Differential effects of spring reacclimation and deacclimation on cell membranes of Norway spruce seedlings

Two-year-old seedlings of Norway spruce (Picea abies) during spring deacclimation were subjected to controlled reacclimation by exposure to low temperatures of 4/ 3°C (day/night) in a cold room. The highest increase in freezing tolerance (by 7°C) was observed after 12 d of low temperature exposure, when shoot water potential (Ψw shoot) decreased to 0.64 MPa. The process of reacclimation was accompanied by an increase in the phospholipid content of needle cell membranes. This increase applied to total (PL) and individual phospholipids: phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylethanolamine (PE) and phosphatidic acid (PA). After being exposed to the low temperature for 18 d, the seedlings were moved into the open air. This caused deacclimation, with an increase in Ψw shoot to 0.36 MPa and a decrease in the total phospholipid content and freezing tolerance of the needles. Significant correlations were observed between freezing tolerance, the membrane permeability (MP) of the needles and the phospholipid content, Ψw shoot and water content of the needles. The results show that during spring deacclimation, Norway spruce seedlings can be subjected to reacclimation, which is reflected in the phospholipid content, the biophysical changes of the membranes, and the freezing tolerance of the seedlings. During both spring deacclimation and reacclimation, water content in the needles plays a critical role in the cold tolerance of spruce seedlings.