Development of freezing tolerance in roots and shoots of Scots pine seedlings at nonfreezing temperatures

The hardening of hydroponically cultured Scots pine (Pinus sylvestris L.) seedlings and their recovery after freezing was studied at the end of the second growing season (LD), after 3 weeks of shea day treatment (SD), after a gradual decrease in temperature to 5 degrees C over 4 weeks (H1), and after 4 weeks at 5 degrees C (H2). Frost hardiness was determined by several methods and the recovery as survival of the seedlings. The highest frost hardiness was achieved in the distal parts of needles (-21 to -27 degrees C) and in the proximal parts of needles (-18 to -250C), followed by woody roots (-7 to -9 degrees C), the 1-year-old and current stem (-8 degrees C), and the fine roots (-5 degrees C), all at the end of H2. Hardening of needles was induced by SD, but the stem and woody roots started to harden later, as a response to low temperature. As a result of frost treatment during LD and SD, potential plasma membrane H+-ATPase activity of roots decreased as electrolyte leakage increased and extracellular resistance decreased, but this relationship was lost during H1 and H2. The present study demonstrates the lack of hardening capacity in the fine roots of Scots pine seedlings at nonfreezing temperatures and the increased mortality of the young seedlings having frost damage on roots.