Chromium toxicity decreased plant weight of lemon balm. It can decrease soil fertility by affecting nutrient availability and uptake by plants. Cr ions can enter plant cells and cause cellular damage. It can disrupt cellular structures and impair various physiological functions, leading to cellular dysfunction and ultimately hindering plant growth. Cr toxicity can induce oxidative stress in plants. High levels of Cr can generate ROS such as superoxide radicals and hydrogen peroxide, leading to oxidative damage to plant cells. Oxidative stress can disrupt cellular membranes, impair enzyme activities, and hinder plant growth (Javed et al. 2021). It can interfere with essential plant nutrients such as phosphorus, iron, and calcium, leading to nutrient imbalances and reduced plant growth. High levels of Cr in soils can inhibit plant growth and reduce crop productivity. Cr can accumulate in plant tissues and eventually transfer to higher trophic levels through consumption. This bioaccumulation and biomagnification phenomenon can pose risks to wildlife and humans who consume contaminated plants or animals, leading to potential health hazards. Therefore, Cr toxicity negatively affects plant growth by impairing seed germination, stunting growth, causing chlorosis, inducing cellular damage, interfering with nutrient uptake and assimilation, disrupting water and nutrient transport, and inducing oxidative stress. These effects collectively reduce plant vigor, biomass production, and overall plant health (Ahmad et al. 2020). Exogenous MT and TiO2 NPs increased plant weight. Melatonin acts as a powerful antioxidant, and its exogenous application can enhance the plant's antioxidant defense system when used in conjunction with TiO2 NPs. This combination can provide added protection against oxidative stress, reducing the damage caused by ROS and improving overall plant health. The synergistic action of MT and TiO2 NPs can boost the plant's ability to scavenge ROS more effectively (Giglou et al. 2023; Sun et al. 2023). Exogenous MT application has been shown to enhance nutrient uptake in plants. By promoting the activity of nutrient transporters and improving root development, melatonin can increase the absorption and translocation of essential nutrients in the presence of TiO2 NPs. This improved nutrient uptake facilitates better plant growth and biomass accumulation. Melatonin has been reported to enhance photosynthesis in plants. When combined with TiO2 NPs, which can also enhance the efficiency of light absorption and energy transfer, exogenous melatonin can further increase photosynthetic activity (Emamverdian et al. 2022). This combination leads to improved carbon assimilation, higher carbohydrate production, and ultimately, enhanced plant growth and weight. In addition, foliar-applied MT can regulate the balance of various plant hormones, including auxins, cytokinins, and gibberellins. This hormone modulation in the presence of TiO2 NPs can influence cell division, elongation, and overall plant growth, resulting in greater biomass accumulation and increased plant weight (Rizwan et al. 2023). Previously, the positive role of TiO2 NPs on Pleioblastus pygmaeus (Emamverdian et al. 2022) and MT on Zea mays (Malik et al. 2021) and Brassica napus (Ayyaz et al. 2021). However, there is no prior evidence on the co-application of MT and TiO2 NPs to improve plant biomass under Cr toxicity that can be presented in this study.
