One of the most devastating abiotic stress factors—salinity by which cultivable land is becoming barren thus reduces total crop production day by day. Oxidative stress is the most dangerous event under salt inundation is imposed by salinity-induced ionic and osmotic stress [10]. Hence, these ionic and osmotic stress both disturb the photosystem, and thus cause excess ROS, such as ¹O₂, O₂⁻, H₂O₂, and OH. Salinity-persuaded acute ROS accumulations, then bother cellular redox followed by cellular damage counting membrane dysfunction, DNA damage, collapse the enzymatic action, along with distraction of the antioxidant defense system [91,92]. At this point, the plant synthesizes cellular AsA and GSH, which act as non-enzymatic antioxidants by involving their enzymatic components to detoxify ROS up to tolerable levels (Table 1).

However, the enzymes of AsA-GSH pathway showed their differential responses intolerant and sensitive varieties due to saline toxicity. Among salt-tolerant (Pokkali) and sensitive (BRRI dhan29) rice cultivars. Pokkali responded by enhancing the enzymatic activities of the AsA-GSH cycle, where, lowered APX and higher DHAR activity along with unchanged MDHAR and GR activities were found from BRRI dhan29. Rahman et al. [91,93] reported about the well involvement of AsA-GSH cycle in salt-stressed O. sativa where ROS generation was extreme. Here, salt exposed rice enhanced the reduced and oxidized GSH content with a lesser amount of AsA by the higher APX, MDHAR, DHAR, and GR activities against overproduced ROS. Vigna radiata was grown under the saline condition [94] and where salt-induced oxidative stress was marked with extreme O₂⁻ and H₂O₂ overgeneration. Salt-stressed V. radiata augmented GSH and GSSG contents along with lowered AsA, whereas up-regulated the activity of all enzymatic antioxidants of AsA-GSH cycle and thus responded with elevated ROS [95]. Salt exposed Lens culinaris up-stimulated both MDHAR and DHAR activities, which resulted in a lesser amount of AsA and indicated the overproduced H₂O₂ detoxification [96]. Recently, Singh et al. [97] disclosed the incremental activity of enzymatic antioxidants, including APX, DHAR, and GR, with lower AsA, GSH, and GSSG contents, because of salt-induced higher ROS accumulation in Solanum lycopersicum. Similarly, 150 mM salt-treated S. lycopersicum also decreased AsA content, which might be used in H₂O₂ detoxification, while better GSH showed its role in lowering H₂O₂. Ahmad et al. [98] also observed higher APX, and GR activities, while MDHAR and DHAR activities again reduced as well as supported AsA-GSH mediated ROS regulation. Ahanger et al. [99] reported the same response of S. lycopersicum upon saline toxicity. Both activities of APX and GR were enhanced in salt-treated Triticum aestivum besides elevated H₂O₂ generation and resulted in higher GSH accumulation [100]. The activity of APX, MDHAR, DHAR, and GR enhanced in salt-stressed S. lycopersicum to check the excessive H₂O₂ generation, which resulted in lowered AsA and GSH contents [92].

The changes in AsA-GSH pathway were investigated in salt-stressed Nitraria tangutorum by applying a varied level of NaCl (100, 200, 300, and 400 mM) [101]. They noticed a gradual enhancement of AsA, DHA, GSH, and GSSG contents by keeping pace with sequential increment of salt-induced H₂O₂. Here, increased MDHAR and DHAR activities in stressed seedlings also contributed to increasing AsA, and higher DHAR and GR were responsible for better GSH and GSSG contents [92,102]. Talaat et al. corroborated these results with salt-exposed Phaseolus vulgaris [103]. Thus, as a part of plant antioxidant defense under salinity, AsA-GSH pathway is very efficient to regulate extra ROS for being tolerant.

