Stress Physiology - Drought, Salt and Water Stress | Agriculture Optional Notes for UPSC PDF Download

Stress Physiology

Plants are exposed to a variety of environmental challenges, including water scarcity, drought, heat, cold, salinity, and air pollution, much like all other species. Stress physiology is the study of how plants behave in unfavorable environmental circumstances. The phrase "biological stress" was first used in reference to plants by Jacob Levitt in 1972. He defined stress as "any change in environmental condition that might adversely change the growth and development of a plant."

Strain is the response that plants have to stress. For instance, a regular plant growing in optimal light settings will experience less photosynthesis if it is exposed to low light levels. As a result, strain is defined as a decrease in photosynthesis while stress is defined as reduced light intensity. Elastic and plastic biological strains are the two categories of biological strains. Elastic biological strain is the term used when a plant's response is transient and it eventually recovers to its initial state. For instance, transient withering. Plastic biological strain is the term used to describe a reaction that is permanent and does not allow the plant to operate normally again. For instance: Constant wilting. Stress does not negatively impact certain plants since they have evolved to withstand it. These kinds of plants are known as stress-tolerant or stress-resistant plants. Consider mangroves. Certain plants are known as stress-enduring plants since they are unable to withstand stress and survive their difficult time in a dormant state. Ephemeral plants are short-lived desert plants that finish their life cycle just before the dry season begins, during the periodic rains. We refer to these transient plants as stress escapers. Plant stress can be categorized using the criteria shown in figure 15.31.

Environmental Stress

Biotic Stresses

These are negative consequences that other living things, like bacteria, fungus, viruses, insects, parasites, weeds, and rival plants, cause to plants.
Man's activities, such as pruning plants and trees and twigs for firewood, feed, and agriculture, can contribute to biotic environmental stress. Potential biotic stressors are those brought on by nematodes, fungus, and bacteria that are always present in the environment. There are two categories for them.
These are:

1. Allelopathy
Allelopathy is the creation of one or more biochemical compounds by an organism that has a significant impact on the germination, growth, and reproduction of other organisms. Allelochemicals are the name given to these biochemicals. Positive allelopathic effects or negative allelopathic effects can result from them. These allelochemicals are extracted from roots as well as leaves that have leached on the ground. Allelopathy was coined in 1937 by Hans Molisch and comes from the Greek words allelon, which means "each other," and pathos, which means "to suffer." Weeds can have an allelopathic influence on crops, and vice versa (Figure 15.32).

The black walnut (Juglans nigrum) is one of the most well-known allelopathic plants. Black walnut contains a chemical called juglone, which inhibits breathing. Solanaceous plants, including eggplant, tomatoes, and capsicums, are prone to juglone. When exposed to these allelochemicals, these plants show signs of death, chlorosis, and wilting.

Allelopathic trees include the Tree of Heaven (Ailanthus altissima), which was added relatively recently. An effective herbicide is ailanthone, an allelochemical that is isolated from the root of Ailanthus. The allelochemical sorgolone exhibits allelopathic activity in the sorghum plant. Most Sorghum species' root exudates contain it. The growth of some weeds, like Chenopodium album and Amaranthus retroflexus, is inhibited by the exudation of maize roots. The oat (Avena fatua) seed exudates have an impact on wheat seedling germination.

2. Pathogenecity
The effect of microbes that cause diseases in plants. Example: Xanthomonas citri

Abiotic Stresses

Abiotic stress can result from soil conditions (edaphic stress) or atmospheric conditions (atmospheric stress). Both excessive and insufficient amounts of light temperature and air pollution can result in atmospheric stresses.

1. Light Stress
The distribution of species is restricted by light. Sciophytes, or plants that prefer shade, grow in low light, while Heliophytes, or plants that prefer high light, grow in high light. Because stomata do not fully open in low light, there is less gas diffusion. Less photosynthesis occurs as a result, and chlorophyll synthesis is also impacted. Additionally, photosynthesis is inhibited by strong light. Flowering is inhibited by changes in photoperiod.

