Mechanisms of impairment of the photosynthetic apparatus in intact leaves by ozone

Tropospheric ozone has been recognised as a limiting factor for plant growt h since late fifties of our century. The decrease in the rate of light satu rated net photosynthesis (A(sat)) was shown to be the major effect of ozone in leaves with negative consequences for plant growth and the development of plant communities. The reasons for the ozone-induced decrease in A(sat) are still under investigation. Possible mechanisms are an increasing stomat al limitation, an increase in mesophyll limitation including a reduction of the CO2 fixation in the Calvin cycle and an impairment of the photochemica l reactions in the grana membranes of chloroplasts. We conclude from the re viewed literature and from our own experiments that a decrease in carboxyla tion efficiency (CE) seems to be an early event caused by ozone leading to a decrease in A(sat). The loss in current photochemical capacity (F-v/F-m) appears with a lag phase of many days and therefore the loss is thought to be a secondary effect due to a decreased demand of 'assimilatory power'.