Environmental Stress Physiology of Plants and Crop Productivity

Environmental stress is one of the major limiting factors for agricultural productivity worldwide. Plants are closely associated with the environment where they grow and adapt to the varying conditions brought about by the huge number of environmental factors resulting in abiotic stress. Abiotic factors or stressors include high or low temperature, drought, flooding, salinity, mineral nutrient deficiency, radiation, gaseous pollutants, and heavy metals. High salinity, drought, cold, and heat are the major factors influencing crop productivity and yield. The negative impact of various abiotic stress factors is the alteration in the plant metabolism, growth, and development and, in severe cases, plant death. Abiotic stress has been becoming a specific concern in agriculture leading to unbearable economic loss to the breeders. Thus, understanding these stresses will help in achieving the long-term goal of crop improvement, therefore, minimizing the loss in crop yield to cope with increasing food requirements. With this chapter, an attempt has been made to present an overview of various environmental factors that are hostile to plant growth and development, thereby leading to great loss in crop yield.

During natural conditions, plants undergo a series of biogenic stressors that are devastating to plant growth. Among these, drought stress is the most common, which alters molecular and morphological parameters in plants and thus has detrimental effects due to environmental injury and physico-chemical disturbances. These have led to the development of technologies that increase the quality and sustainability of crops under deteriorating soil, declining natural resources, and environmental stress. Effective agronomic and genetic methods for crop protection provide best management practices to combat drought conditions. This chapter aims to contribute to the development of approaches for sustainable agricultural management practices suitable for crop production during drought stress.

Salinity is considered a crucial environmental factor that limits the production of the crop in many parts of the world with marginal agricultural soils. It causes a reduction in agricultural productivity globally and renders an estimated onethird of irrigated land of the world unsuitable for the production of crops. A high concentration of salt can kill all the crops and plants. Salinity can affect the yield and growth of most crops, as the higher rate of salinity can cause both hyperosmotic and hyper ionic effects in plants, leading to an increase in the production of activated oxygen species, membrane disorganization, and metabolic toxicity. Its effects on the growth and development of plants include osmotic stress, ion toxicity, mineral deficiencies, biochemical and physiological perturbations, and combinations of these stressors. Salinity reduces Ca2+ availability that in turn decreases the mobility and transport of Ca2+ to growing regions of the plant when dominated by Na+ salts and thereby affects the quality of both reproductive and vegetative organs. The horticultural crops are mostly glycophytes that evolved under the conditions of low salinity of the soil. Nutrient uptake is directly affected by salinity, such as Cl- reducing NO3- uptake or Na+ reducing K+ uptake. The performance of crops may be affected adversely by salinity-induced nutritional disorders. These disorders resulting from salinity may affect the availability of nutrients, transporter partitioning, and competitive uptake within the plant. This chapter will elucidate the deleterious effect of salinity on the growth and development of crop plants.

Rapid urbanization and land-use transition contribute to the rise in the thermal scale of cities as well as small towns and villages worldwide. The equilibrium between the incoming solar energy and the outgoing terrestrial energy regulates the temperature. Nevertheless, the temperature, as we know, varies from place to place, and it also affects the natural processes as well as surrounding flora and fauna. On the other hand, temperature beyond the physiologically optimal limit is called high temperature that adversely affects the growth and development of plants as it has significant impacts on both the vegetative and reproductive phases of the plant life cycle. The extremely high temperature is referred to as heat stress which is reported as one of the devastating abiotic stressors. In plants, heat stress triggers various morphophysiological changes in plants that affect their growth and economic outcomes via accelerating reactive oxygen species generation, reduced carbon assimilation, degradation and denaturation of proteins, lipid peroxidation of membranes, etc. Several conventional and modern strategies have been employed to resolve heat stress-induced damages in plants. Therefore, the present work is an outcome of extensive literature focused on the factors responsible for temperature variations’ patterns, morphophysiological responses of crops, and impacts on the economic yields of edible plants.

Soil acts as a source of nutrient elements, and the availability of nutrients is determined by soil properties. The different elements are grouped according to their biochemical behavior and physiological functions. The nutrients that are needed in moderately large amounts are called macronutrients. The plant macronutrients comprise nitrogen, potassium, phosphorus, calcium, sulfur, and magnesium. The micronutrients are required for plant growth in much smaller quantities than macronutrients. These micronutrients contain boron, copper, iron, manganese, molybdenum, and zinc. All of these nutrients are absorbed through the roots. Water will transfers the nutrients from soil to the roots of the plant. In this chapter, we will discuss the physiological role of essential and non-essential elements and the effects of some general environmental stressors, such as salinity, drought, and metals, on nutrient uptake by plants.

