EFFECT OF FRUIT LOAD ON WHOLE TREE CARBON ASSIMILATION, DARK RESPIRATION, AND WATER RELATIONS IN APPLE

Four-year-old apple (Malus domestica Borkh. cv. Golden Delicious) trees were grown in the field in lysimeters and either left to bear fruits or defruited in August, September or October, and flooded or droughted for 7 days. Net photosynthesis, dark respiration, and water use efficiency were measured for 5 days prior to and after fruit removal, using tree canopy chambers and infrared gas analysis. Scaling up from the porometry measurements at the single leaf level to the canopy resulted in a factor of two-fold on overcast and four-fold on sunny days. Fruiting enhanced both net photosynthesis and dark respiration, with the magnitude increasing with fruit ontogeny. Respiratory losses after fruit removal in October were induced by translocation of carbohydrates from the leaves to the perennial woody parts of the tree and by the onset of leaf senescence. Fruiting reduced the water use efficiency due to a greater increase of transpiration relative to the increase in net photosynthesis.