Protective Role of Silicon and Potassium in Enhancing Root Tolerance of Rice against Ferrous Iron (Fe2+) Toxicity

Ferrous iron (Fe²⁺) toxicity is a potent soil-generated abiotic stress that can severely limit lowland rice (Oryza sativa L.) production via cellular and physio-biochemical disruptions. Silicon (Si) or potassium (K) can enhance plant tolerance to numerous metal and metalloid stresses. The present study focused on evaluating whether exogenous Si/K could alleviate Fe²⁺ toxicity impairments of rice roots. Two independent pot experiments were simultaneously conducted involving two rice genotypes (RD85 [Fe²⁺-tolerant] and RD31 [Fe²⁺-susceptible]) and three Fe²⁺ (FeSO₄.7H₂O) levels (0 [control], 600, and 900 mg L⁻¹). In Experiment 1, four Si doses were applied (0 [control], 30, 60, and 90 kg ha⁻¹) as monosilicic acid. In Experiment 2, four K doses were applied (0 [control], 60, 120, and 180 kg ha⁻¹) as potassium chloride. Root impairments increased in both experiments with increasing Fe²⁺ levels, but exogenous Si/K effectively reduced the impairments across all measured parameters regardless of genotypes. The application of Si alleviated at best 26%, 36%, 16%, 28%, 44%, 33%, 34%, and 35% adverse impacts on root number per plant, root volume, root dry matter, membrane stability index, relative cell death, lipid peroxidation rate, free proline content, and total antioxidant activity, respectively, along with a 59% reduction in Fe content and a 37% increase in Si content in roots. Similarly, K application alleviated at the best 22%, 44%, 33%, 15%, 41%, 30%, 30%, and 86% adverse impacts on the same respective parameters along with a 39% reduction in Fe content and a 50% increase in K content in roots. The end mitigation responses from Si/K were similar; however, the underlying mechanisms were distinct. The major difference in the Fe²⁺ toxicity ameliorative activity between Si and K remained within the amount of Fe²⁺ uptake for the former and within the total antioxidant activity for the latter. Silicon promoted exodermal Casparian band formation in root exodermis for effective Fe²⁺ exclusion, whereas K extensively boosted free radical scavenging capacity to attain similar mitigation responses. The combined or synchronous application of Si and K could provide a promising research direction, considering their distinct mechanisms in mitigating Fe toxicity. The findings of this study would enhance present understanding on Fe²⁺ toxicity alleviation effects of both Si and K as well as help formulate management strategies for successful rice cultivation under Fe²⁺-toxic environments.