About Temperature Stress
Temperature stress includes both high temperature and low temperature stresses.
1. Low temperature
2. High temperature
LOW TEMPERATURE STRESS
Classification of plants - Response to Low Temperature
Chilling sensitive plants: Seriously injured by temperature above 00C, below 150C
Chilling resistant Plants:
(i) Able to tolerate low temperature
(ii) Seriously injured when ice start to form in tissues
Frost Resistant Plants: tolerate exposure to very low temperatures(-500C to -1000C) even when immersed in liquid N2.
Chilling injury
- Occurs at Low temperature but nonfreezing temperatures
- Chilling injury occurs in
- Tropical and subtropical plants at100C to 250C
- Temperate plants at 0 to 150C
- 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 include
- 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)
Some of the more common symptoms of chilling stress are
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 sp skins and degradation of pulp tissue in banana
-
Blackheart of pine apple (Wilson, 1987)
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 irrevesible
- During hardening at low or above zero temp 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 no. of starch grains
- Grana disintegration and increase in size and no.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 ie.,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-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: 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.
Freezing damage occurs primarily due to the formation of ice crystals, which damage cell structure when the temperature falls below 00C.
- Ice usually forms first in the cell walls and intercellular spaces
- Damage occurs when ice crystals grow and puncture into the cytoplasm
Physics of ice formation
- 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
- 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 30C/ min) to a very low temp.C
- Crystallisation / ice formation : Crystallisation of ice occur either extracellularly or intracellularly (gradual cooling /drop in temperature)
Formation of ice intracellularly may be due to
- By internal nucleation (certain large polysaccharides /proteins serve as nucleating agents to form ice)
- By penetration of external ice crystals into the cells
- Intracellular ice formation is very lethal which causes immediate disruption of cells.
Many plants can avoid freezing injury, because they allow deep super cooling
- In plants, the temperature of water will drop below its freezing temperature and will still remain liquid - supercooling
- The liquid in the intercellular space never makes the transition to solid phase, so ice crystals don't form
- Some can supercool down to -350C
- At -40C, ice crystal formation begins spontaneously
- Occurs in hardwoods and some fruit trees
- When the ice crystals formed are very small due to rapid freezing, these crystals melt before they reach a harmful size.
Intracellular Ice Formation
- Spreads from cell to cell through plasmadesmata
- Formed in the cellwall 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.
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