IMPLICATION OF ENDOGENOUS CYTOKININS IN THE GROWTH-INHIBITION OF CUCUMBER PLANTS BY SUPRAOPTIMAL ROOT-ZONE TEMPERATURE

Seedlings of cucumber (Cucumis sativis L., cv. Sharp I) were grown in hydroponic culture at root-zone temperatures of 25, 35, and 38 degrees 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 isopentenyl adenosine besides the above two cytokinins. High root-zone temperature s caused a marked decrease in cytokinin concentrations. The change was gradual at 35 degrees C but very rapid at 38 degrees C, and fluctuate d more in the roots than in the leaves. In plants grown at 38 degrees C, cytokinin concentrations in the roots were very low already at day 5; at day 10 both the roots and leaves contained only negligible conce ntrations of cytokinins. Zeatin riboside was more sensitive to high ro ot-zone temperatures than the other cytokinins in the leaves. Cucumber plants were grown at 38 degrees C for 10 days and treated with 0, 10( -6), 10(-5), and 10(-4) M benzylaminopurine (BAP) to leaves in combina tion with 0, 10(-6), and 10(-5) M abscisic acid (ABA) treatments to ro ots. Treatment of roots with 10(-6) M ABA promoted slightly growth of BAP-untreated plants; but 10(-5) M ABA was inhibitory, irresepective o f 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. L ower BAP concentrations had no significant effects. This growth-promot ing effect of 10(-4) M BAP sprays to leaves was not observed when the roots were supplied with ABA at either concentrations. The results str ongly suggest that inhibition of cytokinin synthesis in the roots and the resultant decrease in endogenous cytokinin concentrations in the l eaves are the bases for growth inhibition of cucumber plants at suprao ptimal root-zone temperature.