Author
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Heagle, Allen |
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STEFANSKI, LEONARD - NORTH CAROLINA STATE UNIV |
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Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/16/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Ground level ozone (O3) causes foliar injury, reduces growth, and decreases yield of many crop plants. Because O3 concentrations vary widely from day to day and from place to place, there is debate as to how to describe O3 exposure (exposure index) in a way that best relates exposure to the amount of stress it causes. The debate centers on relative eimportance of high vs low O3 concentrations, and whether exposure during certain daily or seasonal periods is more important than exposure at other times. Use of one O3 exposure index will facilitate comparison of response to ambient O3 in one area to response in another area. To help identify an appropriate exposure index, we used relative growth response of an O3 sensitive (NC-S) and an O3 resistant (NC-R) white clover clone from a 2-year field experiment at eight locations in the USA. We used O3 concentrations, to calculate 330 exposure indexes for each location. These eindexes either emphasized or did not emphasize relative importance of high vs low O3 concentrations or averaging times. Correlations between each index and plant growth (biomass) were determined. The worst correlations occurred for the 24 h averaging period, much better correlations occurred for the 12 h light period, and the best occurred for periods of 6 h or less during midday. Moreover, it was not necessary to treat low and high O3 concentrations differently when calculating a meaningful O3 exposure index; any exposure form was as good as any other if mid-day averaging times were used. Apparently, O3 exposure caused more stress to clover during mid-day than at other times which is consistent with our observations that uptake of O3 by clover is greatest during mid-day. The results should help to end debate over which exposure index is best related Technical Abstract: Effects of seasonal exposure to tropospheric ozone (O3) are mediated by interactions with physical and genetic factors that complicate attempts to develop a measure of O3 exposure (exposure index) that best relates to plant response. Current debate centers on relative importance of high vs low O3 concentrations and whether exposure during certain daily or seasonal periods is more important than exposure at other times. We tested correlations between 330 O3 exposure indexes and relative biomass response of an O3 sensitive (NC-S) and an O3 resistant (NC-R) white clover clone from a 2-year field experiment at eight locations in the USA. The 330 exposure indices were calculated for each location using exposure forms with different emphasis on the importance of high vs low O3 concentrations combined with different diurnal and seasonal averaging times. Results showed that the diurnal averaging period was more important than exposure form or seasonal averaging period. Poorest correlations between O3 exposure and clover biomass occurred for the 24 h averaging period. Correlations for the 12 h light period were improved, and the best correlations occurred for periods of 6 h or less during midday The results show that any exposure form is as good as any other, if mid-day averaging times are used. Apparently, O3 exposure caused more stress to clover during mid-day than at other times which is consistent wit our observations that clover foliar gas exchange is greatest during mid-day The results should help conclude the debate over which exposure index is best related to plant response by indicating that all may be equally suited when the appropriate daily averaging period is used. |
