Submitted to: Low Temperature Agricultural Science Association
Publication Type: Other
Publication Acceptance Date: 4/20/2002
Publication Date: 5/1/2002
Citation: Livingston, D.P. 2002. Freezing tolerance in plants.. Low Temperature Agricultural Science Association. Interpretive Summary: Winter hardiness is the primary restriction to growing winter cereals. Damage to cell membranes in crucial tissues such as those which support meristems in crowns of winter cereals is the primary cause of injury during winter. Membranes become more stable after a period of acclimation at temperatures just above freezing. This is known as the first phase of cold hardening. Additional hardening has been observed in winter cereals when they were exposed to temperatures just below freezing. This period has been called the second phase of cold hardening. We have found that in addition to sugars, a fructose polymer, known as fructan, and the enzyme which cleaves fructan (fructan exohydrolase) all increase in the spaces between the cells. Research is continuing to see how these changes in frozen plants help them withstand freezing stresses.
Technical Abstract: Winter cereals have a distinct advantage over those which are spring-planted. Because they are planted in the fall, they emerge before it is possible to plant in the spring and are therefore ready to harvest before a spring crop. This allows the fall-planted crop to avoid hot and dry summer environments. In winter cereals, under some conditions, this can result in a yield which is double that of their spring-planted counterparts. In addition, under some conditions an early harvest makes planting a second crop possible. The major restriction to growing fall-planted crops is their susceptibility to freezing conditions during winter. Of all the fall-planted crops, oats are the most winter tender, followed by barley, wheat and finally rye, which is the hardiest. Under controlled conditions the LT50 (temperature at which 50% of a population is killed) of the most winter hardy oat is about -11°C while that of rye is about -20°C. Reducing the LT50 by one or two degrees can allow complete survival of a winter crop in a location where it is now completely killed in most years. Membrane systems of the cell are the first targets of freezing injury in plants. Cold acclimation stabilizes the plasma membrane in rye and other plants by changing their lipid composition. Changes in the electrophoretic patterns of plasma membrane proteins from spring wheat and winter wheat, grown for four weeks in cold chamber have been reported. Adhesions to membranes are the result of a slow rate of freezing such that very small displacements from equilibrium occur (equilibrium freezing). The advancing ice lattice upon reaching the vicinity of the cell wall competes with it for the intervening liquid which causes adhesion between ice and the wall or wall and plasmalemma. As the protoplast shrinks during freezing, adhesions to it can cause considerable damage. This adhesive injury may be relieved by the cell releasing solutes outside the protoplast producing a fluid barrier to adhesions. We revised an apoplastic fluid extraction technique and found that a winter hardy oat releases a significant amount of sugars as well as fructan into the apoplast during 2PH. In addition, a significant increase in the activity of fructan exohydrolase (FEH) and invertase in the apoplast was found during 2PH. The increase in molarity of the apoplast solution during 2PH due to carbohydrate increases was not high enough to lower the freezing point of the apoplast by more than a fraction of a degree however, it has been showed that solute concentration in the apoplast is not uniform and that solutes tend to accumulate in discrete regions termed sumps. In addition to sugars being distributed unevenly in the apoplast, the layer of liquid water into which sugars have apparently been released would be very small due to the presence of apoplastic ice at -3°C. This could lead to regions of considerably higher sugar concentrations in the apoplast of crown tissue than if the sugars were distributed evenly throughout the crown. These regions of high sugar could prevent adhesions and possibly help stabilize membranes thereby increasing freezing tolerance during 2PH.