Apple fruit set is influenced by altered floral bud density but not by reduced carbohydrate reserves

Abstract

The extent to which carbohydrate reserves contribute to apple (Malus × domestica Borkh.) fruit set is unclear. On ‘Royal Gala’/‘M9’ trees, we used postharvest early defoliation (12 days after harvest) to alter carbohydrate reserve accumulation compared with natural leaf fall. Superimposed over this, late winter bud thinning using artificial spur extinction (ASE, 6 buds cm(-1) branch cross sectional area) to manipulate the number of developing sinks in spring was compared with unmodified trees. Reducing bud density using ASE did not affect the concentration of total non-structural or soluble carbohydrates in root or spur tissues sampled at intervals from dormancy to final fruit set. However, ASE increased within-bud fruit set compared with unmodified trees. Early defoliation did not affect fruit set compared with natural defoliation, but reduced concentrations of total non-structural carbohydrates in roots in winter by 44% (264 to 147 mg g(-1) dry weight) and concentrations of starch in spurs in winter by 60% (19 to 7.7 mg g(-1)). Between dormancy and 1 week before bloom (WBB), total non-structural carbohydrate concentration declined in roots of naturally defoliated trees by 31%, and in spurs of naturally and early defoliated trees by 37 and 46% (respectively). In trees defoliated early, concentrations of total non-structural carbohydrates increased from their minimum at 1WBB in spurs and 2 weeks after bloom (WAB) in roots. In naturally defoliated trees total non-structural carbohydrates did not increase in roots during the measurement period, but increased in spurs between 5 and 9 WAB. Lack of differences in fruit set among defoliation treatments despite differences in non-structural carbohydrate concentration in spurs and roots implied that at concentrations observed here, stored carbohydrates in these tissues had little role in fruit set. This was supported by observations that total non-structural carbohydrate concentration declined before full bloom, not during fruit set, and total non-structural carbohydrate concentration increased during the period 2 to 4 WAB, when limitations in carbohydrate resources are thought most likely to affect fruit set.