**1. Introduction**

The salinization of soil has emerged as one of the most significant environmental and socioeconomic problems on a global scale, and it is anticipated that this problem will become much more severe as a result of forecasted changes in the climate. It affects food security, water availability, and human health. Salinization of soil occurs when salt accumulates in the soil, making it unsuitable for crop growth. This can be caused by the accumulation of naturally occurring salts or by the introduction of salt-laden irrigation water. Salinization can also be caused by inadequate drainage, where water accumulates in the soil and dissolves and accumulates salts from the soil. Salinization can reduce crop yields and cause environmental damage such as increasing the salinity of nearby rivers and streams. To prevent salinization, farmers can use soil amendments to reduce the salt content of the soil and use irrigation practices that avoid the accumulation of salt in the soil. This can be caused by over-irrigation, poor drainage, and high levels of evaporation. The economic effects of salinization of soil can be seen in the form of reduced crop yields, increased. It is a process where the salt content in soil increases, which can endanger plant life and lead to soil erosion. Salinization is a result of increased precipitation, flooding, or irrigation that saturates the soil with salt. Salinization can also be the result of the chemical precipitation of salt from the atmosphere [1]. High levels of salt can cause soil structure degradation. It can lead to the formation of a hardpan layer, which can make it difficult for roots to penetrate the soil. This can further reduce the crop yields significantly. Soil salinity

can have a significant adverse effect on crop yield. High levels of salinity can reduce crop yields by reducing germination rates, reducing plant growth, and increasing the susceptibility of plants to disease. Salinity can also reduce the availability of essential nutrients, resulting in stunted growth and reduced yields. Additionally, high salinity levels can result in increased water stress, which can further reduce crop yields. Recent estimates show that salt affects about 1125 million hectares of land around the world and that every year, 1.5 million hectares of land cannot be used for farming because the soil is too salty [2, 3]. It is hard to figure out how much agricultural production is lost, but it is thought that between 25% and 50% of all irrigated land is affected by salt accumulation, leading to decreased yields and the abandonment of agricultural land. It is estimated that up to 1 billion people in the world are affected by salinization and that the problem is growing. The World Bank has estimated that up to \$20 billion a year is lost due to salinization [4]. Due to the extremely complex natural processes, abiotic stress inhibits plant development, reduces agricultural production, and further contributes to excessive soil degradation. Farmers' incomes and other local economies are solely slowed [1].

Plants make substances called secondary metabolites that help them do well in their environment. A variety of responses occur in plants and other species as a result of these tiny chemicals. They signal the continuation of perennial growth or the onset of dormancy, and they are responsible for triggering blooming, fruit set, and abscission. Depending on the circumstances, they can attract or repel microbes. There are over 50,000 distinct secondary metabolites present in plants. Secondary plant metabolites are responsible for the activities of therapeutic plants and a variety of modern pharmaceuticals. Secondary metabolites are produced by plants in response to environmental and biotic stimuli. These include defensive chemicals, attractants, and toxins. They are also responsible for plant growth, development, and health. Universities and drug companies are always looking for new secondary products in plants in the hopes of finding new products or, even better, new ways to treat diseases. As secondary metabolites, once-held hopes that better appreciating natural product distribution might aid in plant taxonomy were also entertained. This supplementary rationale is no longer relevant because plant classification is increasingly done by comparing DNA sequences [5, 6]. In this chapter, we talk in-depth about the fundamental contribution of secondary metabolites that enable plants to sustain soil salinity, increase agricultural production, and have no negative effects on the economy or people's health.

### **1.1 Soil salinity causing factors**

Soil salinization can be caused by a number of factors, but most commonly it is due to improper irrigation practices that allow salts to accumulate in the soil. Other causes of salinization include soil type, water table fluctuations, and high evaporation rates. In some cases, poor agricultural practices, such as excessive fertilization, can also lead to soil salinization. In order to prevent soil salinization, it is important to practice proper irrigation techniques, ensure that the soil has adequate drainage, and make sure to use the appropriate amount of fertilizer for the crops being grown. Additionally, certain soil amendments, such as gypsum, can help reduce the number of salts in the soil [2, 7]. One of the most common and bad effects of soil flooding is that it makes the soil saltier. Salinity is based on how much salt is dissolved in the soil, and it can be changed by a number of things. Poor drainage can also make it easier for certain types of weeds to grow, which can lead to soil erosion and other problems. It can also cause the soil to get harder, which can make it harder for plants to get enough

