SEASONAL CO2 EXCHANGE PATTERNS OF DEVELOPING PEACH (PRUNUS-PERSICA) FRUITS IN RESPONSE TO TEMPERATURE, LIGHT AND CO2 CONCENTRATION

CO2 exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1 200 mumol photons m-2 s-1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO2 evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0.087 +/- 0.06 to 0.003 +/- 0.05 nmol CO2 (g dry weight)-1 s-1 were measured in midseason under optimal temperature (15 and 20-degrees-C) and light (1 200 mumol photons m-2 s-1) conditions. Calculated fruit photosynthetic 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-degrees-C; the mean seasonal Q10 value was 2.03 between 20 and 30-degrees-C. The highest photosynthetic rates were measured at 35-degrees-C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO2 exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO2 concentrations via CO2 refixation. At 15-degrees-C, fruit photosynthetic rates tended to be saturated at about 600 mumol photons m-2 s-1. Young peach fruits responded to increasing ambient CO2 concentrations with decreasing net CO2 exchange rates in light, but more mature fruits did not respond to increases in ambient CO2. Fruit CO2 exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO2 concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO2 response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 mul CO2 l-1. In CO2-free air, fruit photosynthesis was dependent on CO2 refixation since CO2 uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO2-free air and 370 mul CO2 l-1 indicated that young peach fruits were apparently able to take UP CO2 from the external atmosphere. CO2 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 CO2 refixation.