Abstract

Chitosan-based biostimulants offer a promising strategy to enhance crop resilience under environmental stress, though their physiological effects under field conditions remain poorly understood. This study evaluated FF-BR—a foliar-applied chitosan-based formulation (U.S. Patent No. 9,868,677 B2), under development, for its effects on physiological traits and grain yield in sprinkler-irrigated upland rice (Oryza sativa L.) cultivated in the Brazilian Cerrado. Foliar applications at 1.0%, 1.5%, and 2.0% (v/v) were compared to untreated controls under 48-hour irrigation intervals. FF-BR enhanced net CO₂ assimilation (A), stomatal conductance (gs), and intrinsic water use efficiency (iWUE), particularly during the early photoperiod when vapor pressure deficit increased—suggesting improved stomatal regulation and hydraulic integrity. Chlorophyll a fluorescence analysis revealed elevated Y(II) and ETR, moderated NPQ, and enhanced qP at 2.0%, indicating greater photochemical efficiency. Grain yield increased by 589 kg ha^-1 (10.7%) and 620 kg ha^-1 (11.3%) at 1.5% and 2.0%, respectively, due to higher grain number per panicle, reduced spikelet sterility, and increased 1000-grain weight. These enhancements at the physiological level were successfully translated into superior agronomic performance, supporting the priming hypothesis under fluctuating water availability. This study provides the first field-based evidence of FF-BR’s priming capacity in rice, demonstrating its ability to improve water use efficiency, photosynthetic performance, and yield components under realistic production conditions. FF-BR shows strong potential as a climate-resilient biostimulant that promotes yield stability through enhanced physiological plasticity in sprinkler-irrigated rice systems.

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