Histological Changes Induced in Scotch Pine Needles by Sulfur Dioxide

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Abstract
Scotch pine [Pinus sylvestris] cotyledons, primary needles and secondary needles of various ages were exposed to 2620 .mu.g/m3 (1.0 ppm) SO2 for 1, 3, or 5 h. Needles from unexposed (control) and exposed plants were embedded in paraffin, sectioned, stained and examined under a light microscope. Mesophyll collapse was the most common structural change induced by SO2 in all 3 needle types. Resin canal occlusion also was observed in primary and secondary needles exposed to SO2. Increased dosages of SO2 did not induce additional symptoms but intensified those already present.