Photosynthetic Responses of Canola and Wheat to Elevated Levels of CO2, O3 and Water Deficit in Open-Top Chambers

The effects of elevated CO2 (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 (PItotal), an overall parameter calculated from the JIP-test formulae, was reduced by elevated CO2 and O-3 under well-watered conditions. In the absence of any other treatment, water stress caused a decrease of the PItotal, and it was partly eliminated by fumigation with elevated CO2 and CO2 + O-3. This finding was also supported by the biomass results, which revealed a higher biomass under elevated CO2 and CO2 + 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 CO2 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.