Influence of zinc on cadmium-induced toxicity in nodules of pigeonpea (Cajanus cajan L. Millsp.) inoculated with arbuscular mycorrhizal (AM) fungi

Arbuscular mycorrhizal (AM) fungi are known to alleviate heavy-metal stress in plants. The intent of the present work was to analyze accumulation of heavy metals (Cd and Zn) in nodules of two Cajanus cajan (L.) Millsp. genotypes and their subsequent impact on nitrogen fixation, oxidative stress, and non-protein thiols (glutathione and phytochelatins) with and without AM fungus Glomus mosseae. Accumulation of Cd and Zn in nodules resulted in sharp reduction in nodule number, nodule dry mass as well as nitrogen fixation (leghemoglobin and nitrogenase (N₂ase)), although Cd had more pronounced effects than Zn. Cd-induced lipid peroxidation, H₂O₂ accumulation, and electrolyte leakage were largely reversed by Zn supplementation. Zn application significantly altered the negative effects of Cd on the synthesis of non-protein thiols, suggesting antagonistic behaviour of Zn. Higher concentration of Zn was more effective in lessening the negative effects of Cd than its lower concentration. Remarkable genotypic variation was found, with more severe effects of both the metals in P792 than Sel 85N. Glomus mosseae attenuated the phytotoxic effects of metals in nodules by decreasing metal uptake, oxidative stress, and by enhancing defense system ultimately leading to better nitrogen-fixing potential of pigeonpea nodules.