The plants can be classified into three broad categories on the basis of their response to the low temperature conditions.

  1. Chill Sensitive Plants: These plants are seriously injured by temperature above 0°C, below 15°C.
  2. Chill Resistant Plants: These plants are able to tolerate low temperature but get seriously injured when ice start to form in tissues.
  3. Frost Resistant Plants: tolerate exposure to very low temperatures (-50°C to -100°C) even when immersed in liquid N2.

Depending on the category of the plants, when the temperature goes below its range, then two types of injuries happen to the plant, namely the chilling and freezing injuries.

Chilling injury

  • Occurs at low but non-freezing temperatures.
  • Chilling injury occurs in.
  • Tropical and subtropical plants at 10°C to 25°C.
  • Temperate plants at 0 to 15°C.
  • Chilling Effect is manifested by physiological and cytological changes.
  • Cytological changes may be reversible or irreversible depending upon time of exposure to low temperature.

Symptoms of chilling injury

Cellular changes: Changes in membrane structure and composition decreased protoplasmic streaming, electrolyte leakage and plasmolysis.

Altered metabolism: Increased or reduced respiration, depending on severity of stress, production of abnormal metabolites due to anaerobic condition.

Common Symptoms:

  • Reduced plant growth and death.
  • Surface lesions on leaves and fruits.
  • Abnormal curling, lobbing and crinkling of leaves.
  • Water soaking of tissues.
  • Cracking, splitting and dieback of stems.
  • Internal discolouration (vascular browning).
  • Increased susceptibility to decay.
  • Failure to ripen normally.
  • Loss of vigour (potato lose the ability to sprout if chilled).

Rapid wilting followed by water soaked patches, which develop into sunken pits that reflect cells tissue collapse. Following warming, the sunken pits usually dry up, leaving necrotic patches of tissues on the leaf surface.

Chilling symptoms in fruits vary and include

  • Sunken pits in cucumber.
  • Browning of skins and degradation of pulp tissue in banana.
  • Blackheart of pine apple.

Cellular Membranes

  • The first symptom of chilling injury is the phase transition from liquid crytalline phase to solid gel state.
  • Increase in permeability of plasmalemma results in leakage of organic and inorganic substances.
  • Plasmolysis: Plasmolemma- pressed against the tonoplast and deleted into the vacuole as sac like intrusions.
  • Formation of crystalline deposits in root cells, epidermal, mesophyll and vascular cells of leaves -leading to tonoplast disruption.
  • Tonoplast injury is irreversible.

During hardening at low or above zero temperatures, the lipid bodies accumulate in cytoplasm in close association with plasmalemma.

Lipid composition

  • Ratio of unsaturated to saturated fatty acids is higher in chilling resistant plants.
  • Increase in activity of fatty acid de-saturases was found in chilling resistant plants.

Cytological Changes

  • Swelling of plastid membranes and mitochondrial membranes.
  • Swelling of chloroplast thylakoids.
  • Decrease in size and number of starch grains.
  • Grana disintegration and increase in size and number of plastoglobules.
  • Mitochondria with reduced cristae and transparent matrix.
  • Mitochondria- double the volume.
  • Extensive dilation and vesiculation of smooth ER cisternae.
  • Rough ER completely disappears i.e. ribosomes are lost from the membrane.
  • Dilated vesicular ER cisternae – accumulation of cryoprotective substances.
  • Transformation of rough ER into vacuolated smooth ER- represents early stage of chilling.
  • Since ER is the most dynamic structure-full reversibility of ER ultrastructure is possible.
  • Swelling of dictyosomes.
  • Longer exposure to chilling-disintegration of dictyosomes.
  • Freezing Injury Freezing injury in plants can be from two sources.
  • Freezing of soil water.
  • 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. Freezing damage occurs primarily due to the formation of ice crystals, which damage cell structure when the temperature falls below 0°C.

Ice usually forms first in the cell walls and intercellular spaces.

Damage occurs when ice crystals grow and puncture the cytoplasm.

Physics behind formation of ice

For the transition to solid phase to take place, need ice nucleation points.

When becoming solid, ice gives off heat, so the temperature rises.

When all of the water in the cell wall has frozen, then the temperature begins to drop again.

Two types of freezing occur in plant cells and tissues

  1. Vitrification Solidification of the cellular content into non-crystalline state (amorphous state). It occurs by rapid freezing of cells (decrease in temperature by more than 3°C/ min) to a very low temperature.
  2. Crystallisation/ice formation Crystallisation of ice occurs either extracellularly or intracellularly (gradual cooling /drop in temperature).

Formation of ice intracellularly may be due to

Internal nucleation (certain large polysaccharides/proteins serve as nucleating agents to form ice).

Penetration of external ice crystals into the cells.

Intracellular ice formation which is very lethal and causes immediate disruption of cells.

Intracellular Ice Formation

Spreads from cell to cell through plasmadesmata.

Formed in the cell wall adjacent to the intercellular spaces.

Originates spontaneously from centres of nucleation in the cytoplasm.

Membrane changes Intact cellular membranes act as effective barrier to the propagation of ice. This depends upon temperature and cold hardening. Cellular membranes are more susceptible to freezing damage than soluble enzymes. Plasmalemma is the major site of lethal injury. Leakage of ions from thawed tissues occurs due to protoplasmic swelling and alteration in permeability to K+ ions.

Mechanism of tolerance to chilling and freezing

Chilling refers to excessively low temperatures that prevent normal growth in susceptible plants, but without formation of ice crystals (freezing). Many common plants are susceptible to injury by chilling

Crop plants Phaseolus bean, corn (maize), cotton, cucumber, rise, soybean (Glycine max), sweet potato, tobacco and tomato.

Ornamentals Coleus (genus Coleus or Solenostemon), Gloxinia (family Gesneriaceae) and Passiflora (Passion flower).

Plants or seeds collected from and therefore adapted to higher altitudes tend to have better chilling resistance. Resistance to chilling often improves by acclimation following exposure to slowly and gradually increased chilling (hardening). A sudden exposure to temperature around 0°C produces cold shock, whereas exposures well below 0°C lead to overt freezing injury. Some plants can be acclimation to freezing temperatures by gradual cold acclimation