Growth and yield responses of Pakistan rice (Oryza sativa L.) cultivars to O3 and NO2

Two separate experiments were undertaken to determine Pakistan rice (Oryza sativa L.) cultivar responses to air pollution, and specifically to test whether O3 was the pollutant likely to have caused the large effects of filtration on rice yield found in earlier field experiments in Pakistan. In the first experiment (Exp 1), cv. IRRI-6 was exposed to ozone (O3) at a mean concentration of 43 nl l−1 (8 h day−1) for 133 days until final harvest, covering both the vegetative and reproductive stages of growth. Results showed a significant reduction in total grain weight per plant. In the second experiment (Exp 2), fumigation of two rice cultivars with O3 at 40−42 nl l−1 (8 h day−1) and nitrogen dioxide (NO2) at 21–23 nl l−1 (24 h day−1), both singly and in combination, was carried out for 42 days during the vegetative stage of growth to determine if the presence of low concentrations of NO2 might influence the responses of both cultivars to O3. Results showed O3 to be more phytotoxic than NO2 at the concentrations used. No significant O3×NO2 interactions were found at the time of harvest, but O3 significantly reduced the photosynthetic capacity and biomass of both cultivars. Results of both experiments are compared with those reported for an open-top chamber filtration experiment carried out in Pakistan using the same two rice cultivars. Grain yield of cv. IRRI-6 was decreased in the closed chamber experiment by 57%, compared to a 37% decrease in the open-top chamber experiment; different yield components were affected in the open-top and closed chamber experiments which may be the result of differences in environmental conditions between the two studies. The results suggest that O3 may have significant effects on rice yield in Pakistan and indicate that Pakistan rice cultivars have a high sensitivity to O3.