Internal management of non-structural carbohydrate resources in apple leaves and branch wood under a broad range of sink and source manipulations

Abstract

Apple (Malus x domestica Borkh.) grown in a Mediterranean climate depends on regular irrigation throughout the growing season. The objective of the current study was to elucidate the changes in carbohydrate storage and utilization by mature, field-grown apple trees in response to water availability to the trees and to the level of cropping. Fourteen-year-old apple trees cv. 'Golden Delicious' were grown under various combinations of irrigation rate (11, 33 or 77 l day(-)(1) per tree) and crop level ( approximately 100, approximately 300 or >1000 fruits per tree) beginning 47 days after full bloom (DAFB). Non-structural carbohydrate concentrations were measured at 78 (leaves and branch wood), 102 (leaves), 183 (branch wood) and 214 (branch wood) DAFB. Midday stem water potential (SWP) was measured at 2-week intervals between June and October. Trunk cross-sectional area was measured 47 and 265 DAFB. At harvest, 139 DAFB, the fruits of each tree were counted and weighed. SWP at 102 DAFB ranged between -0.6 and -2.7 MPa. Fruit fresh weight at harvest was positively related to SWP measured 37 days before harvest with distinct slopes for light/intermediate and heavy crop levels. Leaf and branch wood starch concentrations 78 and 102 DAFB were positively related to irrigation rate and negatively related to crop level. Mean fruit weight at harvest was positively related to branch wood starch concentration and neared maximum at a concentration of 40 mg g(-)(1) dry weight. Branch wood starch concentration recovered after harvest, especially in water-stressed trees. Sorbitol concentration was negatively related to irrigation rate. The sorbitol-to-starch concentration ratio in leaves at 102 DAFB was closely proportional to SWP. It is suggested that branch wood starch concentration represents the overall balance between carbon sources and sinks and may therefore serve as a reliable indicator of photo-assimilate availability. In water-stressed trees, sorbitol is prioritized over starch, probably to support osmotic adjustment, thereby suppressing fruit growth even further.