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

8. Plant Adaptation and Tolerance to Adverse Environmental Factors
8.5 Salt Tolerance

The Study of plant Salt tolerance is of great practical significance, as 25% of the planet's soils are saline, and one-third of the world's irrigated agricultural land has already become salinized due to poor drainage. According to the research of B. P. Strogonov (1962), based on the degree of salinity, soils are classified into practically non-saline, slightly saline, moderately saline, and solonchaks. The type of salinity is determined by the soil's anion content: chloride, sulfate, sulfate-chloride, chloride-sulfate, and carbonate. Sodium is the predominant cation in such soils, though carbonate-magnesium and chloride-magnesium salinities also occur.

Plants adapted to survive under conditions of excessive salinity are called halophytes (from the Greek galos - salt, phyton - plant). They differ from glycophytes—plants of non-saline habitats and soils—by A number of anatomical, physiological, and biochemical traits. Halophytes protect themselves against excessive salt concentrations in three ways: by absorbing large amounts of salt and concentrating them in the vacuolar sap, which results in a high osmotic pressure; by eliminating absorbed salts from Cells along with Water via specialized salt glands and shedding excess salt with fallen leaves; and by restricted salt absorption by ROOT cells.

All halophytes can be divided into three groups:

1. True halophytes (euhalophytes) are the most salt-tolerant plants, accumulating significant salt concentrations in their vacuoles. They grow on moist saline soils. Due to high intracellular osmotic pressure, these plants possess a strong suction force, enabling water uptake from highly saline soils. Plants of this group are characterized by succulent leaves, a trait that disappears when they are cultivated on non-saline soils. Typical representatives of true halophytes include glasswort (Salicornia herbacea L.) and seepweed (Suaeda maritima (L.) Dumort.).

2. Salt-secreting halophytes (crinohalophytes), upon absorbing salts, do not accumulate them within their Tissues; instead, they excrete them from cells using secretory glands located on the leaves. Salt excretion by these glands is driven by ion pumps and accompanied by The transport of a large volume of water. The salts precipitate as a white crust on the leaves, and some is removed with fallen leaves. These features are characteristic of sea lavender (Statice gmelini Led.), tamarisk (Tamarix speciosa), and others.

3. Salt-excluding halophytes (glycohalophytes) grow on less saline soils. High osmotic pressure in their cells is maintained through Photosynthesis products, while their Cell membranes remain largely impermeable to salts. Typical representatives of this group include wormwood (Artemisia salina) and various species of kochia (Kochia).

Glycophytic plants also exhibit a certain capacity to tolerate excess salt under saline conditions. Among agricultural crops, barley, sugar beet, and cotton are relatively salt-tolerant, and bread wheat is more resilient than durum wheat. Prolonged cultivation under saline conditions has been shown to significantly increase the salt tolerance of barley, millet, and tomatoes without any loss in productivity.

In agricultural practice, the primary method for combating salinity is the amelioration of saline soils, the establishment of reliable drainage, and the leaching of soils after harvest. On solonchaks (soils with a high sodium content), amelioration is carried out via gypsum application, which displaces sodium from the soil adsorption complex and replaces it with calcium.

Applying microelements to the soil improves plant Ion Exchange under saline conditions. Plant salt tolerance is further enhanced by pre-sowing seed hardening. For seeds of cotton, wheat, and sugar beet, a one-hour Treatment in a 3% NaCl solution followed by a 1.5-hour water wash is sufficient.