## STRESS PHYSIOLOGY

### STRESS PHYSIOLOGY

12.      STRESS PHYSIOLOGY

Crop growth and production are dependent on various climatic factors. Both abiotic and biotic stresses have become an integral part of plant growth and development. More emphasis has been given to elaborate on the injury and tolerance mechanisms and growth behaviour in plants against abiotic and biotic stresses.
If there is any change in the surrounding environment it may disrupt the homeostasis (The maintenance of steady-state results in a meta-stable condition called homeostasis).
The modulation of homeostasis due to  Environment is called biological stress.
Plant stress disturbs the physiology.
Plants respond to stress in several different ways.
Plant stress can be divided into two primary categories.
1.    Abiotic stress is physical (e.g., water, temperature and salt) or chemical insult that the environment may impose on a plant.
2.    Biotic stress is a biological insult, (e.g., insects, pathogen) to which a plant may be exposed during its lifetime.
3.    If a plant is injured by stress this indicates that their metabolic functions are disturbed.
The injury may be temporary if the stress is moderate and for a short period of time. The plant may Recover when the stress is removed.
Severe stress may prevent flowering, seed formation, and may induce senescence also and which can ultimately lead to plant death. Some plants escape the stress altogether, such as ephemeral, or short-lived, desert plants.

Ephemeral plants germinate, grow, and flower very quickly following seasonal rains. Ephemeral plant avoids stress. They thus complete their life cycle during a period of adequate moisture and from dormant seeds before the onset of the dry season.
The arctic annuals plants rapidly complete their life cycle during the short arctic summer and survive over winter in the form of seeds.
12.1.    Plant adopt to strategies for stress :
Avoidance Mechanisms of stress; Avoidance mechanisms reduce the impact of stress, even though the stress is present in the environment.
Resistance Mechanism for Stress: plant develops the capacity to tolerate particular stress and hence are considered to be stress resistant. In stress resistance plant show capacity to adjust or to acclimate to the stress.
In stress-tolerant plant species, exposure to particular stress leads to acclimation to that specific stress in a time-dependent manner. Thus, plant stress and plant acclimation are intimately linked with each other. Plants exhibit stress resistance or stress tolerance because of their genetic capacity to adjust or to acclimate to the stress and establish a new homeostatic state over time.
Furthermore, the acclimation process in stress-resistant species is usually reversible upon removal of the external stress.
Plants usually integrate these physiological processes over a short-term as well as a long-term basis. The short-term processes involved in acclimation can be initiated within seconds or minutes upon exposure to stress but may be transient in nature, their activities also disappear rather rapidly. In contrast, long-term processes are less transient and thus usually exhibit a longer lifetime. Such a hierarchy of short- and long-term responses indicates that the attainment of the acclimated state can be considered a complex, time-tested response to stress. Acclimation usually involves the differential expression of specific sets of genes associated with exposure to a particular stress. The remarkable capacity to regulate gene expression in response to environmental change in a time-tested manner is the basis of plant plasticity.

12.2.    Stress physiology due to abiotic factors :
Plants may experience physiological stress when an abiotic factor is deficient or in excess (referred to as an imbalance). Most agricultural crops, for example, are cultivated in regions to which they are not highly adapted. Field crops are estimated to produce only 22% of their genetic potential for yield because of suboptimal climatic and soil conditions. Imbalances of abiotic factors in the environment cause primary and secondary effects in plants. Primary effects such as reduced water potential and cellular dehydration directly alter the physical and biochemical properties of cells, which then lead to secondary effects. These secondary effects, such as reduced metabolic activity, ion cytotoxicity, and the production of reactive oxygen species, initiate and accelerate the disruption of cellular integrity, and may lead ultimately to cell death.
Different abiotic factors may cause similar primary physiological effects because they affect the same cellular processes. This is the case for water deficit, salinity, and freezing, all of which cause reduction in hydrostatic pressure (turgor pressure, Ψp) and cellular dehydration imbalances in many abiotic factors reduce cell proliferation, photosynthesis, membrane integrity, and protein stability, and induce production of reactive oxygen species (ROS), oxidative damage, and cell death.
12.3.    The light-dependent inhibition of photosynthesis :
As photoautotrophs, plants are dependent upon – and exquisitely adapted to – visible light for the maintenance of a positive carbon balance through photosynthesis. Higher energy wavelengths of electromagnetic radiation, especially in the ultraviolet range, can inhibit cellular processes by damaging membranes, proteins, and nucleic acids. However, even in the visible range, irradiances far above the light saturation point of photosynthesis cause high light stress, which can disrupt chloroplast structure and reduce photosynthetic rates, a process known as photoinhibition.
Photoinhibition by high light leads to the production of destructive forms of oxygen
Excess light excitation arriving at the PSII reaction center can lead to its inactivation by the direct damage of the D1 protein. Excess absorption of light energy by photosynthetic pigments also produces excess electrons outpacing the availability of NADP+ to act as an electron sink at PSI. The excess electrons produced by PSI lead to the production of reactive oxygen species (ROS), notably superoxide. Superoxide and other ROS are low-molecular-weight molecules that function in signalling and, in excess, cause oxidative damage to proteins, lipids, RNA, and DNA. The oxidative stress generated by excessive ROS destroys cellular and metabolic functions and leads to cell death.

