How Does Salinization, Which Is So Frightening, Harm Crops?

Saline-alkali stress stands as one of the primary abiotic stressors, second only to drought, that severely impede plant growth and development. This stress not only affects the growth processes of plants but also poses a grave threat to agricultural production and the ecological environment. Understanding the mechanisms of plant salt-alkali tolerance holds immense practical significance for the breeding of salt-alkali tolerant crops and the effective utilization of saline-alkali lands.

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1 The Mechanisms of Metabolic Damage Caused by Saline-Alkali Stress in Plants

1.1 1.1 Stress-Induced Ion Damage

1.2 1.2 Stress-Induced Damage to the Membrane System

1.3 1.3 Stress-Induced Osmotic Damage

2 The Anti-Saline-Alkali Mitigation Mechanisms of Plants under Saline-Alkali Stress

2.1 2.1 Ion Selective Absorption and pH Balance

2.2 2.2 The Protection of Membrane System by Enzyme Activity

2.3 2.3 The Accumulation of Osmotic Adjustment Substances

2.4 2.4 Inducing the Expression of Genes Related to Salt-Alkali Resistance

The Mechanisms of Metabolic Damage Caused by Saline-Alkali Stress in Plants

1.1 Stress-Induced Ion Damage

Ion damage occurs when certain ions accumulate excessively in plants under adverse stress, causing toxicity. For normal growth and development, plants require an environment rich in nutrients. However, saline-alkali stress disrupts the dynamic balance of ions within plants. This disruption mainly affects the distribution of various ions in plant cells, leading to an imbalance in the normal ion dynamics. As a result, there is a large accumulation of Na+ while K+ uptake becomes difficult. The antagonism between Na+ and K+ causes ion toxicity, severely affecting plant growth and development. For example, under saline-alkali stress, a series of damages such as disordered somatic cell metabolism, damaged cell membrane structure, increased permeability, weakened photosynthesis, and inhibited plant growth are all caused by high concentrations of Na+. Additionally, excessive Na+ can replace K+ in plant cells, reducing K+ absorption and interfering with the Na+/K+ ratio balance within the cell. Meanwhile, the large accumulation of Na+ also affects the absorption of Ca2+. Insufficient Ca2+ can damage the selective permeability of the cell membrane and hinder signal transduction on the cell membrane.

1.2 Stress-Induced Damage to the Membrane System

Membrane system damage occurs when plants accumulate excessive harmful ions under adverse stress, resulting in the destruction of membrane permeability, the outflow of a large number of nutrients from cells, an increase in membrane lipid peroxidation and conductivity, thus harming the plants. In saline-alkali soils, the high content of metal ions and pH value, especially the large accumulation of metal ions (especially Na+) in the cytoplasm under high saline-alkali conditions, disrupts the balance of nutrient ions in the cell, causing osmotic stress and generating a large amount of reactive oxygen species, resulting in oxidative stress. At the same time, Na+ toxicity occurs under saline-alkali stress and accumulates in large quantities in cells, leading to an increase in the permeability of the plant cell membrane and a large accumulation of MDA in the cell, causing significant damage to plant root water absorption and the cytoplasmic membrane.

1.3 Stress-Induced Osmotic Damage

Osmotic damage is the high osmotic pressure stress caused by physiological drought in crops under adverse stress, often simply referred to as osmotic stress. High-concentration saline-alkali stress in a saline-alkali environment leads to changes in crop cell turgor pressure and high osmotic potential. The turgor pressure of cells is a mechanical effect. Under certain conditions, the physiological drought stress caused by water deficiency in plants is transformed into mechanical stress and recognized by plants. In 1898, Schimper analyzed the mechanism of salt damage using physiological drought and believed that the inhibition of plant growth under salt stress was caused by physiological water deficiency. This was mainly because the presence of a large number of soluble salts reduced the soil osmotic potential, making it difficult or impossible for plant roots to absorb water. Therefore, in saline-alkali areas, although the soil water content is high, due to the high salt content, plants are prone to physiological drought due to insufficient water absorption, resulting in osmotic damage.