Nitric oxide (NO) is a small-sized, short-lived, highly diffusible, gaseous, and bioactive molecule that regulates various physiological and pathological processes in plants. It also plays a role in development, germination, flowering, senescence as well as response to abiotic stress in plants. In recent years, the role of NO in tolerance of abiotic stress posed by salinity, drought, heat, cold, UV-B, and heavy metals in plants has been identified and gained importance in the field of plant research. Salinity stress triggers the production of reactive oxygen species (ROS) that leads to oxidative stress in plants, resulting in cellular destruction via lipid peroxidation, enzyme inactivation, and DNA damage. To combat this stress and to minimize its harmful effects, certain plants activate various ROS-scavenging enzyme activities. The role of exogenous NO, which acts as an indicator in triggering ROS scavenging enzyme activities and regulates antioxidant activities in plants to alleviate the toxic effects of salt stress, has been well established. NO has been considered to play a key role in reducing the excessive production of ROS in cells, improving secondary defense mechanisms, alleviating osmotic damage, and maintaining cell permeability. Thus, understanding the mechanisms of action of NO that help in effectively utilizing the crop cultivation under saline conditions and support better growth of the plants is the need of an hour. Considering this, the present review focuses upon the recent knowledge of the crucial role of NO in providing tolerance to plants under salt stress.

Plants are exposed to different types of environmental stressors throughout the different developmental stages. Reactive oxygen species (ROS) are found to play key roles in the maintenance of normal plant growth and improving their ability of stress tolerance. ROS as a secondary messenger performs crucial cellular functions, including the proliferation of cells, apoptosis, and necrosis. Both the external environmental factors and intrinsic genetic programs regulate the morphogenesis of plants. ROS are also considered as by-products of the aerobic metabolism of the plant and are formed in certain cellular compartments like mitochondria, chloroplasts, and peroxisomes. Plants form a huge number of ROS species under unfavorable circumstances that are involved in the regulation of different processes, including programmed cell death, pathogen defense, and stomatal behavior. These reactions often exert irreversible or profound effects on the development of organs and tissues, leading to abnormal death or plant growth. Several molecular approaches to understand the signaling and metabolism of ROS have opened novel avenues in comprehending its key role in abiotic stress. Plants possess their own enzymatic and non-enzymatic antioxidant defense system to encounter ROS. The interconnecting activities of these defensive antioxidants reduce oxidative load and regulate the detoxification of ROS in plants. This book chapter will highlight the importance of ROS metabolism and the role of the antioxidant defense mechanism of plants in combating the deleterious effect of oxidative stress under stressful conditions.

Melatonin (N-acetyl-5-methoxytryptamine) is a small (232 daltons), nontoxic, indole molecule first isolated from the pineal gland of cows and later on found in different tissues of plants and bacteria. Melatonin acts as a signaling molecule/messenger, which plays an important role in coping with various biotic and abiotic stress conditions. Biosynthesis of melatonin involves two key cellular organelles viz. mitochondria and chloroplast. The endogenously produced melatonin serves as an antioxidant signaling molecule during the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). The exogenous melatonin also functions in the same way during critical conditions by repairing mitochondria and deal with various stresses. The plants regulate the production of melatonin depending upon the conditions and regulate salt, drought, cold, heat, oxidative, and heavy metalsrelated stresses. Besides that, melatonin acts as a plant hormone and regulates numerous functions in plants, including growth, development, photoperiod, clearing oxygen species, rhizogenesis, photosynthesis, and enhances antioxidase activity. It acts as a multi regulatory molecule by regulating gene expressions as well as by cross-talks with phytohormones (auxin, cytokinin, salicylic acid, and abscisic acid) involved in plant growth and development. Therefore, understanding the mechanism of action of melatonin as a signaling molecule may serve as a novel strategy to combat various stresses in plants and animals. An attempt has been made in this chapter to discuss the important role of melatonin in modulating oxidative stress in plants during stressful conditions.