12.4.    Temperature stress :
Three types of temperature stress: high temperatures, low temperatures above freezing, and temperatures below freezing. Most actively growing tissues of higher plants are tillable to survive extended exposure to temperatures above 45°C or even short exposure to temperatures of 55°C or above. However, nongrowing cells or dehydrated tissues (e.g., seeds and pollen) remain viable at much higher temperatures. Pollen grains of some species can survive 70°C and some dry seeds can tolerate temperatures as high as 120°C.when soil water deficit causes partial stomatal closure or when high relative humidity reduces the gradient driving evaporative cooling.
12.4.1.    Temperature stress can result in damaged membranes and enzymes :
Plant membranes consist of a lipid bilayer interspersed with proteins and sterols. High temperatures cause an increase in the fluidity of membrane lipids and a decrease in the strength of hydrogen bonds and electrostatic interactions between polar groups of proteins within the aqueous phase of the membrane. High temperatures thus modify membrane composition and structure and can cause leakage of ions. High temperatures can also lead to a loss of the three-dimensional structure required for correct function of enzymes or structural cellular components, thereby leading to loss of proper enzyme structure and activity. Misfolded proteins often aggregate and precipitate, creating serious problems within the cell.
12.4.2.    Inhibition of  photosynthesis because of temperature stress :
Photosynthesis and respiration are both inhibited by temperature stress. Typically, photosynthetic rates are inhibited by high temperatures to a greater extent than respiratory rates. Although chloroplast enzymes such as Rubisco, Rubisco activase, NADP-G3P dehydrogenase, and PEP carboxylase become unstable at high temperatures, the temperatures at which these enzymes began to denature and lose activity are distinctly higher than the temperatures at which photosynthetic rates begin to decline. This would indicate that the early stages of heat injury to photosynthesis are more directly related to changes in membrane properties and to uncoupling of the energy transfer mechanisms in chloroplasts.
This imbalance between photosynthesis and respiration is one of the main reasons for the deleterious effects of high temperatures. On an individual plant, leaves growing in the shade have a lower temperature compensation point than leaves that are exposed to the sun (and heat). Reduced photosynthate production may also result from stress-induced stomatal closure, reduction in leaf canopy area, and regulation of assimilate partitioning.
12.5.    Freezing temperatures cause ice crystal formation and dehydration :
Freezing temperatures result in intra- and extracellular ice crystal formation. Intracellular ice formation physically shears membranes and organelles. Extracellular ice crystals, which usually form before the cell contents freeze, may not cause immediate physical damage to cells, but they do cause cellular dehydration. This is because ice formation substantially lowers the water potential (Ψw) in the apoplast, resulting in a gradient from high Ψw in the symplast to low Ψw in the apoplast. Consequently, water moves from the symplast to the apoplast, resulting in cellular dehydration. Cells that are already dehydrated, such as those in seeds and pollen, are relatively less affected by ice crystal formation. Ice usually forms first within the intercellular spaces and in the xylem vessels, along which the ice can quickly propagate. This ice formation is not lethal to hardy plants, and the tissue recovers fully if warmed. However, when plants are exposed to freezing temperatures for an extended period, the growth of extracellular ice crystals leads to physical destruction of membranes and excessive dehydration.
12.6.    Salinity Stress :
Imbalances in the mineral content of soils can affect plant fitness either indirectly, by affecting plant nutritional status or water uptake, or directly, through toxic effects on plant cells. Several anomalies associated with the elemental composition of soils can result in plant stress, including high concentrations of salts (e.g., Na+ and Cl-) and toxic ions (e.g., As and Cd), and low concentrations of essential mineral nutrients, such as Ca2+, Mg2+, N, and P. The term salinity is used to describe excessive accumulation of salt in the soil solution. Salinity stress has two components: nonspecific osmotic stress that causes water deficits, and specific ion effects resulting from the accumulation of toxic ions, which disturb nutrient acquisition and result in cytotoxicity. Salt-tolerant plants genetically adapted to salinity are termed halophytes, while less salt-tolerant plants that are not adapted to salinity are termed glycophytes. High Na+ concentrations that occur in soils (soils in which Na+ occupies $\geq$10% of the cation exchange capacity) not only injure plants but also degrade the soil structure, decreasing porosity and water permeability.
soils (soils in which Na+ occupies $\geq$10% of the cation exchange capacity) not only injure plants but also degrade the soil structure, decreasing porosity and water permeability.
High cytosolic Na+ and Cl- denature proteins and destabilize membranes
The most widespread example of a specific ion effect is the cytotoxic accumulation of Na+ and Cl- ions under saline conditions. Under non-saline conditions, the cytosol of higher plant cells contains about 100 mM K+ and less than 10 mM Na+, an ionic environment in which enzymes are optimally functional. In saline environments, cytosolic Na+ and Cl- increase to more than 100 mM, and these ions become cytotoxic. High concentrations of salt cause protein denaturation and membrane destabilization by reducing the hydration of these macromolecules. However, Na+ is a more potent denaturant than K+. Excess Na+ may also restrict the availability of Ca2+ in the cytosol.
12.7.    Water Imbalancing stress :
Salinity reduces the ability of plants to take up water, and this quickly causes reductions in growth rate, along with a suite of metabolic changes identical to those caused by water stress. The first phase of growth reduction is quickly apparent and is due to the salt outside the roots. It is essentially water stress or osmotic phase, for which there is surprisingly little genotypic variation.  The turgor of young root cells is normally around 0.6 MPa. Turgor pressures in the range of 0.5–0.7 have been measured with a pressure the variation possibly being associated with the developmental stage of the cells and the growing conditions of the plants. If cell turgor is only 0.4 MPa, which is likely for older cells in roots then a one-step transfer to a solution of 100 mM NaCl (0.5 MPa) will plasmolyse cells.
