S V\] MARY Frost injury in three Finnish provenances of acid rain-treated (pH 4 and 3) pine {Pinus svlvestris (L.)] and spruce {Picea ahies (L.) Karst.] seedlings was studied by microscopical methods. Seedlinfj-s from southern and northern pr()\-enanccs were watered for sex'en weeks \\-ith artificially acidified rain during' the hardening period in 1986. A cold treatment on the \\-inter hardened seedlings was carried out in January by lowering the temperature graduallv to --S-S°C. Afterwards the development of injury symptoms was followed visually and microscopically. The most prominent sign of freezing injury was needle discoloration. In addition, the stems and buds of the seedlings showed signs of severe injury. Under the light microscope the injury was exhibited as largened intercellular spaces and increased numbers of disintegrating cells. Diminution of the n-iembraneous structures within the cytoplasm of mesophyll cells and rupturing ot plasmalemma and tonoplast, as well as swelling of chloroplasts, were seen in damaged cells. The symptoms of freezing injury at ultrastructural le\-el differed from those found in needles exposed to air pollutants.
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