Physiological changes on agricultural crops induced by different ambient ozone exposure regimes: I. Effects on photosynthesis and assimilate allocation in spring wheat.

Spring wheat (Triticum aeslivum 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 as well as with ozone peaks, partly under equal dose-conditions, in times of high solar radiation during different stages of development (seedling, late tillering, anthesis). Chlorophyll fluorescence was monitored and amounts of carbohydrates (hexoses, sucrose, starch) and chlorophyll were measured in young leaves (seedling) and flag leaves (late tillering, anthesis) during and after ozone exposure. Although seedlings showed no significant response in photosynthesis, strong effects on photosynthesis 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 fluorescence parameters (F :F ) and electron transport rate decreased and sucrose content of flag leaves increased significantly if ozone at a concentration of 220 μg m was supplied for 4 h, indicating that peak concentrations show stronger effects than constant concentrations. The reaction of wheat plants is dependent on environmental conditions such as preceding heat stress and on the developmental stage during exposure. The results favour the hypothesis that photoinhibition and disturbance of photosynthesis are only secondary effects as a consequence of retarded sucrose export from the leaf, because of damage at the plasma membrane.