Does availability of soluble carbohydrate reserves determine apple fruit set?

Sorbitol is the dominant photoassimilate and an important soluble reserve utilized by reproductive and vegetative sink tissues in apple (Malus × domestica Borkh.). Within these sinks sorbitol dehydrogenase (SDH, EC 1.1.1.14) is the primary enzyme that catalyzes the oxidation of sorbitol to fructose. SDH likely plays a critical role in establishing young apple fruit as sinks during the fruit set phase. A study of expressed sap from shoots subtending fruit indicated that sorbitol concentration was in greatest abundance, followed by glucose, fructose, sucrose, and by low levels of myo-inositol, galactose, raffinose, and stachyose. The levels of the major carbohydrates were high at bloom and 1 week after but declined appreciably in the subsequent 2 to 3 week period. A comparison of SDH activity in cortex of fruit that were clearly abscising to those that were persisting during natural fruit drop indicated no significant differences in cortex SDH activity between the two types, but seed SDH activity was less in dropping fruit. The lack of change in SDH activity of cortex, the major part of the fruit creating sink demand, in dropping fruit indicates that the capacity to utilize sorbitol may not be limiting and that sink demand may persist. In the 3 week period following chemical fruit thinning applications, whole fruit (i.e., cortex) SDH activity was not affected but levels of sorbitol in the xylem sap were lower than in unthinned trees for a longer period of time. Thus, considering both SDH activity and soluble carbohydrate availability, fruit drop at 4-6 weeks after bloom may be due to limited availability of carbohydrates at 2-3 weeks and not to an inability to utilize them, at least by cortex tissue.