Economically important cultivated beets such as fodder beets, sugar beets, garden beets (e.g., red beet) and leaf beets (e.g., Swiss chard) belong to the sub-species Beta vulgaris [28]. All beets originate from a halophytic plant, Beta vulgaris (sea beet or wild beet). Glycinebetaine (GB) is a quaternary ammonium compound naturally synthesized by various plant species. Involvement of GB in the protection of native protein from denaturation, cell membranes from oxidative damage and its contribution to cellular osmotic adjustments under water-limited environment make it a vital plant-osmolyte [29]. It is also involved in the regulation of various biochemical processes via systematic signaling pathways and studies also suggested its positive contribution to carbon, nitrogen reserves and reactive oxygen species neutralization [30]. Although several studies report different responses of Beta vulgaris to environmental stresses, research articles and reviews mostly focus on salt and drought response mechanisms in beets [31,32]. Therefore, we need breeding techniques and agronomic practices for better tolerance to biotic and abiotic stresses in B. vulgaris [33]. Thus, cultivated beets and their wild ancestor are important genetic sources for crop breeding programs and studying abiotic stress tolerance [32]. Sugar beet belongs to the family Chenopodiaceae, and beetroot also contains a significant fraction of antioxidants and other bioactive compounds such as betaine, betalain and ferulic acid [31]. Glycinebetaine was primarily discovered from sugar beet (Beta vulgaris), which accumulates GB up to 100 mM concentration [34]. These compounds can improve agricultural productivity through mitigation of adverse effects of environmental stresses on cultivated crops.

The exogenous application of GB improved plant growth and productivity under different stress conditions. Nowadays, a number of compounds including osmoprotectants such as proline and GB are used with exogenous application to plants to reduce the harmful effects of abiotic stresses including drought stress. GB, a quaternary ammonium substance, is an osmoprotectants that can effectively scavenge ROS in plant tissues [35,36], and improves the photosynthetic rate by maintaining the Rubisco ultra-structure [17]. It is present in different amounts in plant parts including seed, stem, root and flowers [37]. During the early juvenile stage of plant, it is present in small amounts in the roots but later increases in leaves [38]. Different levels of GB can be observed in different plant species under different abiotic stresses depending on plant species, genotype, development stage, application modes and different stress conditions [39]. GB plays an essential role to provide protection from high accumulation of ROS species in plants under water shortage [40] and increases the photosynthetic defensive mechanism [29]. Rapid change in cellular metabolism, inferior level of water potential and ABA recognition sites give rise to accumulation of GB under water stress [17]. Furthermore, exogenously applied GB enhances yield and tolerance level by increasing chlorophyll contents, stimulating antioxidant defensive system, decreasing ROS and stabilizing the photosynthesis ability of photosystem II under drought stress [36]. The application of sugar beet extract also resulted in improvement in drought stress tolerance in okra plants through maintenance of ionic homeostasis which contributed to the better photosynthetic activity and yield attributes [32]. Similarly, improvement in growth and biochemical parameters of drought-stressed pea plants was recorded in response to sugar beet extract application [33]. Interestingly, economically important cereal crops such as wheat, rice, barley and maize do not synthesize or retain GB naturally. As a way forward, exogenous application of sugar beet extract can be tested on major cereal crops to study its effects in abiotic stress tolerance particularly osmotic stress [32]. Moreover, various transgenic plants over-expressing GB biosynthetic genes and enhanced retention also exhibited drought and salinity tolerance.

Ascorbic acid (AsA), also referred to as vitamin C, is a major nonenzymatic antioxidant in plants and plays an important role in alleviating certain oxidative stresses caused by biotic and abiotic stress [82,83]. AsA can enhance the growth of a plant and boost its capacity to withstand stress [84,85,86]. Moreover, AsA is the first line of plant defense against oxidative stress by removing a number of free radicals, such as O₂^•–, HO^•, and H₂O₂, mostly as a substrate of APX, an essential enzyme of the ascorbate–glutathione pathway [17,54,55]. Ascorbate is a cofactor for several cellular enzymes, such as violaxanthin de-epoxidase, which is essential for photoprotection by xanthophyll cycle and other enzymes and is directly involved in the removal of ROS, and the addition of exogenous AsA will inhibit lipid peroxidation and decrease malondialdehyde (MDA) content in plant tissues, thus improving the antioxidant ability of plant tissues [71,83,87,88]. The effect of ascorbic acid on improving the salinity tolerance of potatoes was studied by Sajid and Aftab [89]. They noted that activity of most antioxidant enzymes, such as SOD, POD, CAT and APX, increased significantly under NaCl stress conditions after exogenous application of ascorbic acid, thereby improving plant survival under environmental stresses. Younis et al. [90] also stated that a marked and statistically significant increase in the percentage resistance to salt stress and growth of Vicia faba seedlings was caused by the exogenous addition of 4 mM ascorbic acid with NaCl to the stressful media during experimentation (12 days). Aly et al. [91] observed that addition of 1 mM of ascorbic acid to Egyptian clover (Trifolium alexandrinum L.) seedlings grown in NaCl medium significantly increased seeds germination, carotenoids and chlorophyll and the dry mass of seedlings grown in NaCl medium.

