Photoperiod and light quality shape flowering and photosynthesis in common buckwheat (Fagopyrum esculentum Moench).

Common buckwheat (Fagopyrum esculentum Moench) is cultivated across diverse environments in which photoperiod and light quality influence plant development and productivity. This study evaluated the effects of short-day (10h light/14h dark) and long-day (16h light/8h dark) conditions applied during the first 25 days of growth on flowering, photosynthetic performance, and yield formation in the cultivar 'Panda'. Plants were grown in phytotron chambers under blue- and red-enriched LED illumination and contrasting photoperiods, while plants grown under greenhouse daylight served as a control. All plants were subsequently transferred to the same open-sided foil tunnel and cultivated under natural daylight and temperature conditions until harvest. Long-day conditions accelerated flower bud formation by 10 days and increased flower number more than fivefold compared with short-day conditions. LED-grown plants exhibited enhanced PSII performance, including a higher performance index (PIABS) and number of active reaction centres (RC/CSm), whereas downstream electron transport beyond the primary quinone acceptor (QA-) towards PSI end electron acceptors was less efficient than under greenhouse daylight. Higher net photosynthetic rates (PN) and apparent carboxylation efficiency (ACE) in long-day than in short-day plants indicated greater carbon assimilation under the longer photoperiod. Despite pronounced differences in flowering and photosynthetic traits, most yield-related traits, including seed number and total seed weight per plant, remained unchanged. These findings reveal that early photoperiod-induced changes in flowering and photosynthetic performance can be largely decoupled from final seed yield in 'Panda'. More broadly, flowering responses alone may be insufficient predictors of productivity, an important consideration when designing controlled-environment and speed-breeding protocols intended to accelerate development without compromising seed production.