Influence of post harvest temperatures on leaf gas exchange, carbohydrate reserves and allocations, subsequent budbreak, and fruit yield of 'Braeburn' apple (Malus domestica) trees

Potted 'Braeburn' apple (Malus domestica (Borkh.)) trees were grown after harvest in four controlled temperature conditions for 5 weeks to manipulate differences in carbohydrate reserves. Day/night temperatures ranged from 24/19 to 9/4°C. On several occasions, leaf gas exchange and soil respiration were measured and trees were destructively harvested before and after treatment to measure biomass of component parts. Samples were also taken for carbohydrate analysis. After treatment, the trees were returned outdoors and budbreak and fruit growth were measured in the following spring and crop load was measured at the next harvest. Trees at 24/19°C produced new leaves and flowered profusely whereas those at 9/4°C senesced rapidly but these trees grew new roots. Photosynthesis and respiration were highly dependent on temperature, and after 5 weeks of growth there were marked differences in rates between the various treatments. Non-structural carbohydrate concentrations declined in trees at all treatments but mostly at 24/19°C. However, carbohydrate reserves increased in trees in all treatments except at 24/19°C. Budbreak was delayed by up to 16 days in the trees treated at 24/19°C compared to trees treated at 9/4°C but treatment had no affect on fruit growth. Crop yields were highest in those trees with highest carbohydrate reserves, providing some support for the conclusion that high apple yields in New Zealand are dependent on elevated carbohydrate reserves achieved by favourable photosynthetic conditions and the extended growing season between harvest and leaf senescence.