Co-application of biochar and melatonin enhances rice resilience to combined lead and microplastics stress via modulating antioxidant defense, hormonal regulation, gene expression, and soil quality

Microplastics (MPs) pollution is becoming a serious challenge and poses ecological risks. MPs are known to interact with heavy metals, such as lead (Pb), adversely impacting plant growth. Both biochar (BC) and melatonin (MT) have been extensively utilized in soil remediation efforts. However, the combined effect of BC and MT in alleviating the combined toxicity of Pb and MPs has not been investigated. The study included various treatments: T1: control, T2: Pb (250 mg kg− 1) + MPs (1%), T3: Pb (250 mg kg− 1) + MPs (1%) + BC (2%), T4: Pb (250 mg kg− 1) + MPs (1%) + MT (100 µM) and T5: Pb (250 mg kg− 1) + MPs (1%) + BC (2%) + MT (100 µM). Lead + MPs reduced the rice biomass yield (BY: 25.26%) and grain yield (GY: 41.51%) by increasing the hydrogen peroxide (H2O2), malondialdehyde (MDA), Pb accumulation, and decreasing the indole acetic synthesis (IAA), gibberellic acid synthesis (GA), transpiration rate (Tr), photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 (Ci), soil nitrogen, phosphorous and potassium availability. The combination of BC + MT enhanced rice BY (18.20%) and GY (39.59%) by increasing IAA (36.75%), GA (41.90%), Pn (32.35%), Tr (37.15%), gs (44.44%), and Ci (20.69%) and decreasing the production of MDA and H2O2 by increasing the antioxidants activity (41.17–52.55%). Furthermore, the combined application of BC and MT resulted in the upregulation of antioxidant genes (OsAPx6, OsCAT, OsPOD, and OsSOD), melatonin synthesis gene (OsASMT1), and proline biosynthesis gene (OsP5CS), while concurrently downregulating genes associated with lead uptake (OsHMA9 and OsNRAMP5). The BC + MT treatment also improved soil total nitrogen (TN) by 28.19%, available phosphorus (AP) by 26.73%, and available potassium (AK) by 18.81%, alongside an increase in soil pH. Additionally, it reduced soil lead availability by 37.18%, collectively contributing to enhanced rice biomass and grain yield. Thus, BC + MT can alleviate adversities of Pb + MPs by improving soil fertility, plant functioning, hormonal balance, and gene expression. These findings offer insights to develop measures to enhance the crop in multi-polluted soils.