Growth and flowering responses to temperature and photoperiod in Primula forbesii Franch., a biennial herbaceous plant native to China

• This is the first comprehensive study on the flowering physiology of Primula forbesii , indicating its facultative short-day plant nature. • A temperature of 16°C is critical for flowering and overrides photoperiodic effects. • Photoperiod plays a dual role: long days (16h) promote vegetative growth, whereas short days (8h) strongly accelerate flowering, demonstrating a clear growth–flowering trade-off. • Optimal vegetative growth at 16°C under long days is correlated with enhanced accumulation of soluble proteins and sugars, key metabolites supporting biomass production. • Flowering promotion is associated with a synergistic hormonal balance. • Superior flowering at 16°C correlates with enhanced antioxidant enzyme activities and reduced oxidative stress. Primula forbesii Franch., a species endemic to China, possesses desirable ornamental value for potted and bedding use. This study examined the effects of temperature and photoperiod on the growth and flowering responses of P. forbesii . In the experiment, the plants were subjected to factorial combinations of two temperatures (16°C and 20°C) and three photoperiods (8h, 12h, 16h of daylight). Plants cultivated at 16°C exhibited significantly increased biomass accumulation. Notably, the combination of 16°C and a 16-h photoperiod yielded optimal vegetative growth, markedly increasing leaf number, crown width, plant height, and both stem and root biomass, supported physiologically by the enhanced accumulation of soluble proteins and sugars. In terms of reproductive development, 16°C markedly accelerated floral initiation, hastening visible bud emergence by 28–37 days across all photoperiods compared to 20°C, while the earliest bud appearance occurred under the 8-h photoperiod. Furthermore, flowering was most effectively induced under 16°C combined with 8-h or 12-h photoperiods, which achieved 100% flowering within 62 days and significantly improved flowering quality compared to 20°C. In addition, the 12-h photoperiod under 16°C most strongly promoted the accumulation of photosynthetic pigments and metabolites, enhanced antioxidant enzyme activities, and favorably modulated hormonal balance by promoting gibberellin (GA₃), indole-3-acetic acid (IAA), and Zeatin riboside (ZR) while suppressing abscisic acid (ABA). Collectively, these findings indicate that P. forbesii is a facultative short-day plant exhibiting a dual response: long days promote vegetative growth, whereas short days accelerate flowering. Optimal flowering can be induced in both 8-h and 12-h photoperiod conditions when combined with 16°C. These findings provide a refined environmental management framework for the commercial production of high-quality P. forbesii .