Abstract
Summary The regulation of ribulose-1,5-bisphosphate carboxylase (RuBPCase) by light and hormones has been studied in excised cotyledons of Cucumis sativus L. to determine the relationship between photomorphogenetic and hormonal control. Both content and activity of the enzyme are increased by light, this effeet being mediated by phytochrome, and can be stimulated by external application of 6-benzylaminopurine (BAP) and gibberellic aeid (GA 3 ). With increasing cytokinin concentration the effectiveness of light decreases and saturation by BAP is reached at lower concentrations in the light than in darkness, indicating interactions between photocontrol and cytokinin regulation. However, even at saturating and optimal BAP concentrations light retains a promoting influence which is supposed to be a cytokinin-independent component of light effect. Finally, a two-factor-analysis concerning BAP and GA 3 was performed. A positive correlation between GA 3 concentration and enzyme activity in the absence of BAP and an inhibition of cytokinin-mediated stimulation of RuBPCase by GA 3 indicate interactions between the two hormones.
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