When a terrestňal plant is subjected to a mild drought stress, the leaf net CO2 uptake declines as a result of the stomatal closure. In contrast to previous beliefs the photosynthetic apparatus is vety resistant to dehydration. Particularly the relations between photochemistry, its regulation and leaf CO2 assimilation (f) are identical when F is changed either by increasing leaf water deficit or by decreasing the ambient CO2 concentration. When the stomata close in a leaf under water deficit the CO2 concentration within the leaf declines. As a result photorespiration is favoured, the photochemical yield of open photosystem 2 (PS 2) centres and the activity of some enzymes dechne (sucrose-phosphate synthase, nitráte reductase). The dechne of photochemical yield of PS 2 is consequential to an increase in thermal dissipation of the excitons trapped by PS 2 units. The dechne in CO2 concentration occurring in a desiccating leaf may trigger an integrated response of leaf metabohsm which still remains to be explored.