The following points highlight the two main factors affecting low temperature stress. The factors are: 1. Effects of Chilling Stress 2. Effects of Freezing Stress.
Low Temperature Stress: Factor # 1.
Effects of Chilling Stress:
Chilling stress affects several functions in plants including membrane structure and function, changes in nucleic and protein synthesis, water and nutrient balances, cellular cytoskeletal structure, photosynthetic and respiratory metabolisms. Chilling injury can be divided into a single primary event and several secondary events.
A single primary event has been proposed in which the chilling temperature can cause membrane phase transition from a liquid crystalline phase to the solid gel phase. Such changes can decrease membrane permeability, i.e., fluidity and can affect any of the above-mentioned secondary events, which ultimately lead to the expression of symptoms.
(a) Physiological and Biochemical Effects of Chilling Stress:
Generally enzymes are more labile to high temperatures than they are to low temperatures. However, enzymes having subunits such as pyruvate P1 dikinase and phosphofructokinase, which are involved in carbon fixation reactions in C4 plants and glycolysis respectively are inactivated by chilling temperature as a result of converting them from tetramers to dimers.
Another enzyme affected by chilling stress is K-mediated ATPase, the reduced activity of which leads to K ion leakage from cells. The levels of three adenine nucleotides, viz., ATP, ADP and AMP are reduced by chilling. Chilling affects enzyme kinetics by altering the reaction velocity (Vmax) and the affinity (Km) of the enzyme for its substrate.
Chilling may inhibit photosynthesis by affecting both the light and dark reactions. When a plant is exposed to chilling stress during the day, the injury is probably due to photo-inhibition at the oxidative side of PS II involving D1 protein. Under this condition, normal electron carrier may be disrupted leading to electrons being used to form free-radical species, which may cause degradation of membranes.
Respiration rates can either increase or decrease depending on the severity and duration of chilling stress. Respiratory rates are increased by short-term stress, while long-term or very low temperature causing cell damage and death reduce the rates. Increased rates may be due to the diversion of the normal electron transport to the cyanide-resistant pathway, which is usually not linked with ATP generation.
(b) Acclimation to Cold Temperature:
Chilling-sensitive plants can be acclimated or hardened to low temperature by gradually reducing the temperature over an interval of time. In a natural environment, the process of hardening may be achieved by seasonal changes in temperature. In some cases, plants accumulate sugar alcohol, proline and glycinebetaine that may serve to stabilize membranes.
Low Temperature Stress: Factor # 2.
Effects of Freezing Stress:
Plants generally withstand freezing temperatures either by avoidance or by tolerance. One avoidance mechanism involves the process of super-cooling by means of which solutes accumulate in cells and lower the freezing temperature of the cytoplasm too much below the freezing point of pure water.
This is achieved either by synthesis of solutes, such as sugars, polyols and other osmotic solutes, or by the movement of water from one tissue to another that is less sensitive to freezing. In some plants, avoidance involves the production of metabolic heat to increase temperature in order to prevent freezing.
This can be observed in Cacti and other succulents possessing thick tissue and abundant water content which may avoid freezing stress by storing heat during the day and its slow dissipation during the cooling period.
Tolerance mechanisms include processes that permit ice to form in plant tissues without producing any damage. This process involves extracellular ice formation. This region means apoplast of the cell including cell wall space and intercellular space among cells.
On the other hand, if ice formation is intracellular, i.e., within the cell, it has lethal effects leading to cell death. Ice formation is initiated by ice nucleators restricted to the apoplectic regions and also to the tissue surface. Such ice nucleators include dust particles, bacteria, fungi, insects, wind and ice itself.
It has been observed that once freezing begins in the apoplast, water will move from the cytoplasm to the extracellular space down a vapor pressure gradient that lead to the growth of ice crystals. The rate of ice formation can be related to the potential injury to plant tissue. Generally, the faster the rate of ice formation, the more injury is imposed.
The movement of water from the cell and the formation of extracellular ice lead to dehydration of the cell and accumulation of solutes in the cytoplasm. This has the advantage of lowering the freezing point of cytoplasm, which in turn reduces the possibility of intracellular freezing.
The adverse aspect of this process is concerned with the removal of water from the cell and the increase in ion concentration, which in turn would affect membrane stabilization, protein structure and ultimately, cell function.
As water is removed from the cell and ice forms outside the cell, the cell cytoplasm shrinks as a result of water loss. Consequently, the developing ice leads to the compression of the cell wall against the cytoplasm. Thus, the resistance to freezing stress in plants may relate to the rigidity of the cell wall. A more rigid cell wall would result in less compression of the cytoplasm during development of ice crystals.
The physiological processes that are affected by freezing stress are in many ways similar to those affected by chilling stress, but these are more pronounced. One of the primary differences is related to the development of intracellular ice in case of freezing stress and cavitation of cells that lead to the death of cells and tissues.
It appears that plants growing in cold climate have developed efficient and convenient strategies that take advantage of the limited growth resources that are available to them. Much of the strategy relates to how they trap, store, respire and allocate energy. Plants in these environments grow slowly but have photosynthetic rates that are comparable to plants growing in warmer environment.
Higher photosynthetic rates in cold climate are the result of elevated levels of Rubisco, and this may be regarded as metabolic compensation. Most of these plants show C3 pathway, but as a result of the cold climate in which they grow, photorespiration is kept to a minimum.
Plants growing in cold climate tend to store large portion of their carbohydrate reserves in underground organs and allocate large portions of photosynthetic to the maintenance and replenishment of roots and underground organs. This strategy is important because of low soil nutrient levels and low water availability.
Another advantage of storing carbohydrate reserves in the roots can be related to abnormally cold seasons when little or no growth occurs. Under such condition, the plant depends on storage carbohydrate to sustain it during stress.