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The cellular structures of acid rain-irrigated needles of several provenances of Norway spruce (Picea abies L. Karst) seedlings were studied after winter experimental freezing. Frost injuries and recovery were characterized by visual damage scoring and classification of mesophyll cell alterations, also using histochemical methods for carbohydrate fluorescent staining. The treatment with -30~ during the late dormancy period was sufficient to cause significant injuries and intracellular degradation in the tissues of the green needles. The most affected seedlings in terms of visual injury scoring were found among those treated with clean water or at pH 3, while freezing injury, defined as an occlusion of phenolic substances in the central vacuole of the mesophyll cells, was most abundant in the needles from spruces irrigated either with clean water or at pH 4 or pH 3. Electron microscopy revealed the details of the injury, e.g. thinning out of the cytoplasm and chloroplast stroma, darkening of the chloroplasts and eventually swelling of the chloroplasts and protoplast. PAS and ConA reactions in the needle tissue revealed intense starch accumulation in the mesophyll and transfusion tissues as early as in March, with a tendency to increase, especially in the untreated needles during the recovery period. Plasma membrane disturbances were indicated by histochemical identification of callose deposits in the mesophyll cell walls, these being most abundant in the acid rain-treated needles. All these findings suggest that freezing at -30~ was more deleterious to the seedlings pretreated with acid or clean water than to those not given additional irrigation.
The cellular structures of acid rain-irrigated needles of several provenances of Norway spruce (Picea abies L. Karst) seedlings were studied after winter experimental freezing. Frost injuries and recovery were characterized by visual damage scoring and classification of mesophyll cell alterations, also using histochemical methods for carbohydrate fluorescent staining. The treatment with -30~ during the late dormancy period was sufficient to cause significant injuries and intracellular degradation in the tissues of the green needles. The most affected seedlings in terms of visual injury scoring were found among those treated with clean water or at pH 3, while freezing injury, defined as an occlusion of phenolic substances in the central vacuole of the mesophyll cells, was most abundant in the needles from spruces irrigated either with clean water or at pH 4 or pH 3. Electron microscopy revealed the details of the injury, e.g. thinning out of the cytoplasm and chloroplast stroma, darkening of the chloroplasts and eventually swelling of the chloroplasts and protoplast. PAS and ConA reactions in the needle tissue revealed intense starch accumulation in the mesophyll and transfusion tissues as early as in March, with a tendency to increase, especially in the untreated needles during the recovery period. Plasma membrane disturbances were indicated by histochemical identification of callose deposits in the mesophyll cell walls, these being most abundant in the acid rain-treated needles. All these findings suggest that freezing at -30~ was more deleterious to the seedlings pretreated with acid or clean water than to those not given additional irrigation.
SUMMARYThe mesophyll ultrastructure of needles of Norway spruce [Pkea abies (L.) Karst.] and Scots pine (Pinus sylvestris L.) seedlings was examined in the course of experimental acid rain treatment lasting three years. The pH 3-treated needle mesophyll cells exhibited thicker granal packing in their chloroplasts than the dry control needles after one season's irrigation. Starch occurrence had decreased due to pH treatment when the whole hardening and rest period was considered. Chloroplast shapes were seen to vary from ellipsoid to irregular, especially in the pH 4 and pH 3-treated needles, due to divisions during hardening. The mean area of mesophyll chloroplasts in the hardened state after three seasons' irrigation was largest (10-3 /(m^) in the pH 3-treated needles, which also contained the smallest number of dark-stained plastoglobuii. Starch occurrence in the hardened mesophyll cells in October reflected disturbances in the hardening processes in the pH 3-treated needles.
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