Does ozone exposure protect from photoinhibition?

Tropospheric ozone and high light intensities are two stress factors that o ften occur simultaneously under natural conditions. Ozone is well known to form oxygen radicals in the apoplastic water and long lasting photoinhibiti on can cause photooxidative damage also by formation of several species of oxygen radicals. We were interested whether moderate levels of ozone would be able to modulate the response of leaves to photoinhibitory conditions na turally occurring around noon on a bright day. Cuttings of Populus sp. were cultivated in two separate greenhouse-compartm ents adapted as fumigation chambers. In the two compartments plants were gr own in ambient air containing about 20 nmol mol(-1) ozone and in elevated o zone concentrations supplied for 8 h per day. During the midday of bright d ays F-v/F-m decreased by the same amount in all leaves, indicating photoinh ibition. At the same time F-0 increased in control leaves more than in ozon e-exposed leaves indicating a higher amount of heat-deactivating PSII centr es in the latter. This was confirmed by a higher epoxidation state in ozone -exposed leaves during midday of a bright day. The contents of chlorophyll a and chlorophyll b were significantly decreased in ozone-exposed leaves. I n older leaves the ratios chlorophyll a:b, and xanthophylls: chlorophyll b were increased indicating an adaptation to higher light stress. From this w e conclude that by increasing the amount of heat-deactivating centres ozone seems to protect PSII from photoinhibition.