Effects of root temperature on cytokinin concentrations in roots and root xylem exudate were compared between cucumber (Cucumis sativus L. cvs. ‘Suyô’ and ‘Kurume-ochiai H’) and figleaf gourd (Cucurbita ficifolia Bouchè). The rootchilling tolerance of the latter plants is known to be considerably higher than that of the former plants.
Cytokinin concentrations in root xylem exudate of cucumber cultivars decreased sharply at lower root temperatures, with ‘Suyô’ more greatly affected. In contrast, in figleaf gourd the concentrations were relatively unchanged at 14-23°C, and increased to a strikingly high level at 11°C. Cytokinin concentrations in roots were highest at 23°C in ‘Suyô’ and at 17°C in ‘Kurume-ochiai H’, and decreased at lower root temperatures in both cultivars. In figleaf gourd, on the contrary, the concentrations were significantly higher at 14°C than at higher temperatures, and increased to a still higher level at 12°C. Major cytokinins in the roots of figleaf gourd and probably of cucumber cultivars were zeatin and zeatin riboside, and the composition apparently changed little with root temperature.
These results strongly suggest that figleaf gourd roots respond to low root temperature by stimulating cytokinin synthesis within the roots. However, cytokinin synthesis in cucumber roots may be greatly inhibited by low root temperature. The implications for the difference in root-chilling tolerance between cucumber and figleaf gourd are discussed.