How Frost Damages Plant Cells

An interesting report from the Food and Agriculture Organization of the United Nations states that it is the formation of ice within a plant which causes frost damage, not the sudden change in temperature. A turgid (full to capacity) cell's liquid contents expand as they freeze and can rupture the cell wall. This damage within individual cells is called 'intracellular'. It often occurs when temperatures drop below freezing point very quickly.

If temperatures fall more slowly, the liquid in the cell has more time to be forced outside through the cell wall. If it freezes here, the damage is called 'extracellular'. It has the effect of forcing the plant cells apart and dehydrating the cells.

DROP IN TEMPERATURE IMPORTANT, NOT LENGTH OF FREEZE
Within a reasonable amount of time (less than 24 hours), it is not so much the time endured at freezing temperatures which affects the degree of damage, but the amount of the drop - the lowest temperatures causing the most damage.

DROUGHT-RESISTANCE IMPORTANT
If ice forms outside the cell (extracellular), even though the cell wall remains undamaged the plant can be slowly killed. This is because the saturation vapour pressure is lower over ice than over liquid water, causing water to move out of the cell towards the ice, and thus death by dehydration.

FROST HARDENING
However, if exposed gradually, plants can harden themselves against frost damage in the following ways:

• removal of water from cell contents, making them more concentrated. This has two effects:
• it lowers the freezing point, and acts as a kind of antifreeze
• it makes the cell flaccid, thus pulling the contents away from the wall and thus making room for frost expansion (see diagram)

• decreasing ice-nucleation active (INA) bacteria concentrations during cold periods, thus allowing super-cooling to occur (cooling of contents below freezing point without the formation of ice crystals)

This hardening occurs naturally as a season become progressively colder. However, a warm spell can cause a reversal to occur much quicker than the hardening, especially if growth is resumed.