Cr toxicity reduced Chl content in lemon balm plants. Cr toxicity can disrupt Chl synthesis, which is essential for the production of Chl molecules. Cr interferes with the activity of enzymes involved in the biosynthesis of Chl, such as chlorophyll synthase and magnesium chelatase. This disruption hampers the normal production and turnover of Chl molecules, leading to a decrease in Chl content. Cr toxicity can disrupt the structure and function of photosystem components, specifically photosystem II (PS II). PS II is responsible for capturing light energy and initiating the process of photosynthesis (Bruno et al. 2020). Chromium can impair the activity of PS II reaction centers and reduce the efficiency of light energy conversion, which can reduce the synthesis of Chl molecules. Cr toxicity can promote the generation of ROS in plants, which can damage cell membranes, proteins, and pigments, including chlorophyll. Chromium can disrupt the integrity and permeability of cell membranes in plants (Kumar and Seth 2022; Singh et al. 2023). This disturbance affects the efficiency of nutrient uptake, water transport, and metabolic processes, including Chl synthesis and maintenance. As a result, the plants may experience reduced availability of minerals and impaired photosynthetic machinery, leading to a decrease in Chl content. Exogenous melatonin, when combined with titanium nanoparticles TiO2 NPs, can enhance the efficiency of TiO2 NPs on Chl content under Cr toxicity. Melatonin acts as an antioxidant, protecting chlorophyll molecules from oxidative damage caused by Cr toxicity. It also regulates gene expression and hormone levels related to Chl synthesis and photosynthesis. Additionally, melatonin enhances nutrient uptake, supporting Chl production. The synergistic effects of melatonin and TiO2 NPs lead to a more significant increase in Chl content, providing a more efficient approach to counteract the negative effects of Cr toxicity on Chl content in plants (Raja et al. 2023). Gatasheh et al. (2023) reported that synergistic application of MT and Si NPs alleviates Cr stress in Brassica napus through improving Chl content.
The foliar application of MT and TiO2 NPs decreased MDA content under Cr toxicity. Cr toxicity increases the production of ROS in plants. These ROS attack and oxidize lipids in cell membranes, leading to lipid peroxidation. This peroxidation process results in the formation of MDA, a byproduct of lipid peroxidation. Thus, Cr toxicity increases MDA levels as a consequence of oxidative damage to cell membranes (Kumar et al. 2023). Melatonin and TiO2 NPs possess antioxidant properties and can scavenge ROS generated under Cr toxicity. ROS can cause oxidative damage to cell membranes and increase MDA levels. By neutralizing these ROS, MT and TiO2 NPs prevent lipid peroxidation and decrease MDA production. Cr toxicity can disrupt the integrity and fluidity of cell membranes. This disruption can lead to increased lipid peroxidation and MDA levels. Melatonin and TiO2 NPs can help maintain the integrity of cell membranes by preventing lipid peroxidation and stabilizing membrane structure. This action reduces the release of MDA into the extracellular space and decreases MDA levels. MT and TiO2 NPs can enhance the activity of antioxidant enzymes, which play a crucial role in the detoxification of ROS and the prevention of lipid peroxidation (Chen et al. 2021; Malik et al. 2021; Mao et al. 2023). MT and TiO2 can modulate signaling pathways involved in oxidative stress responses. These substances can regulate the expression of genes related to antioxidant defense systems and lipid metabolism. By influencing these signaling pathways, MT and TiO2 can mitigate the production of MDA under Cr toxicity.
Soil Cr at 100 mg Kg− 1 soil had toxic effects on lemon balm plants. Cr toxicity leads to increased ROS production in cells, which damage cellular components. In response to this oxidative stress, cells upregulate the production of antioxidant enzymes such as SOD and CAT to counteract the detrimental effects of ROS. Cr toxicity can modulate gene expression, resulting in increased synthesis and production of SOD and CAT enzymes (Albqmi et al. 2023). This regulation involves the activation or inhibition of specific transcription factors that control the expression of these enzymes. Cr toxicity can also affect the post-translational modifications of SOD and CAT enzymes, thereby impacting their activity. For instance, Cr can induce phosphorylation, acetylation, or nitration of these enzymes, which can either enhance or impair their catalytic functions. These modifications can alter enzyme stability, substrate specificity, or interaction with cofactors, ultimately affecting their enzymatic activities. In addition, Cr ions can directly interact with SOD and CAT enzymes, impacting their activity. Cr can bind to metal centers within these enzymes, such as the copper and zinc (Cu/Zn) active site of SOD or the iron (Fe) center of CAT, leading to changes in their enzymatic properties. This interaction can either enhance or inhibit the catalytic efficiency and stability of these enzymes, depending on the Cr species and concentration (Tang et al. 2023). SOD and CAT activity was higher overall than in the control, but it can be lower in stressed plants. It could be because TiO2 and MT can control Cr toxicity, resulting in decreased ROS generation. As a result, when ROS levels in the cell are low, enzyme activity is decreased. Foliar application of TiO2 and MT is a powerful antioxidant molecule known for its ability to scavenge ROS and reduce oxidative stress in plants (Deng et al. 2024). It can directly neutralize ROS or indirectly facilitate the activities of antioxidant enzymes like SOD and CAT. Melatonin is believed to stimulate the expression and activity of these enzymes by modulating gene expression pathways (Sun et al. 2023). It can activate specific transcription factors or signaling molecules, leading to upregulation of the SOD and CAT genes. Similar to our results, the decreased antioxidant enzyme activity was obtained in stressful plants when treated with NPs (Zadegan et al. 2023).
