EFFECTS OF NITROGEN DEFICIENCY AND RECOVERY FERTILIZATION ON ULTRASTRUCTURE, GROWTH, AND MINERAL CONCENTRATIONS OF SCOTS PINE NEEDLES

During the first year of a 2-year field experiment, 3-year-old Scots pine (Pinus sylvestris L.) seedlings were exposed to one of three nitrogen fertilization treatments: no nitrogen, 30% optimal nitrogen, or optimal nitrogen. In the second growing period the experiment was continued as a recovery fertilization experiment. Nitrogen concentration of needles was clearly lower in seedlings of both deficiency levels than in the control seedlings by the end of the first growing season. There was a rapid development of external and ultrastructural symptoms in the first-year nitrogen-deficient seedlings. After 10 weeks of treatment, yellowing of needles, decline in growth, and reduction in ribosome and endoplasmic reticulum content at the ultrastructural level were observed compared with the controls. After 16 weeks of treatment, abnormally shaped and narrow chloroplasts were detected in the mesophyll and transfusion parenchyma tissues of current- and previous-year needles. Increased plastoglobuli number and sizes, decreased numbers of thylakoids, and decreasing electron density of chloroplast stroma followed the other changes. In the second growing season during the recovery fertilization phase, the color of the needles and the shape of the chloroplasts rapidly returned to normal, but the cytoplasmic degeneration did not totally recover. The results of the present experiment suggest that early degeneration of cytoplasmic endoplasmic reticulum and ribosomes and changes in the chloroplast shape can be regarded as characteristic symptoms of nitrogen deficiency.