Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications

Abstract

Plant secondary metabolites (PSM) are one of the major sources of industrially important products such as food additives due to their distinctive tastes, smells, and flavors. Unlike primary metabolites such as carbohydrates, lipids, and proteins, these secondary chemicals are not involved in plant growth, development, and reproduction but play a significant role in ecosystem functioning. These secondary biochemicals also play a key role in plant communication and defense, particularly under different environmental stresses. Plants may exhibit a defense response to combat these abiotic environmental stressors by generating a variety of PSMs to minimize cell and tissue damage. Secondary metabolites are very diverse (almost more than 200,000) in nature, majorly classified into terpenoids, phenolic compounds, nitrogen, and sulfur-containing secondary metabolites, separated based on biosynthetic pathways (shikimate pathway, mevalonic pathway, and tricarboxylic acid cycle pathway). This chapter summarizes the stimulating effects of different abiotic stressors (heavy metals, cold and high temperature, light, salinity, and drought) on secondary metabolite production. A major focus is given on the synthesis of secondary metabolite and accumulation in plants under stressful conditions, and their role in the regulation of plant defense.

1. Introduction

Primary and secondary metabolites are two categories of substances synthesized by plants. Primary metabolites that directly contribute to plant growth and development include lipids, proteins, and carbohydrates. On the other hand, secondary metabolites (SMs) are multipurpose metabolites that frequently participate in plant defense and environmental communications [1]. Interspecies communication, controlling the activity of enzymes, signaling, and defense are just a few of the tasks performed by SMs [2].

The SMs play a major role in the plant’s capacity to endure abiotic stressors. The plant defense system has developed under a variety of stress detection systems, including transmembrane recognition, and the creation of SMs, which ultimately help plants to withstand these harsh or stressful conditions [3, 4].

Secondary metabolism also permits ecological interactions between plants and some other species and contributes to the capacity of plants to adjust and survive in response to environmental conditions throughout their lifecycle [5]. Plants are often at risk from a variety of abiotic stresses, such as oxidative and toxic metals, extreme cold, flooding, salinity, and drought [6]. Plants create a variety of low molecular weight chemicals known as secondary metabolites, such as anthocyanins, phenolic compounds, alkaloids, flavonoids, steroids, and terpenes. These phytochemicals are vital for a plant’s protection, adaptation, and environmental adjustment [7].

Plant secondary metabolites are distinctive sources of flavorings, food additives, and medications with significant industrial use [8]. A transition from a rapid growth period to a stationary(inactive) phase in plants is typically due to the highest amounts of SMs production. Secondary metabolism is a crucial component of organic metabolism and biological processes; it mainly relies on primary metabolism for the contribution of the necessary enzymes, ATP, and cellular machinery and has a role to make a significant contribution to the producer organism’s long-term survival. However, their manufactured organisms can grow and develop even without these secondary compounds, recommending that secondary metabolism is not necessary, at least for short-term or temporary survival [9].

Regulating plant tolerance at the cellular level is crucial for controlling stress responses and signal transduction systems [10]. For instance, the capacity of cotton to synthesize flavonoids and derivatives of cinnamic acid under drought stress tolerance indicated that they are highly effective at scavenging ROS [11], whereas the ability of reed plants to produce isoprene during heat-induced stress suggested that they have an effective antioxidant capacity that can quench oxygen [12]. There are more than 200,000 SMs known in plants [13]. Some SMs are unique to certain plant taxa, and their concentrations might differ between populations and individual plants depending on tissue type and specific plant development stage [14]. Although such SMs variations may be a result of genetic variability, their concentrations are also influenced by abiotic factors (growth circumstances) that are anticipated to get worse with climate change (such as heat stress, drought, and UV radiation) [15]. It has been believed that increased synthesis of the majority of SMs by a plant^’s mechanism for chemical defense response is linked to their resistance to stress and that this may be one explanation for the existence of variation in the type and quantity of SMs produced by different plant^’s taxonomic groups [16, 17].

This chapter elaborates on the sources and classification of plant secondary metabolites, their synthesis under unfavorable conditions, and their role in modulating plant growth and abiotic stress tolerance.

