THE STRUCTURE AND HARDENING STATUS OF SCOTS PINE NEEDLES AT DIFFERENTPOTASSIUM AVAILABILITY LEVELS

Scots pine (Pinus sylvestris L.) seedlings were exposed to three level s of potassium (low, medium and high) and their needle morphology, the cellular structure of the mesophyll and transfusion parenchyma, and t he hardening status of the mesophyll cells were examined by light and transmission electron microscopy. The higher the potassium level the g reater was the growth of the needles. The area of the mesophyll tissue increased slightly and those of the phloem, xylem and resin duels dec reased in the needles of the seedlings grown at the high K level. Cell ular studies revealed that swelling of the chloroplast thylakoids, acc umulation of starch in the chloroplasts, translucency of the cytoplasm and plasmolysis in the mesophyll tells were related to a low K level. The hardening status of the mesophyll cells was enhanced after 5 week s of hardening treatment at high K as seen in changes in chloroplast s hape and position and the structure of the endoplasmic reticulum, but the pines showed no major differences in the hardening status of their mesophyll cells between K levels at the end of the experiment, after 9 weeks of hardening. Frost resistance, as shown by the electrolyte le akage test, was nevertheless highest at low K, being related to the in crease in the concentration of polyamine putrescine at this potassium level.