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Uploaded byNistarini College, Purulia (W.B) India
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Plant Health in response to Plant stress Physiology

Stress is unavoidable and every living organisms have the urge to live with sustainability.The plants face a number of challenges due to different emergent conditions. The abiotic factors of the ecosystem are ever changing and these factors very often possess challenges in the life and sustainability. The teat, sunlight, nutrients, U-V rays, ozone , humidity and many other factors may possess stress upon the plants. The plants adapt with these stresses by a number of adaptive features morphological, physiologically and biochemically by the courtesy of their own genetic make-up. This presentation gives a bird's eye view of stress and the mechanisms to adapt in the changing circumstances.

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Dr. Nandadulal Sannigrahi, Associate Professor, Department of Botany, Nistarini college, Purulia D.B. Road, Purulia, INDIA (W.B)
 The “environment” connotes all external conditions affecting the life and development of an organism but very often, the alteration of any factors can cause extra load for the normal physiological process upon its stakeholders.  Plants are subjected to a wide range of environmental stresses which reduces and limits the productivity of agricultural crops.  Stress is unavoidable irrespective to the organisms,  Two types of environmental stresses are encountered to plants which can be categorized as (1) Abiotic stress and (2) Biotic stress.  The abiotic stress causes the loss of major crop plants worldwide and includes radiation, salinity, floods, drought, extremes in temperature, heavy metals, etc.  On the other hand, attacks by various pathogens such as fungi, bacteria, Oomycetes, nematodes and herbivores are included in biotic stresses. As plants are sessile in nature, they have no choice to escape from these environmental cues.  Plants have developed various mechanisms in order to overcome these threats of biotic and abiotic stresses.  They sense the external stress environment, get stimulated and then generate appropriate cellular responses.
 They do this by stimuli received from the sensors located on the cell surface or cytoplasm and transferred to the transcriptional machinery situated in the nucleus, with the help of various signal transduction pathways.  This leads to differential transcriptional changes making the plant tolerant against the stress.  The signaling pathways act as a connecting link and play an important role between sensing the stress environment and generating an appropriate biochemical and physiological response.  Some stresses to the plants injured them as such that plants exhibit several metabolic dysfunctions .  The plants can be recovered from injuries if the stress is mild or of short term as the effect is temporary while as severe stresses leads to death of crop plants by preventing flowering, seed formation and induce senescence. Such plants will be considered to be stress susceptible. However several plants like desert plants (Ephemerals) can escape the stress altogether.
 Plants respond in many ways to abiotic stresses, from gene expression to physiology, from plant architecture to primary, and secondary metabolism. These complex changes allow plants to tolerate and/or adapt to adverse conditions.  The complexity of plant response can be further influenced by the duration and intensity of stress, the plant genotype, the combination of different stresses, the exposed tissue and cell type, and the developmental stage at which plants perceive the stress.  It is therefore important to understand more about how plants perceive stress conditions and how they respond and adapt (both in natural and anthropogenic environments).  The different influences and the occurrences of the different stress factors are very important one to address the different kind of adverse changes found to occur in the plants .
 The stress is an unavoidable issue irrespective of the nature of organisms starting from unicellular prokaryotes to multicellular eukaryotes including plants and animals including human life.  As far as stress management is concerned, the different types of stress management strategies are adopted by plants to address them in the quest of sustainability.  The plant uses multiple methods of biochemical and genetically methods garnished with molecular pathways to address and acclimatize them. The min stress induced causes of plants are as followed:  Temperature stress  Drought stress  Salt and osmotic stress  Ozone, UV and light stress  Nutrient stress  Heavy metal stress  Hypoxia stress
 Temperature stress is a significant factor influencing the sustainable growth and production of crop plants. Temperature stress includes both the high temperature (HT) stress, and low temperature (LT) stress (cold stress or chilling stress and freezing stress).  High temperature stress is the rise in temperature beyond a critical threshold for a period of time sufficient to cause irreversible damage to plant growth and development (Wahid, 2007).  However, cold stress (0–15 °C) is caused by the temperature cool enough to produce injury without forming ice crystals in plant tissues, while freezing stress (<0 °C) results in the formation of ice crystals in plant tissues (Hasanuzzaman et al., 2013).  Loss of fluidity of membranes and leakage of solutes are the distinct effects of cold stress. Plants exposed to cold stress show poor growth and germination and reduced crop yield (Welti et al., 2002; Suzuki et al., 2008). However, susceptibility to cold stress varies among the plants as well as within the species and plants with higher tolerance levels show less membrane injury than the sensitive species (Maali Amiri et al., 2010; Heidarvand et al., 2011).
