Co-application of biochar and plant growth regulators improves maize growth and decreases Cd accumulation in cadmium-contaminated soil

To ensure the sustainable cultivation of agronomic crops in agricultural soils polluted with cadmium (Cd), the integrated application of Cd-remediation strategies at edaphic and foliar levels has been examined. However, the integrated application of biochar (BC) and plant growth regulators (PGRs) for mitigating Cd stress, especially in maize, remains unexplored. Additionally, the expression patterns of PGRs and heavy metal transporters during Cd stress are unknown. To address these gaps, a pot experiment (having four replications) was performed to explore the potential effects of the integrated application of BC (B₀ = no BC added; B₁ = low BC at 2.5% w/w; and B₂ = high BC at 5% w/w) and PGRs (H₀ = control; WS = water spray; GA = gibberellin, and IAA = auxin) on maize growth, Cd remediation, and productivity. The study was conducted following a factorial completely randomized design. The pots were artificially spiked with Cd (30 ppm) by using Cd(NO₃)₂ as a source. The synergistic application of IAA/GA and BC, improved the morphological characteristics, photosynthetic activity, stomatal conductance, and catalase activity of maize under Cd stress while simultaneously decreasing electrolyte leakage and oxidant activity. The leaf area, shoot fresh biomass (SFB), and root fresh biomass (RFB), were increased by (13.4, 11.7%), (52.8, 39.3%), and (89.2, 74.3%) in the synergistic application of PGRs (i.e., IAA and GA) + BC (5% w/w) treatments, respectively, than control. Furthermore, a drastic reduction in Cd uptake was observed in maize leaves by (201.53, 179.24%) under the dual application of PGRs + BC (5% w/w) treatments, compared to control, which altered Cd bioconcentration in plant parts and modified Cd species in the soil. Decreased expression of heavy metal transporter PMPCB gene and YCF1 indicated reduced Cd uptake in 5% w/w BC based on RT-PCR quantification. The synergistic application of BC with PGRs can serve as an environmentally friendly remediation to mitigate the hazardous effects of Cd stress on cultivated maize. This was achieved by reducing the bio-concentration and metal transport coefficients of Cd in plants through BC's absorption properties. Overall, applying BC and foliar application of PGRs could be suggested as a sustainable approach for cultivating crops on Cd-contaminated soils while ensuring safety.