Lower Far-Red Light Levels Improve Tolerance to High Evaporative Demand in Cucumber (Cucumis sativus L.) Seedlings by Increasing Leaf Hydraulic Conductance

Under light with a low proportion of far-red (FR) light, plants perceive themselves as growing in open places, which may lead them to increase leaf hydraulic conductance (K_leaf) to cope with the higher water demand associated with increased light intensities. We evaluated K_leaf of cucumber (Cucumis sativus L.) seedlings that had been acclimatized to light with different proportions of FR. K_leaf tended to increase with decreasing FR light. K_leaf and leaf vein length density were positively correlated, indicating that increased K_leaf caused by low FR light may have been caused by changes in leaf vein structure. To clarify whether acclimatization to low-FR light can improve tolerance to high evaporative demand, we evaluated changes in stomatal conductance (g_s), quantum yield of photosystem II (Φ_PSII), and leaf water potential (Ψ_leaf) when seedlings that had been acclimatized to light with FR light in the same proportion as sunlight (FR+) or light without FR light (FR−) were transferred to a high vapor-pressure deficit (VPD) condition. After transfer to high VPD, g_s and Ψ_leaf of the seedlings decreased in all treatment groups, but the decrease was smaller in the FR− seedlings. After transfer to high VPD, Φ_PSII decreased significantly in the FR+ seedlings, but not in the FR− seedlings. These findings suggest that the changes in stress tolerance induced by FR light may be partly mediated by changes in K_leaf. Our results also indicate a potential new technique for mitigating drought stress in horticultural crops by controlling FR light.