Effect of temperature preconditioning on responses of Fraxinus pennsylvanica seedlings to SO₂

Nine-week-old Fraxinus pennsylvanica Marsh. seedlings were grown for 4 weeks at 15 or 25°C and then exposed to 0, 0·5, 1·0 or 1·5 μl litre^-1 SO₂ for 30 h. The seedlings were subsequently grown in the glasshouse and harvested 4 and 8 weeks later. Sulphur uptake per plant and per unit of leaf dry weight of leaf area was higher in plants preconditioned at 25°C than at 15°C. Leaf injury, which varied with SO₂ dosage, was also higher in the 25°C preconditioned seedlings. In both unpolluted and polluted plants leaf initiation and expansion, dry weight increment of leaves, stems and roots, and leaf abscission were higher in plants preconditioned at 25°C than at 15°C. However, during the last 4 weeks, the seedlings whose growth had been suppressed by the 15°C preconditioning temperature regime exhibited a surge of compensatory growth. Such accelerated growth in plants previously exposed to 15°C, as well as extensive leaf abscission in the plants exposed to 25°C, resulted in higher relative growth rates of the former group. Overall, the effects of the preconditioning temperature on various plant responses were much greater than the effects of SO₂. The data stress the importance of preconditioning environmental regimes on plant responses to SO₂. They also emphasise the complexity of interactions between environmental factors and air pollutants on plant growth.