2. Sources and classification of secondary metabolites

A simple classification of plant secondary metabolites includes three main groups: terpenes (which include plant volatiles, cardiac glycosides, carotenoids, and sterols), phenolics (which include phenolic acids, coumarins, lignans, stilbenes, flavonoids, tannins, and lignin), and nitrogen-containing compounds (which include alkaloids and glucosinolates) [18].

2.1 Terpenoids

Terpenoids are the largest and most diverse collection of natural products and contain thousands of isoprene units ranging from hemiterpenes to rubber, varying in structure from linear to polycyclic molecules. Each terpene is produced by the condensed isoprene units, and all of them are classified based on the number of carbon (5C) units that make up the basic structure [19].

A monoterpene is a compound composed of two isoprene units (C10) and is found in a wide range of plants. Terpenoids were first reported in their volatile forms, where they contributed to abiotic stress tolerance and chemical communications between species [20]. Terpenes have several uses for attracting pollinators, protecting plants from herbivores [21], and also acting as deadly insecticides and insect repellents [22].

2.2 Phenolics

Phenolics are a crucial subclass of aromatic PSMs, with one or many acidic hydroxyl groups linked to phenyl-ring [23, 24]. Phenolics are a family of heterogeneous molecules made up of about 10,000 chemicals that are both soluble in water and organic solvents [25]. Plants, fungi, and bacteria can make these substances [26].

Phenolic compounds are classified based on their structural characteristics (phenols (C6), phenolic acids (C6-C1 or C6-C3), flavonoids (C6-C3-C6), and tannins (C6-C3-C6)). They can occasionally be further divided according to the degree of polymerization: (I) High molecular weight phenolic compounds, such as tannins, are water-soluble molecules that are strongly polymerized and fall under two subfamilies with distinct properties (II). The majority of saprotrophic organisms readily use low molecular weight phenolic compounds, which are discovered in most plants in glycosylated and solubilized form; however, some of them have specific biological activities such as cinnamic acids, in dicot angiosperms, the bulk of hydrolyzable tannins are present. Pro-anthocyanidins is another name for condensed tannins, which are polymers of flavan-3-ols (flavonoids) [27].

They serve significant roles in giving fruits, flowers, and seeds color, smell, and flavor, and they also have powerful antioxidant qualities because of their physical and chemical makeup [28]. Moreover, these substances have an impact on interactions between plants, such as allelopathic inhibition of targeted species growth [29].

2.3 Nitrogen-containing secondary metabolites

Glucosinolates, alkaloids, and cyanogenic glycosides make up a significant class of nitrogen-containing SMs. Alkaloids are basically alkaline compounds, which have low molecular weight and make up a large group of secondary metabolites [30]. The majority of alkaloids occur in their free state as N-oxides or mix with acids to form salts that can easily dissolve in water [24]. Alkaloids are identified due to the presence of at least one nitrogen atom, and they have a noticeable physiological impact on animal behavior; for instance, many of them have poisonous or analgesic effects (such as cocaine or morphine) [21].

Cyanogenic glucosides (CNglcs), also known as α-hydroxy nitrile glucosides, are naturally occurring bioactive plant secondary metabolites generated from amino acids and primarily composed of the sugar D-glucose [31]. According to Picmanová et al. [32], CNglcs are thought to be crucial for plant development, growth, and tolerance against different abiotic stressors [33].

2.4 Sulfur contains secondary metabolites

Phytoalexins, defensins, glutathione (GSH), and allicin are just a few of these substances that have been linked either directly or indirectly to plants’ protection against microbial diseases [34]. The GSH is one of the primary forms of organic sulfur, which is present in soluble parts of plants and is essential for controlling plant development and growth as well as working as an antioxidant in cells under stress [35].

Higher plants synthesize glucosylates (GSL), a type of low molecular weight plant glucosides containing nitrogen and sulfur to boost their resistance against predators, competitors, and parasites. These compounds degrade into toxic or repellent volatile defense chemicals, for example, allylcys sulfoxides in allium and mustard oil and glucosides in Cruciferae [36]. A typical structural characteristic of Brassica phytoalexins is an indole or closely similar ring structure. These secondary metabolites are different compared to other well-known GSLs and appear to exclusively be produced by the plant family Cruciferae. Due to their high value, multiple research teams have looked at cruciferous phytoalexins as well as their biological activities over the past few decades [37].

