EFFECTS OF OZONE ON DRY-MATTER PARTITIONING AND YIELD OF JAPANESE CULTIVARS OF RICE (ORYZA-SATIVA L)

Rice plants were exposed to ozone in field exposure chambers throughout most of the growing seasons of 3 years from 1987 through 1989. Cultivar 'Koshi-hikari' was used for the 1987 and 1989 experiments, and cv. 'Nippon-bare' was used for the 1988 experiment. Ozone exposure was performed for 7 h from 09:00 to 16:00 h each day, and ozone concentration in each of the five chambers was maintained at either 0.5, 1.0, 1.5, 2.0, or 2.75 times ambient ozone level. Throughout the growing seasons, plant samples were taken for measuring leaf area and dry weight of plant parts. At harvest, samples were taken to determine grain yield and yield components. The results of the ozone exposures indicated no consistent effect of ozone on leaf area growth, whereas total dry matter decreased with increased ozone concentration. The effect of ozone on total dry matter was particularly evident after heading. Ozone also affected dry matter partitioning. There was increased dry matter distribution to the leaf blades compared with the leaf sheaths, culms, or roots. Rice yield was significantly reduced by ozone. Although the yield was significantly different among the 3 years, there was no significant interaction between the ozone treatment and the year of the ozone exposures. The relationship between ozone concentration and yield loss of rice in this study was comparable to the relationship reported for rice cultivars in California, but was different from that reported for soybeans, which showed much greater ozone-induced yield loss than rice. Among the yield components, 1000 grain weight was significantly reduced by ozone. Harvest index was not affected by the ozone treatment. The above results indicated that rice yield is reduced by ozone in a realistic range of 20-100 nl 1-1. The results for yield components were compared with other reports. The increased dry matter distribution to leaf blade was discussed with regard to its implications for leaf area growth and lodging.