Cr toxicity at 50 mg kg-1 soil led to the maximum TPC and TFC in lemon balm plants. Moderate Cr toxicity can induce the production of TPC and TFC. This is primarily achieved through the activation of defense responses, including the synthesis of phenolic compounds as part of the plant's antioxidant defense system (Khosropour et al. 2021). These phenolic compounds, such as flavonoids, can scavenge ROS, induce antioxidant enzymes, chelate metal ions, and regulate gene expression pathways. Overall, the increase in TPC and TFC helps plants cope with the toxic effects of Cr by enhancing their antioxidant capacity and protection against oxidative stress. Similarly, the inbreeded TPC and TFC in have been reported on coriander (Baabashpour et al. 2022), summer savory (Alavian et al. 2023), barberry (Khosropour et al. 2021) under moderate amounts of heavy metals. Both MT and TiO2 NPs can stimulate the synthesis of secondary metabolites, including phenolic compounds and flavonoids, in plants. These compounds are synthesized through various biochemical pathways, such as the phenylpropanoid and flavonoid biosynthetic pathways. The application of Ti NPs or melatonin can activate the expression of key enzymes involved in these pathways and subsequently increase TPC and TFC. Melatonin and TiO2 have antioxidant properties and can scavenge ROS in plants. ROS accumulation can result in oxidative stress, leading to the synthesis of phenolic compounds and flavonoids as a defense response (Vafadar et al. 2020). The scavenging of ROS by MT and TiO2 NPs reduces the oxidative stress burden on plants, allowing them to allocate resources towards the synthesis of TPC and TFC. Melatonin and TiO2 can activate specific genes associated with the biosynthesis of phenolic compounds and flavonoids. These molecules can interact with various signaling pathways, including transcription factors and protein kinases, to enhance the expression of genes encoding key enzymes involved in TPC and TFC synthesis. By upregulating the expression of these genes, MT promote the accumulation of TPC and TFC in plants (Tousi et al. 2020; Nourozi et al. 2021). MT and TiO2 NPs and melatonin can modulate the activity of enzymes involved in the biosynthesis of TPC and TFC. For instance, they may enhance the activity of key enzymes such as phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) in the phenylpropanoid and flavonoid pathways, respectively. This increased enzyme activity leads to higher production and accumulation of phenolic compounds and flavonoids, thereby increasing TPC and TFC levels (Vafadar et al. 2020).
Although moderate Cr toxicity increased EO content, its severe stress decreased EO yield. Chromium is a heavy metal that can have both beneficial and detrimental effects on plants, depending on its concentration and the duration of exposure. The response of plants to Cr toxicity can vary, and the effects on EO production may be influenced by multiple factors, including the plant species, growth stage, and Cr concentration. Under moderate levels of Cr toxicity, some studies have suggested that certain plant species may exhibit an increase in EO production as a defense mechanism against the stress induced by Cr (Prasad et al. 2010). This increase in EO production could be a response to counteract oxidative stress and alleviate the toxic effects of Cr. On the other hand, higher concentrations or severe levels of Cr toxicity are known to have detrimental effects on plant growth and development. This can lead to physiological disorders, such as reduced photosynthesis, impaired nutrient uptake, and oxidative damage. In such cases, the plant's overall health and productivity can be negatively affected, potentially leading to a decrease in EO production (Patra et al. 2019).