Phenylpropanoids are a class of secondary metabolites in plants that are derived from aromatic amino acids like tyrosine and phenylalanine. It mainly includes stilbenes, monolignols, coumarins, flavonoids, and phenolic acids. These are considered to play a crucial role in protecting the plants against both abiotic and biotic stress by quenching the generation of reactive oxygen species (ROS) through a wide range of mechanisms. Phenylpropanoids are found widely in the plant kingdom and serve an essential role in the development of plant by acting as an important cell wall component, floral pigments to mediate the interactions of plant–pollinator, antibiotics (phytoalexins) against pathogens and herbivores, and protectants against UV radiation and high light. Several phenylpropanoids are helpful for the plant to fight against microbial diseases and thereby show broad-spectrum antimicrobial activity. The biosynthetic pathway of phenylpropanoid is mostly activated under abiotic stress conditions including salinity, heavy metal, ultraviolet radiations, high/low temperature, and drought and results in the accumulation of different phenolic compounds that are helpful in scavenging the deleterious effect of ROS. A series of enzymes involved in the activation of the biosynthetic pathway are reductases, transferases, lyases, oxygenases, and ligases. Among these, many are encoded by superfamilies of genes, like NADPH-dependent reductase gene family, the 2-oxoglutarate dependent dioxygenase (2-ODD) gene family, the cytochrome P450 membrane-bound monooxygenase (P450) gene family, and the type III polyketide synthase (PKS III) gene family. Thus, keeping in view the importance of phenylpropanoids in plant defense, the present book chapter is focused on unraveling the role of these essential compounds in ameliorating the stressful conditions in plants.

Plants are frequently exposed to different types of stressful environmental conditions, which have adverse effects on their growth, development, and productivity. These conditions, such as salinity, drought, floods, chilling, freezing, UV exposure, pollution, nutritional deficiencies, metal toxicity, etc., are collectively known as abiotic stressors and hinder plants from fully expressing their genetic potential. With advancements in scientific fields such as genetics and molecular biology, it has become easier to understand that under abiotic stress, a myriad of responses are triggered in plants. These changes include alterations in gene expression to changes in cell metabolism to avoid or tolerate the stress. The intensity of these plant responses depends on affected tissue, age of the plant, type of stress posed, duration and severity of stress, etc. It has been observed that plant growth regulators such as auxins, abscisic acid, cytokinins, ethylene, gibberellins, jasmonic acid, brassinosteroids, salicylic acid, polyamines, strigolactones, etc., which influence the growth and differentiation in plants, also have very important roles in regulating the stress tolerance in plants. This chapter is a comprehensive account of literature based on the role of different plant growth regulators in the regulation of tolerance of plants towards abiotic stressors. The contents of this chapter include a brief discussion about different types of abiotic stressors, their effects on plants, and responses developed in plants against them. There is also a detailed discussion about plant growth regulators, their role in the normal functioning of plants, followed by their contribution and underlying mechanisms in building abiotic stress tolerance in plants.

Abiotic stressors such as drought, salination, flooding, cold, heat, ultraviolet radiation, heavy metals, etc., are the paramount cause that reduce crop yield and weaken universal food security as they strongly affect plant growth, physiology, and metabolism. Plants frequently face a large number of environmental stressors and usually generate common responses to deal with these unfavorable conditions. However, crop improvement against abiotic stressors is one of the urgent priorities that need undivided attention, while a huge increase in demand for various plant-derived products will rise in the near future owing to the rising human population. As conventional methods for crop enhancement have limitations, therefore an epoch of omic research has shot up with new and encouraging perspectives in breeding to improve the crops against abiotic stress. In this light, the genomic, proteomic, and metabolomic approaches are emerging as powerful tools for the identification and description of cellular networks through which stress perception, signal transduction, and defensive responses are exhibited. Further advances in omic techniques have permitted a comprehensive investigation of crop genomes and have magnified the perception of convolution of the mechanisms controlling abiotic stressor tolerance and the adaptation to mitigate them. This chapter will give an overview of genomics, proteomics, and metabolic approaches and their usage to enhance the possibility of producing abiotic stressor tolerant crops.

Plants can identify and cope up with biotic and abiotic stressors, thus resulting in a reduction in agricultural production significantly. Currently, the primary goal of plant breeders is to develop the ability to tolerate multiple stress conditions without lowering the productivity of the crop. However, numerous attempts have failed to release these plants due to persistent divergence between growth and resistance to stressors. Such strategies are not appropriate to effectively enhance characteristics and ensure potential environmental impact. Genome editing approaches and RNA interference (RNAi) technique has been used to develop plants resistant to different environmental changes. Newly developed approaches such as CRISPR are used to grow new varieties that have external stressors tolerance and reduce a vital yield loss. This chapter addresses the use of CRISPR/CAS techniques to improve the stress tolerating ability of the plants.