Effects of soil water stress on fruit yield, quality and their relationship with sugar metabolism in 'Gala' apple

The aim of this research is to explore apple quality responses to water stress during the fruit growing season in seven-year-old potted 'Gala' (Maim domestica Borkh.) trees. Three irrigation levels were applied, including normal irrigation (CK, 70-80%0 theta(f)), light water stress (LS, 60-70%theta(f)), medium water stress (MS, 50%-60%theta(f)), theta(f): field capacity. In stage I, water stress was applied from stage of young fruit to mature. In stage II, water stress was utilized from stage of fruit expanding to mature. The results showed that MS and LS treatments improve fruit quality by improving the contents of total soluble solid (TSS) and soluble sugar (fructose, glucose and sorbitol) and reducing titratable acidity (TA) levels. But MS and IS treatments applied in stage I reduced fruit weight, while LS treatment applied in stage II did not affect fruit weight. Interestingly, the enhancing of activity of Sucrose Synthase (SS) and Acid Invertase (AI) in MS and the increasing of activity of SS in LS promoted the conversion from sucrose to fructose and glucose in stage I. In stage II, the enhancing of AI and Sorbitol Oxidase (SOX) in MS and LS promoted the conversion from sucrose to fructose and glucose and the conversion from sorbitol to glucose, respectively. Our observations suggest that different degrees and duration of soil water stress affect different enzymes to promote the increase of fructose and glucose content. In addition, light water control is recommended only during later fruit developmental stages for optimum quality and production.