Assessment of photoprotection mechanisms of grapevines at low temperature

The photosynthetic response of grapevine leaves (Vitis vinifera L. cv. Riesling) to low temperature was studied in the field and laboratory. Light-saturated rates of photosynthetic electron transport were lower and non-photochemical energy dissipation was higher when leaves were subject to low morning temperatures than to high afternoon temperatures under field conditions. These responses to low temperatures occurred without sustained reduction of quantum efficiency of PSII as measured by the variable to maximum chlorophyll fluorescence yield ratio, F_v/F_m, after dark adaptation. The temperature dependence of light-saturated apparent electron transport rate, gas exchange and non-photochemical quenching (NPQ) was also examined in laboratory experiments with glasshouse-grown material. NPQ reached saturation at lower light intensity with decreasing temperature. The relationship between the quantum efficiency of PSII and CO₂ fixation at 25°C (2-21% O₂) and 10°C (2-21% O₂) indicated a decreased dependence of electron transport on both photorespiration and the Mehler reaction at the lower temperature. The calculated percentage of electron flow to the Mehler reaction declined faster than photorespiration at low temperature. Warm- and cold-treated leaf discs under saturating light showed very little photoinhibition as measured by sustained reduction in F_v/F_m, which was linearly related to the percentage of functional PSII reaction centres. However, the addition of dithiothreitol greatly enhanced the rate of photoinhibition, indicating a potentially strong dependence on xanthophyll de-epoxidation for photoprotection at low temperature.