How Plants are Damaged through Stress ? | Plant Physiology

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Stress is a mechanical concept and defined as a force per unit area applied to an object. The object develops a strain in response to a specific stress. Compared with mechanical systems, in biological systems it is difficult to define stress precisely. Even though applicable in mechanical terms, its biological connotations is different.

Plants usually yield to any stress condition (Table 28-1) and its reaction may be elastic or plastic. In the former the reaction is temporary and the plant reverts back to its original state. However, in the latter state the plants are deformed and the changes are not reversible. The stress may be immediately made out in the plant or plants may become resistant when exposed to stress conditions.

This state is called hardening. Sometimes the effect produced is carried over in the subsequent generations. Thus pea or bean plants subjected to low temperature tend to become dwarf and this effect is passed on for several generations.

However, recent years are witnessing studies on genetic basis of resistant strains. Breeders are also making efforts to evolve the genetic lines which are gradually adapted to diverse climatic conditions. The reaction of plants to the stress conditions is highly complex and is manifested in the form of several physiological responses.

The most common stresses to which the plants are exposed to are drought, heat, cold and frost. In addition several other stresses also exist e.g., shade, salt excesses, altitude. In recent times occurrence of excessive pollutants, effluents also give toxic environments to the plants. We shall discuss briefly only some of these stress conditions.

In general, two types of stress resistance are recognized and these are avoidance and tolerance. In the former an internal environment is created within the plant so that its cells are not put under stress. For instance, in a leaf the process of transpiration is built in whereby the leaf is kept cool even though the environmental temperature is high.

Likewise succulents conserve lot of water to avoid internal drought suffrage. In the latter the plant has the capacity to withstand stress. The examples are found in mosses which can endure desiccation conditions. In some species both the characteristics may be present. Plant physiologists are mainly interested in developmental physiological processes which help the plants to tolerate stress conditions.

It is difficult to determine the magnitude of effect of stress in a plant species. Thus in nature duration and intensity of stress are linked together in complex ways and any information from the laboratory conditions may not provide total and complete amount of hardiness. Thus only drought estimates are obtained on hardiness by creating simulating conditions in the laboratory.

Plants experience usually drought which is one of the commonest stresses. Plants have developed several mechanisms to tide over this stress. Development of thick cuticle, sunken stomata, formation of seeds with low water contents, completion of life cycle in short durations by the desert plants are some of the measures adopted by plants against drought.

In some plants water is retained in enough quantities, or the leaves are reduced to scales. It must be said that drought tolerant mechanisms are not well understood. Dehydration leads to loss of water molecules and thus proteins are disrupted.

Water molecule has several functions to perform and one of these is to help keep complex fluids in a stable configuration. Water loss causes concentration of solutes leading to high concentration of cell sap and intercellular fluids cause a greater decrease in the water potential of the fluids.

This causes stress on the protoplasm. Most of the biochemical processes are adversely affected because of the water imbalance. Changes in the cell pH may also be there.

Plants have developed several mechanisms to tolerate drought conditions. One of these is the presence of hydrophilic substances in the protoplasm like high molecular weight proteins, some carbohydrates (e.g., alginic acid). Low molecular compounds like polyhydric alcohols act as hydrophilic compounds.

These attributes are very common in sea weeds which are subjected to high and low tides. Sugars are usually increased in drought conditions in such plants since their presence insolution directly lowers the water potential of cell sap.

This helps the plant to retain water. This devise helps the plant to conserve water and save the protoplasm from desiccation. In sugarcane high amounts of sugars are present but they are drought prone while pineapple has less amount of sugars and the drought resistant. Thus the capacity of a plant to bind water to the proteins is very important.

There is also a suggestion that during stress conditions certain resistance proteins appear in a cell and these resist denaturation. Some physiologists are of the view that drought resistance is associated with the protoplasmic elasticity. In general, drought tolerant plants have small cells, high nucleic acid contents, less starch and very high amount of sugars.

Plants responses to heat and their tolerance levels vary (Table 28-2). Leaves may help the plant to avoid heat due to transpiration but the process is negligible.

Majority of the plant species survive high temperature because of their internal build up. Some of the thermal algae, cactii and several other desert plants experience as high as 70°C and yet survive. High temperature tends to denature proteins and also causes heavy water loses.

One of the attributes is increased enzyme production to compensate for the destruction. It is also reported that in some organisms a rise in temperature slows down some processes and when some compounds are added the process is restarted.

