Molecular Mechanism of Plant Cold Acclimation and Freezing Tolerance

Abstract

Suboptimal temperature is one of the primary stresses limiting growth, productivity and distribution of plants (Boyer 1982). Two types of low temperature stress can be recognized: (i) chilling stress at temperatures above zero (0–10°C) and (ii) freezing stress at subzero temperatures. Depending on the duration and severity of the stress, exposure to these temperatures can lead to extensive damage to the plants. Chilling sensitivity is common in plants originating from tropical and subtropical regions and the injury is mainly a consequence of destabilization of cell membranes (Levitt 1980). In contrast, plants from temperate regions are in general chilling resistant. In addition, plants growing in temperate regions are commonly encountering subzero temperatures during their growth season and consequently, they have evolved mechanisms to survive the freezing stress. Survival of freezing temperatures is achieved through two main mechanisms: avoidance of or tolerance to freezing. Avoidance of freezing is mainly achieved through supercooling of tissue water and is employed e. g. by xylem parenchyma cells of woody angiosperms, which can supercool to −40°C, the temperature where homogeneous ice nucleation of pure water is taking place. However, avoidance of freezing is not regarded as a major survival mechanism in herbaceous plants, where the main survival determinant appears to be tolerance to freezing (Sakai and Larcher 1987). Tolerance to freezing is not a static characteristic but many species from temperate regions can increase their tolerance when exposed to low nonfreezing temperatures, a process called cold acclimation.