Effect of Root-Zone Temperature on Water Relations and Hormone Contents in Cucumber

The cucumber (Cucumis sativus L.) is sensitive to change in cultivating environment. Among others, changes in root-zone temperature as well as air temperature have direct impacts on cucumber cultivation. Sub-optimal or supra-optimal root-zone temperature in the winter or summer season leads to inhibited growth and decreased yield in cucumbers. This study was conducted to identify the impacts of various root-zone temperatures on physiological responses such as leaf water content, proline content, abscisic acid (ABA), and cytokinin in cucumber. Cucumber cultivar of 'Eunseong Baegdadagi' was grown using the deep flow technique where solution temperatures were maintained at 15℃, 20℃, 25℃, 30℃, and 35℃. Up to 14 days' treatment, there were no significant differences in growth among 20℃, 25℃, and 30℃ but there were significantly lower growth at 15℃ and the lowest at 35℃, respectively. The amount of xylem exudate was severely reduced by 50% at 15℃ and 4% at 35℃ compared with the cucumber plants grown at 20℃ after 16 days' treatment. Leaf water content varied with increased or decreased temperature at reference temperature, 20℃. Severe low leaf water content was measured at 35℃. Positively correlated with water stress, proline concentration and leaf osmolality in leaves were increased in 5 days of treatment. Water stress induced by suboptimal and supraoptimal root-zone temperatures resulted in a significant ABA accumulation in leaf tissues. The concentration of cytokinin decreased in leaves of cucumber plants grown at 15℃ and especially at 35℃ than in those of plants grown at 20℃ root-zone temperature. These results suggest that growth inhibition at 15℃ and 35℃ root-zone temperature was affected by low water status and cytokinin concentration and high ABA concentration in cucumber leaves. Although growth was not significantly different at 20℃, 25℃, and 30℃ in 14 days' treatment, higher osmolality and proline content in 25℃ and 30℃ was investigated.