Chilling and grapevine photosynthesis : an investigation into the carbon metabolism and photoprotection of leaves in the field and laboratory

Relatively few studies have described the response of grapevine photosynthesis or photoprotection to low, chilling temperatures (0-15°C). None have demonstrated either a mechanism responsible for chilling-induced limitation of photosynthesis or whether this limitation impacts on grapevine productivity in a field-acclimated situation. The series of studies that constitute my thesis were designed to investigate grapevine photosynthesis and photoprotection, and the mechanisms that govern the efficiency of these processes at low temperature. Short-term responses of photosynthesis of field-grown cool climate grapevines to early morning low temperatures were mediated by both stomatal closure and Rubisco activity above 15°C and RuBP regeneration below 15°C in the absence of sustained net photoinactivation. The photosynthetic response to low temperature caused restriction of total daily carbon gain in field-acclimated grapevines. The daily loss of integrated daily carbon gain accumulated over time to produce large differences in growth even between microsites within a vineyard differing by 1-3°C due to cold air drainage. The response of stomatal conductance to low temperature was further investigated and it was found that dark chilling induces prolonged stomatal closure well into periods where temperatures were no longer limiting, implying that leaf stomatal closure was mediated or prolonged by accumulation of leaf ABA. This closure limited intercellular CO2 partial pressures and light-saturated photosynthetic capacity. During chilling in the light it was also demonstrated that stomatal conductance declined. Vapour pressure difference could not explain this phenomenon. This demonstrates a direct influence of temperature on stomatal function. When intercellular CO2 partial pressures was kept constant despite stomatal closure it was shown that leaf CO2 assimilation still declined. This indicates that a non-stomatal factor was involved in the limitation of photosynthetic rate. As stomatal resistance to CO2 increased during low temperature treatment, Rubisco activity became the limiting factor under saturating light. Loss of Rubisco activity was due to thermodynamic constraints on catalytic turnover and a de-carbamylation of active Rubisco sites. Above 15°C the loss of Rubisco activity and both stomatal and mesophyll conductance caused the decline in photosynthetic rate. Below 15°C, stomatal conductance, mesophyll conductance and Rubisco activity continued to decline; however, restriction of RuBP regeneration became relatively more important in limiting the rate of photosynthesis. The absence of chronic photoinactivation in fieldand glasshouse-grown grapevines, O2 and CO2 insensitivity of photosynthesis and the stimulation of photosynthesis by phosphate feeding demonstrated that limited RuBP regeneration was due to phosphate limitation. This was likely to have been mediated by deactivation of enzymes in end-product synthesis pathways restricting P j recycling. These temperature dependent processes were incorporated into the Farquhar & von Caemmerer photosynthesis model. Introducing temperature dependence to the rate of triose phosphate export in grapevine leaves greatly improved accurate prediction of photosynthetic rate at low temperatures and ambient and elevated CO2 partial pressures. Despite a reduction in energy utilization by . ßw/usco-mediated processes at low temperature causing significant increases in the amount of light excessive to that of photosynthesis, grapevine leaves maintain high intrinsic quantum efficiencies of PSII, as measured by the chlorophyll fluorescence parameter FJFm, in both field and laboratory conditions. FJFm was found to be a linear measure of the percentage of functional PSII reaction centers in grapevine leaves and was used to calculate the quantum yield of net photoinactivation. Grapevine leaves are highly resistant to photoinactivation at all temperatures relative to other species. Xanthophyll-mediated non-photochemical quenching and high intrinsic rates of D1 repair are responsible for conferring this high resistance by reducing the ‘gross’ rate of photoinactivation and the ‘net’ rate of photoinactivation, respectively. A new method of energy dissipation analysis revealed that up to 75% of all absorbed light is dissipated thermally via pH and xanthophyll-mediated non-photochemical quenching at low temperatures and moderate light. Thus, the xanthophyll cycle and the high rate of D1 repair predominantly mitigate against photoinactivation of PSII in grapevine. At low temperatures, grapevine photosynthesis is limited by both stomatal closure and phosphate limitation, which in turn restricts carbon gain and growth in both glasshouse and field-acclimated conditions. Despite the reduction in common energy dissipation processes that include photosynthesis, photorespiration and the water water cycle, grapevines remain highly resilient due to high capacity for xanthophylls-mediated thermal dissipation and high rates of repair of damaged PSII reaction centers.