Plant weight was significantly reduced as a result of Cr and salt. High amounts of Cr are hazardous to plants. Plants' regular physiological functions, including nutrition, photosynthesis, and water control, may be hampered by excessive amounts of Cr exposure. This may lead to yellowing of the leaves, slowed growth, and eventually a drop in plant weight (Chen and Kattab 2024). Similarly, high soil salinity can impede plant development by blocking the uptake of water and nutrients (Lasheen et al. 2023). On plant growth, salt had a more detrimental effect than Cr stress. High Na and Cl levels make it difficult to control and harmful to plant health because they cause significant osmotic stress, dehydration, and severe nutritional deficits (Abdi et al. 2023). However, it appears that the use of TiO2 and HAP NPs, particularly in combination, promotes plant growth. These NPs may increase plant growth more successfully when combined than when used separately. Because of the complementary actions of TiO2 and HAP NPs, plants may absorb nutrients more readily, have better metabolisms, experience less environmental stress, and overall greater growth (Sardar et al. 2022; Wu et al. 2023). There might be a lot of potential applications for this combination in agriculture. This combination may offer promising opportunities for agricultural applications. In line with the current study, Chen and Kattab (2024) showed that TiO2 NPs can reduce Cr toxicity in black cumin by increasing plant biomass.
Salinity and Cr reduced the Chl level whereas TiO2 and HAP NPs raised it. Plants may experience oxidative stress due to both salt and Cr stress, which can cause ROS to be produced and harm Chl molecules. In addition, this stress can damage the photosynthetic apparatus, alter the mineral-nutrient balance necessary for the synthesis of chlorophyll, and limit the activity of enzymes involved in chlorophyll biosynthesis (Kumar et al. 2023; Abdi et al. 2023). Additionally, these stresses can alter gene expression related to Chl biosynthesis and photosynthesis, ultimately leading to a decrease in Chl content in plants (Patra et al. 2024). TiO2 and HAP NPs can enhance Chl content in plants under Cr contamination and salinity stress by acting as antioxidants to neutralize ROS, regulating enzyme activity for Chl biosynthesis, improving nutrient uptake for Chl production, enhancing photosynthetic efficiency, and modulating gene expression related to Chl synthesis and stress responses (Gohari et al. 2020; Mustafa et al. 2022; Esserti et al. 2024). These NPs collectively contribute to maintaining Chl levels and promoting plant growth in challenging environments. Lashkary et al. (2021) showed that TiO2 NPs could enhance Chl content in plants exposed to salinity stress.
TiO2 and HAP NPs modulated salinity and Cr stress via enhancing RWC. Salinity and Cr stress can cause oxidative stress in plants, which damages membranes and other cellular components by releasing ROS (Shah et al. 2021). Because of the disruption of water transport channels and increased permeability of membranes brought on by this oxidative stress, plants may experience a drop in their RWC (Lashkary et al 2021). Conversely, it has been demonstrated that HAP and TiO2 NPs have antioxidant qualities and can increase the activity of antioxidant enzymes in plants. By scavenging ROS and shielding cellular constituents from oxidative damage, these NPs help preserve membrane integrity and enhance plant cell water retention. TiO2 was shown by Mohammadi et al. (2024) to have a beneficial effect on raising RWC in plants under salinity stress. It is uncertain how HAP NPs are used to alter RWC under stressful environmental conditions. The current work has provided further details on how TiO2 and HAP NPs alter RWC to modify salinity and Cr.
Salinity and Cr stress are known to be detrimental to plant health as they can induce lipid peroxidation and damage to cell membranes. This damage leads to an increase in MDA and EL in plants, which are considered markers of oxidative stress and membrane damage caused ROS produced under stressful conditions (Gohari et al. 2020; Soliman et al. 2024). Conversely, TiO2 and HAP NPs have been shown to possess antioxidant properties that can help mitigate oxidative damage in plants (Yousef et al. 2021; Sardar et al. 2022). These NPs have the ability to reduce the impact of stressors on plant cells. When considering the effects of salinity, Cr, TiO2 and HAP NPs on plants, it is essential to evaluate their impact on oxidative stress and membrane damage (Abdalla et al. 2022). The balance between their antioxidant properties and their potential to induce oxidative stress must be carefully weighed to determine their overall benefit or harm to plant health. According to reports, TiO2 has a beneficial effect on lowering salinity stress in maize (Shah et al. 2021), Linum usitatissimum (Singh et al. 2021), and Mentha piperita (Mohammadi at al. 2024). Few research (Alhammad et al. 2022; Wu et al. 2023) have reported on the beneficial effect of HAP on mitigating abiotic stress. The intriguing finding of this study was that TiO2 and HAP NPs had a synergistic effect on mitigating MDA and EL. This effect may have been caused by the enhanced antioxidant qualities of both TiO2 and HAP NPs, which work together to scavenge ROS and protect cell membranes from oxidative damage (Chen and Kattab 2024).
