Gibberellin-Induced Suppression of Chloroplast Starch Formation from Exogenous Sucrose in Isolated Epidermis of Light-Grown Cucumber Hypocotyls

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Abstract
The effects of gibberellin A3(GA3) on chloroplast starch formation from exogenous sucrose in epidermal strips of light-grown cucumber (Cucumis sativus L. cv. Aonagajibai) hypocotyls were studied. Chloroplast starch formation was measured by counting microscopically the number of I2KI stained chloroplasts. In the presence of sucrose (1–50 mM) and in light, chloroplast starch formation occurred rapidly for 5 hr and then the rate declined. GA3 (100 μM) simultaneously supplied with sucrose clearly suppressed both the rate and the degree of starch formation. When the epidermis was preincubated in the dark with sucrose in the presence or absence of GA3 and then post-incubated in light with buffer only in the presence or absence of GA3, starch formation was suppressed to the same degree by GA3 given either in the pre- or post-incubation period. When the epidermis was preincubated in the dark with GA3 alone and then transferred to light for post-incubation with sucrose alone, GA3 suppressed the initial rate of starch formation during the post-incubation period. The degradation of chloroplast starch formed in advance from exogenous sucrose in light was not significantly affected by GA3 These results are discussed in relation to the mechanism of the GA3 action and also to the GA3-induced decrease in the osmotic potential of the cell.

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