**5. Behavior of flavonoid in soil salinity**

In the kingdom of plantae, flavonoids are the secondary metabolites that are present in the widest variety of plant species. These chemicals have a wide variety of physiological and molecular functions in plants, including acting as signaling molecules, contributing to plant defense, influencing the transport of auxin, exhibiting antioxidant activity, and scavenging free radicals [64]. According to Sirin and Aslam [65], among the nonenzymatic antioxidants, phenols and flavonoids make a substantial contribution to removing free radicals in plants, which allows plants to tolerate salt stress by storing the antioxidants in a variety of tissues. Flavonoids are a group of compounds that are found in many plants. These compounds have been studied for their

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

potential health benefits, and many are now known to be beneficial for human health. Flavonoids are a type of polyphenol. They play essential roles in soil salinity regulation [30, 41, 66]. They can reduce the number of ions, such as sodium and chloride, present in the soil solution, which can help maintain soil health and fertility [66]. They have also been shown to promote beneficial microbial activity in the soil, including improving the growth and health of beneficial bacteria and fungi. They also have antioxidant properties, which can help protect plants from environmental stressors such as high levels of salinity. They can also act as signaling molecules, helping plants regulate their response to salt stress. In addition, flavonoids can help promote beneficial microbial activity in the soil, which can improve the soil's fertility and reduce its salt content. This can help boost the soil's fertility, improve water retention and nutrient cycling, and reduce the risk of disease-causing pathogens. They can also help protect plants from environmental stressors such as drought and extreme temperatures [67, 68].

In plant materials, polyphenols can be found in both their free and bound forms. Phenolic acids make up the bulk of the polyphenols that are found in grains and baked goods derived from cereal, and approximately 75% of these are accessible in bound form. Plants' development, reproduction, and eventual grain yield are almost entirely dependent on the leaf proteins [68]. It is not surprising that salt stress causes a decrease in protein content in plant leaves, given that proteins are known to be one of the first targets of reactive oxygen species (ROS) in living organisms. One of the principal sites of ROS damage is the chloroplast, where it leads to the degradation and inactivation of Rubisco, as well as many other changes in the thylakoid and stromal proteins [69].

Because of this, leaf protein concentration is an important indicator of salt stress [70]. One of the most debated topics in the literature is the extent to which genetic diversity affects the protein content of plant leaves when subjected to salt stress. Few studies have looked into how genetics interact with salt stress to affect protein levels. In addition, there is a lot of debate in the scientific literature concerning the way salinity has an effect. For instance, Birhanie et al. [71] found that salt stress similarly reduced total protein concentration in the shoots of two cultivars of wheat (tolerant and sensitive). In contrast, it was discovered that during salt stress, leaf protein concentration varied by genotype and increased [72, 73].

Many studies have been proposed that increasing the accumulation of suitable solutes, such as proline, can improve salt tolerance. Proline, such as other osmolytes, can eventually regulate redox potential through its effects on osmotic adjustment, membrane protection, and enzyme stability in the face of abiotic and biotic stresses [74]. Arabbeigi et al. [75] reported greater proline biosynthesis gene (P5CS) expression in *Ae. cylindrica* may be linked with salt tolerance. It is a bacterial species that is commonly found in salt marshes and coastal habitats. The P5CS gene is known to play a role in proline biosynthesis, which is a process that helps cells to build proteins. The researchers found that *Ae. cylindrica* cells that expressed high levels of the P5CS gene were more resistant to salt stress. Additionally, the study found that deleting the P5CS gene had no impact on the salt tolerance of *Ae. cylindrica* cells. The authors of the study say that the findings suggest that the P5CS gene may play a role in salt tolerance in *Ae. cylindrica*. They say that the findings could help to identify new strategies for preserving salt marsh habitats. In wheat, this finding agrees with that of Kumar et al. [76]. This is congruent to a certain extent with the findings of Ebrahim et al. [77] in barley, who also found that salt stress led to a huge buildup of proline but also found that salt-sensitive genotypes accumulated more proline in their leaves than salt-tolerant ones. Since the osmo-adaptive response includes proline buildup, whether or not it plays a special function in the resistance to abiotic and biotic stresses (such as salinity

and drought) is debatable. Several explanations come to mind for this discrepancy. Variations in genotype or species, stress level or duration, and physiological maturity or development are particularly important to note when comparing research.
