The rotation of earth around its axis leads to environmental changes in light intensity and temperature that predictably define day-night cycle. Most organisms coordinate their life cycle in anticipation of these environmental fluctuations. The measurement of time and synchronization with the environment are achieved by an internal time-keeping mechanism biological clock. The circadian clock is an endogenous mechanism that generates rhythms with an approximately 24-h period and enables plants to adapt to daily and seasonal changes in their environment.