Seasonal CO 2 exchange patterns of developing peach ( Prunus persica ) fruits in response to temperature, light and CO 2 concentration

CO 2 exchange rates per unit dry weight, measured in the field on attached fruits of the late‐maturing Cal Red peach cultivar, at 1200 μmol photons m −2 S −1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO 2 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 CO 2 (g dry weight) −1 S −1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol 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°C; the mean seasonal O 10 value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO 2 exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO 2 concentrations via CO 2 refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m −2 S −1 . Young peach fruits responded to increasing ambient CO 2 concentrations with decreasing net CO 2 exchange rates in light, but more mature fruits did not respond to increases in ambient CO 2 . Fruit CO 2 exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO 2 concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO 2 response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO 2 1 −2 . In CO 2 free air, fruit photosynthesis was dependent on CO 2 refixation since CO 2 uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO 2 ‐free air and 370 μl CO 2 1 −1 indicated that young peach fruits were apparently able to take up CO 2 from the external atmosphere. CO 2 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 2 refixation.