Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

8. Plant adaptation and resistance to adverse environmental factors
8.3 Plant resistance to low temperatures. Chilling tolerance. Frost resistance

Plants from different habitats vary in their tolerance limits to low, damaging temperatures. For instance, plants native to the Far North easily withstand winter cooling down to -60 °C. Central European species that bloom in winter (such as the common daisy Bellis perennis L. or common chickweed Stellaria media (L.) Vill.) can endure freezing and resume their vital activity once temperatures rise. At the same time, most thermophilic plants of southern origin tolerate low positive temperatures poorly (from 10 °C and below). For example, cocoa plants perish at 8 °C, cotton plants die within 24 hours at temperatures between 1 and 3 °C, and corn seed germination and seedling growth are inhibited by soil temperatures below 10 °C.

Consequently, two MAIN TYPES OF Plant Resistance to low temperatures are distinguished: cold hardiness (the resistance of thermophilic plants to low positive temperatures) and frost hardiness, which is the ability of plants to withstand temperatures below 0 °C.

Cold Hardiness. When thermophilic plants are exposed to low positive temperatures, their aerial Organs exhibit a gradual loss of turgor (cucumber leaves lose turgor at 3 °C by the 3rd day, and the plant wilts and dies). Thus, low temperatures can disrupt Water supply to transpiring organs. Some plants show an accelerated Breakdown of Proteins and an accumulation of soluble nitrogen forms in their Tissues. The primary cause of the NEGATIVE IMPACT OF low positive temperatures on thermophilic plants is the disruption of membrane functional activity. The saturated Fatty acids composing these membranes transition from a liquid-crystalline state to a gel state. This leads to detrimental Metabolic Disorders and, under prolonged exposure to low temperatures, to the death of the plant.

The cold hardiness of thermophilic agricultural crops can be enhanced through pre-sowing seed hardening. Sprouted seeds of thermophilic crops (cucumbers, tomatoes, melons, etc.) are subjected to alternating low positive (1–5 °C) and higher (10–20 °C) temperatures every 12 hours for several days. This same method can subsequently be used to harden seedlings as well. Cold hardiness is also improved by soaking seeds in 0.25% solutions of Trace Elements or ammonium nitrate (for 20 hours in the case of cotton).

Frosts with an average annual minimum air Temperature below -20 °C are common across 42% of the Earth's land surface; therefore, frost hardiness is of paramount importance for agricultural production in these regions. In our country, significant contributions to The Study of frost hardiness were made by N. A. Maksimov, I. I. Tumanov, and other researchers. Rapid temperature drops under experimental conditions lead to intracellular ice formation and, in most cases, Cell death. A gradual temperature decrease (0.5–1 °C/h), as occurs under natural conditions, induces ice formation in the intercellular spaces. Furthermore, the growing ice crystals displace air from these intercellular spaces, causing the frozen tissue to become translucent. Upon thawing, the intercellular spaces fill with water, which is subsequently reabsorbed by the Cells unless they have been killed by the frost.

The primary causes of cell death at low sub-zero temperatures are cellular dehydration and the mechanical pressure of ice, which damages cellular structures. Dehydration occurs as water is drawn out of the cells into the intercellular spaces due to ice crystal formation. The most common symptoms of freezing injury include the loss of cellular turgor, infiltration of intercellular spaces with water, and the leaching of ions from cells. Notably, no significant disruptions are observed in the lipid phase of the membranes. The leakage of sugars and K⁺ ions from cells is evidently associated with damage to membrane systems, particularly their Active Transport MECHANISMS (based on ATPases).