### *Plant Adaptation to Salinity Stress: Significance of Major Metabolites DOI: http://dx.doi.org/10.5772/intechopen.111600*

oxygen and nutrients. Soil salinization occurs when seawater or other salt water floods a field or other piece of land. When water evaporates, salt and other minerals are left behind. These can build up in the soil and make it less fertile. This can cause long-term problems with soil salinity in places that flood often [7]. To mitigate the effects of soil salinity caused by flooding, farmers and landowners should improve drainage, plant salt-tolerant crops, and restrict the amount of irrigation water permitted to pool on the land. When soil is inundated, water from rivers, streams, and other sources can carry enormous quantities of dissolved salts. High concentrations of nutrients, such as nitrogen and phosphorus, can also contribute to an increase in salinity. Inadequate drainage can also raise the water table, leading to flooding and other water-related problems [8, 9]. Salt can build up in the soil as a result of over-irrigation. This can cause a variety of issues, including decreased crop yields, soil structure damage, and plant toxicity. Salt buildup can also cause water logging, which reduces soil oxygen levels and damages plant roots. Farmers should employ water-saving irrigation techniques, such as drip irrigation, to mitigate the impacts of soil salinity caused by over-irrigation. Farmers should also utilize soil tests to detect the salinity of the soil and alter their irrigation operations accordingly. Finally, the use of soil supplements, such as gypsum, can aid in the reduction of salt accumulation in the soil [10]. Climate change and natural disasters (such as a tsunami) that make the soil saltier are still problems for agriculture. When the Earth's temperature rises, the water that falls as rain or snow becomes increasingly saline. An increase in saline water can dissolve salts in the soil, making plant growth more challenging. Also, when the Earth's seas warm, more salt is released into the atmosphere. This rise in salinity can kill marine life, making it impossible for them to live. Finally, as the Earth's surface dries up, salts collect in the soil more easily [11]. Drought, excessive salinity, and cold temperatures are all climatic factors that have a negative impact on plant growth and crop yield. The growth of a plant and the production of secondary metabolites are both affected by environmental conditions such as temperature, humidity, and the intensity of the light, as well as the availability of water, minerals, and carbon dioxide. Cellular dehydration brought on by exposure to salt generates osmotic stress, which in turn leads to the loss of cytoplasmic water and a consequent shrinking of the cytosol and vacuoles. Depending on the severity of the salt stress, plants may either accumulate or decrease a number of different secondary metabolites [12]. Generally, salt-stressed plants are known to accumulate a wide range of secondary metabolites, including polyamines, polyphenols, and flavonoids, which act as antioxidants and chelators of metal ions, thereby helping to protect the plants from oxidative damage caused by the salt stress. In addition, the accumulation of these compounds can also help to increase osmotic pressure, allowing plants to cope with the osmotic stress caused by salt. On the other hand, plants under salt stress may also decrease the synthesis of certain secondary metabolites, such as terpenes and alkaloids, which can be toxic to plants when present in high concentrations. Therefore, the accumulation or decrease of secondary metabolites in salt-stressed plants is largely dependent on the severity of the salt stress.

### **1.2 Plant defenses against salt stress and metabolic alertness**

There are several different ways in which plants react when they are subjected to salt stress. The physiological and the biochemical are the two categories that can be used to classify these responses (**Figure 1**). Alterations in the growth rate of the plant are one type of physiological reaction. Other types of physiological reactions include changes in photosynthesis, respiration, transpiration, water uptake, nutrient uptake,

