PHYSIOLOGICAL-CHANGES ON AGRICULTURAL CROPS INDUCED BY DIFFERENT AMBIENT OZONE EXPOSURE REGIMES .1. EFFECTS ON PHOTOSYNTHESIS AND ASSIMILATE ALLOCATION IN SPRING WHEAT

Spring wheat (Triticum aestivum cv. Nandu) cultivated under glasshouse conditions was exposed to ozone in large fumigation chambers for 2 wk . Different exposure regimes were applied as constant concentrations a s well as with ozone peaks, partly under equal dose-conditions, in tim es of high solar radiation during different stages of development (see dling, late tillering, anthesis). Chlorophyll fluorescence was monitor ed and amounts of carbohydrates (hexoses, sucrose, starch) and chlorop hyll were measured in young leaves (seedling) and flag leaves (late ti llering, anthesis) during and after ozone exposure. Although seedlings showed no significant response in photosynthesis, strong effects on p hotosynthesis and carbohydrate accumulation were measured when plants were fumigated during anthesis, especially after a heat stress period preceding ozone treatments. Under equal dose conditions chlorophyll fl uorescence parameters (F-v:F-m) and electron transport rate decreased and sucrose content of flag leaves increased significantly if ozone at a concentration of 220 mu g m(-3) was supplied for 4 h, indicating th at peak concentrations show stronger effects than constant concentrati ons. The reaction of wheat plants is dependent on environmental condit ions such as preceding heat stress and on the developmental stage duri ng exposure. The results favour the hypothesis that photoinhibition an d disturbance of photosynthesis are only secondary effects as a conseq uence of retarded sucrose export from the leaf, because of damage at t he plasma membrane.