A time-concentration study on the effects of ozone on spring wheat (Triticum aestivum L) .2. A comparison of indices

Several indices to describe the relationship between time-concentration of ozone and the resulting changes in grain yield were evaluated. In addition, hourly ozone concentrations were weighted with simultaneous sunshine hour values in an attempt to incorporate a measure of absorbed dose into current indices. Sunshine hour values were chosen as an environmental variable which would approximately reflect stomatal conductance at the time of exposure. Index performance was evaluated by fitting a polynomial model to the exposure-response data. Treatment effects on grain yield were unrelated to characterisations of ozone exposure obtained using either mean values or cumulative totals. Progressively better fits of the data were obtained, however, when cumulative totals were calculated above progressively increasing threshold concentrations. Good fits were obtained when indices which summed concentrations above thresholds of 50, 60 and 70 ppb were used. Cumulation of radiation weighted ozone concentrations above progressively increasing threshold concentrations also resulted in improving fits. However, those indices which used hourly ozone concentrations weighted with sunshine hour values did not perform as well as those which only used hourly ozone concentrations. Testing of the relationship on more extensive data is recommended.