In stressed S. canadensis L. plants, salt and Cr stress increased SOD and CAT activity, whereas TiO2 and HAP NPs alleviate it. By encouraging the generation of ROS, salinity and Cr stress can cause oxidative stress in plants. Plants initiate antioxidant defense systems in response to elevated ROS levels, safeguarding themselves against oxidative damage (Ma et al. 2024). Upregulating the activity of antioxidant enzymes like SOD and CAT is one of the main ways plants respond to oxidative stress. Superoxide radicals undergo dismutation into hydrogen peroxide by the action of SOD, whereas hydrogen peroxide is transformed into water and oxygen by CAT. SOD and CAT are essential for preserving cellular redox equilibrium and reducing oxidative stress-related damage because they remove hydrogen peroxide and superoxide radicals (Moezzi and Javanshir Khoei 2024). However, it has been discovered that the activity of these enzymes in stressed plants is affected differently by TiO2 and HAP NPs (Soliman et al. 2024). According to some study, TiO2 and HAP NPs may be antioxidants in and of themselves, which would lessen the requirement for plants to increase their SOD and CAT activities in response to stress (Wu et al. 2023; Ma et al. 2024). The complex biochemical reactions of plants to environmental stresses are highlighted by the interaction of salinity, Cr stress, NPs, and enzyme activity. Similarity, the increased SOD and CAT under Cr toxicity and lowered by TiO2 has been reported on lemon balm (Soliman et al. 2024).
Cr stress along with TiO2 and HAP NPs led to reach the maximum TPC and TFC. One possible mechanism by which Cr and TiO2 and HAP NPs can enhance TPC and TFC is through the activation of the phenylpropanoid pathway. This pathway is responsible for the biosynthesis of phenolic compounds and flavonoids, which are important antioxidants in plants. Exposure to Cr and TiO2 and HAP NPs may upregulate the expression of genes encoding key enzymes involved in the phenylpropanoid pathway, leading to increased production of phenolic compounds and flavonoids (Soliman et al. 2024). Furthermore, Cr and TiO2 and HAP NPs can act as stressors that trigger signaling pathways involved in the production of secondary metabolites, including phenolic compounds and flavonoids. In addition, the presence of these NPs may directly interact with plant cells and organelles, potentially affecting cellular processes involved in the synthesis and accumulation of TPC and TFC (Jampílek and Kráľová 2021). TiO2 and HAP NPs can penetrate plant tissues and cell walls, where they may trigger physiological responses and metabolic changes that lead to alterations in TPC and TFC levels (Soliman et al. 2024). It has previously been documented that a variety of medicinal plants exhibit increased TPC and TFC in response to mild abiotic stress and decreased TPC and TFC in response to severe abiotic stress (Babashpour-Asl et al. 2022; Alawamleh et al. 2023). Accordingly, salinity has a tendency of severe stress in the current study, while Cr has a trend of mild stress.
Cr exposure can enhance the production of EOs in plants through biochemical pathways involving the activation of the terpenoid biosynthetic pathway, modulation of gene expression, and regulation of signaling molecules (Ma et al. 2022). Cr-induced oxidative stress and the influence on plant hormones may also play a role in stimulating the biosynthesis of EOs. One possible mechanism by which TiO2 and HAP NPs enhance EO production is through their ability to induce stress responses in plants (Gohari et al. 2020). The NPs exposure can trigger oxidative stress and the production ROS, which can act as signaling molecules to activate pathways involved in secondary metabolite biosynthesis, including EOs. The plant's response to NP-induced stress may involve the upregulation of enzymes and genes involved in EO biosynthesis (Jafari et al. 2022). Additionally, TiO2 and HAP NPs have been shown to modulate plant hormone levels and signaling pathways, which can impact the production of EOs (Kumar et al. 2023). The NPs may influence the expression of genes encoding enzymes in the terpenoid biosynthetic pathway, leading to increased accumulation of EOs in plants (Sardar et al. 2022). In this context, Soliman et al. (2024) demonstrated the beneficial function of TiO2 NPs in raising the yield and EO % of lemon balm. Furthermore, Jafari et al. (2022) showed the beneficial effects of TiO2 NPs in raising marjoram EO production in the face of salt stress. Exposure to Cr and salinity can decrease plant weight, which in turn reduces the overall yield of EOs. This is because EO yield is calculated as a percentage of the plant material's weight. Environmental stressors like salinity and heavy metal contamination can negatively impact plant growth and development, resulting in lower biomass production and ultimately leading to a decrease in EO yield (Babashpour-Asl et al. 2022; Soliman et al. 2024).