EFFECTS OF OZONE AND WATER DEFICIT ON THE GROWTH AND PHYSIOLOGY OF FAGUS-SYLVATICA

Elevated concentrations of ozone tend to be associated with periods of water deficit in plants. Ozone is highly episodic in its occurrence in the UK but plant responses to intermittent exposure to ozone and water deficit are poorly understood. Fagus sylvatica saplings were exposed to intermittent episodes of ozone and simultaneous water deficit during one summer. Dose-response relationships were determined by using six ozone treatments ranging from 20 to 48 ppb (cumulative 8-h mean concentration) over the experimental period. Gas exchange measurements were made at regular intervals through the summer and the final destructive harvest was made in the spring of the following year. There were significant ozone-water deficit interactions seen in both plant growth and gas exchange. Root weight measured in the subsequent spring was reduced by ozone in well-watered plants, but increased by ozone in plants which experienced water deficit. Therefore, ozone reduced the negative impact of water deficit on root weight, Ozone-induced increases in photosynthetic rate and leaf conductance were observed on occasions in water-stressed plants, and this may be at least a partial explanation of the observed effects on root weight. These results demonstrate that Fagus sylvatica responds to ozone at concentrations typical of British summers and that these responses can be modified by water availability.