Cold Injury and Cold Resistance in Plants!
Under natural and agricultural conditions, higher plants are also affected or stressed by cold or very low temperatures in certain parts of the year especially during autumn and winter.
There are two types of cold injuries in plants:
(i) Chilling injury and
(ii) Freezing (frost) injury.
Injuries to plants caused by low temperatures (chilling temperatures) well above their freezing point are called as chilling injuries while injury caused by freezing temperatures (below the freezing point of water) that results in ice formation in plant tissues is called as freezing or frost injury.
There is varying degree of tolerance to cold temperatures among different plants. Some plants may be more sensitive to cold than others, whereas some species may be cold resistant altogether. Chilling and frost damage is an important hazard in crop plants all over the world.
A. Chilling Injury and Chilling Resistance:
Many species of plants especially those native to tropical and subtropical regions are susceptible to chilling injury. For instance, exposure to a temperature of 1°C to 5°C for 24 to 36 hours is fatal or markedly injurious to crop plants such as rice, velvet beans, cotton and peanut, but less injurious to maize, tomatoes, sorghums and pumpkins. Among ornamentals, Coleus, Croton, Passiflora, Diffenbachia etc., are examples of chill-sensitive plants. Arabidopsis is a chill-tolerant plant.
As a result of chilling injury, the plants invariably show:
(i) Reduced growth,
(ii) Chlorosis and lesions on leaves,
(iii) Appearance of foliage as if soaked in water for long and
(iv) In extreme cases, wilting of the plant and its death.
The causes of such pronounced effects lie in disorders which are induced in the metabolic activities and physiological conditions within the plant cells such as:
(i) Loss of membrane function,
(ii) Inhibition of photosynthesis and translocation of carbohydrates,
(iii) Slower respiration,
(iv) Inhibition of protein synthesis and
(v) Increased degradation of existing proteins.
In general, larger the exposure of chill- sensitive plants to chilling temperatures, the greater is the resulting injury. Chill-sensitive plants may be acclimated or hardened by prior exposure to low but non- injurious temperatures. Plants are better acclimated to chilling by slow and gradual exposure to prior chilling temperature. Sudden exposure to low temperature around 0°C (which is called as ‘cold shock’) is markedly injurious to plants. The plants native to temperate regions or those glowing at high altitudes, are usually genetically adapted to colder temperatures and are therefore, resistant to chilling injuries.
One of the reasons of some plants being chill-sensitive and others chill-resistant lies in the proportion of saturated and unsaturated fatty acids components in the lipid bilayers of their cell membranes. The cell membranes of chill-sensitive species have higher proportion of saturated fatty acids (with higher melting points) while those of chill-resistant species have higher proportion of unsaturated fatty acids in their lipid bilayers.
This view is further supported by the findings of Williams et al, (1988) and Palta et al, (1993), who observed increased activities of the enzymes desaturases (the enzymes converting saturated fatty acids into unsaturated fatty acids) and an increase in the level of unsaturated fatty acids during acclimation to low temperatures.
Thus, at chilling temperatures, the saturated lipids in the membranes will solidify soon disrupting the membrane activity in chill sensitive species. Higher proportion of unsaturated lipids in chill-resistant or chill-hardened species on the other hand, allows the membrane to remain fluid at low temperatures and function normally. The importance of membrane lipids in acclimation to low temperatures has also been demonstrated by different workers using mutant and transgenic strains in plants such as Arabidopsis and tobacco.
B. Freezing (Frost) Injury and Freezing (Frost) Resistance:
Frost-sensitive plants are killed or seriously injured when exposed to temperatures low enough to cause ice formation in them. This type of injury is called as frost or freezing injury and is frequently encountered in temperate zone plants.
Ice formation in plant tissues does not take place at freezing point of water, but only after the plant tissue has under-cooled several degrees below its freezing point. This under-cooling has also been called as super-cooling. Under natural conditions, ice formation is a gradual process and takes place in intercellular spaces and xylem vessels. The beginning of ice crystal formation is called as ice nucleation and takes place around some large polysaccharides and proteins present in cell walls which act as ice nucleators.
As water continues to freeze in intercellular spaces, the ice crystal gradually proliferates (enlarges), drawing water from the adjoining cells which get dehydrated. Water may also move from more distant cells towards proliferating ice crystals along a water potential gradient. In general, the lower the temperature, the greater is the proportion of water that freezes in intercellular spaces and the larger the ice crystals become.
Dehydration of the protoplasm of these cells induces various dis-organising effects in its structure resulting in freezing injury. Under certain conditions, freezing injury occurs not at the time of ice formation in intercellular spaces, but during subsequent thawing (liquefying) when ice melts and water re-enters into the adjoining cells. If thawing is rapid, death of cells may occur due to mechanical distortions of their protoplasm. Although under natural conditions, intracellular freezing (or ice formation inside protoplasm) is uncommon in plants, but when it does occur, it is invariably lethal.
Some plants are freeze-sensitive and are injured if exposed to freezing temperatures even for a short period. On the other hand, some temperate zone woody plants are freeze-tolerant which can survive long exposures to temperatures of -20°C to -30°C or even lower without apparent injury. Degree of freezing tolerance varies among different plants, different organs of the same plant and their age.
Acclimation (hardening) of plants to freezing can be done by prior exposure of plants to cold temperatures. Abscisic acid (ABA) is also known to increase freezing tolerance of some plants.
The basis of frost resistance of plants lies in certain properties of their protoplasm. During acclimation of plants to freezing, certain proteins are synthesized in the cells which are called as antifreeze proteins. These proteins bind to the surfaces of ice crystals in intercellular spaces and check their proliferation. Some of these proteins are also believed to stabilize other proteins and cell membranes during dehydration of cells induced by cold temperatures.
During acclimation of winter cereals such as wheat, sugars (chiefly sucrose) are known to accumulate in cell walls where they may check enlargement of ice crystals. Because these proteins and sugars have cryoprotective function, these are called as cryprotectants. Besides this, frost tolerant species have greater capacity of super-cooling and dehydration of their cells is slow, when ice forms extracellular in their tissues.