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Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/4/2001 Publication Date: N/A Citation: N/A Interpretive Summary: Exposure of soybean plants to elevated ground level ozone levels (60-100 nL ozone L-1) results in yield losses of as much as 10-30% and an estimated net loss to agricultural productivity in the billions of dollars yearly in the U.S. and Canada. A Beltsville scientist is examining the relationship between the ability of soybean cultivars to withstand chronic exposure to environmentally relevant, elevated ozone levels and the foliar synthesis o the antioxidant, ascorbic acid (ASC) and the ASC:dehydroascorbate (DHA) oxidation-reduction turnover. He previously reported that an ozone- tolerant soybean cultivar (cv Essex) was able to maintain a higher leaf ASC level and ASC:DHA status than did an ozone-sensitive cultivar (cv Forrest) during periods of elevated ozone exposure in the field. However, in this new study, which compared a different ozone tolerant soybean cultivar, cv Hood, and a different ozone sensitive cultivar, cv Hark, it was clear that under environmentally relevant elevated ozone, the reductio in dry mass per plant was similar. The unifoliates, but not the trifoliate leaflets of cv Hark displayed lower ASC levels and lower ASC: DHA oxidation reduction status than cv Hood. The study is important because it demonstrates to soybean growers and breeders that it is necessary to assess the impact of ozone on crop plants using environmentally relevant ozone levels, because the use of acute ozone exposure produces misleading information concerning ozone tolerance potential. Soybean breeders and geneticists, whose mission is to develop soybeans that will have maximal yield even when ozone levels are high, should test and compare ozone tolerance in new cultivars by comparing not only visual damage, but also vegetative yield as well as grain yield. Technical Abstract: The purpose of this study was to compare the effects of chronic elevated ozone levels on vegetative growth and foliar ascorbic acid relations of two soybean cultivars, cv Hood and cv Hark. In studies done during the 1970s and early 1980s, cv Hood was judged to be an ozone tolerant cultivar and cv Hark a very ozone sensitive cultivar, based on short episodes of very high (acute) ozone levels. In the current study, the effect of exposure of these two cultivars to environmentally relevant, moderately elevated ozone levels was examined. Also these cultivars were used to examine the potential role of ascorbate (ASC) accumulation and redox turnover in conferring ozone tolerance. Studies were conducted in May, June, and July 2000 in open-top chambers fitted into the greenhouse. Prolonged ozone exposure resulted in a similar reduction in vegetative production in both cv Hood and cv Hark in three separate studies. In newly ymatured trifoliate leaflets, ascorbate (ASC) levels were similar or only slightly lower, and ascorbate (ASC): dehydroascorbate (DHA) redox status was always lower in cv Hood than in cv Hark plants after a period of 22-26 d PE and 17 d of treatment with 64-76 nL ozone L-1 air. In contrast, at 14 d PE and 5 d ozone treatment, prior to visible necrosis on the unfoliates. ASC levels as well as ASC redox status was lower in cv Hark in both control and ozone-treated plants, especially in the early afternoon. Thus ASC may be accumulating more rapidly and ASC: DHA redox turnover may be more rapid in the unifoliate leaflets of ozone-treated cv Hood than in ozone-treated cv Hark. The unifoliate leaflets of cv Hark and cv Hood appear to be the most useful for the assessment of the role of ASC synthesis and ASC: DHA oxidation-reduction turnover in tolerance to elevated ozone. |
