Zinc Oxide Nanoparticle Alleviates the Inhibition of <i>Dendrobium huoshanense</i> Photosynthesis by Cadmium through Enhancing Antioxidant Enzyme System

Heavy metal pollution has become a pervasive environmental issue affecting numerous regions worldwide. Recently, there has been significant attention given to the application of nano-enabled technologies with the purpose of enhancing plant development and alleviating heavy metal stress. This study aimed to illustrate the potential of zinc oxide nanoparticles (ZnO-NPs) to enhance the morphological traits of D. huoshenense exposed to cadmium (Cd) stress. The chemical structure and elemental composition of the ZnO-NPs were characterised by a series of analytical methods, including X-ray diffraction, UV-Vis spectrometry, XPS, and TEM. Plant samples used were collected at 0, 5, and 15 days in order to assess physiological and biochemical parameters under different Cd treatments. ZnO-NPs administered in pot experiments have been shown to enhance plant proliferation through the modulation of Cd enrichment levels. The results revealed that ZnO-NPs enhanced plant growth by increasing soluble sugars and proline levels, enhancing activities of antioxidant enzymes (SOD, POD, CAT, APX) and reducing electrolyte leakage (EL) and malondialdehyde (MDA) content. Furthermore, ZnO-NPs enhanced the net photosynthetic rate, transpiration, stomatal conductance, and chlorophyll content in leaves subjected to Cd stress at the 10-day sampling stage. Exogenous ZnO-NPs significantly elevated the expression of genes associated with flavonoid biosynthesis, potentially facilitating the accumulation of medicinal compounds to mitigate Cd stress. Taken together, these findings provide a novel perspective on the strategies employed by medicinal plants in response to Cd.