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Effects of Low Levels of Ozone on Growth of Crimson Clover and Annual Ryegrass¹

Studies have been conducted on short term exposure of plants to broad ranges of O₃ concentration. Our purpose was to investigate long term exposure of plants to O₃ levels that were below those normally causing visible injury. Annual ryegrass (Lolium multiflorum Lam.) and crimson clover (Trifolium incarnatum L.) were grown for 6 weeks in chambers within a greenhouse and fumigated for 8 hours/day with 0.03 and 0.09 ppm O₃ in air. Treated plants showed less growth and yield than the controls despite the absence of visible injury. Both total dry weight and leaf area of treated plants were as much as 22% lower than the controls, but percent dry weight was significantly higher. Root growth was impaired the most, followed by stems and leaves, respectively, as evidenced by a lower root/shoot ratio and a slightly higher leaf weight ratio with O₃ treatment. Both species showed significantly lower specific leaf areas and leaf area ratios in O₃ compared to the control, but higher net assimilation rates led to relative growth rates which were unaffected. It was concluded that although the relative size of the leaf system was very sensitive to prolonged exposure to low O₃, compensation in growth parameters occurred to counteract the stress response.

Key Words: Growth analysis • Trifolium incarnatum L. • Lolium multiflorum Lam. • Root/shott ratio • Specific leaf area • Relative growth rate • Net assimilation rate • Air pollutants

² Assistant professor, Dep. of Vegetable Crops, Univ. of California, Davis, CA 95616, and professor, Dep. of Plant Science, Univ. of British Columbia, Vancouver, B.C., Canada.

Received for publication September 7, 1976.