LONG-TERM RESISTANCE OF SPRUCE TO SO2 REQUIRES MOBILIZATION OF CATIONS

Oxidative detoxification of atmospheric SO2 inside the needles of spru ce yields sulfuric acid. Protection of needle cells against acidificat ion requires the uptake of cations. At the boundary between apoplasm a nd symplasm cations can either be exchanged against protons or taken u p together with anions, which on degradation (e.g. malate) or reductio n (e.g. nitrate) yield hydroxyl ions. Cations are supplied from the ro ot system by the transpiration stream. As long as transferable cations become available, low atmospheric levels of SO2 can be tolerated for long times while sulfate accumulates in the plant.