Day-length perception and the photoperiodic regulation of flowering in Arabidopsis

The flowering of Arabidopsis plants is accelerated by long-day photoperiods , and recent genetic studies have identified elements of the photoperiodic timing mechanism. These elements comprise genes that regulate the function of the circadian clock, photoreceptors, and downstream components of light signaling pathways. These results provide evidence for the role of the circ adian clock in photoperiodic time measurement and suggest that photoperiod perception may follow Pittendrigh's external coincidence model. T-cycle exp eriments indicated that changes in the timing of circadian rhythms, relativ e to dawn and dusk, correlated with altered flowering time. Thus, the perce ption of photoperiod may be mediated by adjustments in the phase of the cir cadian cycle that arise upon re-entrainment to a different light-dark cycle . The nature of the rhythm underlying the floral response is not known, but candidate molecules have been identified.