EFFECTS OF OZONE ON THE LIGHT USE OF RICE (ORYZA-SATIVA L) PLANTS

Effects of ozone on rice growth processes were addressed in terms of light use of plants exposed to ozone in field exposure chambers in experiments in 1987, 1988 and 1989. Incident, reflected and transmitted light fluxes were measured with light sensors set above and below the rice canopy in the exposure chambers, Light-use efficiency (LUE) was calculated from the total dry weight and the accumulated amount of light absorbed by the rice canopies. The results showed increase of light absorption with the increase of leaf area index (LAI) during the vegetative growth, but light absorption was almost constant after heading despite the decrease of LAI due to senescence. While no effects of ozone on light absorption were found, the LUE was decreased by ozone. A quadratic function was fitted to the relationship between mean ozone concentration and LUE during the vegetative growth, and a linear function was fitted to the relationship for the reproductive growth. The effect of ozone on LUE was much greater in the reproductive than in the vegetative stage. Some mechanisms were discussed with regard to the enhancement of the ozone impact on LUE in the reproductive stage.