Drought is another most important abiotic stress, which generates excess ROS accumulation and thus causes variation in the enzymatic activities of AsA-GSH pathway for ROS detoxification. The enzymatic responses of AsA-GSH pathways varied, depending upon plant species, plant age, drought intensity, and duration [10]. Commonly, drought up-regulated the enzymatic antioxidant activities of AsA-GSH pool [10,104]. Plant tolerance to drought stress is categorized based on stress-induced endogenous antioxidants contents along with enzymatic activities (Table 2). Dendranthema grandiflorum responded differentially according to their tolerant and sensitive varieties, where tolerant one comparatively displayed better enzyme activity of antioxidants than the sensitive ones [105]. Lou et al. [106] demonstrated how T. aestivum responded upon drought exposure. Hence, they noticed that the AsA-GSH cycle responded considerably with excess ROS generation by significant variation of GSH/GSSG and AsA/DHA redox along with the steady increment of H₂O₂. Their team also observed the enzymatic up-stimulation of AsA-GSH pathway to alleviate stress by scavenging excess ROS in T. aestivum spike. Thus, T. aestivum showed higher participation of AsA with higher APX activity in drought exposure for scavenging extra H₂O₂, as well as higher enzymatic activity to run the AsA-GSH pathway systematically [107].

Drought-stressed A. thaliana enhanced GSH and GSSG content along with the higher GR activity [108]. Hence, Arabidopsis showed the GSH dependent H₂O₂ detoxification to attain tolerance. Higher total AsA was accumulated in Cajanus cajan upon complete water restriction conditions for up to nine days to defend against excess H₂O₂ toxicity [109]. Hence, drought enhanced the enzymatic activity of APX, DHAR, and GR for decreasing GSH/GSSG, as well as controlling ROS levels.

Similarly, the tolerant genotype VA13 of Amaranthus tricolor showed comparatively better tolerance under drought stress than the sensitive one (VA15) by expressing differential responses of the enzymatic and non-enzymatic ROS detoxification pathways [110]. Hence, VA13 expressed a remarkable increment in AsA-GSH redox by accelerating the enzymatic antioxidative actions by which increased non-enzymatic antioxidants (AsA and GSH) accumulation, which are vital for ROS detoxification.

Vigna radiata responded differently regarding different drought intensities [111] to control diverse levels of ROS. Moderate drought imposed by 10% polyethylene glycol (PEG) induced comparatively lowered ROS than severe drought (by 20% PEG). Therefore, severe drought-stressed Brassica showed a larger use of AsA-GSH pathways against higher H₂O₂ generation than moderate stress. Here, higher stress caused a higher increase of APX activity along with the lowest MDHAR and DHAR activity, while GR activity reduced differently than lower stress exposure to rapeseeds seedlings. Additionally, Hasanuzzaman et al. [52] also observed AsA and GSH both antioxidants contents reduced under severe drought condition, but increased under moderate stress. Bhuiyan et al. [104] found increased AsA content in B. rapa under drought (20% PEG). They also observed increased APX activity in drought-stressed seedlings, which assisted in efficiently scavenging the H₂O₂. Another two enzymes related to AsA regeneration MDHAR and DHAR also upregulated, as a result the AsA level was increased and strongly maintained its redox balance during oxidative stress situation. Nahar et al. [111] narrated the function of AsA as ROS detoxifier under drought stress where AsA content reduced in V. radiata with the increasing of ROS generation. Here, drought-induced higher APX activity enhanced the oxidation of AsA by scavenging H₂O_2, and improved GR activity increased the supply of GSH for involving ROS detoxification. Anacardium occidentale also showed the active participation of AsA-GSH cycle by integrative responses of both non-enzymatic and enzymatic antioxidants for drought-induced excess ROS regulation, where the higher accumulation of AsA and GSH, along with APX activity, coordinately reduced the overproduced H₂O₂ [112]. Thus, the AsA-GSH pathways involve in ROS detoxification as well as ROS homeostasis by eliminating excess ROS for keeping them up to the requirement of functioning cell signals.

Due to the fast industrialization of the modern world and unrestrained anthropogenic activities, toxic metals/metalloids stresses have become a gargantuan problem for the plant growth and development [121]. Plants experience toxic metals/metalloids stress try to survive to some extent by using their well-established antioxidant defense system. But, the activity and performance of defense system differ with stress concentration, stress duration, plant type, and age of the plant.

