When the temperature is lower than 15 ℃ in autumn, the plants start the preparation for winter, that is to prevent the appearance of crystal nuclei and reduce the freezing point concentration.
For many years, in order to prevent frost damage to plants in agricultural production, smog is often used to make ice crystals form in the air instead of on the leaf surface, which is very effective. Scientists found that there is a kind of bacteria on the surface of plants that can become crystal nuclei, so a new variety was cultivated in the 1960s. This variety will not form ice nuclei, which prevents them from being used as crystal nuclei and has achieved great success in agricultural production.
Some plants have their own way to secrete a wonderful protein called antifreeze protein on their leaves. This protein was first found in the blood of polar marine fish. It not only reduces the freezing point of water, but also allows water molecules to arrange orderly around them, forming only two-dimensional rather than three-dimensional ice crystals. Scientists speculate that this may be the reason why the blood of polar marine fish can't freeze. Later, molecular proteins with similar structures were also found on the leaf surface and plant body of oats. It is speculated that they can prevent the ice crystals on the leaf surface from expanding into the leaf tissue through pores.
In order to reduce the freezing point of solution inside and outside the cell, some plants accumulate more potassium ions, calcium ions, antioxidants, sugar, starch, hydrophilic amino acids, hydrophilic proteins and so on in the vacuole. Cells also secrete a lot of substances to the cell wall and regulate the state of solution in the cell wall. For example, the famous Canadian maple sugar is the sugar that accumulates a large amount of starch in autumn, freezes in the bark and turns into in spring.
In autumn, plants also increase the composition of unsaturated fatty acids in cell plasma membrane and all biofilms, improve their elasticity and fluidity, and are conducive to withstand the toss of mass wall separation caused by day and night freezing and thawing, although these preparations make plants pay the price of slowing down the growth and reproduction rate. Sometimes the temperature in autumn is capricious. When the plants are not ready for cooling and freezing, the plants will suffer unprecedented serious damage.
Ice crystal is a solid hydrate formed by the condensation and growth of water vapor on the ice core. Whether the small ice crystal outside the cell can become larger depends on whether the cell wall can be compressed. Herbaceous plants have more cellulose and less lignin in the cell wall. Small ice crystals are easy to merge into large ice blocks, so their frost resistance is poor. The bark of trees in the North may be split into ice cubes, and the bark of trees in the North may be too large. Some trees in Siberia have a hard cell wall with a lot of lignin. They simply don't give small ice crystals the chance to freeze and crack the bark, so they can survive at - 60 ° to - 50 ° C.
In addition, the place where ice can be found in plant tissue is the base outside the dormant flower bud. The water inside the dormant flower buds of apple, pear and peach trees has the ability of cold resistance. Ice crystals will not form at - 20 ℃ or even - 40 ℃, which is called organ supercooling. In the severe winter, the trunk is frozen and cracked, but the dormant flower buds on the fruit trees avoid the damage of ice, ensuring that the trees bloom and bear fruit in the coming spring.