Plasmolysis starts when the osmotic pressure of the solution is increased above that of the cells, causing the protoplast to shrink, and the plasma membrane separates from the wall. Large gaps created between the plasma membrane and the wall may fill with solution and allow an artefactual apoplastic pathway for salts to move radially across the root. During this time, the plasma membrane is stretched into strands that remain tethered to the wall at particular sites. Plasma membranes and plasmodesmata can be repaired, but in the meantime, an unregulated flux of solutes in or out of the protoplast occurs. It is likely that signal transduction pathways that arise from calcium or proton fluxes across the plasma membrane may not be functioning normally.
12.8.    Developmental and physiological mechanisms against Abiotic stress :
Plants can modify their life cycles to avoid abiotic stress.
Plants can adapt to extreme environmental conditions is through modification of their life cycles. For example, annual desert plants have short life cycles: they complete them during the periods when water is available, and are dormant (as seeds) during dry periods. Deciduous trees of the temperate zone shed their leaves before the winter so that sensitive leaf tissue is not damaged by cold temperatures.
12.8.1.    Phenotypic changes in leaf structure and behaviour are important to stress responses :
Because of their roles in photosynthesis, leaves (or their equivalent) are crucial to the survival of a plant. Plants have thus evolved various mechanisms that enable them to avoid or mitigate the effects of abiotic extremes to leaves. Such mechanisms include changes in leaf area, leaf orientation, trichomes, and the cuticle. Turgor reduction is the earliest significant biophysical effect of water deficit. As a result, turgor-dependent processes such as leaf expansion and root elongation are the most sensitive to water deficits. Inhibition of cell expansion results in a slowing of leaf expansion early in the development of water deficits. The resulting smaller leaf area transpires less water, Altering leaf shape is another way that plants can reduce leaf area. For protection against overheating during water deficit, the leaves of some plants may orient themselves away from the sun. Leaf orientation may also change in response to low oxygen availability.
12.8.2.    Plants can regulate stomatal aperture in response to dehydration stress :
The ability to control stomatal aperture allows plants to respond quickly to a changing environment, for example, to avoid excessive water loss or limit uptake of liquid or gaseous pollutants through stomata. Stomatal opening and closing are modulated by uptake and loss of water in guard cells, which changes their turgor pressure. Although guard cells can lose turgor as a result of a direct loss of water by evaporation to the atmosphere.
12.8.3.    Plants adjust osmotically to drying soil by accumulating solutes :
Osmotic adjustment is the capacity of plant cells to accumulate solutes and use them to lower Ψw during periods of osmotic stress. The adjustment involves a net increase in solute content per cell that is independent of the volume changes that result from loss of water. The decrease in ΨS (= osmotic potential) is typically limited to about 0.2 to 0.8 MPa, except in plants adapted to extremely dry conditions.
12.8.4.    Heat shock proteins can be induced by different environmental conditions :
Under environmental extremes, protein structure is sensitive to disruption. Plants have several mechanisms to limit or avoid such problems, including osmotic adjustment for maintenance of hydration and chaperone proteins that physically interact with other proteins to facilitate protein folding, reduce misfolding and aggregation, and stabilize protein tertiary structure. In response to sudden 5 to 10°C increases in temperature, plants produce a unique set of chaperone proteins referred to as heat shock proteins (HSPs). Cells that have been induced to synthesize HSPs show improved thermal tolerance and can tolerate subsequent exposure to temperatures that otherwise would be lethal.
12.8.5.    Stress physiology due to Biotic Factors :
Biotic factors refer to the living organisms, both macro- and micro-organisms, including the various ways in which they affect plant growth and development. These organisms are the living components of the environment which influence the manifestation of the genetic factor on phenotypic expression. Macro-organisms refer to animals such as humans and other mammals, birds, insects, arachnids, molluscs, and plants while microorganisms include fungi, bacteria, virus and nematodes.
The effects of these living factors on plant expression may be advantageous or disadvantageous, depending on how they interact with the plant. These interactions include mutualism, herbivory, parasitism. Insects and molluscs feed on plant parts. Herbivores with significant deleterious effects on crop growth and yield are called a pest. Damage caused by these biotic factors is varied such as the death of the entire plant or organs, reduced root, stem, leaf or inflorescence mass, total defoliation, bores and holes on plant parts, and other marks of feeding. Insects are associated with plant diseases that may reduce crop yield or kill the entire plant. The aphids, mealy bugs, and scale insects are associated with the sooty mould. Parasitism is an interaction between two organisms in which one organism, called a parasite, is a benefit but causes harm to another, called host. The parasite steals its food from the host. Microorganisms such as fungi, bacteria and virus injure crops by causing diseases and are called pathogens. Examples of parasitic plants are the dodder, mistletoe, Rafflesia, and some orchids.
When the cause of a plant health problem is not readily diagnosed, it's important to take a systematic approach and carefully consider site conditions, weather conditions, care of the plant, and the known biotic disease agents of that plant. The first important step is to determine the identity of the plant and its requirements for healthy growth. Biotic disease problems are more limited to a certain species. The fungi that cause tomato leaf blight do not cause damage on sweet corn, for example.

