In vivo photochemical activity of photosystem II (PSII) was measured by Chlorophyll a fluorescence intensity of pea leaves exposed to different types of stresses of daily time course. Pea plants grown at 22 °C show a dramatical decrease in the activity of PSII when exposed to higher temperatures. High light exacerbates the damage of PSII when imposed together with high temperature. But the low light acts as an efficient protector of the photochemical activity against its inactivation by heat. The thermal tolerance of PSII was also triggered by exposing the leaves to moderately elevated temperature and low light (30 °C and 30 Wm^-2) before exposing them to higher temperatures. The increase in thermo-tolerance was more than 5 to 10 °C. Pre-heat treatment causes an upward shift in the photoinhibitory light. The stability of PSII against heat was increased strongly in water stressed leaf discs. The experimental results show the existence of an adaptive mechanism of land plants to protect themselves against heat and strong light, the usual changes during the diurnal cycle of the day. A comparable behavior has not been found in unicellular organisms. Our results demonstrate the existence of an antagonism between different stresses (heat, light, water deficit) which are associated to the time course of the day.