Effects of water stress on quality and sugar metabolism in ‘Gala’ apple fruit

Sugar plays an important role in apple fruit development, appearance and quality as well as contributing to a plant's water stress response. Trehalose and the trehalose biosynthetic metabolic pathways are part of the sugar signaling system in plants, which are important regulator of water stress response in apple. The effect of water stress treatments applied to apple trees and the corresponding effects of ABA on developmental fruit quality were examined for indicators of fruit quality during fruit development. The results indicated that the severe water stress treatment (W2) occurring after the last stage of fruit cell division caused a decrease in the color and size of fruit. The moderate water stress (W1) occurring after the last stage of fruit cell enlargement (S2) caused an increase in the content of fructose and sorbitol while the apple fruit shape was not affected. These changes in sugar are related to the activity of sugar metabolic enzymes. While the enzymatic activity of vacuolar acid invertase (vAINV) was higher, that of sucrose-phosphate synthase (SPS) was lower in water stress treated fruit throughout the developmental period. This indicates that enhanced sucrose degradation and reduced sucrose synthesis leads to an overall reduced sucrose content during times of drought. Thus, water stress reduced sucrose content. Whereas the content of endogenous trehalose and ABA were the highest in water stress treated fruit. A moderate water stress (W1) imposed on apple trees via water restriction (60%–65% of field capacity) after the fruit cell enlargement phase of fruit development yielded sweeter fruit of higher economic value.