Effects of arbuscular mycorrhizal fungi and TiO₂ nanoparticles on the photosynthetic physiology and cadmium uptake of Hemerocallis fulva under cadmium pollution

Studying the combined remediation effects of microorganisms and nanomaterials on heavy metal-contaminated soil is of considerable significance for the prevention and control of heavy metal pollution. In this study, we conducted an indoor pot experiment to investigate the effects of 50 mg·kg⁻¹ cadmium (Cd) pollution on the growth, chlorophyll content, chlorophyll fluorescence and gas exchange parameters, Cd uptake using Hemerocallis fulva (Golden Doll). Additionally, we also investigated the antioxidant system of H. fulva treated with an inoculation of arbuscular mycorrhizal fungi (AMF) and root application of T₁ (100 mg·L⁻¹), T₂ (200 mg·L⁻¹), T₃ (400 mg·L⁻¹) concentrations of titanium dioxide nanoparticles (TiO₂ NPs), as both independent and combined treatments. The aim of the study was to establish the effects of these two treatments on improving the photosynthetic physiology of H. fulva and promoting Cd uptake in Cd-contaminated soils. The results revealed that compared with the control (CK) treatment, there were significant reductions in the height, number of leaves, and aboveground and root biomasses of H. fulva under Cd pollution. Root application of different concentrations of TiO₂ NPs and the combined TiO₂ NPs + AMF treatment were found to alleviate the Cd-induced growth inhibition of H. fulva. Compared with the Cd treatment, the Cd + T₂ + AMF treatment increased chlorophyll a, chlorophyll b, chlorophyll a + b, chlorophyll a/b, and carotenoid contents of H. fulva by 53.8%, 29.7%, 48.6%, 18.4%, and 25.8%, respectively. Among the assessed chlorophyll fluorescence parameters, the maximum photochemical efficiency (F_v/F_m), potential activity (F_v/F_o), and photochemical quenching coefficient (qP) of photosystem II were increased by 14.6%, 60.7%, and 67.6%, respectively, whereas the non-photochemical quenching coefficient (NPQ) showed a declining trend. Cd pollution inhibited the net photosynthetic rate (P_n), transpiration rate, and stomatal conductance of H. fulva, whereas the Cd + T₂ + AMF treatment restored the value of P_n to a level comparable to that of the CK treatment in the absence of Cd pollution. Furthermore, root application of different concentrations of TiO₂ NPs and the combined TiO₂ NP + AMF treatments were observed to reduce Cd contents in the aboveground and belowground parts of H. fulva plants, increase the Cd transport coefficient, reduce the enrichment coefficient, promote the activities of antioxidant enzymes, including peroxidase, catalase, and glutathione reductase, and increase the contents of non-enzymatic antioxidants, such as ascorbic acid, glutathione, and total flavonoids (TF) to alleviate oxidative stress on H. fulva. On the basis of the findings in this study, we established that the Cd + T₂ + AMF treatment was the most effective regarding the reduction of the adverse effects of Cd pollution on H. fulva.