The structure and hardening status of Scots pine needles at different potassium availability levels

Abstract

Scots pine (Pinus sylvestris L.) seedlings were exposed to three levels of potassium (low, medium and high) and their needle morphology, the cellular structure of the mesophyll and transfusion parenchyma, and the hardening status of the mesophyll cells were examined by light and transmission electron microscopy. The higher the potassium level the greater was the growth of the needles. The area of the mesophyll tissue increased slightly and those of the phloem, xylem and resin ducts decreased in the needles of the seedlings grown at the high K level. Cellular studies revealed that swelling of the chloroplast thylakoids, accumulation of starch in the chloroplasts, translucency of the cytoplasm and plasmolysis in the mesophyll cells were related to a low K level. The hardening status of the mesophyll cells was enhanced after 5 weeks of hardening treatment at high K as seen in changes in chloroplast shape 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 leakage test, was nevertheless highest at low K, being related to the increase in the concentration of polyamine putrescine at this potassium level.