Roles of Starch and Sorbitol in Apple Leaves and Stems under Water Stress and Crop Load Combinations

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 roles of major carbohydrate components in mature, field-grown apple trees in response to water availability and crop load. Fourteen-year-old 'Golden Delicious' apple trees were grown under various combinations of irrigation rate (11, 33 or 77 liters/day per tree) and crop level (~100, ~300 or >1000 fruit per tree), beginning 47 days after full bloom (DAFB). In a second experiment, similar 'Golden Delicious' trees with high (>1000 fruit per tree) or low (<200 fruit per tree) crop loads were irrigated at 77 or 11 liters/day per tree).  Fruit count and weight, photosynthetic assimilation, stomatal conductance, midday stem and leaf water potentials, osmotic potential and concentrations of non-structural carbohydrates in leaves and stems were determined. Leaf and stem starch contents were positively related to irrigation rate and negatively related to crop level. Mean fruit weight at harvest was positively related to stem starch content and neared maximum at a content of 4% dry weight. Starch content recovered after harvest, especially in water-stressed trees. Sorbitol concentration was inversely related to both irrigation rate and crop level. Sorbitol in leaves averaged at 366 and 618 mmol·Kg^-1 in high and low irrigation regimes, respectively, implying the existence of an osmotic modification mechanism in apple. In average, soluble carbohydrates accounted for 96% of leaf osmotic potential with sorbitol alone accounting for 70% to 80% of this term. Stem starch content appears to represent the overall balance between carbon sources and sinks and may therefore be used as a reliable indicator of photo-assimilate availability for vegetative and reproductive growth in apple trees. Sorbitol, on the other hand, serves a dual role in apple trees, both as a primary agent of carbon translocation and as an efficient osmotic regulator. The competition between those two processes is exacerbated in heavily cropping, deficiently irrigated trees.