EXCESS NITROGEN-FERTILIZATION AND THE STRUCTURE OF SCOTS-PINE NEEDLES

The effects of excess nitrogen fertilization on the structure of Scots -pine needles (Pinus sylvestris) were examined in Finnish Lapland. The trees were fertilized with calcium ammonium-nitrate, and the levels o f nitrogen applied were 0, 75, 150, 250, 500 and 1000 kg N/ha, respect ively. The thickness and width of the needle and thicknesses of the ti ssues were measured, and the conditions of various celltypes were clas sified at the light-microscopy level. In addition, the areas of the ce ll organelles were measured and the conditions of the chloroplasts and other cell compartments observed at the ultrastructural level. Larger needles and a thicker adaxial mesophyll were observed following exces s N application (250-1000 kg N/ha). The natural xeromorphic structure of the conifer needles became more mesomorphic, possibly influencing t he protection afforded against environmental stresses. A high level of nitrogen fertilization caused changes in the central cylinder, especi ally in the vascular bundle and the scler-enchyma. The needle nitrogen concentration was high in the highly fertilized trees and the boron c oncentration was simultaneously extremely low. As a consequence of the changes In the central cylinder, photoassimilate transport from the m esophyll to the phloem may be disturbed at high fertilization levels. The changes at the ultrastructural level, i.e. injuries to the chlorop last thylakoids and the plasma membrane, and the formation of lipid ac cumulations, were indicators of the adverse effects of excess nitrogen . With an increasing atmospheric-nitrogen load, lower nitrogen fertili zation levels in pine forests naturally adapted to low soil N should b e considered in forest management.