Ascorbic acid, other vitamins are some of these compounds. Recently it has been demonstrated that addition of adenine bestowed tolerance in some plants. In general in plants which are temperature tolerant, process enzymes are more stable to high temperature. In fact in these enzymes some isozymes develop at high temperatures.

In summary it may be stated that most of the plants are heat tolerant because they possess the capacity to produce heat-stable proteins. They also have the ability to replace thermal denatured proteins immediately. However, the precise mechanism of stabilization and synthesis of such proteins is not clear.

Like the heat tolerance, plant species also possess the capability to resist freezing. Several processes may be involved in causing freeze injury (Table-3) to the plant. In general plants growing under tropical climate are more chilling-prone. Hence chilling causes damage to their tissues or organs.

Such plants also are sensitive to low temperature like 12-13°C while low temperature like 0-5-C is lethal. Obviously proteins are sensitive to low temperature. On the contrary, most of the alpine and arctic plants do not experience any damaging effect at this low temperature. In these plants the danger is that their tissues may not undergo water formation in their cells.

Seeds, pollen, embryos can be stored at low temperature; 190°C obtained through liquid nitrogen. Freezing damage is done in two ways. First, there is formation of ice crystals and the damage is due to mechanical effects. There is disruption of membranes and even cell organization is disturbed.

Second, ice formation reduces the water amount in the cell leading to drought situation. However, the intercellular water has high potential whereas water in the cell cytoplasm or vacuole has nearly negative water Ψ. To begin with it is in the intercellular spaces that the ice crystals are formed and then with the freezing continuing, water leaves the cell cytoplasm. In plants which are freezing-hardy, water tends to remain in the intercellular spaces.

In brief, following is a set of events which take place: Ice crystals formed in the intercellular spaces; protoplast solutes become concentrated due to removal of water from there. There is precipitation of solutes in the protoplast which causes abrupt shifts in the cell pH.

If the temperature is lowered down still further (e.g.—35 to—40°C), all water in the tissues is crystallized. Gradually the crystals enlarge in size and hence there is mechanical damage to the cell. The freezing may be slow or rapid and different plants exhibit different responses.

Chilling-sensitive plants usually have higher proportion of saturated fatty acids (Table 28-4) and hence higher transition temperature. Chilling-resistant species lower proportion of saturated fatty acids and hence lower transition temperatures. During the time the plant gets adjusted to low temperature, the proportion of unsaturated fatty acids enhances and transition temperature decreases.

On the contrary high temperature exposure is accompanied by low soil moisture, high potential transpiration rates etc. High temperature affects membrane and metabolism considerably. For instance several of the enzymes are denatured.

The membrane comes to possess higher proportion of saturated fatty acids in the membrane lipids in the high temperature tolerant species. The fluidity of the membrane increases at high temperature and thus affect permeability and catalytic functions of membrane proteins.

Thylakoid membranes are most sensitive to high temperature damage and thus efficiency of photosynthesis is curtailed. Photosystem II and associated oxygen-evolving complex is grossly affected by high temperatures though activities of Rubisco and other carbon fixing enzymes may get less affected at high temperatures.

Exposure of plants to high temperatures suppresses the synthesis of most proteins but stimulates the synthesis of specific new proteins which are of low molecular mass and known as heat shock proteins (HSPs). Initially discovered in Drosophilla now reported from diverse plants and animals exposed to high temperatures.

There are three classes of HSPs (see Table 28-5) based on their molecular mass. The genetic aspect of these proteins is being well understood but their precise mode of function is still conjuctural. Chaperonin are a class of proteins which direct the assembly of multimeric protein aggregates.

It may be stated that induction of new proteins is not just confined to heat shock, several other stresses like low temperature, water deficit, salinity, anoxia, osomotic stress also induce synthesis of new family of proteins.

Some of these stress proteins resemble HSPs. In summary, it may be mentioned that the synthesis of new proteins appears to be a common response to stress though no universal proteins have been identified.

High concentrations of salt in the plant’s surroundings may vary considerably and plant may experience salt stress. In fact large ha of soils are saline. Desert soils have high concentrations of Na, CI, Ca, SO₄ and carbonates.

High salinity is also found near coastal areas, sea shores and even agricultural lands that are heavily irrigated. The salinization of agriculture land has lot of implications on productivity. Salt stress can damage plants at three different levels; alter the soil structure; cause water deficit (physiological drought), and toxic effects of specific ions especially Na, CI.

