THERMOINHIBITION OF PHOTOSYNTHESIS AS ANALYZED BY GAS-EXCHANGE AND CHLOROPHYLL FLUORESCENCE

Carbon dioxide uptake and fluorescence parameters were measured in sun flower (Helianthus annuus L.) leaves at different temperatures up to 4 5 degrees C. In the dark, steady-state fluorescence F increased with t emperature but part of it could be reversed by far-red light (F-0) Pul se-saturated fluorescence F,decreased and approached F at high tempera ture. In the light, a close relationship existed between the quantum y ields of electron transport calculated from fluorescence and from net CO2 uptake, considering photorespiration and dark respiration in the l ight. 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 dive rted to alternative pathways, while the absolute value of the alternat ive electron flow changed less. The results are interpreted to show th at the major factor causing thermoinhibition of photosynthesis is prim arily photosystem II damage, which causes a decrease in electron trans port, increased alternative electron transport, and, consequently, ina ctivation of Rubisco as a result of decreased electron transport throu gh the photosystem I acceptor side and deceased ATP production.