The enzymes of AsA-GSH pathway confirmed their differential responses to different toxic metals/metalloids stress (Table 2). Mahmud et al. [122] confirmed that due to Cr stress, the few components of AsA-GSH pathway increased their amount or activity in B. juncea L. cv. BARI Sharisha-11. They found five days duration of 0.15 mM and 0.3 mM K₂CrO₄ treatment decreased the content of AsA, but did not change the GSH content. Moreover, activities of APX and GR were enhanced; however, the activities of MDHAR and DHAR were diminished. The higher APX and GR activity might play a function in scavenging excess ROS. A similar upregulation of APX and GR was also recorded in B. napus L. cv. Binasharisha-3 due to Cd treatment [123]. From two separate experiments, they also found Cd stress (0.5 mM and 1.0 mM CdCl₂) for 48 h decreased the AsA content, but increased GSH content only under 0.5 mM CdCl₂ treatment. Exposure of Gossypium to 50 and 100 μM Pb(NO₃)₂ for six weeks increased the H₂O₂ content and APX activity [124]. The addition of 150 μM NiCl₂·6H₂O in growing media of B. juncea L. for one week increased the H₂O₂ content. Moreover, Ni stress decreased the AsA level but augmented the content of GSH and GSSG. Nickel also diminished the function of DHAR and MDHAR, however enhanced APX and GR activity [125]. Similar differential responses of AsA-GSH pathway components were also observed under As [126] and Al [50] toxicity. It can be stated that overproduced ROS plays the signaling role to some extent and inaugurate the higher activity of AsA-GSH enzymes under metals/metalloids toxicity. The upregulation of enzymes plays a significant role in maintaining the redox balance of AsA-GSH pathway under stress condition.

Along with the rise in average global temperature, HT stress has been turned into a topic to be concerned about among environmentalists and researchers worldwide. In general, a 5 °C temperature rise above the optimum temperature of growth is considered to be extreme temperature stress or HT stress or heat shock to any plant species [142,143]. Heat stress causes denaturation of protein and membrane lipids, enzyme inactivation, inhibited protein synthesis, and loss of membrane integrity [144], which results from the disruption of cellular homeostasis through the ROS formed in a mass amount under heat stress [143,145]. Focusing on the role of AsA-GSH pathway to scavenge these ROS, different crop species under different levels of extreme or HT stress have been studied (Table 3).

Khanna-Chopra and Chauhan [146] selected a warmer season to induce HT stress to two different cultivars of wheat (T. aestivum), which are Hindi62 (heat-tolerant) and PBW343 (heat-sensitive). They sowed the wheat seeds in mid-January and considered it as heat stress environment, while the control plants were sown in mid-November and considered as the non-stress environment. Data were collected at seven days interval up to 35 days after anthesis (DAA), and the results showed a sharp increase in H₂O₂ content up to 14 days, but then declined. Whereas, MDHAR and DHAR enzymes’ activity only increased in Hindi62, but APX and GR activities showed a fluctuating pattern of alteration in both cultivars [146]. Another cereal Z. mays when experimented similarly with two different cultivars; LM-11 (heat-sensitive) and CML-32 (heat-tolerant), exposed to 40 °C for 72 h, resulted in higher APX and GR activities in CML-32 roots, while a reduction occurred in the shoot. In LM-11, none of the enzyme activity or AsA content was affected [147]. Higher levels of O₂⁻ production rate and H₂O₂ content were observed in Ficus concinna seedlings under 48 h of HT (35 °C and 40 °C) stress condition, where AsA and GSH contents were unaffected at 35 °C, while declining AsA at 40 °C temperature [148]. The activity of APX, MDHAR, DHAR, and GR enzymes increased at 35 °C, but then again reduced at 40 °C to the level of control plants [148]. Under similar heat stress condition (40 °C, 48 h), V. radiata seedlings resulted in decreased GSH content and MDHAR-DHAR activities, but higher APX-GR activities [50]. Kiwi fruit (Actinidia deliciosa) seedlings, when exposed to 45 °C in an incubator for 8 h, resulted in higher AsA content and enhanced activity of all the AsA-GSH cycle enzymes [143]. Tomato seedlings were studied in two different aspects: short-term heat shock (40 °C, 9 h) [149] and long-term heat stress (38/28 °C day/night, seven days) [150]. In both experiments, the enhancement of O₂⁻ generation rate and H₂O₂ content were recorded, but enzyme (APX and GR) activity was only increased at short-term stress condition [149], while the long-term heat exposure reduced all four enzymes activities and GSH content [150]. Similar enzymatic activity was observed in Nicotiana tabacum seedlings after seven days of heat (35 °C) stress [151]. From the above discussion, it can be stated that heat stress prevailing for a longer duration is less likely to have the capability to modulate AsA-GSH pathway as compared to short-term heat stress.

