Phenological weighting of ozone exposures in the calculation of critical levels for wheat, bean and plantain

This paper presents phenological weighting factors to be applied to AOT40 ( accumulated ozone exposure above a threshold of 40 nl l(-1)) ozone exposure -response relationships for crops at different growth stages. The quantific ation of such factors represents a step-forward in the derivation of Level II critical levels for ozone. The weighting factors presented are derived f rom published literature on the sensitivity of wheat (Triticum aestivum), b ean (Phaseolus vulgaris) and plantain (Plantago major) to ozone at differen t growth stages. Weighting functions were calculated using either multiple linear regression or the reciprocal residual mean square (RMS-1). The resul ting weights were transformed into multiplication factors to be applied to the monthly AOT40 during the 3-month assessment period of critical level ex ceedance. Interspecific differences were too large to allow for the develop ment of a unified weighting function for the three species considered. For wheat grain yield, the derived multiplication factors varied by almost four -fold (0.40, 1.06, 1.54), while those for bean pod yield varied by only abo ut 25% (0.85, 1.01, 1.14), The available data for plantain were restricted to short-term studies conducted under controlled conditions. These data wer e not suitable for the derivation of weighting factors comparable to those derived for bean and wheat. Based on known differences in wheat development and phenology across Europe, the need for a geographic differentiation of the time period for the calculation of the critical level exceedances is al so discussed and examples provided of the adoption of the derived weighting s in the mapping of critical level exceedances. Differences between critica l level exceedance maps using weighted and unweighted AOT40 calculations ar e discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.