USE OF CHLOROPHYLL FLUORESCENCE TO ASSESS HEAT STRESS IN APPLE FRUIT

Chlorophyll fluorescence of apple fruit (Malus × domestica Borkh. ‘Fuji’) was measured under the high solar radiation and temperatures prevailing in Washington State, USA, during 2003 and 2004. Under these conditions, the fruit surface temperatures (FST) of apples fully exposed to sunlight were often between 38 to 46°C from 13:00 to 18:00 hours on hot sunny days. Fruit whose FST was decreased with evaporative cooling (EC) had higher Fv/Fm (0.54) as compared with non EC (0.45). As fruit became more severely sunburned, the Fv/Fm values decreased significantly. Fv/Fm decreased following an increase of FST after 13:00 hours in suddenly exposed fruits in sunlight following a pruning treatment. However, shaded fruit did not exceed 35°C and showed no change in fluorescence. We suggest that chlorophyll fluorescence can be used as a tool to non-destructively detect heat stress in apple fruit.