Comparison Between the Effect of the Fruit and of Exogenous GA3-Applications on Source-Sink Relationships in Citrus Sprouts

Abstract

Inflorescence leaves of citrus (Citrus sinensis L. Osbeck) accumulate carbohydrate reserves at the beginning of the fruit set period. This effect is mimicked by exogenous GA3 applications in deflorate inflorescences. The effect of the hormone on two processes that could result in carbohydrate accumulation, i.e. increased ribulose bisphosphate carboxylase (RuBPCase) activity and changes in the import/export balance of assimilates in the sprouts, was studied. Neither RuBPCase activity nor respiration rates were affected by the hormonal treatment. On the other hand, blocking of the phloem connections with the rest of the plant induced an increase in carbohydrate contents in the leaves that was proportionally lower in GA3-treated sprouts than in control deflorate inflorescences. This increase was minimal in leaves from inflorescences at later stages of fruit development. This was in spite of a lower fruit growth rate that, in turn, brought about a decrease in total dry weight of the sprout. In contrast, total dry weight was higher in girdled deflorate inflorescences, whether or not they were treated with the hormone. The presence of the fruit induced a thickening of the stems of intact inflorescences that was also seen in ungirdled GA2-treated sprouts. This increase in dry weight per unit length, however, had a different chemical basis depending upon whether it was caused by the fruit or the hormone. From these data, the possible causes for the enhancement of carbohydrate accumulation in inflorescence leaves, and the involvement of gibberellin in the effect of fruit on the sprouts is discussed.