Modelling the seasonal changes in the gas exchange response to CO2 in relation to short-term leaf temperature changes in Vitis vinifera cv. Shiraz grapevines grown in outdoor conditions

Effects of temperature on the photosynthetic response of Vitis vinifera cv. Shiraz leaves to CO2 were investigated across the growing season and modelling was used to determine relationships between photosynthesis and seasonal climate. Results indicated that photosynthetic rates declined from spring to summer, conforming to the deciduous habit of grapevines. Rates of ribulose 1,5-bisphosphate (RuBP) carboxylation and regeneration increased in a temperature dependent pattern throughout the season. However, the maximum rates decreased as the season progressed. There were also marked decreases in temperature sensitivity for each of these processes, consistent with the decreases occurring faster at high compared to low temperatures. There were no correlations between the seasonal climate and each of these photosynthetic processes but the effect of day was significant in all cases. CO2 saturated rates of photosynthesis (A(max)) across the season were highly correlated with the maximum rates of RuBP carboxylation and regeneration. The transition temperature between RuBP regeneration and RuBP carboxylation-limited assimilation varied across the growing season, from 23 degrees C in spring, 35 degrees C in midsummer and 30 degrees C at harvest and were highly correlated with mean day temperature. This suggested dynamic control of assimilation by carboxylation and regeneration processes occurred in these grapevines in tune with the seasonal climate.