Effects of Low Concentrations of SO2 on the Growth of Sunflower Plants

Abstract

One-week-old seedlings of sunflower (Helianthus annuusL. cv. Russian Mammoth) which were sown in artificially-lighted growth cabinets were subsequently exposed to 0.05 or 0.1 ppm SO2for 5 weeks. The plants were harvested at intervals of 7 days, and relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR), leaf weight ratio (LWR) and specific leaf area (SLA) were obtained. No remarkable changes were caused by SO2exposure in the dry weight of stem, root and whole plant. However, the leaf area and the leaf dry weight of plants exposed to 0.05 and 0.1 ppm SO2were markedly increased from harvesting on the 3rd week (2 weeks' exposure) . Exposure to 0.1 ppm SO2resulted in 20-25% reduction of NAR during the later stage (4 to 6 weeks), whereas RGR was slightly decreased. Exposure to 0.05 and 0.1 ppm SO2 caused the increase in LAR and LWR but not SLA. Same treatments also brought about the acceleration of leaf senescence and the depression of flower bud development and stem elongation.