Impaired Photosynthesis in Cucumber (Cucumis sativus L.) by High Root-zone Temperature Involves ABA-induced Stomatal Closure and Reduction in Ribulose-1,5-bisphosphate Carboxylase/oxygenase Activity

Abstract

Effects of high root-zone temperature (RT) on ABA content, photosynthesis and its related parameters in cucumber (Cucumis sativus L., cv. Suyo) were investigated. Plants were grown in water culture at a RT of either 30°C or 3°C for 10 days. Other growth conditions were 30/25°C air temperatures (day/night), a 14-hr photoperiod, and a light intensity of 250 μmol·m·s-1. Plant growth was severely inhibited at 38°C RT as compared with that at 30°C RT. At 38°C RT, leaves did not wilt and the leaf color was normal green, but showed a marked increase in ABA content after day 6 with a significant decrease in stomatal leaf conductance. In both RT treatments, leaf water potential declined at day 6, which was more pronounced at 38°C RT than at 30°C RT, but the difference was relatively small. The photochemical activity of chloroplasts was not affected by high RT. However, the CO2 exchange rate (CER) of leaves decreased linearly after day 4 in the high RT treatment. Peeling the epidermis of the abaxial leaf surface to eliminate the stomatal limitation to CER resulted in a significant but not complete recovery of CER, indicating that the high RT limited CER through a decrease in not only stomatal aperture but also photosynthetic capacity of mesophyll cells. After day 6 of high RT treatment, ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) exhibited a gradual decrease in the initial level and the activation state. The results indicate that ABA-induced stomatal closure and reduction in the activation state of Rubisco are responsible for impaired photosynthesis in cucumber grown at elevated RT.