Light-driven chlorophyll a fluorescence intensity was used to measure the photochemical activity of photosystem II (PSII) after heat and light stress Pea plants grown at 22°C show a dramatical decrease in the activity of PSII when exposed to high temperature (above 40°C). High light exacerbates the damage of PSII when exposed together with high temperature. But the low light strongly affect the response of the photosystem, acting as an efficient protector of the photochemical activity against its inactivation by heat. Heat protection was also triggered by exposing the leaves to moderately elevated temperature (30°C) before exposing them to high temperature (40°C). Photoinhibition resistance was induced by exposing leaves to high temperature for 15 min before illuminating them to high light intensity. 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 diumal cycle of the day. A comparable behaviour has not been found in algae. Aquatic higher plants respond individually. Some were able to protect themselves with low light against heat stress but this phenomena was totally absent in others.