EFFECTS OF OZONE AND MILD DROUGHT STRESS ON GAS-EXCHANGE, ANTIOXIDANTS AND CHLOROPLAST PIGMENTS IN CURRENT-YEAR NEEDLES OF YOUNG NORWAY SPRUCE [PICEA-ABIES (L.) KARST.]

To investigate the effects of ozone exposure and soil drought, singly and in combination, on gas-exchange, antioxidant contents and pigments in current-year needles of Norway spruce [Picea abies (L.) Karst.] 4- year-old seedlings were fumigated in growth chambers with either charc oal-filtered air or with 100 nl l(-1) ozone for 106 days. After 3 week s a 20% reduction in gas exchange was observed In ozone-treated seedli ngs. However, no further decrease occurred in spite of continued ozone exposure. Whole needle ascorbate and apoplastic ascorbate increased u ntil the end of the experiment and contents were 62% and 82%, respecti vely, higher than in ozone-free controls. This increase in ascorbate m ight have protected net photosynthesis from Further decline. Ozone pre -treated plants and ozone-free controls were subjected to soil drought for 38 days which caused stomatal narrowing. Thereby ozone uptake was reduced when compared to well watered seedlings. At the end of the ex periment drought alone, and even more in combination with ozone, had a lso caused an increase in ascorbate. Glutathione increased only in dro ught-stressed seedlings. The redox states of the ascorbate and the glu tathione pools were not affected by any treatment. Superoxide dismutas e activity declined under both stresses but was most reduced by ozone alone. While chlorophyll and neoxanthin contents remained unchanged, c arotenes were significantly decreased upon drought. The combination of O-3 and drought induced increased lutein contents, an increased pool size of the xanthophyll cycle as well as an increased epoxidation stat us of the xanthophyll cycle. These results suggest that spruce needles seem To be able to acclimate to ozone stress but also to drought stre ss by increasing their ascorbate pools and protecting pigments.