Thermoinhibition of photosynthesis as analyzed by gas exchange and chlorophyll fluorescence.

Carbon dioxide uptake and fluorescence parameters were measured in sunflower (Helianthus annuus L.) leaves at different temperatures up to 45 degrees C. In the dark, steady-state fluorescence F increased with temperature but part of it could be reversed by far-red light (F-0) Pulse-saturated fluorescence F,decreased and approached F at high temperature. In the light, a close relationship existed between the quantum yields of electron transport calculated from fluorescence and from net CO2 uptake, considering photorespiration and dark respiration in the light. The difference between these two values of electron transport is interpreted as alternative electron transport to accepters other than CO2. With increasing temperature, relatively more electrons were diverted to alternative pathways, while the absolute value of the alternative electron flow changed less. The results are interpreted to show that the major factor causing thermoinhibition of photosynthesis is primarily photosystem II damage, which causes a decrease in electron transport, increased alternative electron transport, and, consequently, inactivation of Rubisco as a result of decreased electron transport through the photosystem I acceptor side and deceased ATP production.