Photosynthetic Responses of Canola and Wheat to Elevated Levels of CO(2), O(3) and Water Deficit in Open-Top Chambers

The effects of elevated CO(2) (700 ppm) and O(3) (80 ppb) alone and in combination on the photosynthetic efficiency of canola and wheat plants were investigated in open-top chambers (OTCs). The plants were fumigated for four weeks under well-watered and water-stressed (water deficit) conditions. The fast chlorophyll a fluorescence transients were measured after 2 and 4 weeks of fumigation, as well as in control plants, and analyzed by the JIP-test, which is a non-destructive, non-invasive, informative, very fast and inexpensive technique used to evaluate the changes in photosynthetic efficiency. Biomass measurements were taken only after 4 weeks of fumigation. The performance index (PI(total)), an overall parameter calculated from the JIP-test formulae, was reduced by elevated CO(2) and O(3) under well-watered conditions. In the absence of any other treatment, water stress caused a decrease of the PI(total), and it was partly eliminated by fumigation with elevated CO(2) and CO(2) + O(3). This finding was also supported by the biomass results, which revealed a higher biomass under elevated CO(2) and CO(2) + O(3). The decrease in biomass induced by elevated O(3) was likely caused by the decline of photosynthetic efficiency. Our findings suggest that elevated CO(2) reduces the drought effect both in the absence and presence of O(3) in canola and wheat plants. The study also indicates that elevated O(3) would pose a threat in future to agricultural crops.