Intra-canopy variability of fruit growth rate in peach trees grafted on rootstocks with different vigour-control capacity

The aim of the present research was to study intra-canopy variability in fruit growth under conditions of low fruit-to-fruit competition in peach trees grafted on rootstocks with differing size-controlling capacity. The experiment was conducted on adult peach trees of two cultivars, each grafted on five rootstocks. Tree canopies were divided vertically into five layers. The diameter of 12 fruit per canopy layer was measured early in the growing season and at harvest. At harvest, the fresh weight of each selected fruit was also determined. After harvest, thirty shoots per tree bearing the selected fruit were harvested, and several parameters related to local carbon source availability, competition between vegetative and reproductive organs, vigour of the fruiting shoot, and position of the fruit on the fruiting shoot were measured. The results demonstrated that fruit variability within peach trees is very large, even when fruit-to-fruit competition is low. Fruit size at harvest decreased significantly and progressively from the top layers to the bottom layers of the canopy. Fruit growth was positively correlated with specific leaf weight, leaf area, and vigour of the fruiting shoot, and was negatively correlated with the vigour of current-season shoot growth on the fruiting shoot. However, the relative importance of each parameter depended on the cultivar. Rootstock affected the relationship between fruit growth and the measured parameters, and some rootstocks appeared to limit the maximum potential fruit growth rate. The measured parameters did not explain all the intra-canopy variability in fruit growth, suggesting that other factors also played a role. Interestingly, fruit size within 1 month of full bloom increased progressively from the top to the bottom of the canopy.