#### **Figure 1.**

 *Salt stress and metabolic reactions are physiological and biochemical: salt stress intensity affects secondary metabolite accumulation in salt-stressed plants. Salt stress increases osmotic potential, decreases water potential, increases photosynthetic rate, and decreases stomatal conductance. Salt stress increases antioxidant enzyme activity, stress-related gene expression, proline buildup, and soluble sugar accumulation. Plants need these responses to stay healthy and avoid salt stress.* 

and hormone production. Plants respond to salt stress by increasing the concentration of compatible solutes in their cells, which helps them maintain their turgor pressure and prevent dehydration [ 13 ]. Compatible solutes are small molecules that can be accumulated in the cell without disrupting its osmotic balance. Examples of compatible solutes include sugars, amino acids, and polyols. Plants also respond to salt stress by activating various stress-response pathways, such as the MAPK, calcium, and ABA pathways, which help them cope with the stress and protect them from damage [ 14 ]. Stomatal movement also gets affected under this salt stress. Plants close their stomata to reduce the amount of water lost through transpiration. This helps them conserve water and reduce the amount of salt that enters the plant. Stomata are small pores on the surface of leaves that open and close to regulate the exchange of gases and water vapor. When the stomata are closed, the plant reduces the amount of water vapor that is released into the atmosphere. This helps the plant conserve water and reduce the amount of salt that enters the plant [ 15 , 16 ]. In some cases, plants may shed their leaves and can cause root growth inhibition, which reduces the amount of water and nutrients taken up by the plant to cut the amount of salt uptake. Salt stress led to antioxidant enzymes (such as superoxide dismutase, catalase, and peroxidase), osmoprotectants (such as proline, glycine betaine, and polyols to help them maintain their turgor pressure and prevent dehydration), stress hormones (abscisic acid and jasmonic acid), and phytohormones (gibberellins and cytokinins) production [ 17 , 18 ]. Other types of physiological responses include changes in the structure of the plant such as an increase in the production of root hairs and thicker cell walls. Changes in the plant's metabolism, such as the creation of enzymes and other proteins that assist the plant in coping with salt stress, are examples of biochemical responses that take

#### *Plant Adaptation to Salinity Stress: Significance of Major Metabolites DOI: http://dx.doi.org/10.5772/intechopen.111600*

place as a result of salt stress. In addition, plants may create compounds that assist them in absorbing and storing salt, or they may develop compounds that assist them in excreting excess salt as a defense mechanism against the effects of salt stress.

Plants can not get away from potentially dangerous situations, so they have evolved to have different defense mechanisms to protect themselves. These include physical barriers such as thorns, chemical defenses such as toxins and poisons, and camouflage to blend in with their environment. Some plants also produce chemicals that attract predators of the herbivores that would otherwise feed on them. Some plants, such as certain species of grasses, are able to tolerate high levels of salinity. Other plants may be able to survive in the short term but suffer long-term damage. In extreme cases, the plants may die. There are several strategies that can be employed to help plants cope with high levels of salinity such as using salt-tolerant varieties of plants, avoiding over-irrigation, and using soil amendments to improve the soil's structure and drainage [19]. Metabolites are used for a variety of purposes in plants. Plants can use these metabolites to help regulate their internal water balance, allowing them to cope with salt stress [19, 20]. Such compounds help plants detoxify and eliminate excess salt from their cells. They can be used to protect the plant's cells from the damaging effects of salt. They can be used as a source of energy, to help regulate growth and development, to produce hormones, to protect against environmental stress, to produce pigments, to aid in defense against pathogens, and to help in the synthesis of other molecules [21]. Some plants have evolved mechanisms to cope with soil salinity. These mechanisms involve the production of metabolites, such as proline, glycine betaine, and trehalose, which help the plant to maintain its water balance and protect its cells from the damaging effects of salt. Additionally, some plants produce special root structures that help to reduce the uptake of salt from the soil.

The prospects of metabolic alertness in crop production against salinity stress are very promising. By understanding the role of metabolic alertness in the response to salt stress, researchers can develop new strategies to improve crop performance under salinity. It refers to the ability of a crop to detect and respond to salt stress. It could improve the drought tolerance of crops by reducing the amount of water lost through transpiration. It helps crops adjust their metabolic processes to adapt to the salinity environment. Various studies have shown that crops with higher metabolic alertness respond better to salinity stress than crops with lower alertness. This is done by increasing the production of certain proteins that are involved in salt tolerance. It also involves the upregulation of certain genes that are involved in salt tolerance. Finally, metabolic alertness can be used to improve the nutrient content of crops by increasing the uptake of minerals and other nutrients.
