Circadian (around the day ) is a complex of two Latin words circa [around] and dies [day] they are endogenous and adjusted to the local environment by the daylight cues called Zeitgebers ZT (Germany for time giver) and the study of other biological rhythms is called chronobiology .most of metabolic and physiological processes in plants and animals including humans are going around an internal oscillator in a continuous period of 24h called circadian rhythm and is a diurnal rhythm this biological clock central loop is composed of the circadian clock components .in plants this circadian rhythm controls processes like photosynthesis , photoperiodism and photo protection by adjusting the crop with the environment that is why is referred to the master regulator of the plant life. Many researchers have proved that well understanding of the biological clocks can be useful in development of agricultural improved crops .in Arabidopsis as a model plant in plant breeding and development this biological clock has been used to identify some clock related genes which are responsible to the plant growth regulation, germination ,flowering ,biotic and abiotic stress responses and others studies have revealed that tissues and even every single cell is governed by circadian clock systems .in this review we are discussing our current understanding of the clock and its interactions with light and temperature signaling pathways, the history of clock research development, the circadian clock in Arabidopsis thaliana as a model plant ,its role during biotic and abiotic stresses ,plant hormones and homeostasis and our future perspectives about circadian research.
Keywords: biological clock, circadian rhythm, homeostasis, plant metabolism
