Abstract
Seedlings of cucumber (Cucumis sativus L., cv. Sharp I) were grown in hydroponic culture at root-zone temperatures of 25, 35, and 38°C for 10 days. Cytokinins in the leaves and roots were quantified at day 5 and 10. Zeatin and its riboside were the main cytokinins in the leaves, while the roots contained a considerable amount of isopentenyladenosine besides the above two cytokinins. High root-zone temperatures caused a marked decrease in cytokinin concentrations. The change was gradual at 35°C but very rapid at 38°C, and fluctuated more in the roots than in the leaves. In plants grown at 38°C, cytokinin concentrations in the roots were very low already at day 5; at day 10 both the roots and leaves contained only negligible concentrations of cytokinins. Zeatin riboside was more sensitive to high root-zone temperatures than the other cytokinins in the leaves.
Cucumber plants were grown at 38°C for 10 days and treated with 0, 10-6, 10-5, and 10-4 M benzylaminopurine (BAP) to leaves in combination with 0, 10-6, and 10-5 M abscisic acid (ABA) treatments to roots. Treatment of roots with 10-6 M ABA promoted slightly growth of BAP-untreated plants; but 10 5 M ABA was inhibitory, irresepective of BAP treatments. On the other hand, foliar sprays of BAP at 10-4 M caused a marked increase in the growth rate of ABA-untreated plants. Lower BAP concentrations had no significant effects. This growth-promoting effect of 10-4 M BAP sprays to leaves was not observed when the roots were supplied with ABA at either concentrations.
The results strongly suggest that inhibition of cytokinin synthesis in the roots and the resultant decrease in endogenous cytokinin concentrations in the leaves are the bases for growth inhibition of cucumber plants at supraoptimal root-zone temperature.
