Exogenous nitric oxide enhances cadmium tolerance and uptake in <i>Ricinus communis</i> L. through improved photosynthesis and antioxidant defense

Cadmium (Cd), a highly toxic heavy metal(loid) (HM), poses significant environmental challenges, demanding effective remediation strategies. <i>Ricinus communis</i> L. (castor), recognized for HM accumulation, offers promise for phytoremediation. This study examines the role of nitric oxide (NO), applied as 100 µM sodium nitroprusside (SNP) (NO donor), in mitigating Cd stress (500 µM CdCl₂) in 7-day-old seedlings of contrasting castor genotypes: Western Maruti (WM), Cd-tolerant, and Gujarat Castor Hybrid-2 (GCH-2), Cd-sensitive, for 21 days. Physiological parameters and antioxidant enzyme activity were analyzed under Cd and NO treatments. SNP mitigated Cd toxicity in GCH-2, boosting root length (137.88%), shoot length (83.53%), dry biomass (root-125.95%; stem-169% and leaves-70.14%), chlorophyll content (117.1%), photosynthetic rate (119.28%), stomatal conductance (129.11%) and transpiration rate (167.3%), compared to plants under Cd treatment. SNP treatment increased Cd accumulation in the root and stem of GCH-2 (4.4% and 13.7%) and WM (7.5% and 29.3%), respectively. This study hypothesizes that exogenous application of NO in <i>R. communis</i> reduces the cellular toxicity due to Cd stress, particularly in the sensitive genotype, as well as enhances Cd tolerance and accumulation by modulating growth, photosynthesis, and antioxidant defense. Thus, NO may be a viable candidate for improving phytoremediation in Cd-contaminated soils.