Photosynthetic Activity of Scots Pine (Pinus-Sylvestris L) Needles During Winter Is Affected by Exposure to So2 and No2 During Summer
New Phytologist: 1993 123:133-141
A 50-year-old stand of Scots pine (Pinus sylvestris L.) with a mean height of 12 m, located 60 km northwest of Umea in northern Sweden, was fumigated with SO2 and NO2 from June to September in 1988 and 1989. SO2 and NO2 were released from vertical vent-pipes arranged in a circle with a diameter of 60 m. During fumigation, the average concentrations of both SO2 and NO2 in the centre of the circle were 10-15 nl l-1. Measurements of O2 evolution and chlorophyll a fluorescence of needles were made under laboratory conditions. For samples collected in january of 1989 and 1990, the light- and CO2-saturated rate of O2 evolution at 20-degrees-C in needles of trees exposed to SO2 + NO2 was lower than in needles of trees exposed to ambient clean air. Furthermore, the photochemical efficiency of photosystem II, as indicated by the ratio of variable to maximum fluorescence was lower for needles of fumigated trees, in samples collected in December 1989 and January 1990. These results suggest that low concentrations of SO2 + NO2 may have significant long-term effects on the photosynthetic apparatus in needles of Scots pine. The observed responses may be triggered by unfavourable environmental conditions during autumn and winter. However, no evidence for a greater sensitivity to prolonged freezing stress at -19-degrees-C was found in needles of trees exposed to SO2 + NO2 when measured in December of January.
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