The effect of atmospheric NH3 deposition on freezing tolerance of needles of Scots pine was investigated. Needles from 3 month old seedlings and from 20-30 year old field-grown Scots pine (Pinus sylvestris L.) trees were studied. Seedlings were exposed to NH3 during the first stage of hardening in the laboratory. Field-grown trees were growing in a "clean" or an NH3 polluted area. In the 3 month old seedlings, exposure to 1000 ml 1** (-1) NH3 for 4 weeks resulted in a 40 increase of total nitrogen content. Freezing tolerance of the needles was significantly enhanced in the NH3 exposed seedlings. In the field-grown trees, current and one year old needles were studied. Freezing tolerance of the current year needles did not differ significantly between the two forest stands. However, one year old needles from the NH3 polluted stand were significantly less frost hardened from October until February. Total nitrogen content was higher in both current and one year old needles from the NH3 polluted stand. At the same time, potassium content of these needles was lower compared to the needles from the clean stand. The content of calcium, magnesium and phosphorus did not differ significantly. It was concluded that a decrease of freezing tolerance upon atmospheric NH3 was not directly related to an increased nitrogen content, but might probably be due to the nutrient imbalance of the needles.

Among a variety of stress indicators for early identification of Norway spruce (Picea abies (L.) Karsten) decline the sulphur content, photosynthetic pigment content, activity of the enzyme peroxidase, watersoluble thiols and ascorbic acid in 4 age classes of 5 spruce needles were analysed. The 10 sampling sites were selected according to the degree of forest decline, sea level, distances from the Thermal Power Plant Sostanj and air pollution.

It was our aim to investigate whether the cytokinin status of Norway spruce seedlings (Picea abies L. Karst.) was changed when the plants were grown under nutritive stress. Cytokinins in shoots and roots from spruce seedlings, grown in various hydrocultural media with complete (control) or poor nutrient supply (stress), were determined by an indirect competitive enzyme-liked immunosorbent assay. The shoots of spruce seedlings grown in a poor acidic culture medium, to which aluminium ions (AlCl3, 0.8 mM) were added, showed up to 5-fold higher concentrations of zeatin riboside and isopentenyladenosine (iPa). When adding naphtylacetic acid to the nutrient medium also higher levels of cytokinin ribosides were measured in the shoots. In the roots however cytokinin riboside levels did not increase significantly under Al- or auxin treatment. Feeding experiments using tritiated iPA have shown that the metabolism of this cytokinin is strongly retarded in the roots of stressed seedlings. The experiments show that acid conditions and nutrient shortage together with aluminium ions in the rhizosphere increase the number of lateral roots in Norway spruce seedlings. It is assumed that the increased number of sites of cytokinin biosynthesis and the reduced metabolism of iPA result in the higher content of cytokinin ribosides in the shoots of seedlings.

Cytokinins were analysed by a combined HPLC-ELISA method in needles of Norway spruce seedlings. The seedlings were grown in vitro on sterile or nonsterile soil substrates from two differently polluted forest research plots. Difference were predominantly found in the isopentenyladenine-type of cytokinins. These were elevated in seedlings, grown on polluted soils in comparison to those, grown on soils from the unpolluted plot. A possible explanation might be in the change of the metabolism in the roots due to pollution stress (when grown on sterilized substrates) or in change of the mycorrhizosphere organisms (when grown on nonsterile soil substrates). The model system using Norway spruce seedlings as tester organisms for soil pollution is discussed.

Mechanisms of air pollution toxicity are very complex and depend on various physiological and biochemical properties of plants. These mechanisms (including formation of free radicals) are still poorly understood. In addition, physiological responses of forest plants to air pollution stress can be modified by various biotic (e.g., insects, pathogens, mycorrhizae associations, genetic variation) and abiotic (e.g., increasing CO2 concentrations, ultraviolet-B radiation, nitrogen desposition, nutrient deficiencies, drought) factors. An example of air pollution effects on forest trees may be responses of ponderosa pine seedlings to elevated concentrations of ozone: various physiological changes in trees (lowered net photosynthesis, altered carbon allocation, deterioration of photosynthetic pigments, etc.) have led to reduced growth and biomass of plants.