Influence of Preharvest AVG Application on the Activity of Sorbitol Dehydrogenase in Apple Fruit

Abstract

Sorbitol dehydrogenase (SDH) is a key enzyme in apple fruit converting sorbitol into fructose. SDH activity in `Fuji' apple was reported to increase close to harvest, perhaps as part of the ripening process. Aminoethoxyvinylglycine (AVG) is used to delay fruit ripening and prevent fruit drop, though its effect on sorbitol metabolism is not known. To determine if the late season increase in SDH activity is common among apple cultivars and if AVG use affects SDH expression and activity, AVG was applied to `Lodi', `Redchief Delicious', and `Red Fuji' trees 4 weeks before harvest. Control and AVG-treated fruit were collected 1 week prior to, at, and 1 week after the normal harvest date for assessment of ethylene production over time after harvest and SDH presence and activity at harvest. Ethylene production in control fruit increased after harvest and AVG reduced it in all cultivars. `Redchief Delicious' fruit had the highest ethylene production of the treated samples. The levels of SDH activity in controls were similar across dates for `Redchief Delicious' and showed no consistent pattern in the other cultivars. `Redchief Delicious' and `Red Fuji' showed the highest and lowest levels of SDH activity, respectively. AVG did not affect SDH activity in `Redchief Delicious', and substantially increased SDH activity in `Red Fuji' on each of the three harvest dates, and, in `Lodi', only 1 week after normal harvest. SDH presence was confirmed through immunoblotting for all cultivars and harvest dates. Overall, fruit with the greatest reduction in ethylene production in response to AVG also showed changes in SDH activity.