SLIGHTLY ELEVATED OZONE EXPOSURE CAUSES CELL STRUCTURAL-CHANGES IN NEEDLES AND ROOTS OF SCOTS PINE

Scots pine (Pinus sylvestris L.) seedlings were fumigated with 1.2-1.5 x ambient ozone over 2 seasons in an open-air experiment. Fumigation started in the early spring and continued into late autumn during both years. Needle and root cell structures were analyzed in the summer, a utumn and early winter following the second fumigation period. Under t he light microscope an increase in the intercellular space and disinte grating cells in the mesophyll tissue near the stomata and stomatal ca vities were observed in the ozone-exposed needles. Darkening of chloro plast stroma, increased plastoglobulus size and decreased chloroplast size were characteristic ultrastructural changes associated with ozone exposure. In addition, less dense grouping of the chloroplasts in the needles of elevated ozone-exposed seedlings as compared to the contro ls (background ozone) was observed in the early winter. Fewer starch g rains and an increased accumulation of tannin-like substances were det ected in both mycorrhizal and uninfected roots of ozone-exposed seedli ngs as compared to the control seedlings. For the first time, we were able to show that the ozone-induced darkening of needle chloroplast st roma is a reversible symptom. An increased frequency of frost injury s ymptoms indicated that the winter hardening process was disturbed in t he needles of ozone-treated seedlings.