PHOTOINHIBITION IN VITIS-CALIFORNICA - THE ROLE OF TEMPERATURE DURING HIGH-LIGHT TREATMENT

Leaves of Vitis californica Benth. (California wild grape) exposed to a photon flux density (PFD) equivalent to full sun exhibited temperature-dependent reductions in the rates or efficiencies of component photosynthetic processes. During high-PFD exposure, net CO2 uptake, photon yield of oxygen evolution, and photosystem II chlorophyll fluorescence at 77 Kelvin (FM, FV, and FV/FM) were more severely inhibited at high and low temperatures than at intermediate temperatures. Sun leaves tolerated high PFD more than growth chamber-grown leaves but exhibited qualitatively similar temperature-dependent responses to high-PFD exposures. Photosystem II fluorescence and net CO2 uptake exhibited different sensitivities to PFD and temperature. Fluorescence and gas exchange kinetics during exposure to high PFD suggested an interaction of multiple, temperature-dependent processes, involving both regulation of energy distribution and damage to photosynthetic components. Comparison of FV/FM to photon yield of oxygen evolution yielded a single, curvilinear relationship, regardless of growth condition or treatment temperature, whereas the relationship between FM (or FV) and photon yield varied with growth conditions. This indicated that FV/FM was the most reliable fluorescence indicator of PSII photochemical efficiency for leaves of different growth conditions and treatments.