Changes in global climate result in the frequent or unexpected occurrence of heavy rainfall in different regions of the globe, which causes a sudden flood and disrupts the normal ecosystem [2]. Such changes in the ecosystem may cause the extinction of plants species and imbalance in the natural environment [2]. Flooding-induced production of ROS and subsequent cellular damage has been authenticated in many studies so far [152,153,154]. Following are the discussion regarding crop species facing flooding stresses and modulation of their AsA-GSH pathway by flooding stress (Table 3).

Pigeon pea (C. cajan) seedlings that are exposed to waterlogged condition for six days revealed that tolerant cultivar could increase APX and GR activities, but a susceptible one cannot [155]. They also observed that, unlike other cases, waterlogging caused a lower accumulation of H₂O₂ and O₂⁻ [155]. In another experiment with V. radiata, Sairam et al. [156] showed that waterlogging similarly reduced the H₂O₂ and O₂⁻production rate in susceptible cultivar, while the tolerant ones remained unaffected. However, both APX and GR enzymes’ activity increased in tolerant genotypes, while the susceptible one got reduced [156]. The enhanced production rate of O₂⁻and H₂O₂ content under flooding stress has been reported in cotton [154], Welsh onion [157], and clover [158] plants. Cotton (G. hirsutum cv. Siza) plants after three and six days of flood exposure raised the AsA content but reduced the activity of APX, MDHAR, and GR [154]. A similar reduction in APX and GR enzymes activities was also recorded in Welsh onion (Allium fistulosum L.) after 10 days of waterlogging stress [157]. When Z. mays seedlings were waterlogged for 21 h at their root portions, they resulted in reduced AsA content and increased APX activity [159]. On the other hand, under long duration (14 days) flooding stress, Glycine max L. plants showed a reduction of GSH activity in roots and GR activity in the shoot, but the GSH in shoot and GR in root were not affected [153]. In case of complete submergence of O. sativa L. plants for two, four, or eight days, elevated levels GR enzyme activity was recorded, while APX enzyme activity increased only in tolerant cultivar [152]. Accordingly, the discussion reveals that the impact of flooding stress on AsA-GSH pathway varies depending upon the plant species and duration.

Atmospheric pollutants are the substances that are assembled in the air to a level or magnitude that is dangerous for living beings. Plants that are grown under different levels of atmospheric pollution have shown their oxidative stress responses and AsA-GSH pathway regulation in different manners (Table 3).

