The discovery of twilight length sensing in plants and its implications for models of plant photoperiodism.

For centuries, we have known that plants exhibit different phenologies and growth patterns under differing photoperiods, accounting for both seasonality and traditional latitudinal limits in their cultivation. Decades of research in plant circadian biology have deciphered the role of the clock in controlling photoperiodic flowering through what is known as the 'external coincidence model'. However, past work has largely neglected to include another aspect of natural photoperiod change in experimentation - twilight length. In natural environments, photoperiod change is accompanied by changes in the duration of twilight, the period of gradual change in light intensity at day-night transitions. It has recently been revealed that twilight length impacts both the growth and reproductive transition in plants, via the circadian clock. This Tansley Insights explores how this recent discovery fits with recent work on metabolic daylength measurement, leading to a higher resolution understanding of plant photoperiodism at the molecular level.