Synergistic effects of polystyrene nanoplastics and arsenic on growth and biochemistry of in vitro-grown Oryza sativa: ameliorative role of humic acid

Nanoplastics have emerged as pervasive environmental contaminants with growing concern over their ecological and biological impacts. Several studies have demonstrated the negative impact of nanoplastic exposure on plants; however, limited research has focused on the synergistic effects of nanoplastic and heavy metal exposure. This study investigates the individual and combined toxicity of polystyrene nanoplastics (PS-NPs) and arsenic (As) on the growth and physiological responses of Oryza sativa and evaluates potential mitigation strategies using humic acid (HA). In-vitro rice plantlets were exposed to varying concentrations of PS (5, 25, 50 µg/mL), As (10, 50, 100 µg/mL), urea (10, 50, 100 µg/mL), and HA (10, 50, 100 µg/mL), as well as to combined treatments of PS: HA (5:10, 25:50, 50:100 µg/mL), PS: As (5:10, 25:50, 50:100 µg/mL), and PS: As: HA (5:10:10, 25:50:50, 50:100:100 µg/mL). Plant growth, chlorophyll content, and biochemical markers, including carbohydrates, proteins, lipids, and nitrate levels, in root and shoot tissues were assessed. Results showed that exposure to PS nanoplastics inhibited rice growth and decreased chlorophyll and biochemical content. Co-exposure with arsenic exacerbated these toxic effects, notably enhancing arsenic translocation from roots to shoots. Importantly, the addition of HA in PS: As: HA treatments significantly alleviated toxicity, suggesting a potential ameliorative role of HA in mitigating co-contaminant stress. These findings underscore the synergistic toxicity of nanoplastics and heavy metals in rice and highlight the potential of humic acid as a sustainable strategy for environmental remediation in contaminated agroecosystems.