Erythrina orientalis plants were grown in three different locations of Philippines: La Mesa (a non-polluted area); and, Makati and Quezon (highly air-polluted cities). The results revealed that plants grown in the non-polluted area had lower activities of APX and GR as compared to the ones grown in highly polluted areas [160]. A similar increase in APX activity along with higher AsA content was recorded in Prosopis juliflora plants grown under polluted industrial region [161]. In a recent experiment, Lucas et al. [162] studied Lolium perenne plants that were grown under two different areas of Spain, Madrid, and Ciudad Real, where Madrid was considered to be more polluted than Ciudad Real. The findings indicated that the pollens of L. perenne accumulated a higher concentration of H₂O₂ and in shoots APX and DHAR activity declined, but the activity of MDHAR and GR increased in the shoot of L. perenne plants that were grown in Madrid [162]. When rice seedlings were exposed to continuous O₃ treatment, the results showed a remarkable increase in both O₂⁻ generation rate and H₂O₂ content. In addition, contents of AsA and GSH reduced, while APX, MDHAR, DHAR, and GR activity increased up to 70 days of O₃ exposure in SY63 cultivar and up to 79 days of O₃ exposure in WXJ14 cultivar [163]. Ascorbate and GSH contents were not affected by O₃ exposure in the Populus seedlings, but DHAR activity was lower, while the activity of GR and MDHAR was higher after 17 days of O₃ treatment [164]. Young strawberry (Fragaria x anansa) seedlings were exposed to three different levels of CO, NO_x, and SO₂, which are as follows: CO @ 133, 267, and 533 ppm, NO_x and SO₂ @ 25, 50, and 199 ppm corresponding to low, medium, and high dose, respectively. As a result of exposure to these atmospheric pollutants, H₂O₂ content as well as O₂⁻ generation rate increased. However, at low and medium doses of their exposure APX and GR activity increased, while at a high dose that decreased [165]. All sorts of atmospheric pollutants have a remarkable effect on AsA-GSH pathway, but further studies are required to demonstrate that those pollutants completely induced the modification of the AsA-GSH pathway.

Conklin et al. confirmed the positive role of AsA in protecting plants from ultraviolet (UV) radiation [159], where they found that Vit-C deficient mutant of A. thaliana was suffered by stress-induced damages than that of wild type. AsA-deficient mutants also showed sensitivity to O₃ stress due to a lower biosynthesis of AsA [171]. Gao and Zhang [172] reported that vitc1 mutants of A. thaliana showed physiological disorders and greater oxidative damages than the wild type, which was due to lower activities of antioxidant enzymes. Mutant plants also showed lower GSH/GSSG and higher DHA/(AsA+DHA) ratio than the wild type. Singh et al. [173] observed a decrease in AsA-GSH cycle enzymes in UV-exposed plants, which in turn affected the plants with oxidative stress. Similar to higher plants, marine macroalga Ulva fasciata also showed a positive correlation between enhanced the functions of AsA-GSH cycle and better tolerance of plants to UV radiation [174]. In their study, the scavenging of H₂O₂ was regulated by AsA-GSH cycle components, especially APX and GR. Noshi et al. [175] reported that AsA-GSH redox pool provided better protection of Arabidopsis from high-light mediated oxidative stress, which was mainly attained due to the higher activities of DHAR. However, both AsA and GSH were found to be responsible for conferring high light (HL) stress [175]. Later, Zheng et al. [176] that susceptibility of Arabidopsis mutant was to HL stress was related to the deficiency of AsA and GSH. When AsA deficient A. thaliana mutant (vtc2-1) was exposed to HL, they generated a high level of H₂O₂ (an oxidative stress marker) than the wild type, which was highly and negatively correlated with the total AsA content. The lack of AsA also resulted in lower chlorophyll (chl) content, chl fluorescence parameters, and PSII photochemistry [176]. Recently, Choudhury et al. [177] studied the metabolomics of A. thaliana grown under HL and found that the increased biosynthesis of GSH supports the photochemistry that supports Arabidopsis better survival under HL stress.

The pivotal role of the AsA-GSH cycle was observed in low pH stress also. Bhuyan et al. [170] tested five spring wheat cultivars at different levels of low pH stress. Their observation exhibited that low-pH stress resulted in elevated O₂⁻and H₂O₂ generation. A decrease in AsA content with increased DHA content was observed, although the APX activity decreased. Increased MDHAR activity was observed, but the ratio of AsA/DHA was not increased. Decreased GSH content and increased GSSG content were found where DHAR and GR activity decreased, resulting in a drop in the GSH/GSSG ratio.