3. Production sites and biosynthesis of secondary metabolites

Within plant cells, many secondary compounds are biosynthesized from primary metabolites, and their production can be stimulated under many abiotic elicitors and signaling molecules [38]. Under stressful environments, specific SMs conditionally accumulate in different plant sections [39]. The Krebs cycle and the Shikimate pathway are run where the precursors of metabolites are formed. The important starting materials for SMs are primary metabolites. Based on their chemical makeup, intended use, and distribution in plants, primary and SMs can be separated from one another. In most plants, the basic biosynthesis routes of metabolites remain conserved and most of the primary metabolites are present in all tissue types. Several basic metabolic frameworks have emerged because of the preservation of this metabolic core. Many modifications in fundamental structures result from regular glycosylation, methylation, hydroxylation, acylation, oxidation, phosphorylation, and prenylation, as well as from fewer chemical modifications brought on by the specialized enzymes.

The three main categories of SMs can be separated based on biosynthetic pathways. Phenolic substances are synthesized through the shikimate pathway. Terpenes are synthesized through the mevalonic pathway, while the N-containing substances are synthesized through tricarboxylic acid cycle pathway [40]. The precursor of the shikimate pathway (from the pentose phosphate pathway) is shikimic acid, which is produced when erythrose 4-phosphate and phosphoenolpyruvate from the glycolytic pathway combine. Phenylalanine, tyrosine, and tryptophan are the precursors for PSMs like phenolics, and N-containing compounds are produced via the shikimate pathway and also serve as the building blocks for protein synthesis [39]. In contrast to tryptophan (which is the precursor of alkaloids, phytoalexins, indole glucosinolates, and plant hormones like auxin), tyrosine further produces isoquinoline alkaloids, pigment betalains, and quinones (such as tocochromanols and plastoquinone) [41].

4. Metabolomic approaches for quantifying secondary metabolites

Understanding the kinds and quantities of SMs generated by the kingdom of plants is beneficial for plant research since this production reveals how plants have evolved in nature to meet various challenges [42]. We need to use a wide range of various analysis tools to get better handling of metabolites for plant metabolomics due to the wide variety of chemical classes and characteristics as well as their enormous dynamic range of metabolite concentrations in plants.

After the organic substrate has been extracted using a non-polar solvent, the only approach for terpene analysis is through GCMS (gas chromatography-mass spectroscopy). The principle behind the technique is the specific absorption of near-infrared electromagnetic radiation by different OCs (organic compounds), which is known as near-infrared spectrometry (NIRS). This method creates spectra reflecting the material’s organic composition (nitrogen, lignin, cellulose, hemicelluloses, etc.) [43]. Another recently created comprehensive method is called “metabolomics,” which enables quantitative determination and all metabolites’ identifications in different tissue by using a variety of analytical methods such as gas/liquid chromatography (GC/LC) combined with mass spectrometry (MS) or nuclear magnetic resonance (NMR) spectroscopy. Metabolomics provides a full snapshot of the chemical makeup of a tissue (such as a leaf) at a specific moment, enabling somebody to focus on the least understood chemicals and evaluate or recognize metabolites that have insufficient information [44].

Folin-Ciocalteu method, used to calculate the concentration of OH⁻ phenolic compounds that are bound to the benzene ring is the method most frequently employed to examine the phenolic fraction of plants [45]. The Folin-Ciocalteu reagent reacts with phenolic groups to produce a blue complex, which is the basis for this procedure [46].

The STME (Silicone tubing microextraction) is a technique created by Mohney et al. [47] and is one of the newer techniques that open up new opportunities for studying secondary metabolites in soils. Direct placement of sorbent microtubes in the soil allows for in situ monitoring of allelochemicals released from roots [48].

5. Secondary metabolite accumulation under various environmental factors

The production and accumulation of PSMs in tissues are tightly regulated in a spatiotemporal way and influenced by many abiotic factors [49]. Environmental factors affect how PSMs produce and accumulate in different plants [50]. The alkaloid content of Catharanthus roseus seedlings under salt stress and water stress (drought) was significantly higher than when it is under control conditions [51]. Different abiotic stress conditions also have a substantial impact on the formation of phenolic chemicals [52]. Oryza sativa stimulates phenolics secretions in the stele and epidermis of roots in alkaline conditions, which effectively boosts ion absorption and reduces iron-deficiency reactions [53].

