**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

*Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications DOI: http://dx.doi.org/10.5772/intechopen.111696*

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**).

#### **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


*Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications DOI: http://dx.doi.org/10.5772/intechopen.111696*

#### **Table 1.**

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

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**).


#### **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

*Plant Secondary Metabolites and Abiotic Stress Tolerance: Overview and Implications DOI: http://dx.doi.org/10.5772/intechopen.111696*

*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.

### **Acknowledgements**

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.
