Summer pruning effects on fruit size, fruit quality, return bloom and fine root survival in apple trees

Abstract

SummaryWhile many undesirable effects of summer pruning on apple (Malus domestica Borkh.) growth and development have been reported, the results are inconsistent and difficult to interpret. This study resolves the inconsistency by supplying a model that integrates pruning effects with tree physiological crop load, i.e. canopy net carbohydrate exchange rate per fruit. Our previous study suggests that the potential impact of reducing canopy photosynthesis after summer pruning depends on the balance of carbohydrate supply and demand. To test the hypothesis that summer pruning affects carbohydrate balance, we measured fruit growth, fruit quality, return bloom, and root growth in 20 year old slender spindle ‘Empire’/M.9 apple trees in response to different severities of summer pruning. Results were interpreted in relation to pruning severity, fruit number per tree, and the physiological crop load. Within commercial cropping ranges, light and moderate summer pruning had slight influences on fruit size and fresh weight. Summer pruning did not affect fruit colour, soluble solids content, starch, firmness, and internal breakdown after storage. Summer pruning alone did not affect return bloom or root growth. However, the potential negative effects of summer pruning on fruit growth, return bloom, and fine root survival can be predicted through their relationships with physiological crop load. This study suggests that the carbohydrate supply and demand balance model feasibly explains summer pruning influences. In addition, the impact of carbohydrate shortage after summer pruning is likely to be mediated by the reduction in canopy transpiration. However, the interaction between canopy carbon balance and water status after summer pruning is also likely to be manipulated by annual weather pattern.