Circadian Rhythm
Circadian Rhythm:-
> A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours.
> Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (is entrained by the environment).
> Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
> Its primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximise the fitness of an individual.
> The term circadian comes from the Latin circa, meaning "approximately", and dies, meaning "day".
Circadian Rhythm in Plants:-
- Plant circadian rhythms tell the plant what season it is and when to flower for the best chance of attracting pollinators.
- Behaviors showing rhythms include leaf movement (Nyctinasty), growth, germination, stomatal/gas exchange, enzyme activity, photosynthetic activity, and fragrance emission, among others.
- Circadian rhythms occur as a plant entrains to synchronize with the light cycle of its surrounding environment.
- These rhythms are endogenously generated, self-sustaining and are relatively constant over a range of ambient temperatures. I
-mportant features include two interacting transcription-translation feedback loops: proteins containing
i. PAS domains, which facilitate protein-protein interactions.
ii. Several photoreceptors, that fine-tune the clock to different light conditions.
- A better understanding of plant circadian rhythms has applications in agriculture, such as helping farmers stagger crop harvests to extend crop availability and securing against massive losses due to weather.
- Light is the signal by which plants synchronize their internal clocks to their environment and is sensed by a wide variety of photoreceptors.
- Red and blue light are absorbed through several phytochromes and cryptochromes.
- Phytochrome A, phyA, is light labile and allows germination and de-etiolation when light is scarce.
- Phytochromes B–E are more stable with phyB, the main phytochrome in seedlings grown in the light.
- The cryptochrome (cry) gene is also a light-sensitive component of the circadian clock and is thought to be involved both as a photoreceptor and as part of the clock's endogenous pacemaker mechanism.
- Cryptochromes 1–2 (involved in blue–UVA) help to maintain the period length in the clock through a whole range of light conditions.