR+ and high VPD. Under FR–, particularly at high VPD, the increase in photosynthesis was outweighed by an increase in stomatal conductance (g_s), leading to lower WUE_i. This increased g_s may be attributable to improved leaf hydraulic conductance induced by low-FR light. Our results reveal the complex interactions between light quality and evaporative demand through their effects on plant responses, which suggest possible trade-offs between increasing photosynthetic performance and maintaining water-use efficiency across combinations of FR light and VPD. Such insights are critical for optimizing growth conditions in controlled environments for transplant production to improve post-transplant growth.