Effects of continuous light and skeleton photoperiods on photosynthesis and circadian rhythm in celery

Photoperiod plays a pivotal role in regulating plant physiological cycles and photosynthetic processes via the circadian clock. Celery is an important leafy vegetable with high sensitivity to photoperiod. However, how photosynthesis and circadian rhythms respond to different photoperiod patterns remains to be fully explored in this species. To investigate the effects of different photoperiods on photosynthesis and circadian rhythm in celery, we applied equinoctial photoperiod (12 h of light/12 h of dark, 12L12D), continuous light (24 h of light, 24L) and two skeleton photoperiods (6 h of light/6 h of dark, 6L6D and 3 h of light/3 h of dark, 3L3D). Under the 12L12D, stomatal aperture, chlorophyll content, photosynthetic parameters, and the expression pattens of circadian rhythm-related genes all exhibited robust 24-h rhythmic patterns. Continuous light (24L) increased the daily mean chlorophyll and nitrogen contents to 1.1-fold than those observed under 12L12D, while stabilizing photosynthetic parameters and stomatal apertures with minimal daily variation. Conversely, skeletal photoperiods (6L6D and 3L3D) rapidly disrupted these established patterns, the expression of circadian rhythm-related genes and photosynthetic parameters exhibited arrhythmic fluctuations. Our findings demonstrated that extended light exposure within a 24-h framework optimized photosynthetic performance in celery, whereas skeleton photoperiods impaired circadian rhythm and photosynthesis-related output.