RESPONSES OF LEAF PROCESSES IN A SENSITIVE BIRCH (BETULA-PENDULA ROTH) CLONE TO OZONE COMBINED WITH DROUGHT

Saplings of an ozone sensitive clone of birch (Betula pendula Roth, KL -5-M) were well-watered or exposed to mild drought-stress combined wit h ambient or elevated (1.5 x the ambient) ozone for 11 weeks in open-f ield conditions in central Finland. Stomatal response, visible injury, chlorophyll and nutrient content, and changes in cellular anatomy and plant growth were studied. Drought stress alone, in ambient ozone, re duced stomatal density and stomatal conductance. Drought stress and oz one effects were additive, reducing total leaf number, foliage area an d starch formation in mesophyll cells. Drought stress and ozone effect s were additive, increasing the N concentration in the leaves, the thi ckness of the upper epidermal cell wall, the number of pectinaceous pr ojections of mesophyll ci:ll walls, and the vacuolar tannin-like depos itions and phenolic droplets, regarded as signs of activated stress de fence mechanisms. The increase in specific foliage mass, cytoplasmic l ipids (younger leaves), and a condensed appearance of the upper epider mal mucilaginous layer were caused by both drought and ozone, but were not additive. The results show that combined drought stress contribut ed to birch responses to 1 5 x current ambient ozone concentrations, c orresponding to critical-level ozone exposure. The only beneficial eff ect of drought stress was the slight reduction of visible leaf symptom s induced by ozone in autumnal leaves. (C) 1998 Annals of Botany Compa ny.