Circadian Oscillations in Branchlet Angle and Irradiance-Dependent Acclimation in Chara braunii Under Continuous Light.

Endogenous biological rhythms are central to temporal organization in land plants, yet their evolutionary origins in streptophyte algae remain unresolved. We investigated whether rhythmic morpho-physiological processes in Chara braunii are consistent with endogenous regulation and how continuous light intensity modulates these responses. In C. braunii, time-lapse imaging, pigment quantification, chlorophyll fluorescence measurements, and metabolomic profiling were conducted under continuous low (dim) light and continuous higher (regular) light conditions. Circadian oscillations in branchlet angle persisted under continuous dim light but were attenuated under continuous regular light, whereas internodal elongation showed only limited temporal structure. Pigment and metabolite measurements, obtained once per day under two different continuous light conditions, revealed intensity-dependent physiological adjustment, including changes in chlorophyll and carotenoid contents and altered accumulation of tricarboxylic acid cycle intermediates such as malate and fumarate. Chlorophyll fluorescence parameters showed no detectable temporal variation under the applied sampling scheme. Comparative genomic analyses identified putative homologs of several canonical land-plant clock-associated components, whereas canonical TOC1 and ZTL orthologs were not detected. Together, these results are consistent with the presence of endogenous regulatory components governing rhythmic outputs in C. braunii, while highlighting irradiance as a major modulator capable of suppressing these outputs. The parallel attenuation of rhythmic outputs and the shift toward a stress-associated metabolic profile under higher irradiance suggest that higher irradiance affects both endogenous regulatory outputs and physiological acclimation responses, although whether these processes are causally linked remains to be determined.