Molecular biology of salt tolerance is attracting attention recently. Salt-induced gene expression was studied till recently in cell suspension cultures and intact root systems. However, salt tolerant genes have been isolated from rice cv growing in coastal areas and also from mangroves and cloned in rice and tomato.

An increase in the level of osmotin is a striking observation. Cloning of salt tolerant genes will be an important step in raising crop species which grow in saline soils. One of the ways to bolster plant tolerance to salinity is by transferring genes encoding protective proteins or enzymes from other organisms.

Main approaches currently being followed are engineered alterations in the amounts of osmolytes and osmoprotectants, saturation levels of membrane fatty acids and rate of scavenging of reactive oxygen intermediates.

A variety of genes coding for proteins/enzymes involved in stress relief have been introduced into various plant species with reasonable degrees of tolerance to abiotic stresses. These include: low molecular mass osmoprotectants and osmolytes (quaternary amines—betaines), amino acids (proline), sugar alcohols (mannitol, sorbitol) (Fig. 28-5).

There are as many as six theories available to account for the effects of frost and also frost resistance. These are discussed below:

Denaturation of proteins at low temperature:

It is believed that at low temperature in the frost- hardy plants there is formation of low temperature resistant proteins. This may be due to increased concentrations of electrolytes which protect tissue water against its removal by the intercellular ice. Frost-hardy plants have high sugar amount and obviously frost-hardiness involves synthesis of more sugars.

Plant tissues may be made frost-hardy by placing in sugar solution. Thus even though new frost-resistant proteins might be synthesized these proteins must be resistant to high sugar concentrations. Some physiologists believe that shrinkage causes the protein molecules to come together. It is assumed that frost resistance proteins have more hydrophilic bonds.

Thus several factors may be contributory to freezing resistance and these are: formation of low-temperature resistant proteins; dehydration effects; concentration of elctrolytes; high levels of sugars; ice crystal formation and lastly steric effects.

Process of frost hardening is highly complex and much remains to be understood about it. In several plant species it seems to be associated with photoperiod and in some it is essential that tissues experience some pretreatments to attain maximum hardiness. These may include dormancy, photoperiod, etc. Frost hardiness also seems to be associated with the occurrence of metabolism specific inhibitors or even some amount of starvation.

Inadequate light excessive shading of plants causes deformity and starvation. Plants growing in the shade adjust their leaf area, blade thickness, chlorophyll contents, number and orientation of chloroplasts in them. Similarly when ¹⁴CO₂ was provided in high amounts there was strong reaction on the photosynthesis. Several instances are known where radiations have been shown to affect diverse metabolic processes.

Sometimes toxic materials may be present in the soil in large amounts and they affect several mechanisms of the plants. Mangroves are usually referred to as salt regulators since they do not absorb salts but possess the general ability to exclude them from their roots.

On the other hand, there are plant species e.g. Atriplex which are salt accumulators and have cell sap with low and are thus able to absorb salt water of high concentration. Internally such plant species are able to tolerate high concentrations of salt. Such plants also possess special glands on their leaves which excrete high salt contents.

The plants growing at high altitudes also undergo complex stress including climatic conditions. Here much of the plant response depends upon weather conditions, surface microclimate, especially topographical features. In addition radiations also affect their growth.

Plants are also exposed to insects and potentially pathogenic microorganisms though some plants exhibit resistance against disease. Plants adopt several strategies to overcome such adversities. In some plants once the pathogen attacks it, it responds by altering the physical properties of the cell wall and also biosynthesis of secondary metabolites that limit the spread of the attacking pathogen. These responses are known as hypersensitive reactions.

Such reactions are activated by fungi, viruses, nematodes and generally occur outside the pathogen’s specificity range. Such hypersensitive reactions are highly complex and are associated with the type of pathogen which attacks it.

Initially there is activation of defense- related genes and synthesis of their products, pathogenesis-related proteins (PR). These proteins include proteinase inhibitors that inhibit proteolytic enzymes secreted by the pathogen and activate chitinase which degrade microbial cell walls. Some genes are activated which synthesize isoflavonoids and other phytoalexins and hence restrict the growth of pathogens.

Lignin, suberin and callose are deposited in the cell wall along with glycoproteins and thus structural support is provided to the cell wall. The invading insects find it tough to perculate such walls. The invaded cells start programmed death resulting in the formation of necrotic lesions at the site of infection. In this way pathogen is isolated, and spread of pathogen is restricted and slowed down.

