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

Plant responses to 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. 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 O3 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 O3 concentrations, and low ratios indicated high concentrations. We used these results in attempts to identify the relative suitability of several exposure indexes in defining O3 exposure–forage biomass relationships. Indexes were calculated using combinations of O3 exposure forms (SUM00, SUM06, W95, W126, and AOT04), diurnal and seasonal accumulating times and harvests. Squared correlations (r2ʹs) 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 r2ʹs 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 r2ʹs occurred for the 24 h accumulating period, much higher r2ʹs occurred for the 12 h daylight period, and the highest r2ʹs 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 r2ʹs. All forms, including SUM00, produced similarly high r2ʹs for 6, 5, and 4 h midday accumulating times.