The Anti-Saline-Alkali Mitigation Mechanisms of Plants under Saline-Alkali Stress

2.1 Ion Selective Absorption and pH Balance

For normal growth and metabolism, plants must maintain a high K+/low Na+ ratio balance within cells. Under saline-alkali stress, the Na+ content in plants rises sharply while the K+ content decreases, disrupting the ion balance within the cell. Therefore, plants can only adjust themselves to absorb and accumulate organic acids and inorganic anions to balance excessive cations such as Na+, thereby maintaining the ion balance within the cell and the stability of the internal pH, and improving their own salt-alkali resistance.

2.2 The Protection of Membrane System by Enzyme Activity

The protection system of plants under adverse stress is divided into two types. The first is the enzyme protection system, mainly including catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD). The second is the non-enzyme protection system, mainly including ascorbic acid (ASA), glutathione (GSH), etc., which act as the main redox buffers and are distributed in different parts of the cell to regulate the balance of reactive oxygen species in the cell. The scavenging of reactive oxygen species in plant cells depends on the action of POD. It can catalyze the redox reaction with H2O2 as the oxidant, reduce H2O2 to H2O, scavenge excess H2O2 in the cell, and maintain the stability and integrity of the cell membrane, thus greatly improving the salt-alkali tolerance of plants. Under salt stress conditions, the increase in the activities of enzymes such as SOD, POD, and CAT in plants and the scavenging of reactive oxygen free radicals play a very important role in improving plant salt-alkali tolerance.

2.3 The Accumulation of Osmotic Adjustment Substances

Under saline-alkali stress, in order to maintain normal physiological metabolism, in addition to inorganic ions such as K+, NO3-, and Cl- that are involved in salt-alkali resistance, plants also synthesize small organic substances such as proline, organic acids, and betaine as osmotic adjustment substances. Through these substances, plants respond to saline-alkali stress by increasing cell osmotic pressure and reducing water potential, improving their salt-alkali resistance and reducing the damage caused by saline-alkali stress. Different types of plants accumulate a large amount of proline, betaine, and different types of sugars and organic acids under saline-alkali stress, compensating for the deficiency of anions in the cell and neutralizing excessive cations to maintain the charge balance of the internal environment, regulating the osmotic adjustment substances in plants and the rhizosphere microenvironment, stabilizing the cell structure and inclusions, and improving the adaptability of plants to saline-alkali stress.

2.4 Inducing the Expression of Genes Related to Salt-Alkali Resistance

The salt-alkali tolerance of plants is a quantitative genetic trait with the coordinated expression of multiple genes. These genes are divided into the abscisic acid pathway, the Na+ stress defense pathway, and the protein kinase pathway according to different saline-alkali stress signal transduction pathways. Among them, the abscisic acid pathway and the protein kinase pathway are mainly used for the transduction of osmotic stress signals, while the Na+ stress defense pathway is mainly used for the transduction of ion stress signals. When plants are subjected to salt stress, abscisic acid is rapidly synthesized, and its receptor binds to PYR/PYL and PP2C in sequence to form a complex. Subsequently, SnRK2 is released and its activity is enhanced. Finally, bZIP binds to the response element AREB, activating and expressing the abscisic acid response genes, directly or indirectly regulating plant salt tolerance.

In general, salt stress and alkali stress are two different but related abiotic stresses that often occur simultaneously. Osmotic stress and ion toxicity are the main harmful factors of salt stress, while alkali stress also involves high pH stress in addition to these factors. Saline-alkali stress not only changes the effective nutrient ion components such as K+ and Ca2+ in the soil, reduces the soil osmotic potential, but also disrupts the dynamic balance between ions, resulting in disordered plant physiological metabolism, inhibited growth, and ultimately affecting its yield and quality. The saline-alkali soils in China have complex components, being a mixture of salinization and alkalization, and the degree of salinization and alkalization varies. Currently, breeding salt-alkali tolerant plant resources, screening new salt-alkali resistant crop varieties, and improving their salt-alkali resistance are the main measures to solve the problem of plant salt-alkali resistance. It is also an effective way to expand the cultivated land area, ensure grain and oil security, improve the ecological environment, and ensure the sustainable development of agriculture. The research on plant salt-alkali resistance is also one of the popular topics among agricultural experts and scholars at home and abroad.