MT and TiO2 NPs have been reported to stimulate the synthesis of secondary metabolites, including essential oils, in plants. These compounds are produced through various biochemical pathways, such as the terpenoid and phenylpropanoid pathways. The application of MT and TiO2 NPs can activate the expression of key enzymes involved in these pathways, leading to an increase in essential oil production. MT and TiO2 can activate specific genes associated with the biosynthesis of essential oils. They can interact with various signaling pathways, such as transcription factors and protein kinases, to enhance the expression of genes encoding enzymes involved in essential oil synthesis (Gohari et al. 2020). By upregulating these genes, MT and TiO2 and melatonin promote the accumulation of essential oils in plants. TiO2 NPs and melatonin can induce the release of volatile compounds, including essential oils, from plant tissues. This could be attributed to their influence on the regulation of stomatal opening and closure, which can affect the release of essential oils. The increased release of volatile compounds leads to a higher concentration of essential oils in plants treated with co-application of MT and TiO2 (Bidabadi et al. 2020; Gohari et al. 2020).
The moderate Cr toxicity had a positive impact but severe Cr stress possessed a negative impact on the production and accumulation of rosmarinic acid in lemon balm. Cr is a heavy metal that can be taken up by plant roots and subsequently transported to various plant tissues, where it can disrupt normal cellular processes and biochemical pathways. One way in which Cr toxicity can reduce rosmarinic acid in lemon balm is by interfering with the activity of key enzymes involved in its biosynthesis (Adamczyk-Szabela et al. 2023). Rosmarinic acid is synthesized through the phenylpropanoid pathway, which begins with the conversion of phenylalanine into cinnamic acid by the enzyme phenylalanine ammonia-lyase (PAL). The increased PAL and rosmarinic acid were reported under salinity stress in lemon balm by Ghasemian et al. (2021). Studies have shown that severe Cr can inhibit the activity of PAL and other enzymes involved in this pathway, thereby reducing the synthesis of rosmarinic acid (Gho et al. 2020). Furthermore, Cr toxicity can disrupt the antioxidant defense system in lemon balm. Rosmarinic acid acts as a potent antioxidant, protecting the plant against oxidative stress. However, heavy metals like Cr can ROS and impair the plant's ability to scavenge and neutralize them, causing oxidative damage (Adamczyk-Szabela et al. 2023). This oxidative stress can lead to a reduced production of rosmarinic acid, as the plant allocates its resources towards combating Cr-induced oxidative damage. Cr ions can compete with essential nutrients like nitrogen, phosphorus, and iron, leading to nutrient deficiencies and impaired rosmarinic acid production. Abiotic stress can interfere with gene expression and transcription factors involved in the biosynthesis of rosmarinic acid. Studies have shown that abiotic stresses can downregulate the expression of genes encoding key enzymes like PAL, affecting the overall synthesis of rosmarinic acid (Ghasemian et al. 2021). Foliar application of MT and TiO2 NPs has been investigated for their potential to enhance rosmarinic acid production in lemon balm and other plant species. MT and TiO2 NPs can stimulate the phenylpropanoid pathway and activate key enzymes involved in rosmarinic acid synthesis, while MT acts as a signaling molecule to enhance gene expression in the phenylpropanoid pathway. Both MT and also have antioxidant properties, reducing oxidative stress and redirecting plant resources towards secondary metabolite production (Vafadar et al. 2020). The combination of MT and TiO2 NPS has shown synergistic effects on rosmarinic acid production. However, the optimal concentration and application method should be determined for efficient results without negative impacts on plant growth and development.