A GAS-EXCHANGE STUDY OF THE DIFFERENTIAL RESPONSE OF QUERCUS SPECIES TO LONG-TERM FUMIGATIONS WITH A GRADIENT OF SULFUR-DIOXIDE

Young oak plants (Qeuercus pubescens Willd. and 9. cerris L.) were exp osed for 23 weeks to a gradient of sulphur dioxide concentrations (28, 61, 93 ppb in vol., compared with charcoal filtered-air ventilated co ntrols) in fumigation chambers. Foliar gas exchange was analyzed by an IRGA methodology after 11 weeks and chlorophyll was assessed in a non -destructive way. At the end, foliar biomass produced during the exper imental period was determined and its total sulphur and carbohydrate c ontent analyzed. Visible foliar injury, as well as reduction in chloro phyll content was not observed. Carbon dioxide assimilation was linear ly ;I depressed by sulphur dioxide in both the species, with a maximum depression of 25% in e. pubescens and 44% trend. The two species show ed a similar behaviour in water use efficiency, this parameter being r educed in a in Q. cern's. Stomatal conductance and transpiration were negatively affected only in e. pubescens, with a linear linear fashion . The intracellular/ambient carbon dioxide concentration ratio was aff ected in a differential way: while in e. pubescens no alteration was e vident, a linear increase was shown by e. cerris. Total foliar sulphur content was dramatically increased in both the species. Average leaf area was unaffected in both the species. Reduction in foliar dry weigh t was only observed in e. cerris (-30% at the highest sulphur dioxide concentration). We speculate that in Q. pubescens photosynthetic limit ations are mainly of stomatal nature, and in Q. cerris the major const raints are probably associated with mesophyll disturbances. Foliar sta rch content was linearly increased by the pollutant.