 Whereas, irrespective of tolerance and sensitivity of genotypes to cold stress, NMs such as TiO2 possess the capability to alleviate the harmful effects of cold stress by lessening the membrane damages and electrolyte leakage but sensitive genotype accumulate more TiO2 than the tolerant one (Mohammadi et al., 2013) this is because of thinner epidermis and large and more opened stoma in sensitive genotypes than that of tolerant (Giacomo et al., 2010).  Plants possess a number of adaptive, avoidance, or acclimation mechanisms to cope with HT situations.  In addition, major tolerance mechanisms that employ ion transporters, proteins, osmoprotectants, antioxidants, and other factors involved in signaling cascades and transcriptional control are activated to offset stress- induced biochemical and physiological alterations.  Plant survival under HT stress depends on the ability to perceive the HT stimulus, generate and transmit the signal, and initiate appropriate physiological and biochemical changes. HT-induced gene expression and metabolite synthesis also substantially improve tolerance.
 A prolonged period of abnormally low rainfall leading to severe and temporary shortage of water in the soil with wide degree of impact to be treated as drought,  Drought stress, being the inevitable factor that exists in various environments without recognizing borders and no clear warning thereby hampering plant biomass production, quality, and energy.  It is the key important environmental stress that occurs due to temperature dynamics, light intensity, and low rainfall.  Despite this, its cumulative, not obvious impact and multidimensional nature severely affects the plant morphological, physiological, biochemical and molecular attributes with adverse impact on photosynthetic capacity. Coping with water scarcity, plants evolve various complex resistance and adaptation mechanisms including physiological and biochemical responses, which differ with species level.  The sophisticated adaptation mechanisms and regularity network that improves the water stress tolerance and adaptation in plants are some of the practices exercise by the plants to overcome this stress issue.
 Growth pattern and structural dynamics, reduction in transpiration loss through altering stomata conductance and distribution, leaf rolling, root to shoot ratio dynamics, root length increment, accumulation of compatible solutes, enhancement in transpiration efficiency, osmotic and hormonal regulation, and delayed senescence are the strategies that are adopted by plants under water deficit condition.  Approaches for drought stress alleviations are breeding strategies, molecular and genomics perspectives with special emphasis on the omics technology alteration i.e., metabolomics, proteomics, genomics, transcriptomics, glyomics and phenomics that improve the stress tolerance in plants.  For drought stress induction, seed priming, growth hormones, osmoprotectants, silicon (Si), selenium (Se) and potassium application are worth using under drought stress conditions in plant.  In addition, drought adaptation through microbes, hydrogel, nanoparticles applications and metabolic engineering techniques that regulate the antioxidant enzymes activity for adaptation to drought stress in plants, enhancing plant tolerance through maintenance in cell homeostasis and ameliorates the adverse effects of water stress are of great potential in agriculture.
MORPHOLOGICAL CHANGE PHYSIOLOGICAL CHANGES BIOCHEMICAL CHANGES Dwarf plants Closure stomata RUBISCO efficiency decreased Early maturity Photosynthesis stop Photochemical efficient decreases Leaf area reduction Oxidative stress increases ROS production Leaf rolling Cell wall integrity changes Oxidative damage Stomata position Reduction in leaf water potential Antioxidant defense Leaf color, hairs, waxiness Decrease in stomatal conductance ABA generation Leaf orientation High proline accumulation Chlorophyll contents decline High to root : shoot Translocation ceases Polyamine generation Leaf longevity reduction Assimilation stops ABA accumulation Leaf small size Internal temperature increases Carbohydrate production
 Soil salinization inhibits plant growth and seriously restricts food security and agricultural development.  Excessive salt can cause ionic stress, osmotic stress, and ultimately oxidative stress in plants.  Plants exclude excess salt from their cells to help maintain ionic homeostasis and stimulate phytohormone signaling pathways, thereby balancing growth and stress tolerance to enhance their survival.  Plants are classified as halophytic or glycophytic according to their salt tolerance. Most crops are glycophytic and sensitive to high salt concentrations in the soil.  Improper use of fertilizers, excess irrigation, and industrial pollution are the primary causes of widespread soil salinity , which poses a serious threat to agricultural productivity and food security for both humans and livestock.  High concentrations of Na+ and Cl− in the soil result in both osmotic and ionic stress, which decrease plants’ capacity to take up water and nutrients.
