Effects of Girdling and Leaf Removal on Fruit Set and Vegetative Growth in Grape

Abstract

The effect of the carbohydrate supply on fruit set and on the development of the inflorescence, from flowering to three weeks after flowering, was investigated using field-grown Sauvignon blanc grapevines (Vitis vinifera L.). Carbohydrate supply in individual shoots was manipulated by leaf removal and by isolating them from the parent vine using a steam girdle at their bases. Shoots were either not girdled and had no leaves removed (control) or girdled with 0, 1, 2, 4, 6, 8, 10, or 12 leaves removed. The fresh and dry weight of each remaining leaf and internode were used as indicators of carbohydrate supply. Effects of carbohydrate supply on reproductive growth were assessed by recording the dimensions and fresh and dry weights of the rachis and berries. Percentage fruit set was determined from flower and berry counts. Fruit set and vegetative growth responded in an exponential and linear manner, respectively, to changes in carbohydrate supply. Elevated carbohydrate 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 accumulation in the basal part of the shoot declined to levels similar to those in the ungirdled control. The reduction in fruit set, despite an apparent increase in available carbohydrates, indicates the importance of the adjacent basal leaves to the developing cluster. The wide range of fruit sets achieved in this experiment, and their exponential responses to carbohydrate supply, indicate that carbohydrate is the major determinant of fruit set in 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 related to problems of nitrogen nutrition or of mineral imbalance.