The influence of needle age on sensitivity of Scotch pine to acute doses of SO₂

Young Scotch pine seedlings in the cotyledon, primary, and secondary needle stages were exposed to 2620 μg/m³ (1.0 ppm) SO₂ for 1, 3, or 5 hours. Exposures were conducted at time of needle emergence, and when the current foliage was 1, 3, 5, and 7 weeks of age. Secondary needles were generally more sensitive than were primary needles. Cotyledons were generally more tolerant than the other two needle types. The sensitivity of all three needle types was at a minimum at time of needle emergence, increased through 3 to 5 weeks, and then remained constant or decreased from 5 to 7 weeks. All three needle types exhibited needle tip necrosis as the common symptom.