Introductory Crop Physiology (1+1)

Freezing damage

Freezing Injury

• Like the heat tolerance, plant species also possess the capability to resist freezing. Several processes may be involved in causing freezing injury to the plants.
• In general plants growing under tropical under tropical climate are more chilling prone. Hence chilling causes damage to their tissue or organs.
• Such plants also are sensitive to low temperature +12 to 13oC while, low temperature like -0 to -5oC is lethal.
• Obviously proteins are sensitive to low temperature on the contrary most of the alpine and arctic plants don’t experience any damaging effect at this low temperature.
• In these plants the danger is that their tissues may not undergoes water formation in their cells. Seeds, pollens and embryos can be stored at low temperature such as -190oC obtained through liquid nitrogen.

Freezing damage is done in two ways

1. There is formation of ice crystals and the damage is due to mechanical effects. There is disruption of membranes and even cell organization.
2. Ice formation reduces the water amount in the cells leading to drought situation.

However, the intercellular water has high potentials whereas, water in the cell cytoplasm or vacuole has nearly negative water potential.
To begain with it is the intercellular space that the ice crystals are formed and then with the freezing continuing, water leaves the cell cytoplasm in the plants which are freezing hardy, water tends to remain in the intercellular spaces.

In brief following is a set of events which takes place

• Crystals formed in the intercellular spaces and protoplast solutes become concentrated due to removal of water from these.
• This is precipitation of solutes in the protoplast which causes abrupt shifts in the cell pH.
• If the temperature is lowered down still further (eg -35 to -45oC) all water in the tissue is crystallized.
• Gradually, the crystals emerge in size and hence there is mechanical damage to the cell.
• The freezing may be slow or rapid and different plant exhibit different response.
• At low temperature in the frost-hardy plants there is formation of low temperature resistant proteins.
• This may be due to increased concentration of electrolytes which protect tissue water against its removal by the intercellular ice.
• Frost hardy plant have high sugar amount and obviously frost hardiness involves synthesis of more sugars.
• Plant tissue may be made frost-hardy by placing in sugar solution.
• Thus even though new frost resistant proteins might be synthesized these protein must be resistant to high sugar concentration.
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