Far-Red Light Regulates the Circadian Rhythm Pathway to Accelerate Rice Flowering

Early flowering is a key element of the rice speed-breeding protocol that enables improved genetic gain and accelerates the cultivation of new varieties. Although far-red light (FR) is commonly used to modulate plant developmental processes, the mechanisms by which it influences flowering and growth in rice are poorly understood. In this study, the control treatment (CK) consisted of red-blue-green composite light at 300 μmol m−2 s−1, while two additional treatments were applied: one with the photon flux density (PFD) increased to 350 μmol m−2 s−1 (HI—high intensity) under the same light spectrum as CK, and the other supplemented with 50 μmol m−2 s−1 of FR based on CK. The results demonstrated that both elevated PFD and supplemental FR significantly enhanced vegetative growth, as evidenced by increased plant height, tiller number, leaf area, and biomass accumulation, along with improved photosynthetic capacity and chlorophyll fluorescence. Under the FR treatment, flowering occurred 53 days after transplanting, which was 12 days and 9 days earlier than in the CK and HI treatments, respectively. Physiological profiling revealed that FR enrichment significantly increased leaf soluble sugar and starch levels, while simultaneously decreasing chlorophyll and carotenoid concentrations. FR also reshaped the endogenous hormonal profile, which was marked by elevated levels of gibberellin (GA3) and abscisic acid (ABA), and reduced auxin (IAA) content. Transcriptomic profiling revealed that FR enrichment activated the circadian rhythm pathway and upregulated genes associated with photoperiodic flowering and inflorescence development. In summary, FR promotes rice growth and early flowering through the integrated regulation of leaf area expansion, enhanced photosynthetic efficiency, hormonal rebalancing, and activation of flowering gene expression. This study provides a theoretical foundation and technical support for optimizing light environments and improving the economic viability of crop speed breeding systems in controlled environmental facilities.