GAS-EXCHANGE PARAMETERS, WATER RELATIONS AND CARBOHYDRATE PARTITIONING IN LEAVES OF FIELD-GROWN PRUNUS-DOMESTICA FOLLOWING FRUIT REMOVAL

The effect of fruit removal on gas exchange, water relations, chlorophyll and non-structural carbohydrate content of leaves from mature, field-grown plum trees (Prunus domestica L. cv. Stanley) was determined over 2 consecutive growing seasons. Removal of fruits during stage II of fruit development decreased CO2 assimilation rate within 24 h from 12.6 to 8.5-mu-mol m-2 s-1 in 1986, and from 12.1 to 10.2-mu-mol m-2 s-1 in 1987. Depression of net photosynthesis persisted for at least 5 days and was greatest in the early afternoon. Recovery of the CO2 assimilation rate to pretreatment levels coincided in defruited trees with vegetative growth that was more than 5-fold that of fruiting trees in the first 6 weeks after fruit removal in 1986. Estimated photorespiration was similar in both fruiting and defruited trees. The stomatal contribution to the decrease of CO2 assimilation rate, calculated from assimilation/intercellular CO2 curves, ranged from 31 to 46%. Defruiting did not affect leaf water potential, but decreased leaf osmotic potential. Leaf levels of chlorophyll, fructose, glucose, sorbitol and sucrose were not affected by defruiting, whereas starch content increased up to 51% in leaves of defruited trees within 24 h after fruit removal. However, because of the small starch pool present in plum leaves (< 1.9% dry weight) it is unlikely that starch accumulation was responsible for the observed decline in CO2 assimilation rate after fruit removal. The decrease of CO2 assimilation rate is discussed in relation to the hypothesis of assimilate demand regulating photosynthesis through a feedback mechanism.