The effect of exposure to NO2 and SO2 on frost hardiness in Calluna vulgaris

Heather (Calluna vulgaris) was grown in pots in a natural heathland soil and exposed in outdoor fumigation chambers ('solardomes') to 40 nl l(-1) of both nitrogen dioxide (NO2) and sulphur dioxide (SO2) simultaneously. Plants exposed to NO2 and SO2 for 8 months over a growing season (February-November) showed increases in the growth of shoots (+ 37%) and the whole plant (+ 15%) and a raised ratio of root to shoot dry matter in comparison with control (charcoal-filtered air) plants. Fumigation raised the average foliar concentrations of nitrogen (+ 34%) and sulphur (+ 173%). The improvements in growth due to pollutant exposure were countered by reductions in tolerance to experimental acute frost. Cellular damage of heather shoots was assessed using measurements of electrolyte leakage from cut shoots following controlled over-night frosts. The rates of leakage were consistently increased in those plants that had been exposed to NO2 and SO2 for 5 months or more in comparison with non-fumigated controls. In some cases a greater leakage rate was recorded in fumigated plants than in controls even in the absence of freezing temperatures. The pollutants caused a similar reduction in frost tolerance whether exposure was given during the hardening period (August-January) or the de-hardening stage (November-April). These results support the hypothesis that low concentrations of air pollutants can reduce the tolerance of plants to freezing stress. (C) 2000 Elsevier Science B.V. All rights reserved.