Seasonal CO2 exchange patterns of developing peach {Prunus persica) fruits in response to temperature, light and CO2 concentration

CO, exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 nmol photons m s ' and in dark, and photosynthetic rates, calculated by the difference between the rates of CO, evolution in light and dark, deelined over the growing season. Caleulated phetosynthetic rates per fruit increased over the season with increasing fruii dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. SUght net fruit photosynthetic rates ranging from 0.087 t 0.06 to O.IK)3 ± 0.05 nmol CO: (g dry weight) ' s ' were measured in midseason under optimal temperature {15 and 20°C) and light (1200 (imol photons m -' s ') conditions. Calculated fruit photosynthetie rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal Om value was 2,03 between 20 and M)°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rales increased at high temperatures to a greater extent than CO, exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO, concentrations via CO, refixation. At 15°C. fniit photosynthetic rates tended lo be saturated at about 600 [.imol photons m"s ' Young peach fruits responded to increasing ambient CO, concentrations with decreasing net CO, exchange rates in light, but more mature fruits did not respond to increases in ambient CO,, Fruit CO, exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO, concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO, response of young and mature peach fruits. Photosynthetic rales of younger peach fruits apparently approached saturation at 370 til CO, I ', In CO,-frec air. fruit photosynthesis was dependent on CO, refixation since CO, uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO,-free air and 370 fil CO, I ' indicated that young peach fruits were apparently able to take up CO, from the external atmosphere. CO; uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO, refixation.