Foliar silicon enhances drought resilience and productivity in buckwheat by stabilizing photosynthetic performance

Drought stress is a major constraint to buckwheat (Fagopyrum esculentum Moench) productivity, primarily by impairing photosynthesis, growth, and reproductive output. This study investigated foliar-applied silicon (Si; 0.5, 1.0, and 2.0 mM) in mitigating drought stress. Drought reduced growth traits and seed yield, with decreased CO2 assimilation, electron transport rate (ETR), Photosystem II efficiency, and relative water content (RWC), while electrolyte leakage (EL) increased. The OJIP transient showed reduced maximum fluorescence (Fm), indicating photoinhibition. Additionally, the difference kinetics (ΔW) analysis revealed over-reduction of the plastoquinone (PQ) pool, restricted QA-QB electron transport, and partial PSI acceptor limitations under drought. Foliar Si, particularly at 1.0 mM, alleviated these effects by maintaining PSII function, stabilizing ETR, reducing PQ over-reduction, and improving light use. Si enhanced water relations by lowering EL and sustaining RWC, supporting cellular stability. Consequently, 1.0 mM Si restored biomass and seed yield to levels comparable to well-watered plants, whereas 0.5 and 2.0 mM Si provided partial mitigation. Following re-irrigation, most parameters partially or fully recovered, highlighting buckwheat resilience. The results indicate that Si acts as a physiological modulator integrating photochemical protection, water retention, and growth stabilization, with optimal concentration crucial for maximizing drought resilience and productivity.