High growth temperatures induced a substantial increase in the thermal stability of the photosynthetic apparatus of Atriplex lentiformis. This was manifested as a much reduced inhibition of light-saturated photosynthesis and the initial slope of the light-dependence curves by exposure to high temperatures in high as compared to moderate temperature-grown plants. Heat treatment at 46 C of leaves from moderate temperature-grown plants resulted in a marked reduction in photosystem II activities of chloroplasts isolated from them. In contrast, heat treatment of leaves from high temperature-grown plants resulted in no reduction of photosystem II activities. In vivo estimates of photosystem II functioning, the 515 nm light-induced absorbance change, and the ratio initial to maximum fluorescence (F(0)/F(max)) indicated a similar increase in the thermal stability of photosystem II in high temperature-grown plants.