The Impact of Ozone and Drought on the Water Relations of Ash Trees ( Fraxinus excelsior L.)

Abstract

Two year old potted saplings of ash (Fraxinus excelsior L.) were exposed to ozone episodes in open-top chambers. Ozone exposure concentrations were 0 ppb and 150 ppb, and there were 22 exposures of 8 h, beginning in May 1994. Additionally half of the plants of each pollution treatment were subjected to three drought cycles of 7-14 days, while the rest of the plants were well-watered. Stomatal conductance and radial increment at the stembase and at the base of the new shoot growth were investigated. Ringwidth and cell number per ringwidth of the annual ring produced in 1994 were determined microscopically, using samples taken from the stembase. The ozone episodes were shown to have an impact on the stomatal responsiveness of the plants, restricting stomatal aperture of the droughted ozone-exposed treatment after drought cycles when all plants were maintained at a high soil moisture. The drought cycles alone, however, left the functioning of the stomata unimpaired. The restriction of stomatal aperture, which was found for the droughted, ozone-exposed treatment, was concomitant with significantly decreased radial growth at the stembase of those plants. Microscopical analysis of the annual rings showed that this was caused by a reduction of cell numbers in xylem tissue. Radial increment at the base of the new shoot growth was less affected by the pollution treatment, but the drought treatment alone caused a significant, but smaller growth reduction. This response to ozone could lead to less water uptake, and thus water supply to the crown might become limiting.