Plants that live in regions where chilling is common undergo a period of hardening in the autumn that prepares them for chilling. Hardened plants do not suffer chilling injury therefore analyzing the differences between hardened and non-hardened members of the same species can yield clues to the nature of the injury. Comparison of tropical plants, which generally are incapable of hardening, to those that are chill-resistant also provides information of a similar type.
The most common site implicated for chilling injury is the plasma membrane. Many hypotheses concerning phase changes or phase separations have been offered, with the consequences of this change leading to cell leakage or disruption. Changes in membrane permeability are often invoked as a cause of the loss of cell turgor but the kinetics of the process suggest that altered permeability is not the cause. The activation energy of membrane-bound enzymes has also been studied extensively, with much work going toward looking for discontinuities in the linearity of Arrhenius plots.
The primary cause of chilling injury in some plants has been found to be the opening (and locking) of the leaf stomata when the permeability of the roots to water is low. Thus the leaves lose water faster than it can be replaced and they become dehydrated. Cold hardening alters the behavior of the stomata so that they close under the same conditions; the root permeability is also increased. Interestingly, drought hardening conditions the stomata similarly but the root permeability is significantly decreased, yet the plant is still able to survive chilling without leaf injury.
In other plants, the stomata behave properly at chilling temperatures and the injury is thought to be metabolic. A decrease in respiration, photosynthesis and fatty acid synthesis may all contribute to the chill-starvation of some plants. Cold hardening affects the lipid content of cell membranes and has been found to lower the optimum temperature for photosynthesis and respiration.
The small flowering plants are able to solve both problems by spending the winter as seeds. The vegetative growth renews itself annually and dies off in the autumn. The seeds have a low water content and are able to avoid extremes in temperature by laying on the ground under the insulated snow-cover.
Deciduous trees lose their leaves in the fall, thereby stopping transpiration for the winter so that they are not affected by the freeze-induced drought. The leaves are also the most susceptible regions of the tree to freeze-injury.
Environmental freezing in plants is usually extracellular. As there are no efficient nucleators within most plants, ice is usually transferred through the epidermis from frost that forms on the outer surface. Bacteria with ice nucleating proteins in their cell walls are often the sites of frost growth on the plants. Once ice growth occurs within the plant, the normal osmotic and chemical effects will result in cellular injury.