EFFECTS OF CHRONIC OZONE EXPOSURE ON GROWTH, ROOT RESPIRATION AND NUTRIENT-UPTAKE OF RICE PLANTS

To clarify the response of growth and root functions to low concentrations of ozone (O3), rice plants (Oryza sativa L.) were exposed to O3 at 0.0 (control), 0.05 and 0.10 ppm for 8 weeks from vegetative to early heading stages. Exposure to 0.05 ppm O3 tended to slightly stimulate the dry weight of whole plants up to 5 weeks and then slightly decrease the dry weight of whole plants. However, these effects were statistically significant only at 6 weeks. Exposure to 0.10 ppm O3 reduced the dry weight of whole plants by 50% at 5 and 6 weeks, and thereafter the reduction of the dry weight of whole plants was gradually alleviated. Those changes in dry weight can be accounted for by a decrease or increase in the relative growth rate (RGR). The changes in the RGR caused by 0.05 and 0.10 ppm O3 could be mainly attributed to the effect of O3 on the net assimilation rate. Root/shoot ratio was lowered by both 0.05 and 0.10 ppm O3 throughout the exposure period. The root/shoot ratio which had severely decreased at 0.10 ppm O3 in the first half period of exposure (1-4 weeks) became close to the control in the latter part of exposure (5-8 weeks). Time-course changes in NH4-N root uptake rate were similar to those in the root/shoot ratio especially for 0.10 ppm O3. On the other hand, root respiration increased from the middle to later periods. Since it is to be supposed that plants grown under stressed conditions change the ratio of plant organ weight to achieve balance between the proportion of shoots to roots in the plant and their activity for maintaining plant growth, these changes in root/shoot ratio and nitrogen uptake rate under long-term exposure can be considered to be an adaptive response to maintain rice growth under O3 stress.