Polyphenols that are also called flavonoids are antioxidants and necessary for plant tolerance against different abiotic stresses [54]. In model plants, excessive accumulation of various flavonoids such as kaempferol, quercetin, and cyanidin is well known [55]. Heat and salt stress increased flavonoid accumulation in O. sativa, which enhanced tolerance in rice against these stresses [56]. Short-wavelength radiation causes a number of flavanol glycosides including quercetin and kaempferol glycosides, which strengthen plant defenses against different stresses [57]. In plants of Brassicaceae family, glucosinolates are significant precursors to several active components [58]. Strong light, high temperatures, and drought caused more accumulation of glucosinolates in Brassica rapa [59]. Brassica oleracea show strong tolerance against chilling and freezing, and it is suggested that the defensive mechanism enabling this tolerance involves the glucosinolates concentrations induced by low temperature [60].

6. Plant secondary metabolites and abiotic stress tolerance

Metabolic processes that lead to the accumulation of natural products are affected by the concentrations of various PSMs, which greatly affect growth conditions. Numerous typical reactions occur when abiotic stresses affect the stimulation of PSMs.

6.1 Heavy metals

Zinc, manganese, nickel, and iron are essential for the development of photosystems (I & II) and different enzymes in plant cells [61]. An excess of various metals especially toxic metals is harmful to plants; as a result, plant cells have systems in place to prevent these metals’ poisonous buildup. Recent research has focused on the development of SMs within plants under heavy metal stress [62]. The formation of different photosynthetic pigments, sugars, proteins, and non-protein thiols is affected by heavy metals at physiological and metabolic levels in plants. By altering certain parts of secondary metabolism, metals can change how bioactive molecules are produced [63].

Secondary metabolite synthesis is also regulated by metal ions (europium, silver, lanthanum, and cadmium) and oxalates [64]. Urease enzyme, which is an essential component of the trace metal nickel (Ni), is required for the development of plants [65]. It has been demonstrated that Cu²⁺ and Cd²⁺ increase the yields of secondary metabolites such as shikonin [66]. Babula and colleagues [67] examined the physiological reactions of Hypericum perforatum plants to cadmium stress in several tissues, specifically in the shoots and the roots. Their findings revealed an increase in phenolic acids (ferulic acid), and on the other hand, there was a decrease in flavonoids (epicatechin and procyanidin) in shoots as well as in roots. It is interesting to note that PAL (phenylalanine ammonia lyase), the first gene to intervene in the phenylpropanoid pathway, was found to be directly correlated with heavy metal accumulation [68]. This showed how heavy metals affect the genes that are responsible for producing phenylpropanoids and explained why phenolic acids build up in heavy metal-stressed plant cells. To conserve energy plants, produce phenolic acids (hydroxycinnamic acids), and may prefer to invest in the first steps of the pathway rather than activating the genes that would otherwise interfere with the proceeding steps (which result in the synthesis of flavonoids and anthocyanins). It was previously mentioned that phenolic compounds are produced as a defensive mechanism in response to toxic metal stress. Phenolics are powerful Cd chelators and the roots of Matricaria chamomilla produce more of these compounds than other plants [69].

In plant cells, oxidants and antioxidants coexist in a dynamic balance that prevents ROS buildup [70]. Secondary metabolites play a well-established role in reducing ROS stress [71]. The plant secondary metabolites that can combat ROS and prevent oxidative stress are polyphenols and terpenes [72]. Their scavenging abilities are also caused by these molecules, it is important to note that the antioxidant characteristic of flavonoids is determined by their OH⁻ groups that provide electrons and hydrogen to radicals to stabilize them [73]. A CdSO₄ treatment led to an increase in protective soluble phenolic compounds within the woody species Populus x canescens [74]. In contrast to the wood, where ROS were created at a faster rate these chemicals were more prevalent in the bark. These findings showed that different organs of the same tree exhibit diverse responses to heavy metals and that these responses are correlated with the capability of plants to produce different SMs [75].

