A gas-exchange study of the differential response of Quercus species to long-term fumigations with a gradient of sulphur dioxide

Abstract

Young oak plants (Quercus pubescens Willd. and Q. cerris L.) were exposed for 23 weeks to a gradient of sulphur dioxide concentrations (28, 61, 93 ppb in vol., compared with charcoal filtered-air ventilated controls) 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 experimental period was determined and its total sulphur and carbohydrate content analyzed. Visible foliar injury, as well as reduction in chlorophyll content was not observed. Carbon dioxide assimilation was linearly depressed by sulphur dioxide in both the species, with a maximum depression of 25% in Q. pubescens and 44% in Q. cerris. Stomatal conductance and transpiration were negatively affected only in Q. pubescens, with a linear trend. The two species showed a similar behaviour in water use efficiency, this parameter being reduced in a linear fashion. The intracellular/ambient carbon dioxide concentration ratio was affected in a differential way: while in Q. pubescens no alteration was evident, a linear increase was shown by Q. 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 weight was only observed in Q. cerris (−30% at the highest sulphur dioxide concentration). We speculate that in Q. pubescens photosynthetic limitations are mainly of stomatal nature, and in Q. cerris the major constraints are probably associated with mesophyll disturbances. Foliar starch content was linearly increased by the pollutant.