Relationships between ambient ozone regimes and white clover forage production using different ozone exposure indexes

Plant responses to seasonal exposure to tropospheric ozone (O-3) are mediated by interactions with physical and genetic factors that complicate attempts to develop a measure of O-3 exposure (exposure index) that best relates to plant response. Dozens of exposure indexes have been tested for best fit to yield response data from open-top chamber studies. These tests have limited applicability because of possible confounding caused by variability in experimental protocols used in chamber dose-response studies. A 2-yr study in ambient air at eight locations in the USA measured relative effects of ambient O-3 On forage weight of a sensitive (NC-S) and a resistant (NC-R) clone of white clover. Protocols included uniform growth medium, irrigation, exposure duration and genetics (clones). Plants were harvested to determine NC-S/NC-R forage biomass ratios after each of four 28-day periods. High ratios indicated low O-3 concentrations, and low ratios indicated high concentrations. We used these results in attempts to identify the relative suitability of several exposure indexes in defining O-3 exposure-forage biomass relationships. Indexes were calculated using combinations of O-3 exposure forms (SUM00, SUM06, W95, W126, and AOT04), diurnal and seasonal accumulating times and harvests. Squared correlations (r(2)'s) between the index and biomass ratio were used as a general indication of relative suitability of the different indexes. Squared correlations were much higher for indexes coupled with harvests 2, 3 and 4, than for harvest 1. Even higher r(2)'s occurred for indexes coupled with the combined mean forage ratio for harvests 2, 3 and 4. Squared correlations were most sensitive to the choice of hourly averaging times. Lowest r(2)'s occurred for the 24 h accumulating period, much higher r(2)'s occurred for the 12 h daylight period, and the highest r(2)'s occurred for periods of 6 h or less during midday, regardless of all other factors. The exposure form was important only for 24 h indexes for which SUM00 gave the lowest r(2)'s. All forms, including SUM00, produced similarly high r(2)'s for 6, 5, and 4 h midday accumulating times. (C) 2000 Elsevier Science Ltd. All rights reserved.