6.2 Temperatures (cold and high temperatures)

The temperature has a significant impact on plant ontology and metabolic activity, and extreme heat can expedite the senescence of leaves. Thermal treatments were observed to marginally reduce carotenoids in Brassicaceae, including β-carotene [76]. Temperatures and the phenological stage had an impact on the production of SMs in Rhodiola rosea clones [77], and increased levels of toxic metals boosted SMs production with a synergistic action associated [78]. Within suitable temperature ranges, plants can grow and develop more effectively. The development and production of plants may be negatively impacted by low and high temperatures [79]. Heat stress affects plants that are growing in hot environments. Stomatal conductance and net CO₂ fixation drop due to heat stress are linked to decreased plant growth and yield. Heat stress in plants and SMs biosynthesis are related to one another [80]. A decrease in the photochemical efficiency of photosystem II is seen in plants developing under heat stress. A review of the literature found that plants under heat stress often produce more SMs, but some studies also showed a decrease in SMs production under elevated temperatures; ginsenoside levels were increased in Panax quinquefolius plants that were cultivated under elevated temperature stress [65].

Among the most detrimental abiotic stressors affecting temperate plants is low-temperature stress. Due to seasonal temperature changes, several species’ metabolisms have modified in the fall to contain more of a variety of cryo-protective compounds for enhancing their capacity to survive cold temperatures [81]. When a temperate plant overwinters, its metabolism is switched to the synthesis of molecules that act as cryoprotectants, such as sugar alcohols, and low molecular weight nitrogenous compounds [81]. Low-temperature stress inhibits metabolic processes, water absorption, and cellular dehydration in many plants [82]. Freezing temperatures caused photosynthesis in Capsicum annuum plants ultimately reducing the plant growth. Cold acclimation occurs when plants that are growing at low temperatures show significant changes in a variety of physiochemical and molecular mechanisms allowing plants to withstand these cold temperature stresses. Moreover, information about the decline in photosynthetic pigments and total soluble protein content in plants during cold temperatures has been documented in the literature [83]. The production and storage of SMs were noticeably decreased under low-temperature stress [80]. Phenolic synthesis is also observed to be increased by cold stress [84]. The relationship between temperature and the production of alkaloids has been observed, particularly with high temperatures being preferred to trigger alkaloid production. At low temperatures, the accumulation of the alkaloids was constrained in dry Papaver somniferum [85]. In contrast to the control, the overall phenolic acid content and isoflavonoids (genistein, daidzein, and genistin) in soybean (Glycine max) roots increased when these roots were treated at a cold temperature for 24 hours with genistin showing the greatest rise of 310% [86]. Christie et al. [87] documented the development of anthocyanins during low-temperature stress. Pinus pinaster undergoes modifications in its endogenous jasmonates because of cold and water stressors [88].

According to Lei et al. [89], melatonin protects carrot suspension cells against cold-induced apoptosis via upregulating the polyamines (putrescine and spermine). According to a recent study by Zhao et al. [90], melatonin has been shown to increase the longevity of Rhodiola crenulata cryopreserved callus. Kovacs et al. [91] observed that when wheat (Triticum aestivum L.) leaves are subjected to low temperatures putrescine accumulates (6–9 times), spermidine accumulates less, and spermine declines little. Moreover, under low-temperature stress, alfalfa (Medicago sativa L.) also accumulates putrescine [92]. According to Hummel et al. [93], agmatine and putrescine levels have been linked to enhanced levels of cold tolerance, and they may serve as a useful indicator of this trait in P. antiscorbutica seedlings. Perilla frutescens suspension cultures showed a striking reduction in anthocyanin production at an elevated temperature of 28°C, which was the greatest at 25°C [94]. Similar findings on anthocyanin productivity at its maximum production level in Daucus carota cell suspension cultures were described [95]. Under the influence of various temperatures, Beta vulgaris hairy root cultures were observed and examined for a release of these pigments [96]. The ideal temperature ranges for each plant species and cultivar are unique, and any variation from those limits may have an impact on biomass and the production of SMs.

6.3 Salinity stress

Salinity stress affects plant growth and the production of bioactive compounds [97, 98]. Many plant species create phenolics to defend themselves against different abiotic stress conditions, including salinity, and their buildup is correlated with plant species’ antioxidant capacities [99]. Proline levels in the roots of salt-tolerant alfalfa plants increased quickly according to research by Petrusa and Winicov, the increase was gradual in salt-sensitive plants [100]. Many plants have also observed an increase in polyphenol content in various tissues under salt stress [101]. Navarro et al. [102] found that red peppers had an enhanced total phenolic content at a moderate salinity level. It has been demonstrated that plant polyamines influence how plants react to salinity. There have been reports of alterations in polyamine levels caused by salinity in Helianthus annuus L. (sunflower) roots [103]. The effects of KCl treatment on amounts of total phenolics and flavonoids in C. cardunculus and Cardunculus var. altilis leaves were more pronounced than those of the other two chloride salts (NaCl and CaCl₂) [104] (Table 1).

