Seasonal changes in the photosynthetic response to CO2 and temperature in apple (Malus domestica cv. 'Red Gala') leaves during a growing season with a high temperature event

Changes in the photosynthetic responses of Malus domestica cv. 'Red Gala' leaves to internal CO2 concentrations at leaf temperatures from 15°C to 40°C were followed across the growing season in field-grown trees exposed to a high temperature event in late summer. Light and CO2-saturated photosynthesis (Amax), maximum rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax) and maximum rates of electron transport (Jmax) were all highest in spring, although Amax was maximal at 30°C, and Vcmax and Jmax were maximal at 40°C. All attributes declined in late summer and reached minima that coincided with the occurrence of the high temperature event. Modelling suggested many of the changes were correlated with the seasonal climate, although the measurement or current temperature had an overriding effect, especially on stomatal conductance. Marked changes in the temperature-dependency of Vcmax and Jmax occurred across the season and appeared to relate to the seasonal changes in climate, especially temperature. The reduction in photosynthesis at high temperatures was, however, partly attributable to a stomatal limitation of up to 60% at the high temperatures. Non-stomatal reductions in photosynthesis during the heat event could be attributed to detrimental effect on RuBP carboxylation and on RuBP regeneration.