Fig.: Cellular and molecular interactions between pathogen and plant cells during recognition and responses to infection. Host resistance depends on the induction of one or more response pathways. Pathogen virulence depends on a lack of host-recognised avirulence gene products.
12.8.6.    Developmental and physiological mechanisms against Biotic stress :
Biotic diseases can spread throughout one plant and also may spread to neighbouring plants of the same species. Wind-blown rain is a common way for disease agents to spread from plant to plant. Biotic diseases sometimes show physical evidence (signs) of the pathogen, such as fungal growth, bacterial ooze, or nematode cysts, or the presence of mites or insects. Despite these observations, ABA can also have a positive effect on pathogen defence systems. ABA-induced stomatal closure was a defensive strategy of plants to prevent microbial invasion through open stomata, a process that also required intact SA signalling. Furthermore, ABA is necessary for β-aminobutyric acid (BABA)-induced callose deposition during defence against fungal pathogens, although interestingly ABA can block bacterial-induced callose production
ABA is now considered a global regulator of stress responses that can dominantly suppress pathogen defence pathways, thus controlling the switch in priority between the response to biotic or abiotic stress and allowing plants to respond to the most severe threat. To prevent damage from herbivores, plants have evolved anti-herbivory defences including chemical and physical strategies. Various plants produce toxic compounds such as nicotine, morphine, caffeine, cyanogenic substances like atropine; mustard oils, terpenoids and phenylpropanes. Some plants produce high amounts of tannins and resins in leaves that prevent the digestion of food. Others have spines, as in cacti, and stinging hairs.
In a summarized manner, Biotic and abiotic stress signal transduction results from a complex arrangement of interacting factors. Certain gene products are crucial to both biotic and abiotic stress signalling, and may, therefore, control the specificity of the response to multiple stresses.