Brrrrrrr! Here in the upper Midwest, it is bone chilling cold! Fortunately, we humans can move indoors as we please but those poor plants are stuck outside. Fortunately for them, they have been able to adapt to the cold over the past few million years.
Perhaps the basic adaptation to the cold weather has been for deciduous trees to drop their leaves in the fall. In response to shorter day lengths, plants produce a hormone called abscisic acid which helps them to form an abscission layer at the bottom of the petiole or leaf stem. That effectively closes off the connection and allows the leaf to drop off without any loss of sap from the tree. Pretty sharp since the leaves are tender and would never survive the cold of the winter.
Way back in your last biology class, you probably learned that one thing that distinguishes plant cells from animal cells is that plants have rigid cell walls while in the animal, the walls are more flexible. Thus, if the contents of a plant cell freeze and expand enough, the cell wall will burst and that is it for that cell. If enough cells burst, the tissue and the related organ i.e. roots, stems, leaves, flowers, will die.
To counteract this, many trees and other woody plants have developed a way to allow their cells to dry out a little in the autumn. By reducing the amount of water in the cell, they also increase the concentration of salts and other chemicals in the cell. Just like adding a teaspoon of salt to a glass of water, this results in a lower freezing point. It is the same as adding anti-freeze to your car radiator. Neat little trick.
More later.
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