Introduction Photoperiod is a major environmental cue for flowering. Many plant species reproduce at favorable times of the year, by measuring seasonal changes in day length, and responding either to long days(LDS) or short days(SDS). Day-length measurement is thought to be achieved through the integration of endogenous circadian rhythms with external light signals(Bunning, 1936, Pittendrigh and Minis, 1964). Light plays 2 distinct roles in photoperiodic flowering: (1) by resetting the circadian clock that regulates the circadian phase