Nitrogen forms in bark, wood and foliage of nitrogen-fertilized Pinus sylvestris

Cycling of soluble non-protein N compounds is thought to be indicative of t he N-nutritional status of trees. We determined the major N forms in bark, wood and foliage and estimated the dependence of prevalent N forms on N ava ilability in Pinus sylvestris L. trees from northern Sweden. Trees subjecte d to severe N limitation and trees that had been fertilized with an average 64 kg N ha(-1) year(-1) for 25 years were analyzed. Bark and wood samples were collected by tangentially cryo-sectioning the trunk into 30-mum thick sections, from the bark to the functional xylem. Soluble amino compounds we re extracted from the sections for analysis. Sap samples from twigs were ob tained by centrifugation, and bark samples from twigs were obtained by tiss ue extraction. In both needles and bark, arginine dominated the amino-N pool. Because argi nine concentrations in needles increased with N fertilization, arginine dom inance of the amino-N pool in needles was higher in N-fertilized trees than in control trees. In bark, N fertilization resulted in a large increase in glutamine concentration, so that glutamine accounted for a larger proporti on of the amino-N pool in bark in N-fertilized trees than in control trees. Glutamine dominated the amino-N pool in wood of control trees. Nitrogen fe rtilization resulted In an increased proportion of arginine in the wood ami no-N pool. We conclude that the composition of the amino-N pools in bark, w ood and foliage is highly sensitive to N supply. The composition of the ami no-N pools can contribute to the regulation of tree N-nutritional status, w hich is mediated by shoot to root signalling by long-distance transport of amino compounds.