Modelling photosynthetic responses to temperature of grapevine (Vitis vinifera cv. Semillon) leaves on vines grown in a hot climate.

Field measurements of photosynthesis of Vitis vinifera cv. Semillon leaves in relation to a hot climate, and responses to photon flux densities (PFDs) and internal CO(2) concentrations (c(i) ) at leaf temperatures from 20 to 40 °C were undertaken. Average rates of photosynthesis measured in situ decreased with increasing temperature and were 60% inhibited at 45 °C compared with 25 °C. This reduction in photosynthesis was attributed to 15-30% stomatal closure. Light response curves at different temperatures revealed light-saturated photosynthesis optimal at 30 °C but also PFDs saturating photosynthesis increased from 550 to 1200 µmol (photons) m(-2)s(-1) as temperatures increased. Photosynthesis under saturating CO(2) concentrations was optimal at 36 °C while maximum rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (V(cmax)) and potential maximum electron transport rates (J(max)) were also optimal at 39 and 36 °C, respectively. Furthermore, the high temperature-induced reduction in photosynthesis at ambient CO(2) was largely eliminated. The chloroplast CO(2) concentration at the transition from RuBP regeneration to RuBP carboxylation-limited assimilation increased steeply with an increase in leaf temperature. Semillon assimilation in situ was limited by RuBP regeneration below 30 °C and above limited by RuBP carboxylation, suggesting high temperatures are detrimental to carbon fixation in this species.