Chapter 12.3

Plants at Low Temperatures

Chilling Injury

Plant chilling injury refers to an injury that is caused by a temperature drop to below 15°C but above the freezing point. The symptoms of chilling injury are usually a rapid wilting of the leaves and the development of water soaked patches that go on to form sunken pits due to cell collapse. Warming will lead to these damaged areas becoming brown and necrotic while continued chilling will eventually lead to the death of the plant.

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.

Freezing Injury

Freezing injury in plants can be from two sources: 1. Freezing of soil water, and; 2. Freezing of the fluids within the plant. The soil water that is available to plants is found in the porous regions between soil particles. It freezes at about -2°C, depriving the plant of its source of water. Freezing of water within the plant is a more serious threat, as it can cause disruption of structure and function of cells and tissues.

Cold Adaptation

For plants that live in Arctic or temperate regions where temperatures are likely to fall as low as -40°C during the winter, any vegetative structures that are subjected to these temperatures must deal with the potential for internal freezing. They will also have to survive the extended drought that is imposed by soil freezing.

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.

Freeze Avoidance

One strategy for avoiding ice formation within the plant is to exist in a supercooled state. This is not an overwintering strategy for most small plants as they can only supercool to about 4-6°C below the freezing point, sufficient only to avoid a few early frosts. In woody plants, the cells of the xylem tissue remain supercooled throughout the winter although extracellular freezing occurs in other tissues. There is a waterproof barrier that prevents ice growth into the xylem rays (and also prevent ice-driven dehydration of the xylem). Several apocryphal accounts of exploding trees exist, initiated by an axe cut at very low temperatures (below -40°C). In some conifers, the vegetative buds also supercool.

Freeze Tolerance

Plants that have a freeze tolerance will harden as winter approaches, undergoing changes that are necessary for surviving ice growth within the tissues. These changes must occur before the temperature gets too low so that the metabolic demands of tissue remodeling can be met. The alterations that commonly occur consist of an increase in cytoplasmic solutes both to act as a cryoprotectant as well as to buffer any freeze- concentration of other solutes that could be toxic in high concentrations, as well as membrane alterations that increase the fluidity of membranes at low temperatures and increase the stability of the membranes to low water contents.