Abstract
Two-year-old seedlings of Norway spruce (<em>Picea abies</em>) 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 (Ψ<sub>w shoot</sub>) 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 Ψ<sub>w shoot</sub> 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, Ψ<sub>w shoot</sub> 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.
Highlights
Plant tolerance to freezing is genetically conditioned to a large extent, but it is dynamic, changing seasonally [1,2]
membrane permeability (MP) and water relations during reacclimation/deacclimation At the beginning of this study, the spruce seedlings were in the spring deacclimation period
Repeated decreases in air temperature at night to several degrees below 0°C can lead to reacclimation or can delay the deacclimation of plants, as shown in the present study
Summary
Plant tolerance to freezing is genetically conditioned to a large extent, but it is dynamic, changing seasonally [1,2]. Environmental factors, such as temperature, water deficit, and light, regulate the cellular level of expression of genes associated with plant dormancy, cold hardiness, and development. At the end of winter, day length and mean air temperatures increase, while freezing tolerance declines, gradually releasing ecodormancy [4]. This release starts the process of deacclimation in trees.
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