One of the most characteristic changes associated with cold acclimation are increases in the degree of saturation of membrane fatty acids, leading to greater membrane fluidity and an increase the functionality of membrane-bound substrate transporters and membrane-bound proteins at low temperatures. Moreover, cold-induced modifications in gene expression, subsequent changes in the abundance and activity of proteins, and alterations in other processes such as source-sink relationships and accumulation of metabolic intermediates, collectively lead to increases in the concentration of many metabolites. Thus, sustained exposure to cold leads to major modifications to the metabolome of plants. Enhanced concentrations of soluble sugars are foremost amongst these, the most studied being the monosaccharides glucose and fructose, the disaccharide sucrose, and the trisaccharide raffinose; levels of each rise within a few hours of the onset of a cold treatment. Starch also accumulates in cold-exposed leaves. The increase in sucrose is particularly rapid, despite the fact that cold often has strong inhibitory effect on one of the enzymes (sucrose phosphate synthase, SPS) responsible for its synthesis. Levels of sucrose continue to rise during the first few days of cold acclimation, aided by a long-term increase in the abundance and activity of SPS. Moreover, overall sugar levels generally remain elevated in plants exposed to sustained cold; this accumulation reflects shifts in source:sink relationships, underpinned by changes in the balance between carbon uptake by photosynthesis, carbon use by catabolic processes (e.g. respiration) and carbon export to other parts of the plant. In some plants, increased concentrations of sugars may convey cryoprotective properties, reducing the incidence of membrane lesions and increasing survival during freezing (see Section 14.6).
Cold treatment also leads to the accumulation of a range of other metabolites, including compatible solutes (i.e. small, highly soluble molecules that are non-toxic at high concentrations). The most studied of these is proline, which accumulates dramatically following cold exposure. Proline appears to act as a cryoprotectant, as evidenced by the fact that freezing tolerant mutants of Arabidopsis accumulate proline to very high levels, even in the absence of a cold stimulus.