 Osmoregulation is important for plants due to the following reasons-  Enables plant to grow and develop along with to perform primary metabolic activities even in  The habitat is dry, hot and desert like,  Rainfall is scarce at certain times,  Adequate water is not available during photosynthesis,  Hydration of the cell contents,  Habitat is completely aquatic,  Salinity of the habitat is high,  Sandy/rocky soil does not hold much water,  It regulates the balance of water and salt uptake to maintain homeostasis,  To overcome thee different stress condition emerges due to high degree of salinity to maintain dynamic equilibrium,
 According to osmoregulatory activities, the plants are broadly classified under four categories as follows:  Halophytes-Mangroves  Hydrophytes- Water lily  Xerophytes- Cactus  Mesophytes – Mango.  The stress arises due to osmo-regulation issues may produce multiple effects on the plants irrespective of morphological, biochemical and qualitative and quantitative effects of crop productivity by the following methods-  Inhibited growth due to root structures and lack of the chlorophyll like pigments synthesis,  Stomata closure, increase ROS activity, increase antioxidant activity,  Pollen sterility and reduction in crop yeild.
 Ozone and UV-B stress are significant environmental factors that negatively affect plant health, particularly by inducing oxidative stress, reducing photosynthesis, and suppressing crop yields.  While stratospheric "good" ozone shields the Earth, ground-level (troposphere) ozone is a "bad" pollutant that enters plants through stomata, leading to chlorosis, necrosis, and premature senescence.  Ozone is deposited from the atmosphere onto plant canopies by diffusion and enters the leaf through stomata. Environmental, biological, and cultural (e.g., irrigation) factors that promote stomata opening in crease the risk of O3 injury to plants .  Ozone causes negative effects on a number of plant processes, including photosynthesis, water use efficiency, rate of senescence, dry matter production, flowering, pollen tube extension, and yield.  Mechanism of Damage:  Upon entering the leaf, is converted into reactive oxygen species (ROS) in the apoplast , damaging chloroplasts and blocking photosynthetic electron transport.
 Physiological Effects:  Disrupts the antioxidant defense system, leading to lipid per oxidation and ion leakage. It causes stomata closure, which reduces fixation  Impact on Yield:  Chronic exposure causes growth reduction, reduced root biomass, and premature leaf senescence. This is particularly damaging to crops like soybeans, wheat, and rice, causing significant yield losses.  Defense Mechanisms: Plants defend themselves by activating enzymatic (SOD, CAT) and non-enzymatic (ascorbate, glutathione) antioxidant defenses.  Mitigation:  Strategies to reduce impact include using chemical protectants like ethylenediurea (EDU) to boost antioxidant activity, managing nutrient levels (e.g., proper Nitrogen/Phosphorus ratios), and selecting resistant crop varieties.
 Nutrients are obtained from the soil through plant roots, but there are many factors that might affect nutrient acquisition, including soil texture and moisture, pH, temperature and organic matter.  Both nutrient limitation (or low bioavailability) and nutrient excess might cause stress to the plant and negatively affect physiological and metabolic processes essential for plant growth, development, reproduction and yield. In this scenario, plants must actively respond to maintain nutrient levels within the optimal range.  Plants adapt to nutrient stress, i.e., a deficiency or excess of nutrients, by altering numerous physiological, biochemical, and molecular processes.  Plant nutrients include macronutrients, which are required in relatively large amounts, and micronutrients, which are required in much smaller amounts. The macronutrients that plants need include nitrogen (N), phosphorus (P), and potassium (K).  N serves as a fundamental constituent in key molecules, like amino acids, nucleic acids, and proteins. The efficiency of N acquisition (or nitrogen use efficiency, NUE) is determined by the activity of Ntransporters involved in N uptake and by the root architecture.