In the recognition of the potential pathogen it is presumed that a signal detection and transduction chain is involved. The general view is that disease has an underlying genetic basis. Both pathogens and host carry genes that determine the nature of their interaction i.e. whether the disease will occur (virulent) or not (avirulent).

One of the hypothesis is based on gene-for-gene assumption which implies that pathogenic microorganisms carry avirulence genes (avr) while the host plant carries corresponding resistance (R) genes. Disease will take place if the pathogen lacks avr genes or the host plant carries recessive, alleles at the R locus. Thus, hypersensitive reaction is initiated when a matching pair of pathogen avr genes and dominant plant R genes interact.

Several avr genes have been isolated from bacteria and fungi, the specific function of their products is still elusive. There is a possibility that avr gene(s) encode enzyme for the production of elicitors and R gene encodes receptors that recognize elicitors. Elicitors are metabolites which have been isolated from pathogens that evoke a response in host plant.

Several elicitors have been identified and they are usually extracellular microbial products generally associated with cell walls of bacteria or fungi. These are β-glucans, chitosan, arachidonic acid, glycoproteins, polysaccharides, small peptides, pectic fragments.

Recognition of elicitors by the plant cell must take place at the plasma membrane. Several signalling agents have been suggested and these include changes in pH, ion fluxes, transient uptake of Ca²⁺. Intracellular Ca²⁺ levels appear to regulate expression of defence of response genes.

Thus by activating Ca²⁺ ionophores or blocking Ca²⁺ channels it is possible to activate defence responses. The role of protein phosphorylation and production of active oxygen are also shown to be concerned with elicitor-treated cells. Much remains to be understood regarding the exact role of these signals and their interaction with signal cascade.

These are some secondary metabolites which appear to be associated with the hypersensitive reaction and constitute early warning system, eminating signals to other cell and/or tissues in order to prepare themselves for the secondary infection. The studies have shown that initially the plant reacts to the initial infection by slowly developing a general immune capacity.

This is known as systemic acquired resistance (SAR). This aspect is still in its infancy but salicylic acid appears to be involved in such a signalling pathway (Fig. 28-6). This phenolic acid is secondary metabolite having analgesic properties.

During 1990’s a relationship between salicylic acid (SA) and resistance to pathogenes was shown and it was demonstrated that application of SA or its derivative aspirin could induce PR genes expression and enhance. The resistance to TMV. Over the years it has been repeatedly demonstrated that following infection, the level of SA increases in the host and the increase could be as high as 20-fold over the controls.

Such a burst in SA was also followed by the formation of PR proteins. In Arabidopsis plants and also mutants, a parallel between SA, pathogenicity and resistance was demonstrated. On the contrary plants with low levels of SA or such mutants having low levels of SA failed to have SAR. In plants with suppressed phenyllanine ammonia lyase activity the level of SA was low and hence they displayed decreased resistance to the pathogens.

However when the SA was applied externally the resistance was introduced. Evidently, SA appears to have a pivotal role in plant defence responses. Very recent studies point out the role of jasmonates and their derivatives in imparting insect and disease resistance in plants. Jasmonates are shown to be universally distributed in plants and in actively growing cells/tissues their level is very high.

Jasmonic acid is also reported to induce/activate several genes encoding proteins with antifungal properties. In fact several similarities in the action of SA and jasmonates with respect to insect and fimgal resistance have been suggested though some distinctions have also been demonstrated. It appears that there are two defense mechanisms, one mediated by SA and the other mediated by jasmonic acid.

The precise role of jasmonates in activation of genes is still obscure. However, jasmonic acid is shown to be synthesized from linolenic acid and some authors have suggested its role as a second messenger. Since plant membranes are rich in linolenic acid as phospholipids they may function as elicitors and then bind with a receptor in the plasma membrane.

Thus elicitor-receptor complex activates a membrane-bound phospholipase releasing linolenic acid. Subsequently linolenic acid is oxidized to jasmonic acid which in turn acts to modulate gene expression. It may be stated that action of jasmonates is not restricted to insects or fungi, they modulate several physiological processes including seed germination, pollen germination, vegetative protein storage, root development, tendril coiling, etc. In such processes jasmonates are shown to work in collaboration with ethylene. Some workers have proposed a status of plant hormone to jasmonates.

In recent years environmental pollutants have added another new set of stresses for the plants. Pollution stress is mainly chemical in nature and includes toxic effects of heavy metals, airborne oxides of carbon, nitrogen, sulphur and photochemical products. Many plant species have the potential to detoxify heavy metals by binding them with small sulphur-rich poly-peptides known as phytochelatins.