The role of photoperiod (daylength) in controlling seasonal responses was noted early in the 20th century (Tournois, 1912; Klebs, 1913). Garner and Allard (1920) demonstrated that many plants flower in response to changes in daylength. The connection between photoperiodism and the circadian clock was first noted by Bünning (1936) and was developed into the external coincidence model, in which a rhythmic process that controls the photoperiodic response is sensitive to light at certain times of day (Pittindrigh and Minis, 1964).

Flowering of Arabidopsis is accelerated in long days, and the mechanism by which this occurs is becoming clear (for review, see Corbesier and Coupland, 2005). Briefly, a key promoter of flowering is CONSTANS (CO). The transcription of the CO gene is clock regulated so that CO mRNA only accumulates late in the day. At least in part, this is because the clock-regulated F-box protein FKF controls the stability of a cycling Dof transcription factor, CDF1, which is a repressor of CO transcription (Imaizumi et al., 2005). FKF has a photoactive LOV domain and likely serves as a photoperiodic blue-light receptor (Imaizumi et al., 2003). Once CDF1 is degraded, CO transcription ensues. However, CO protein is unstable and fails to accumulate in the dark. Light perception via CRY2 and PHYA stabilizes CO (Valverde et al., 2004) in long days when CO mRNA accumulates and is translated in the light but not in short days when CO mRNA only accumulates and is translated after dusk. Thus, CO protein accumulates to activate its target, FLOWERING LOCUS T, in long but not in short days (Suárez-López et al., 2001; Yanovsky and Kay, 2002). Flowering is only promoted inArabidopsis when there is the proper coincidence of the internal oscillation in COtranscription and subsequent translation with the external oscillation in light. Excitingly, this model applies to rice, a short-day plant—the salient difference seems to be that CO serves as a floral repressor in that species (Hayama and Coupland, 2004