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 CO2 concentrations (ci) at leaf temperatures from 20 to 40 degrees C were undertaken. Average rates of photosynthesis measured in situ decreased with increasing temperature and were 60% inhibited at 45 degrees C compared with 25 degrees 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 degrees C but also PFDs saturating photosynthesis increased from 550 to 1200 mu mol (photons) m-2 s-1 as temperatures increased. Photosynthesis under saturating CO2 concentrations was optimal at 36 degrees C while maximum rates of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax) and potential maximum electron transport rates (Jmax) were also optimal at 39 and 36 degrees C, respectively. Furthermore, the high temperature-induced reduction in photosynthesis at ambient CO2 was largely eliminated. The chloroplast CO2 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 degrees C and above limited by RuBP carboxylation, suggesting high temperatures are detrimental to carbon fixation in this species.