Saplings of an ozone sensitive clone of birch (Betula pendula Roth, KL-5-M) were well-watered or exposed to mild drought-stress combined with ambient or elevated (1n5ithe ambient) ozone for 11 weeks in open-field 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, reduced stomatal density and stomatal conductance. Drought stress and ozone effects were additive, reducing total leaf number, foliage area and starch formation in mesophyll cells. Drought stress and ozone effects were additive, increasing the N concentration in the leaves, the thickness of the upper epidermal cell wall, the number of pectinaceous projections of mesophyll cell walls, and the vacuolar tannin-like depositions and phenolic droplets, regarded as signs of activated stress defence mechanisms. The increase in specific foliage mass, cytoplasmic lipids (younger leaves), and a condensed appearance of the upper epidermal mucilaginous layer were caused by both drought and ozone, but were not additive. The results show that combined drought stress contributed to birch responses to 1n5icurrent ambient ozone concentrations, corresponding to critical-level ozone exposure. The only beneficial effect of drought stress was the slight reduction of visible leaf symptoms induced by ozone in autumnal leaves.# 1998 Annals of Botany Company

No abstract

No abstract

SUMMARYIn field experiments three clones of two-year-old birch (Betula pendula Roth.) were exposed for one growing season to an ozone concentration \-2 times higher than amhient. At the end of August, leaf and stem material was analyzed for a wide range of primary and secondary metabolites. Although most of these metabolites were not significantly affected by ozone exposure, ozone-treated leaves contained larger concentrations of total sugars and reduced amounts of the phenolic glucoside, dehydrosalidroside. In the stems, greater amounts of catechin pentoside, hyperoside and papyriferic acid were found. The results indicated considerable inter-and intra-clona! variation in the production of phytochemicals in both leaves and stems.

The physiological, stomatal and ultrastructural responses to ozone and drought of ozone-sensitive and more ozonetolerant birch (Betula pendula Roth.) clones were studied singly and in combination, in a high-stress chamber experiment and in a low-stress open-field experiment. In the chamber experiment, well watered (WW), moderately watered (MW) or drought-stressed (DS) saplings were exposed for 36 d to 0 or 130 nmol mol ∠1 ozone. In the open-field experiment, well watered or drought-stressed saplings were grown for one growing season in ambient air or exposed to 1·8 × ambient ozone. Drought stress reduced growth rate, stomatal conductance, stomatal density and the proportion of starch and thylakoids in chloroplasts, but stimulated net photosynthesis, Rubisco and chlorophyll quantity at the end of the growing season, and increased the size and density of plastoglobuli. Ozone fumigations caused more variable, clone-and exposuredependent responses in growth, decreased stomatal conductance and net photosynthesis, an increased number of stomata, visible and ultrastructural chloroplast injuries, and enhanced autumn yellowing of the leaves. Ozoneinduced changes in plastoglobuli, starch and thylakoids resembled drought responses. The two experiments revealed that, depending on the experimental conditions and the variable, the response to drought and ozone stress can be independent, additive or interactive. Drought protected the plants from ozone injuries under high-stress conditions in the chamber experiment. In the low-stress, open-field experiment, however, enhanced ozone damage was observed in birch saplings grown under restricted water supply.