Late‐season temperature effects on the carbon economy and tree performance of ‘Royal Gala’ apple (Malusdomestica) trees

Abstract

Potted ‘Royal Gala’ apple on M.9 rootstock (Malus domestica) trees were grown after fruit harvest at constant temperature conditions (18/ 8°C day/night) for 3, 6, and 9 weeks to manipulate their carbohydrate reserves. On several occasions, leaf gas exchange was measured and selected trees were destructively harvested before and after each treatment to measure the dry weight of the component parts. Samples were also taken for carbohydrate analysis. After treatment, the remaining trees were returned outdoors and, in spring, selected trees from each treatment were destructively harvested for dry weight and carbohydrate analysis. Budbreak was then measured and fruit weight and crop load determined next autumn and the trees destructively harvested for dry weight and carbohydrate analysis. There was no change in tree dry weight after each late‐season temperature treatment. Carbohydrate concentrations, averaged over the whole tree, increased by 10–15% compared to the pre‐treatment trees, consistent with a net increase in carbon acquisition. Over winter, the total carbohydrate concentration had declined by 25–40%. In spring, time of budbreak differed significantly; trees exposed for 3 weeks to 18/8°C in late‐season broke buds 9–19 days earlier than the other treatments in the following spring. The early budbreak was associated with not only the largest increase in carbohydrate concentration during treatment but also the greatest decrease thereafter. Although there is some support for the conclusion that high carbohydrate reserves confer a direct benefit on the budbreak process, it remains an open question if it was the increase in reserves, or their subsequent consumption that advanced budbreak.