As part of an analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C3 grass, the response of electron transport and photophosphorylation to temperature in isolated chloroplast thylakoids has been examined. The A number of investigations on the temperature sensitivity of the electron transport chain have been reported (1, 4, 6, 7, 9-11, 13-15, 19-23, 27), but few have attempted to correlate the temperature sensitivity of carbon fixation in intact leaves with the temperature sensitivity of the light reactions in isolated chloroplast membranes (1,4). In this report, the temperature dependence of electron transport and photophosphorylation in isolated chloroplast thylakoids from A. smithii was investigated to test the hypothesis that temperature effect on electron transport and photophosphorylation contribute to the decline in whole leaf net photosynthesis at temperatures above 35°C.
MATERIALS AND METHODSPlant Material. Sources ofAgropyron smithii Rybd. and growth conditions are described elsewhere (12).Preparation of Chloroplast Thylakoids. Thylakoids were prepared using 3-to 10-d-old leaves from native plants. Five g of leaves were cut into 3-to 5-cm segments. The segments were washed with deionized H20 and placed in 100 ml of ice-cold grinding solution consisting of 0.33 M sorbitol, 5 mM MgCl2, 20 mM Hepes (pH 7.6) and 0.2% BSA. Leaf segments were blended for 3 to 5 s at the number five setting with a Polytron tissue homogenizer. The resulting homogenate was filtered through a 200-,um net, and the filtrate was placed in 50-ml tubes and centrifuged at 500g for 5 min in a bench top centrifuge at 4°C. The supernatant was poured from the pellets, and the pellets were resuspended in a minimal amount of grinding buffer. A typical thylakoid preparation contained 0.5 mg Chl/ml in 1 to 2 ml total volume.
Measurement of Electron