THE IMPACT OF OZONE AND DROUGHT ON THE WATER RELATIONS OF ASH TREES (FRAXINUS-EXCELSIOR L)

Two year old potted saplings of ash (Fraxinus excelsior L.) were expos ed to ozone episodes in open-top chambers. Ozone exposure concentratio ns were 0 ppb and 150 ppb, and there were 22 exposures of 8 h, beginni ng in May 1994. Additionally half of the plants of each pollution trea tment were subjected to three drought cycles of 7-14 days, while the r est of the plants were well-watered. Stomatal conductance and radial i ncrement at the stembase and at the base of the new shoot growth were investigated. Ringwidth and cell number per ringwidth of the annual ri ng produced in 1994 were determined microscopically, using samples tak en from the stembase. The ozone episodes were shown to have an impact. on the stomatal responsiveness of the plants, restricting stomatal ap erture of the droughted ozone-exposed treatment after drought cycles w hen all plants were maintained at a high soil moisture. The drought cy cles alone, however, left the functioning of the stomata unimpaired. T he restriction of stomatal aperture, which was found for the droughted , ozone-exposed treatment, was concomitant with significantly decrease d radial growth at the stembase of those plants. Microscopical analysi s of the annual rings showed that this was caused by a reduction of ce ll numbers in xylem tissue. Radial increment at the base of the new sh oot growth was less affected by the pollution treatment, but the droug ht treatment alone caused a significant, but smaller growth reduction. This response to ozone could lead to less water uptake, and thus wate r supply to the crown might become limiting.