Cumulative Responses of Semillon Grapevines to Late Season Perturbation of Carbohydrate Reserve Status

Abstract

The response of grapevine carbohydrate reserves and seasonal growth and development to defruiting at the onset of ripening or complete defoliation at commercial harvest was examined at four sites in two hot, inland regions of New South Wales, Australia. Early defruiting over two consecutive seasons increased total nonstructural carbohydrates (TNC) in the roots, and to a lesser extent in the wood, and resulted in yield increases of up to 60% in the third season when fruit was allowed to remain on the vine until harvest. In contrast, defoliation at harvest caused a decline in TNC concentrations and reduced fruit production by up to 22% following one season of treatment and by 50% following two seasons. The higher yields developed after the two proceeding seasons of early defruiting were associated with depletion of the initially high carbohydrate reserves between budbreak and harvest, and reserve restoration did not take place until after harvest. However, in the lower yielding, previously defoliated vines, carbohydrate reserves were restored before fruit maturity. These findings show that carbon demands of ripening fruit, and photoassimilation capacity after harvest, can both limit the restoration of carbohydrate reserves to preseason levels. Marked differences between sites in the seasonal maxima of nonstructural carbohydrate concentrations in perennial tissues and aspects of floral and vegetative development are attributed to water deficits. The interaction between carbohydrate reserves and fruit and shoot growth suggests a feedback process whereby homeostasis, following environmental or cultural perturbations, is restored to a level determined by the capacity of the grapevine to assimilate and store carbohydrates.