Co-application of titanium dioxide and hydroxyapatite nanoparticles modulated chromium and salinity stress via modifying physio-biochemical attributes in Solidago Canadensis L.

Abstract Climate change and human activity have led to an increase in salinity levels and the toxicity of chromium (Cr). One promising approach to modifying these stressors in plants is to use effective nanoparticles (NPs). It is unclear which well-known NPs are most effective in controlling salinity and Cr, as well as which one of salinity and Cr is more dangerous for plants. Although titanium dioxide nanoparticles (TiO₂ NPs) have been used recently to decrease salt and Cr toxicity, it is unclear how TiO₂ NPs compare to and function in conjunction with hydroxyapatite nanoparticles (HAP NPs). The objective of the current study was to determine the effects of foliar-applied TiO₂ NPs (15 mg L⁻ ¹) and HAP NPs (250 mg L⁻ ¹) separately and in combination on growth, photosynthesis, water content, lipid peroxidation, antioxidant capacity, phenolic content, and essential oils (EOs) of Solidago Canadensis L. under salinity (100 mM NaCl) and Cr toxicity (100 mg kg⁻ ¹ soil). Salinity was more deleterious than Cr by decreasing plant weight, chlorophyll (Chl), relative water content (RWC), EO yield, and increasing malondialdehyde (MDA), electrolyte leakage (EL), superoxide dismutase (SOD) activity, and catalase (CAT) activity. The co-application of TiO₂ and HAP NPs proved to be more successful. This was evidenced by the increased shoot weight (36%), root weight (29%), Chl a + b (23%), RWC (15%), total phenolic content (TPC, 34%), total flavonoid content (TFC, 28%), and EO yield (56%), as well as the decreased MDA (21%), EL (11%), SOD (22%) and CAT activity (38%) in salt-exposed plants. The results demonstrated the effective strategy of combining these NPs to modify abiotic stress.