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Leaching of solutes by the action of acidic rain: a comparison of efflux from twigs and single needles of Picea abies (L. Karst.)


Lehrstuhl Botanik I der universität, D-8700 Würzburg, Mittlerer Dallenberweg 64, F.R.G.


Abstract. Leaching of inorganic cations (K+, Mg2+) and in some cases of inorganic anions and sugars from detached twigs and single needles of spruce Picea abies L. Karst.) in the presence of acid rain (H2SO4, 1 mol m−3) or salt solutions (Na2 SO4, 1 mol m−3) was examined under laboratory conditions. Cation leaching (as percentage of the total water soluble ion content of the tissue per hour) was: K+: 0.01-0.02%; Mg2+: 0.005-0.01%; Ca2+: 0.1-0.2%. Leaching rates of anions were even lower than that and concentrations in the leachate were often below the detection limit of anion chromatography. Spraying with H2SO4 (pH 2.95, 1 mol m−3) increased leaching only transiently. Similar effects were found when Na2SO4 was used instead of H2SO4. The transiently enhanced leaching was apparently due to H+/cation or cation/cation exchange at the twig or leaf surfaces. Feeding of K+ or Al3+ through the stems increased leaching of all cations within a few hours, again demonstrating rapid ion exchange in the apoplast. Leaching of potassium and magnesium from single needles occurred at similar relative rates as from twigs. Loss of Ca2+ ions, however, was even smaller from needles than from twigs. Apparently, a large part of the Ca2+ lost from twigs originated from the bark and not from the needles. Efflux of ions from longitudinal needle sections was about 1000 times taster than the rates obtained with intact needles, indicating that the cuticle was the main barrier Preventing solute loss. In relation to the total amount of mineral nutrients in trees, leaching is considered to be too small to be the primary cause of damage to trees stressed by acid rain, as has been suggested in the literature.