An ozone response relationship for four Phleum pratense genotypes based on modelling of the phytotoxic ozone dose (POD)

Abstract

In the last decade extensive research has focused on the development of dose–response relationships based on stomatal plant ozone uptake (phytotoxic ozone dose, POD). So far most work has concentrated on crops and forest trees. This study provides a flux-based dose–response function for timothy (Phleum pratense), a widespread grassland species, which can be used in risk assessment for ground-level ozone. In 1996 and 2001 timothy was exposed in open-top chambers to ozone concentrations ranging from around 10nmolmol−1 in the charcoal filtered treatments up to 60nmolmol−1 in the fumigated treatments (08:00–20:00)in. In 1996 there was a negative effect of ozone on biomass production in the non-filtered treatment while in 2001 no such ozone effect in the non-filtered treatment could be seen. Measurements of stomatal conductance on four timothy genotypes in 2001 were used to calibrate a Jarvis-type multiplicative stomatal conductance model. The maximum conductance varied between the genotypes, from 477 to 589mmolO3m−2s−1 (projected leaf area). The model includes functions describing the reduction of stomatal conductance of senescing leaves and the direct effects on stomatal conductance by light, temperature and water vapour pressure deficit. A function describing ozone induced senescence of the leaves was included since exposure to ozone is known to cause premature senescence. The function for ozone was applied when it suggested ozone to be more limiting to stomatal conductance than phenology. To avoid overestimation of stomatal conductance in days with high VPD, a function reflecting the effect on leaf water potential on stomatal conductance was included. Comparison between modelled and measured conductance for the four timothy genotypes resulted in an r2 value at 0.57 and a very small average deviation of observed from modelled values. The calibrated stomatal conductance model was used to estimate the accumulated POD, i.e. the accumulated stomatal flux of ozone, of the plants in the 1996 and 2001 experiments. The strongest relationship between ozone relative effects on biomass was obtained when POD was accumulated from 105 degree days after emergence to 1000 degree days after emergence, and integrated using an uptake rate threshold of 7nmolm−2s−1 (POD7). The response relationship between biomass and POD7 resulted in an r2 value of 0.71 over all four genotypes. This r2 value was somewhat higher than for the corresponding relationship based on the accumulated ozone exposure over 40nmolmol−1 (AOT40; r2=0.66). With an uptake rate threshold at 7nmolm−2s−1, ozone concentrations above ∼20nmolmol−1, contribute to reduce the biomass production of timothy if meteorological conditions promote maximum stomatal conductance.