Proton Neutralization in the Leaves of English Oak (Quercus roburL.) Exposed to Sulphur Dioxide

Abstract

Reactions to the input of acidic gases were investigated in leaves of Quercus robur L. exposed to different concentrations of SO2 (80, 120, and 160 nl I−1) for 32 to 70 d. Two-year-old oaks were grown in nutrient solutions with varied nitrogen form and were fumigated in closed chambers. An attempt was made to identify the mechanisms of proton neutralization by considering the uptake of nitrogen, the increase in sulphur and carboxylate contents, and the excretion of hydroxyl ions or protons. In addition, nitrate reductase activity was determined in the leaves. The reduction of sulphur was not involved in the neutralization of protons generated by SO2-uptake, whereas organic acid metabolism played a decisive role. Depending on SO2-concentration, duration of fumigation and nitrogen supply, oaks reacted with a reduction in the size of the carboxylate pool in the leaves, and/or with an increase in proton excretion (or a decrease in hydroxyl ion excretion). Nitrate reductase activity increased in the leaves of nitrate-grown oaks exposed to the highest SO2-concentration (160 nl l−1) for 42 d. The capacity of the mechanisms considered is sufficient for the neutralization of the calculated amounts of protons resulting from SO2-uptake.