The big thaw after our “Winter Wallop” damaged some plants until they were salvaged, while others seem to have sailed through without any adverse effects. It is the physiology of a plant that determines whether it is hardy, freeze-resistant, or killed in the first frost. We are witnessing a laboratory for frost damage to living plant physiology.
Plant physiology is the area of biology that studies the natural functions of the organs, tissues and cells of a plant. The organs of a plant are roots, shoots, flowers and fruits, collections of plant tissues made up of cells that work together to perform a common function.
Freezing damage occurs at the cellular level, but the damage is expressed at the tissue and organ level. Freezing events may seem straightforward processes, but freezing in plant cells is far from straightforward.
Today’s column provides a brief explanation of how cells in some plants freeze and suffer damage at 32 degrees Celsius, while cells in other plants can reach extremely low temperatures without damage.
The freezing point of pure water, called “free” water in physiology, is 32 degrees F. The cells of delicate plants are filled with sap, which consists mainly of pure water. Such cells experience freezing events at 32 degrees Fahrenheit that cause severe damage.
A critical distinction between non-hardy tender plants and hardy plants is that hardy plants are genetically programmed to produce cells that will not freeze at 32 ° F. The cells of hardy plants are filled with sap that does not consist mainly of free water. Instead, the content of the sap of hardy plant cells contains dissolved organic substances. The cell sap of the harder plants contains high concentrations of organic substances such as sugars, acids, amino acids, proteins, salts and other molecules that form solutions in the cell sap.
Now to the really remarkable aspect of the physiology of such plant cells: the organic molecules that accumulate in the cell sap act as types of “antifreeze”. The presence of the organic molecules in solution changes the concentration of the solute in the cell sap and lowers the freezing point of the cell contents, as water molecules are bound to organic compounds.
The result: With the lowered freezing point, the cells are resistant to cold and can withstand temperatures of 32 ° F or much lower temperatures without damage.
Look at salt water and pure, clear water. The freezing point of pure water is 32 degrees Fahrenheit, while salt water can be cooled to much lower temperatures without freezing. This is the principle behind the use of salt on the streets of Texas. Last week, more than 1,400 tons of salt mixed with sand were used on the streets of Lubbock during our “Winter Wallop” to keep our street ice-free (Lubbock City Streets Department).
Ellen Peffley taught college horticulture for 28 years, 25 of them at Texas Tech. During this time she developed two types of onions. She is now the sole owner of From the Garden, a gardening farmette. You can email her at Gardens@suddenlink.net