Plant species	Salinity levels	Effects on the concentrations of SMs	References
Solanum nigrum	0, 50, 100, & 150 mM	Salinity stress increased the expression of the flavonoid genes, which in turn increased the synthesis of quercetin 3-d-glucoside and lutein. Moreover, certain carotenoid-related genes, such as phytoene synthase 2 and lycopene cyclase were just overexpressed in response to salt stress.	[105]
Zea Mays	0 & 100 mM	Salinity stress increased the total phenolic and flavonoid levels.	[106]
O. sativa	0 & 25 mM	Grain nutritional quality, including antioxidant activities, anthocyanins, and total phenolics, was significantly improved under salinity stress.	[107]
Triticum astivum	0 & 150 mM	Salinity stress increased phenolics, particularly at the booting stage.	[108]
Lepidium sativum	30, 60, 90, & 120 mM	Accumulation of flavonoids and phenolic compounds in L. sativum was increased under salinity stress.	[109]
Saccharum officinarum	12.5 & 6.8 dS/m	Salinity stress altered the production of flavones, anthocyanins, and soluble phenolics in two sugarcane clones, CP-4333 and HSF-240.	[110]
Fagopyrum esculentum	0,50,100, & 200 mM	Carotenoids, phenolics, and antioxidant activity changed noticeably when buckwheat plants were exposed to different salinity levels. Comparing plants grown under non-saline conditions to plants growing under various salinity doses, it is evident that the concentration of phenolic chemicals increased significantly.	[111]
Salvia mirzayanii	4.5, 6.8, & 9.1 dS/m	S. mirzayanii’s antioxidant activity, phenolic content, and volatile compounds (Bicyclogermacrene, 1,8-cineole, and -terpinyl acetate) were increased under salinity stress.	[112]

Table 1.

Influence of salinity stress on the biosynthesis and accumulation of different PSMs.

6.4 Drought stress

Drought stress is one of the major abiotic stresses that affect plant development and growth [93, 113]. Drought disrupts cellular homeostasis by affecting proteins, carbohydrates, lipids, and DNA. It has an impact on the plant’s height, root growth, and leaf area (LA) [114, 115]. Moreover, drought has a significant impact on the physiology of plants, including osmotic potential, stomatal conductance, rate of photosynthesis, pressure potential, and transpiration rates [116]. Drought stress poses a serious threat to sustainable agriculture since it has a negative impact on crop yield globally. However, in response to drought stress, plants have evolved several morphological, physiological, biochemical, and phonological mechanisms [117].

Willow (Salix) leaves were shown to contain more flavonoids and phenolic acids during a drought, which frequently results in oxidative stress [118]. Changes in the ratio of chlorophylls “a” & “b” and carotenoids were affected by the drought [119]. Cotton under drought stress was shown to have less chlorophyll [120], as was in C. roseus [121]. In Chenopodium quinoa, drought circumstances reduced the amount of saponins from 0.46% dry weight (dw) in plants growing in low water deficit settings to 0.38% in plants growing in high water deficit situations [121]. A number of SMs generated by plants are beneficial for fostering drought resistance [96, 122].

A different study found that applying drought stress improved the quality of significant SMs in Artemisia annua [80]. Similarly, Glechoma longituba grown in drought conditions showed an increase in total flavonoids [123]. Significant changes were seen in the contents of several macronutrients, proline, carbohydrates, and essential oils in Ocimum americanum and Ocimum basilicum under water-limited circumstances [124] (Table 2).

Plant species	Drought levels	Effects on the concentrations of SMs	References
Achillea filipendulina	25, 50,75, &100% (field capacity)	Drought increased total phenolic and flavonoid concentrations.	[125]
Zea mays	PEG-induced (0.6 MPA)	Reduced phenolic compounds and decreased plant biomass occurred under drought.	[126]
O. sativa	25, 45, 65, & 85% Soil moisture	Drought increased the production of flavonoids, phenolics particularly in tolerant genotypes.	[127]
G. max	Control (−15 to −20); Drought (−90 & -100KPA)	Total phenolics and lignin levels significantly increased under drought	[128]
Carthamus tinctorius	25 and 50% water deficit	Plants under mild water scarcity produced more phenolic compounds, whereas plants under severe drought showed a clear drop in phenolics. Likewise, plants exposed to moderate drought had much higher levels of carotenoids.	[129]
Vitis vinifera	30 & 70% Soil moisture	In plants under drought stress, the amounts of the phenolic compounds significantly dropped.	[130]

Table 2.

