Effects of chronic exposures to ozone on the growth of sunflower plants.

Abstract

Fourteen-day-old sunflower plants (Helianthus annuus L. cv. Russian Mammoth) were exposed to 0.1 or 0.2 ppm ozone (O3) for 12 days in an artificially-lighted growth cabinet. Plants were harvested 0, 6 and 12 days after the start of exposure, and the growth analysis was performed. White fleck of injury developed on many leaves after the exposure to 0.1 or 0.2 ppm O3 for 1-2 days, and subsequently withering of old leaves was accelerated. Twelve days after the start of exposure, the dry weight of whole plant was reduced by 11 % and 32% of the control by 0.1 and 0.2 ppm O3, respectively. Root growth was markedly inhibited by O3, while leaf growth was slightly inhibited. Relative growth rate (RGR) and net assimilation rate (NAR) were reduced by 0.1 ppm O3 for the first 6 days, but were not affected for the following 6 days. The RGR and the NAR were reduced by 0.2 ppm O3 throughout the exposure period. For the last 6 days, the RGR was less affected by 0.2 ppm O3 than the NAR, owing to the increase in leaf area ratio (LAR) . Leaf weight ratio (LWR) was also increased by O3 exposure, whereas stem weight ratio (SWR) and root weight ratio (RWR) were reduced. These changes in growth parameters suggest that the chronic exposures to low concentrations of O3 should affect the net photosynthesis and the partitioning ratio of photosynthates in sunflower plants.