I. What is Plant Stress Resistance?
Plant stress tolerance refers to certain traits that plants possess to resist adverse environments. In nature, it’s extremely difficult for excellent stress – resistant traits of one plant species to transfer to other species under natural conditions, mainly due to the existence of reproductive isolation among different plant species.
Owing to various factors such as different geographical locations, climatic conditions, and human activities, diverse adverse environments are created in nature. These environments exceed the range that plants can tolerate for normal growth and development, causing plants to be damaged or even die. The environments that harm plants are called stress or stressors.
Stressors come in a wide variety, including physical, chemical, and biological factors, and can be classified into two major categories: biotic stress and abiotic stress. The main abiotic stressors that significantly impact plants include physical and chemical stresses such as water (drought and waterlogging), temperature (high and low temperatures), salinity – alkalinity, and environmental pollution. Biotic stress mainly includes diseases, pests, and weeds.
Physical and chemical stresses are often interconnected. For example, water deficiency is usually accompanied by salinity – alkalinity and high – temperature stress. Water stress, low – temperature stress, pests and diseases, and air pollution can all cause reactive oxygen species damage.
II. The Formation Reasons of Plant Stress Resistance
When plants are stressed, some are damaged and die, while the physiological activities of others are affected to varying degrees, but they can survive. If plants live in such a stressed environment for a long time, through natural selection, favorable traits are retained and continuously strengthened, while unfavorable traits are gradually eliminated.
In this way, during the long – term evolution and adaptation of plants, plants growing in different environmental conditions will develop the ability to adapt to certain environmental factors, that is, they can adopt different ways to resist various stress factors. The ability of plants to resist various stress (or adversity) factors is called stress resistance, abbreviated as resistance.
III. The Classification of Plant Stress Resistance
Plants can enhance their resistance to various adversities by adjusting their life cycles, changing their morphological and physiological characteristics. Plant stress resistance can be roughly divided into three forms: stress avoidance, stress tolerance, and stress resistance.
It is manifested in that plants adjust their growth and development cycles so that they do not encounter stress in time, avoiding the interference of stress. This way is very important in plant evolution. For example, when there is water, some desert plants will germinate rapidly and complete the entire life process from germination, growth, development to flowering and fruiting in a short time.
It is manifested in that plants resist the influence of stress factors through specific morphological structures, enabling them to still carry out basic normal physiological activities under adversity. When stress occurs, the stress factors do not enter the tissue, and there are no corresponding changes in the plant body with environmental changes. For example, drought – tolerant plants resist drought through morphological characteristics such as developed roots, small leaves, thick cuticles, low transpiration, and developed vascular tissues.
It refers to that when plants are stressed by the environment, they prevent, reduce, or repair the damage caused by adversity through metabolic reactions, so that they can still maintain normal physiological activities. At this time, the stress has entered the plant body. Plants improve the resistance of cells to various stresses by forming stress – induced proteins, increasing the content of osmoregulatory substances and abscisic acid.
Taking osmoregulation as an example, under environments such as drought, high temperature, low temperature, or salinity, cells will passively lose some water, the content of bound water will relatively increase, while the content of free water will relatively decrease. Bound water is not easy to freeze and transpire, which is conducive to plants to resist adversity.
In addition, adversity will also induce the expression of genes involved in osmoregulation, forming some osmoregulatory substances, increasing the solute concentration in cells, reducing the water potential, maintaining osmotic balance with the environment, enabling plants to continue to absorb water from the outside and maintain normal growth. The main osmoregulatory substances include sugars, organic acids, and ions (especially K+). Among them, proline is the most effective osmoregulatory substance. Under any adversity, plants accumulate proline, especially during drought, which can increase dozens to hundreds of times compared to the original content.
IV. The Self - regulation Ways of Crops under Adversity
Although plants are affected by adversity, they resist it through physiological reactions. If the adversity exceeds the tolerable range and the plant’s self – repair ability, the damage will become irreversible, and the plant will be damaged or even die. Plants resist adversity with cells and the entire organism, mainly manifested in four aspects:
A variety of different environmental stresses acting on plants can cause water stress to plants.
Under adversity, the stomata of plants close, and photosynthesis shows a downward trend, with a reduced supply of assimilates.
During freezing damage, heat damage, salt damage, and waterlogging, the respiration rate of plants decreases significantly; during cold damage and drought damage, the respiration rate of plants first increases and then decreases; when plants are diseased, the respiration rate of plants increases significantly. In addition, adversity will also affect the activities of various respiratory metabolism pathways.
Under various adversities, the decomposition of substances in plants is greater than synthesis.
V. The Morphological Changes of Plants under Adversity
For example, drought can cause leaves and young stems to wilt, and the stomatal aperture to decrease or even close; waterlogging can cause leaves to turn yellow and wither, roots to turn brown and even rot; under high temperatures, leaves turn brown, dead spots appear, and the bark cracks; when infected by pathogens, leaves show disease spots.
If the adversity exceeds the range that crops can tolerate for normal growth and development, it will seriously affect crop growth, such as root necrosis, poor growth, slow growth, yellowing of leaves, weak tree bodies, late maturity, premature senescence, pest invasion, frequent diseases, low yield, poor quality, and even the death of crops.
VI. Osmoregulation and Stress Resistance
A variety of adversities will cause water stress to plants. When under water stress, plants accumulate various organic and inorganic substances in their bodies to increase the concentration of cell sap and reduce its osmotic potential. In this way, plants can maintain their internal water and adapt to the water – stress environment. This regulatory effect manifested by increasing the cell – sap concentration and reducing the osmotic potential is called osmoregulation.
There are many types of osmoregulatory substances, which can be roughly divided into two categories. One is inorganic ions that enter cells from the outside, and the other is organic substances synthesized in cells.
1. Inorganic Ions
Under adversity, cells often accumulate inorganic ions to regulate the osmotic potential, especially halophytes mainly rely on the accumulation of inorganic ions in cells for osmoregulation. The absorption of inorganic ions by plants is an active process, so the concentration of inorganic ions in cells can be much higher than that in the external medium.
In wheat and oats, it has been found that this absorption and accumulation are related to the activity of ATPase. After inorganic ions enter cells, they mainly accumulate in vacuoles and become important osmoregulatory substances in vacuoles.
2. Amino Acids
Amino acids are the most important and effective organic osmoregulatory substances.
Almost all adversities, such as drought, low – temperature, high – temperature, freezing, salinity, low pH, poor nutrition, diseases, and air pollution, will cause the accumulation of amino acids in plants. Especially during drought stress, the accumulation of amino acids is the most, which can be dozens or even hundreds of times higher than the initial content.
Amino acids have two functions in stress resistance: one is as an osmoregulatory substance to maintain the osmotic balance between the protoplasm and the environment. It can form polymers with some compounds in the cell, similar to hydrophilic colloids, to prevent water loss; the other is to maintain the integrity of the membrane structure. The interaction between amino acids and proteins can increase the solubility of proteins and reduce the precipitation of soluble proteins, enhancing the hydration of proteins.
3. Glycine Betaine
Betaine is a cytoplasmic osmotic substance and also a kind of quaternary ammonium compound. Its chemical name is N – methyl – amino acid, with the general formula R4·N·X. Plants accumulate betaine under drought and salinity conditions, mainly distributed in the cytoplasm. In normal plants, the content of betaine is about 10 times higher than that of proline; during water deficiency, the accumulation of betaine is slower than that of proline, and when the water – stress is relieved, the degradation of betaine is also slower than that of proline.