Growth and yield responses of Pakistan rice (Oryza sativa L.) cultivars toO-3 and NO2

Two separate experiments were undertaken to determine Pakistan rice (Oryza sativa L.) cultivar responses to air pollution, and specifically to test wh ether O-3 was the pollutant likely to have caused the large effects of filt ration on rice yield found in earlier field experiments in Pakistan. In the first experiment (Exp 1), cv. IRRI-6 was exposed to ozone (O-3) at a mean concentration of 43 nl 1(-1) (8 h day(-1)) for 133 days until final harvest , covering both the vegetative and reproductive stages of growth. Results s howed a significant reduction in total grain weight per plant. In the secon d experiment (Exp 2), fumigation of two rice cultivars with O-3 at 40-42 nl 1(-1) (8 h day(-1)) and nitrogen dioxide (NO2) at 21-23 nl 1(-1) (24 h day (-1)), both singly and in combination, was carried out for 42 days during t he vegetative stage of growth to determine if the presence of low concentra tions of NO2 might influence the responses of both cultivars to O-3 Results showed O-3 to be more phytotoxic than NO2 at the concentrations used. No s ignificant O3XNO2 interactions were found at the time of harvest, but O-3 s ignificantly reduced the photosynthetic capacity and biomass of both cultiv ars. Results of both experiments are compared with those reported for an op en-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 chamb er experiment; different yield components were affected in the open-top and closed chamber experiments which may be the result of differences in envir onmental conditions between the two studies. The results suggest that O-3 m ay have significant effects on rice yield in Pakistan and indicate that Pak istan rice cultivars have a high sensitivity to O-3. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.