EXPOSURE OF NORWAY SPRUCE TO OZONE INCREASES THE SENSITIVITY OF CURRENT YEAR NEEDLES TO PHOTOINHIBITION AND DESICCATION

Physiological effects of ozone exposure over three consecutive growing seasons on current year needles of Norway spruce were studied in open -top chambers, during daily fumigation cycles in the summer, and after the termination of ozone fumigation in autumn 1990. The trees were ex posed to two levels of ozone: charcoal filtered air and non-filtered a ir to which 30 nl l(-1) of ozone was added in three consecutive years from 1988 to 1990, daily from May to September (8 hours a day). Photos ynthesis, stomatal conductance, transpiration and chlorophyll fluoresc ence were studied on selected days. Significant decreases in net photo synthesis and chlorophyll fluorescence (F-v/F-m) were found during per iods with co-occurrence of high ozone concentrations and high light in tensities, indicating interactions between effects of ozone and photoi nhibition. After termination of fumigation enhanced rates of photosynt hesis were seen in the trees which had been exposed to ozone. A signif icant decrease in F-v/F-m was found for twigs from ozone treated trees when exposed to severe desiccation.