 Regarding P, it is a component in key molecules, such as nucleic acids, phospholipids, and adenosine triphosphate (ATP).  It also plays vital roles in enzymatic reactions and signal transduction cascades, particularly protein phosphorylation. P is absorbed through the roots in the form of inorganic phosphate (Pi), and although the overall content of P in the soil is generally high, its low bioavailability represents a limiting factor for plant growth in most agricultural soils.  Furthermore, although considerable progress has been made in our understanding of the mechanisms underlying the adaptation of plants to Pi limiting conditions during the past few years, less is known about adaptive mechanisms to Pi excess conditions.  As for K, it is involved in different cellular processes controlling photosynthesis and chloroplast function, stomata-opening control, and turgor pressure regulation.  Despite the abundance of K in most soils, only a small portion can be readily absorbed by plants. Detection of K deficiency triggers the activation of K channels and transporters in the plant, which is sometimes accompanied by the induction of other cellular processes that are also regulated by Ca2+, hormones, and ROS.
 Anthropogenic perturbations of biosphere manifested in a broad array of global phenomena including accelerated rate of industrialization, intensive agriculture, and extensive mining accompanied by burgeoning population and rapid urbanization have not only wreaked the havoc on the availability of natural resources but also caused widespread and grave contamination of essential components of life on the plane  Human-induced disturbance of natural biogeochemical cycles, accentuated accumulation of heavy metals (HMs) is a problem of paramount importance for ecological, nutritional, and environmental reasons  Unprecedented bioaccumulation and biomagnifications of heavy metals (HMs) in the environment have become a dilemma for all living organisms including plants.  HMs at toxic levels have the capability to interact with several vital cellular biomolecules such as nuclear proteins and DNA, leading to excessive augmentation of reactive oxygen species (ROS).  This would inflict serious morphological, metabolic, and physiological anomalies in plants ranging from chlorosis of shoot to lipid peroxidation and protein degradation. In response, plants are equipped with a repertoire of mechanisms to counteract heavy metal (HM) toxicity.
 Ms belong to group of non-biodegradable, persistent inorganic chemical constituents with the atomic mass over 20 and the density higher than 5 g·cm−3 that have cytotoxic, genotoxic, and mutagenic effects on humans or animals and plants through influencing and tainting food chains, soil, irrigation or potable water, aquifers, and surrounding atmosphere.  There are two kinds of metals found in soils, which are referred to as essential micronutrients for normal plant growth (Fe, Mn, Zn, Cu, Mg, Mo, and Ni) and nonessential elements with unknown biological and physiological function (Cd, Sb, Cr, Pb, As, Co, Ag, Se, and Hg).  The essential elements play a pivotal role in the structure of enzymes and proteins. Plants require them in tiny quantities for their growth, metabolism, and development; however, the concentration of both essential and nonessential metals is one single important factor in the growing process of plants so that their presence in excess can lead to the reduction and inhibition of growth in plants.  HMs at toxic levels hamper normal plant functioning and act as an impediment to metabolic processes.
 Disturbance or displacement of building blocks of protein structure, which arises from the formation of bonds between HMs and Sulf-hydryl groups ,  hindering functional groups of important cellular molecules , superseding or disrupting functionality of essential metals in biomolecules such as pigments or enzymes , and adversely affecting the integrity of the cytoplasmic membrane , resulting in the repression of vital events in plants such as photosynthesis, respiration, and enzymatic activities.  On the other hand, elevated levels of HMs are associated with the increased generation of reactive oxygen species (ROS), such as superoxide free radicals (O2 •−), hydroxyl free radicals (OH•−), or non-free radical species (molecular forms) such as singlet oxygen (O2) and hydrogen peroxide (H2O2) as well as cytotoxic compounds like methylglyoxal (MG), which can cause oxidative stress via disturbing the equilibrium between pro-oxidant and antioxidant homeostasis within the plant cells,  This condition implicates the causation of multiple deteriorative disorders such as, oxidation of protein and lipids, ion leakage, oxidative DNA attack, red-ox imbalance, and denature of cell structure and membrane, ultimately resulting in the activation of programmed cell death (PCD) pathways.