2. Temperature
Plants are adapted to a particular region and they face temperature stress in another region.
(a) High temperature: Drought in the soil and atmosphere is caused by high temperatures. In soil drought, plants wilt permanently; in atmospheric drought, they wilt temporarily. Generally speaking, plants perish above 44°C. A cyanobacterium called Mastigocladus, on the other hand, grows well in hot springs between 85 and 90 degrees Celsius. Normal protein synthesis stops at 42°C, and a new class of protein known as Heat Shock Proteins (HSPs) emerges. Following their discovery in fruit flies (Drosophila melanogaster), these proteins have subsequently been found in animals, plants, and microorganisms. Every physiological process slows down in hot weather. Respiration rises and photosynthesis falls. Thus, there is a deficiency of organic materials for plants.

(b) Low Temperature: Plants suffer greatly from low temperature stress, and temperatures close to freezing point permanently harm them, making it impossible for them to withstand extremely cold temperatures. Cold-resistant plants, on the other hand, are those that thrive in cold climates and are found in alpine and arctic regions. Frost stress is the stress brought on by freezing temperatures. Lower than 10°C temperatures increase ion leakage, inhibit root growth, and produce more ethylene.

3. Air pollutants
Significant air pollutants that are common in the Indian subcontinent include fluoride, H ₂S, CO₂, CO, SO₂, NO₂, and O₃. These pollutants cause hidden harm instead of obvious harm. Visible damage, such as chlorotic and necrotic spots appearing on leaves, as well as the inhibition of photosynthetic carbon metabolism and biomass formation, occur when the concentration of these pollutants rises. Low concentrations of certain pollutants promote the growth of plants. For instance, SO₂, NO₂, and NO. Air pollution can affect both photorespiration and respiration. Respiration is stimulated at lower concentrations of air pollutants, but is inhibited at higher concentrations. While NO₂ lowers pigment content during acute exposure, nitrogenous air pollutants cause an increase in chlorophyll content during chronic exposure.

4. Edaphic Stress
They are divided into two types. They are water stress and salt stress:

(a) Water stress: Water stress is a common stress condition caused by either too much or too little water. Stresses related to water scarcity and abundance are referred to as drought stress and flood stress, respectively.

I. Flood Stress: Soil-borne microorganisms and the roots of plants experience an oxygen shortage when they are temporarily submerged by floodwaters. The following are some effects of flooding: Reduced nitrogen turnover in the soil; increased formation of ethylene, abscisic acid, and ethylene precursors; stimulation of partial stomatal closure, epiphany, and abscission in leaves; Enzymes become partially inhibited, cellular membrane systems degrade, mitochondria and microbodies disintegrate. Some plants that grow on permanently wet soils are resistant to flooding. Examples include hydrophytes, shore plants, and marsh plants. Tree species that are common in areas that flood also tolerate flooding. Mangroves, palms, and Taxodium disticum are a few examples of plants that can withstand flooding.

II. Drought Stress: The word "drought" refers to a time when there is little to no precipitation and the soil's water content is so low that plants experience water shortage. The following are some effects of drought: The cells shrink as a result of decreased cellular growth and synthesis of cell wall components; Certain enzymes' reduced activity reduces both nitrogen fixation and reduction; Transpiration decreases as a result of an increase in abscisic acid levels, which eventually shut down the stomatal apparatus to the minimum; The production of chlorophyll is suppressed and the photosynthetic process is slowed down; increases in proline levels; Assimilate translocation and respiration both decline; When water is lost, hydrolytic enzyme activity rises, which is followed by RNA oxidation and protein disruption; Mature leaf wilting is linked to mobilization export-induced carbohydrate depletion and leaf senescence.

