Responses of antioxidant defense enzymes and thiols in root nodules of black gram following foliar application of Kappaphycus alvarezii seaweed based biostimulant

A pot experiment was conducted in a split-plot design with five concentrations (0, 5, 10, 15, and 20%) of Kappaphycus alvarezii seaweed extract (KSWE) as main plots and two black gram varieties (TAU–1 and DBGV–5) as subplots, with treatment each replicated thrice. Activities of superoxide dismutase (SOD), catalase (CAT), glutathione reductase (GR), and ascorbate peroxidase (APX), protein, non-protein and total thiol contents in black gram root nodules were evaluated at 23–24 days after sowing (DAS), 72–96 h after a single foliar application of the extract at 20 DAS. SOD and GR activities were consistently higher in DBGV–5 than in TAU–1, with SOD declining at elevated KSWE concentrations and GR showing a pronounced reduction at 15% KSWE in 2020. APX activity was reduced by KSWE in 2019 but remained unaffected in 2020, whereas CAT activity was stable across treatments and years. Overall, KSWE modulated nodule antioxidant enzyme responses in a concentration-, variety-, and year-dependent manner, with DBGV–5 exhibiting greater enzymatic sensitivity. Thiol metabolism was likewise influenced by KSWE and genotype, with significant increases in total and protein thiols, particularly at 5% KSWE, and consistently higher levels in DBGV–5 than in TAU–1. Non-protein thiols showed significant variety × dose interactions in 2019 but remained unchanged in 2020, indicating a context-dependent adjustment of cellular redox status. The present study represents one of the earliest evaluations of the effects of seaweed biostimulants on antioxidant responses in root nodules.