THE EFFECTS OF LONG-TERM ELEVATED OZONE CONCENTRATIONS ON THE GROWTH AND PHOTOSYNTHESIS OF SPHAGNUM-RECURVUM AND POLYTRICHUM-COMMUNE

Although ozone is generally accepted to be an important phytotoxic air pollutant, little is known of its effects on natural and semi-natural vegetation. In this study two bryophyte species, Sphagnum recurvum P. Beauv. and Polytrichum commune Hedw., were exposed to long-term chron ic ozone concentrations, i.e. 70-80 ppb (nl l(-1)), for 6-9 wk in summ er conditions (mean 15 degrees C) and in winter conditions (mean 6.4 d egrees C) in open-top chambers (OTC), in the field, and at 15 degrees C and 5 degrees C in controlled-environment chambers, under laboratory conditions. Fumigation with ozone caused a significant reduction in t he growth of treated plants in comparison with that of control plants. However, there were differences between species. S. recurvum was more affected than P. commune. In the controlled-environment chambers grow th of S. recurvum was reduced under both winter and summer conditions by 21 and 15%, respectively. In the OTCs, S. recurvum growth was reduc ed (by 17%) only under summer conditions. P. commune growth was only s ignificantly reduced following ozone exposure in the OTCs under winter conditions. Here the growth reduction was by 22%. Net CO2 assimilatio n of the two species was also significantly reduced by ozone exposure but only in the OTCs under winter conditions. Net CO2 assimilation of S. recurvum and P. commune was reduced by 60 and 80%, respectively. Th e ratio of variable to maximal fluorescence (F-v/F-m ratio) was the le ast affected of the parameters measured. Only for P. commune fumigated in the OTCs under winter conditions was the F-v/F-m ratio significant ly reduced (by 4.4%). Bryophytes, by the very nature of their morpholo gy and physiology are, perhaps, more likely to be susceptible to ozone pollution than higher plants; these results are discussed in relation to their possible ecological significance.