A STUDY OF THE MECHANISM OF FROST INJURY TO PLANTS

Abstract

Observations in a micro-freezing apparatus of isolated tissues of the cortex of hardy and non-hardy plants of Catalpa and Cornus species, and of the epidermis of red cabbage, reveal that there are two modes of freezing of plant cells, intracellular and extracellular.In intracellular freezing, ice crystals form first in the protoplasm and then in the vacuole. In extracellular freezing, ice forms outside the cells from water in the cells. The resulting dehydration of the cell causes its collapse, the opposite walls coming together and squeezing the contents to the periphery. Intracellular freezing is fatal to all cells by visible mechanical disruption of the protoplasm and vacuole. It is facilitated by rapid freezing and occurs less easily and less frequently in hardy tissues and in trees and shrubs than in non-hardy and herbaceous tissues. Extracellular freezing induced through slow cooling is fatal to all unhardy cells in trees and herbs at all temperatures below the freezing point, and to cells of hardy cabbage only at − 10 °C. to − 15 °C., but not to cells of hardy trees and shrubs.Both types of ice formation have been observed in intact plants of red cabbage frozen in a refrigerator.The behavior of hardened plants shows that intracellular freezing tends to be prevented in them by an increased permeability to water. In regard to extracellular freezing, from the behavior of the cells on freezing and in micrurgy, a mechanical injury hypothesis is presented.