Estimating the photosynthetic contribution of developing peach {Prunus persica) fruits to their growth and maintenance carbohydrate requirements

CO, exchange rales (CO, evolution) of late-maturing cv. Cai Red peaches, exposed lo different photon flux densities, were simulated from 24 days after flowering {DAF) until harvest by using light and temperature response curves measured on attached fruits in the field at biweekly intervals. The daily patterns of dark respiration rales per unil dry weight indicated their dependence on temperatures. Frui! CO, exchange rates in light were also affected hy photosynthelic photon flux densities. Dai]\ photosynthetic rales per unit dry weight and per fruil were significantlv low'er in shaded fruits receiving 7% of the full daily sunlight compared lo fruits expo.sed lo 35°/r. sunlighl. However, the difference in photosynthetic rates in peach fruits receiving 21 and 35% of total daily sunlight was small. Within the last 4 weeks before harvest, weekly carbohydrate requirements for the production of dry matter rose rapidly in cv. Cal Red peaches and were related to high carbohydrate accumulations, especially of sucrose, in the peach mesocarp. Weekly photosynthetic contribution of late-maturing cv Cal Red peaches to these carbohydrate accumulations increased up to 115 DAF. A decline in photosynthetic contributions between 115 DAF and harvest was related to decreasing photosynthetic activities in association with declining chlorophyll contenls. Photosynthesis of lale-maturing cv. Cal Red peaches provided 3-9% of the weekly fruit carbohydrate requirements early in the season and 8-15% in the midseason depending on fruit exposure to light. Photosynthesis of mature fruils contributed 3-5% of the total fruit carbohydrate requirements Since fruil photosynthetic rates approach saturation at a photosynthetic photon flux density of about 6(K1 [imol m s '. the difference in weekly photosynthetic contributions was small hetween exposed and partially exposed (35 and 21 % suniight. respectively) peach fruits. However, a shaded fruit (7% sunlight) supplied significantly less of its weekly carbohydrate requirements through photosynthesis compared to exposed fruits. During the growing period of 24 DAF untii harvest, dry matter accumulation of latematuring cv. Cal Red peaches accounted for 78% of the total carbohydrate requirements and 22% was used in respiration. Fruit photosynthesis of shaded peach fruit, partially exposed fruil and exposed fruit (receiving 7.21 and 35% of full sunlight over the day. respectively) contdbuled 5. 8 and 9%. respectively, of the total growth and maintenance carbohydrate requirements during the growing season.