HEAT INJURY THRESHOLDS IN APPLES MEASURED USING CHLOROPHYLL FLUORESCENCE ARE INFLUENCED BY ORCHARD HEAT REDUCTION TECHNOLOGIES

Chlorophyll fluorescence (Fv/Fm) was used to investigate thresholds of heat injury in apple fruit grown under orchard heat reduction technologies, i.e. evaporative cooling (EC) and 20% black shade netting. Picked apples were exposed to oven temperatures of 35, 40, 45, 50 or 55°C for two, four, six or eight hours and fruit surface temperature (FST) and Fv/Fm measured immediately following heat treatment, and after a 12-hour recovery period. In all trials, permanent injury occurred at 48-53°C FST irrespective of duration, with permanent injury possible at 42-47°C depending on duration. Non-EC ‘Cripps’ Pink’ were more resistant to heat injury at 45°C than EC fruit. Fruit from three cultivars under netting (‘Royal Gala’, ‘Fuji’, and ‘Cripps’ Pink’) had lower Fv/Fm at 43°C than non-netted fruit. The results indicate clear temperature thresholds for heat injury, with duration of exposure and orchard acclimation responses becoming important at sub-lethal surface temperature of c. 45°C.