ALUMINUM-INDUCED INCREASE OF ZEATIN RIBOSIDE AND DIHYDROZEATIN RIBOSIDE IN PHASEOLUS-VULGARIS L CULTIVARS

Toxic effects of aluminium (Al) on root tips are considered to decreas e export of cytokinins to shoots, and deficiency of cytokinins has bee n made responsible for Al-induced inhibition of shoot growth. But no e xperimental data on the influence of Al on endogenous cytokinin levels in higher plants have been reported. In this study, the endogenous le vels of zeatin riboside (ZR) and dihydrozeatin riboside (DHZR) of root s, stems, and leaves of two bean cultivars (Phaseolus vulgaris L. cv C ontender and cv Strike) exposed to Al in continuously flowing nutrient solution (pH 4.5) was analysed. The supply of a high Al concentration (sum of monomeric Al species, 127 mu M) caused severe inhibition of r oot elongation in both cultivars. The cv Strike was more affected by b oth Al-induced mineral nutrient disorders and: Al-induced alteration o f leaf water relationships. In both cultivars Al-supply significantly increased ZR and DHZR. Leaves of Al-treated plants exhibited a more th an three times higher concentration of ribosylated cytokinins than con trols. Nevertheless, stomatal resistance was significantly increased b y Al in both cultivars. Our results support the hypothesis that Al aff ects plants not by inducing deficiency of cytokinins but of some other factor necessary for the manifestation of cytokinin action.