Effects of girdling and leaf removal on fruit set and vegetative growth ingrape

The effect of the carbohydrate supply on fruit set and on the development o f the inflorescence, from flowering to three weeks after flowering, was inv estigated using field-grown Sauvignon blanc grapevines (Vitis vinifera L.). Carbohydrate supply in individual shoots was manipulated by leaf removal a nd by isolating them from the parent vine using a steam girdle at their bas es. Shoots were either not girdled and had no leaves removed (control) or g irdled with 0, 1, 2, 4, 6, 8, 10, or 12 leaves removed. The fresh and dry w eight of each remaining leaf and internode were used as indicators of carbo hydrate supply. Effects of carbohydrate supply on reproductive growth were assessed by recording the dimensions and fresh and dry weights of the rachi s and berries. Percentage fruit set was determined from flower and berry co unts. Fruit set and vegetative growth responded in an exponential and linea r manner, respectively, to changes in carbohydrate supply. Elevated carbohy drate supply increased fruit set to 55% compared with 33% in the controls. There was no fruit set in the harshest treatments which had 10 or 12 leaves removed. Removing only one or two basal leaves substantially reduced fruit set, although up to four leaves had to be removed before the dry matter ac cumulation in the basal part of the shoot declined to levels similar to tho se in the ungirdled control. The reduction in fruit set, despite an apparen t increase in available carbohydrates, indicates the importance of the adja cent basal leaves to the developing cluster. The wide range of fruit sets a chieved in this experiment, and their exponential responses to carbohydrate supply, indicate that carbohydrate is the major determinant of fruit set i n grapes. Moreover, symptoms such as abortion of branches of the rachis and similar to those of the disorder early bunchstem necrosis (EBSN) increased as the level of carbohydrate supply declined. This indicates that EBSN can be caused by inadequate carbohydrate supply, and is not necessarily relate d to problems of nitrogen nutrition or of mineral imbalance.