Biochar coating promoted rice growth under drought stress through modulating photosynthetic apparatus, chloroplast ultrastructure, stomatal traits and ROS homeostasis.

Drought hampers agricultural production by constraining crop growth and development. Nevertheless, there has been limited exploration regarding the effect of biochar coating in enhancing seed germination under drought conditions and understanding its underlying mechanisms. To fill this gap and clarify the pathway to drought resistance, the current research investigated the protective effectiveness of BC on seedling establishment and subsequent growth of rice under drought conditions. Results showed that BC notably elevated emergence rate (5.5%), shoot length (27.4%), root length (33.4%), plant height (19.6/10.3%), leaf area (69.8/71.7%), and plant biomass (85.7/67.9%) after 15/30 days under drought conditions compared to the control. Biochar coating facilitated the maintenance of a stable chloroplast structure, reduced chlorophyll degradation, and sustained cell expansion. This contributed to the improvement of stomatal characteristics on both adaxial and abaxial leaf surfaces during drought stress, encompassing enhancements in stomatal density and aperture. The preservation of stomatal opening led to an increased photosynthetic capacity, thereby fostering elevated photosynthetic activity and heightened plant biomass under stressful conditions. Simultaneously, BC treatment significantly diminished the production of reactive oxygen species, preserved cell membrane integrity, and augmented the accumulation of osmotic protectants. These outcomes signify that biochar coating mitigates the deleterious impacts of drought stress on photosynthesis, stomatal aperture, chloroplast ultrastructure, osmotic regulation, and redox homeostasis in plants through specific water and nutrient regulation. Consequently, this enhances the tolerance and growth of rice under drought stress.