Being a cofactor of various enzymes involved in phytohormone-dependent signaling cascades [92,93], it acts as a signaling molecule in various cellular and sub-cellular processes [94]. It can efficiently quench reactive oxygen species and thereby protect membrane structures and vital bio-molecules from oxidative stress [95]. The diverse involvement of ascorbic acid in the regulation of plant growth, physio-biochemical responses, flowering and most importantly stress sensing, signalling and regulation of ascorbate-glutathione cycle is well documented [96]. Sweet oranges are cultivated as the largest citrus fruit, and its global cultivation produces about 70% of total annual citrus yield [97]. The cultivation and production of oranges in Pakistan is ranked amongst the top suppliers. Sweet oranges are borne on a small flowering evergreen tree (7.5 to 15 m height) from the Rutaceae or citrus family and are rich source of vitamin C, and contain trace quantities of other vitamins and minerals including Ca, K, Mg, folate, thiamin and niacin [98]. Its juice is a good source of vitamin C, folate and polyphenols. The exogenous application of vitamin C improves stress tolerance among plants via regulation of cell expansion, ion transport, phytohormone signaling and reactive free radicals [71,99]. The use of Citrus sinensis extracts could potentially be an eco-friendly approach to induce multi-stress tolerance in plants and future studies should investigate its involvement and efficacy to regulate crop responses.

Melia azedarach is a deciduous tree of the Melia genus, which also commonly known as the purple flower tree, forest tree, and golden Lingzi. It is a fast-growing and high-quality timber tree; it is also a good nectar plant and a vital plant pesticide [100]. The timber, which resembles mahogany, is used to manufacture agricultural implements, furniture, plywood, etc. Melia azedarach is also of value for the health care and pharmaceutical industries, an effective composition due to its analgesic, anticancer, antiviral, antimalarial, antibacterial, antifeedant, and antifertility activity [101]. Furthermore, it is an important afforestation tree species, as are the surrounding greening tree species. Melia azedarach is widely distributed. It is native to tropical Asia and has been introduced to the Philippines, United States of America, Brazil, Argentina, African and Arab countries [100]. In China, it is concentrated in the south and southwest, with a relatively concentrated distribution in the east and central regions, and a marginal distribution area in the north, southwest, and southern Shanxi and Gansu [102]. For this reason, Melia azedarach, as a tree native to China, has diverse provenances [103].

Various naturally occurring secondary metabolites including terpenoids play developmental and regulatory roles among plants. Terpenoids are derived from isoprene units and such compounds serve as pigment molecules, vitamins, hormones and non-enzymatic antioxidants [104]. The diverse involvement of terpenoids in plant physio-biochemical functioning and regulation of stress tolerance is documented. The M. azedarach (Persian lilac or Chinaberry) is a deciduous tree from Meliaceae family is rich in terpenoids [100]. Different plant parts including fruit, root, bark, stem and leaf contain diverse chemical compounds such as azedarachins, trichillins, limonoids and meliacarpns. It is widely distributed in sub-continent countries including Pakistan, Nepal, Bangladesh, Sri Lanka and exhibit excellent medicinal properties [103]. Certain phenolic compounds also contribute to higher antioxidant activity of Melia [105]. Extracts of M. azedarach fruit were effective in controlling chickpea blight. Similarly, a pathogenic fungus, Sclerotium rolfsii was found to be controlled by the application of Melia extract [106]. Antifungal and antibacterial properties of the M. azedarach extract on pathogenic fungal species including Fusarium oxysporum, Fusarium solani, Fusarium sambucinum, Fusarium oxysporum, Alternaria alternate, Botrytis cinerea and bacteria including Enterococcus faecalis, Escherichia coli and Bacillus subtilis were prominent [107]. The application of M. azedarach leaf extract was reported to enhance salinity tolerance of pea plants [108]. The inhibitory effects of M. azedarach extracts were also recorded on germination and biochemical traits of radish [107] and future studies should investigate the crop-specific effect of M. azedarach extracts to potentiate its applications at larger agricultural scale.

5. Azadirachta indica—Source of Secondary Metabolites