 Plants employ various inherent and extrinsic defense strategies for tolerance or detoxification whenever confronted with the stressful condition caused by the high concentrations of HMs.  As a first step towards dealing with metal intoxication, plants adopt avoidance strategy to preclude the onset of stress via restricting metal uptake from soil or excluding it, preventing metal entry into plant root,  This can be achieved by some mechanisms such as immobilization of metals by mycorrhizal association, metal sequestration, or complexation by exuding organic compounds from root ,  One of the mechanisms adopted by plants to detoxify HMs is the production of short-chain thiol-rich repetitions of peptides of low- molecular weight synthesized from sulfur-rich glutathione (GSH) by the enzyme phytochelatin synthase (PCS) with the general structure of (γ- glutamyl-cysteinyl) n-glycine (n = 2 to 11) that have a high affinity to bind to HMs when they are at toxic levels, by metal homeostasis and detoxification,
 Metallothionins, small cysteine-rich, low-molecular-weight cytoplasmic metal-binding proteins or polypeptides that are found in a wide variety of eukaryotic organisms including fungi, invertebrates, mammals, and plants and plants in particular for Cd detoxification,  Proline, is a proteinogenic five-carbon α-amino acid that acts as a compatible and metabolic osmolyte, a constituent of cell wall, free radical scavenger, antioxidant, and macromolecules stabilizer that promoting embryo/seed evolvement, extending stem length as well as moving plants from vegetative growth to reproductive stage,  Symbiotic mycorrhizal fungi such as AM form a mutualistic symbiosis with roots of most vascular plant species under different climatic conditions in which they are beneficiary of photosynthetic assimilations provided by plants and in return they improve the mineral nutrition status of plants and can also enhance their tolerance towards some stresses and pollutants, adopted by mycorrhizal fungi to cancel out impacts of HM stress on plants include acting as a barrier by depositing metals within cortical cells,
 Plants are aerobic organisms that require oxygen for their respiration. Hypoxia arises due to the insufficient availability of oxygen, and is sensed by plants, which adapt their growth and metabolism accordingly.  Plant hypoxia can occur as a result of excessive rain and soil water logging, thus constraining plant growth.  In order to keep the level of oxygen under control, cells sense the oxygen levels and react to insufficient oxygen (hypoxia) by adopting survival strategies ranging from gene regulation to morphological adaptive responses,  Hypoxia occurs when the oxygen level limits aerobic respiration (usually between 1% and 5%), while anoxia takes place when oxygen is absent in the environment . It is also important to highlight the difference between acute and chronic hypoxia,  The term acute hypoxia is used when the drop in oxygen availability is transitory in plants, which can be due to adverse environmental conditions such as flooding events or when unusual, transient increases in oxygen consumption occur in a plant tissue.
 Acute hypoxia is perceived by the plants as stressful. On the other hand, chronic hypoxia is a constitutive and, usually, non-stressful condition where the oxygen level is maintained low only in a given group of cells and not in the entire plant.  In other words, chronic hypoxia can be a physiological condition in specific plant tissues.  In plants, proteins belonging to group VII of the Ethylene Response Factors (ERF-VIIs) are destabilized by oxygen and become stable only under hypoxia, with a mechanism conceptually similar to the one that regulating HIF-1. ERF-VIIs are transcription factors characterized by a Cys residue at their N-terminal,  The plants always try to adapt in the changing circumstances irrespective to oxygen deficiency by altering the biochemical pathways to ensure sustainability in the long run.
 ACKNOWLEDGEMENTS: a. Google for images, b. Different websites for enriching the course content, c. Science Direct pages, d. Plant Pathology- Dr. B. P. Pandey, e. Plant diseases, insects and pests- NCBI-NH, f. Topics in Mycology and Pathology-L.N. Nair g. www.nature.com h. www.spruce.com i. www.aphis.usda.gov j. Puregreensaz.com k. www.fao.org l. Many more websites for references.  This presentation has been designed to address the academic fraternity without any financial interest. This is absolutely free to use . The author does not claim any kind of financial benefits from this content.