Drought-induced alterations in the biosynthesis and storage of PSMs.

6.5 Light

The physiological reactions of various plant species and even cultivars to exposure to light conditions, such as photoperiod or small durations connected to the generation of SMs [131]. Light is a physical element that is widely established to have an impact on metabolite synthesis. In Z. officinale callus cultivation, light can enhance the formation of such secondary metabolites as gingerol and zingiberene [5]. Hence, the amounts of phenolics have been found to increase in direct proportion to light intensity.

Due to shorter light duration, many plant portions have significantly lower endogenous levels of coumarins. Furthermore, the prolonged period of light markedly enhanced the number of coumarins [132]. American ginseng (P. quinquefolius) plants that were exposed to direct sunlight for a longer period produced more ginsenoside in their roots than those that were exposed for a shorter time [133].

Blue light was found to have the greatest impact on SMs in Scutellaria laterifora shoot cultures, and their connection with PGRs (Plant growth regulators) was found [134]. The effects of various light spectra from light-emitting diode sources on the production of SMs were seen when Peucedanum japonicum callus cultures were exposed to them. The red and blue light was shown to be the most effective [135]. Based on the length of the cell suspension cultures of Artemisia absinthium, light and dark incubation conditions had a significant impact on the generation of biomass [136]. Different species have different effects on how light affects plant growth and development [131].

According to Liang et al. [137], UV-B radiation may cause a decrease in chlorophyll content while increasing flavonoid content and PAL activity. Root flavonoids in Pisum sativum plants were elevated by UV light (300–400 nm) [138]. Recent research showed that photoperiod regimes influence endogenous indoleamines (serotonin and melatonin) in farmed green algae Dunaliella bardawil [139]. In primary and secondary metabolism as well as a number of plant developmental processes, light is widely known to be essential [140]. Several studies have revealed that light sources directly induced the synthesis of crucial secondary metabolites, such as anthocyanins, artemisinin, caffeic acid derivatives, and flavonoids [141]. Regvar et al. [142] compared the effects of UV irradiation on different concentrations of rutin, catechin, and quercetin in Fagopyrum esculentum and F. tataricum, and they discovered that F. esculentum was found to have more quercetin when exposed to the elevated UV irradiation. Markham et al. [143] investigated the C-glycosyl flavone content of various rice cultivars under UV-B light and discovered that C-glycosyl flavones were enriched in a UV-tolerant rice cultivar but lacking in a sensitive cultivar.

7. Conclusions

This chapter explains the importance of secondary metabolites in plants’ defense against abiotic stresses such as heavy metals, flooding, salinity, and drought. These metabolites are produced in response to environmental stressors and are regulated depending upon growth circumstances and developmental stage. There are three main groups of secondary metabolites: terpenoids, phenolics, and nitrogen-containing compounds. Higher plants synthesize GSL (N & S containing secondary metabolites) to boost their resistance against predators, competitors, and parasites. The biosynthetic pathways of these SMs are distinct and use different precursors, with the shikimate pathway producing phenolic substances, the mevalonic pathway producing terpenes, and the tricarboxylic acid cycle pathway producing nitrogen-containing compounds. Understanding the types and quantities of secondary metabolites in plants is important for plant research, as it reveals how plants have evolved to cope with various challenges. Metabolomics is a comprehensive method used to identify and quantify all metabolites in different tissues. Flavonoids and glucosinolates are two examples of secondary metabolites that are important for plant tolerance against different abiotic stresses. Plant breeders have the potential to develop new plant varieties with increased tolerance to various abiotic stresses by selectively incorporating specific secondary metabolites. In the context of climate change, where plants will face more extreme environmental conditions, this could be particularly valuable.

Acknowledgments

The authors acknowledge the support from Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP-HC2022/4), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Conflict of interest

The authors declare that there is no conflict of interest.