Mechanism of drought resistance

Xerophytes are drought-tolerant due to the following reasons:
These plants' protoplasm can withstand prolonged or severe desiccation (dehydration) without dying; therefore, it can withstand such conditions. For instance, the lethal level of desiccation in most plants is below 50–70%, but the creosote bush (Larrea tridentata) can withstand water content drops of up to 30%. These plants are able to avoid or delay this level of desiccation because they have evolved structural or physiological adaptations. Plants that avert or delay desiccation have created the following mechanisms in order to evolve along a different path: enhanced absorption of water by roots that reach the water source deeply; efficient water conduction through the production of more xylem elements, dense leaf venation, and a reduction in the transport distance (short internodes) through the expansion and enlargement of conductive tissues; limitation of transpiration caused by densely trichome-covered stomata that are only found on the lower epidermis; By reducing the transpiring surface, rolling leaves also aids in lowering water absorption; It has been discovered that Agave americana and other CAM plants use water sparingly when storing it in their succulent tissue.

The induced gene expression of stress proteins (dehydrin and osmotin) is a crucial defense mechanism that stabilizes the cell structure during drought stress. These proteins guard against denaturation the cytoskeleton's (biomembrane) macromolecules, which are found in the cytoplasm and nucleus. A high tolerance to desiccation suggests that when water becomes available, the protoplasm rehydrates. Most plants that thrive in arid environments and deserts are resistant to drought.

Salt Stress

Plant growth and development are restricted when there is a high concentration of salt in the soil. Salt stress most frequently affects plants that are found close to estuaries and the coast. An estimated one-third of the world's irrigated land suffers from salt stress. The ions Na⁺, Cl^-, K⁺, Ca⁺⁺, and Mg⁺⁺ typically contribute to the salinity of soil.
Growing in such areas presents two challenges for plants:

• Absorption of water from the soil with negative water potential
• Interaction with high concentration of toxic sodium carbonate and chloride ions.

On the basis of salt tolerance, they are grouped into two categories:

• Halophytes
• Non-halophytes or glycophytes

Saline soils are the natural habitat of halophytes. Halophytes with a broad range of resistance are referred to as euryhaline, while those with a narrow range of resistance are known as stenohaline. Unlike halophytes, non-halophytes are unable to withstand salts. High Mn²⁺ ions are tolerated by Helianthus annus.
Those that live in salty environments deal with two issues:

• One is high concentration of salts in soil water leads to decrease in water potential so they grow in opposite direction. Example: Salicornia.
• Injuries in salt affected plants caused by both osmotic effects and specification effects. Accumulation of chloride ions reduces water absorption and transpiration.

Deficits in K, P, S, Fe, Mo, Zn, Mg, and Mn mineral elements lead to salt stress, which in turn causes physiological disorders that lower yields and growth.

• To maintain their cells' negative osmotic potential throughout the growing region, salt accumulators take in and hold onto salts.
• Excess salt is excluded from the leaf surface of certain salt-tolerant plants. Certain plants have salt glands that release salt, primarily NaCl. Water is hygroscopically absorbed from the atmosphere by the exuded salt.
• Some plants shed their too-salted leaves or allow the excess salt to seep into the ground.
• Salt-tolerant plants, also known as true halophytes, produce significant amounts of the amino acids proline, galactosyl glycerol, and certain organic acids, all of which are involved in osmotic adjustments.

Mechanism of salt tolerance

The issue of too many dissolved salts in the solution affects plants that grow in salty environments, such as halophytes. Because too much salt has a relatively higher negative osmotic potential, plants are more likely to lose water to the surrounding medium. The plants in these circumstances usually only start to lose water when their water potential starts to decrease. Only if they take in excess salt and store it in their cell saps to maintain concentrations that are at least as high as those of other plants will this be feasible.

The drawbacks:

• Salt accumulates in the vacuoles
• The plants become succulents
• Accumulated salt dehydrates the cytoplasm
• Sodium chloride cannot be tolerated in the cytoplasm and it denatures several enzymes

Thus, the issue cannot be resolved by the absorption or accumulation of inorganic salts. On the other hand, plants create organic compounds that can withstand high salt concentrations without denaturing the enzymes, allowing them to withstand the stress caused by salt. We refer to these organic substances as nontoxic organic osmotica. Proline and betalin are two examples of osmoregulators.