Circadian Rhythm, Vernalization, Seed dormancy
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.
Vernalization:- It is the artificial exposure of plants or seeds to low temperatures in order to stimulate flowering or to enhance seed production. Gibberellin is a hormone that replaces vernalization. The metabolically active apical meristems are the sites of perception of temperature to initiate flowering. The younger leaves are more susceptible to the process of vernalization. The shoot apex of mature stems or embryo of seeds receives low temperature stimulus.
Mechanism of Vernalization:- Through vernalization, there is an advancement in the process of blooming as a result of the delayed period of low temperatures, for instance, that which is attained in winter. To describe the mechanism of vernalization, there are two main hypotheses –
a. Phasic development theory
b. Hormonal theories
a. Phasic Development Theory:- As per this hypothesis, there is organization of stages in the plant’s improvement. Each stage is under the impact of environmental elements such as light, temperature etc. Here, in turn, there are two main stages –
i.Thermostage:- It depends on temperature, wherein vernalization accelerates thermostat. Thermostage is the vegetative phase requiring low heat, aeration and enough dampness
ii. Photostage:- It necessitates high temperature. Here, vernalin assists in producing florigen.
b. Hormonal theories:- As per this hypothesis, the freezing treatment propels the development of a floral hormone referred to as vernalin. Such a hormone is imparted to various parts of the plant. The vernalin hormone diffuses from the vernalized plants to the unvernalized plants, prompting blooming.
Photoperiodism versus Vernalization:-
i. Definition:- Photoperiodism is defined as the period of day and night for flowering, while vernalization requires cold temperature treatment for flowering.
ii. Hormones:- Florigen hormone is responsible for photoperiodism whereas vernalin hormone is responsible for vernalization.
iii. Pigments:- Phytochrome pigment is involved in photoperiodism whereas no pigment is involved in vernalization.
iv. Stimulus:- In photoperiodism, the stimulus is recognized by leaves, whereas in vernalization, the stimulus is recognized by meristematic cells of shoot tip and embryonal cells.
v. Flowering:- In photoperiodism, flowers produce after specific photoperiod, whereas, in vernalization, flowering is received after previous cold treatment.
Seed dormancy:- A condition in which seeds are prevented from germinating even under the favourable environmental conditions for germination. Seed require a period of rest before being capable of germination.
Causes of seed dormancy:-
> Light
> Temperature
> Hard Seed Coat
> Period after ripening
> Germination inhibitors
> Immaturity of the seed embryo
> Impermeability of seed coat to water
> Impermeability of seed coat to oxygen
> Mechanically resistant seed coat
> Presence of high concentrate solutes
Types of seed dormancy:-
i. Primary dormancy:- The seed is released from the plant, which is already in a dormant state.
ii. Secondary dormancy:- The released seed from a plant becomes dormant due to environmental conditions.
Treatment to break dormancy in seeds:-
a. Seed coat treatment:- These treatments make a hard seed coat permeable to water or gases either by softening or cracking. This process is called scarification. The treatment can be either chemical or physical in nature.
b. Embryo treatments:-
i. Stratification:- The incubation of seeds at an appropriate low temperature over a moist layer before transferring to a temperature suitable for germination.
ii. High-temperature treatment:- Incubation at 40-50 °C for a few hours to a few days may have an effect in overcoming dormancy in some species. For instance, rice seeds treated with hot water at 40°C for at least 4 hours.
iii. Chemical treatments:- Plant growth regulators or other chemicals can be used to induce seed germination.
Advantages of seed dormancy:-
i. In temperate zones, the dormancy of seeds helps the plants to tide over severe cold which may be injurious for their vegetative and reproductive growth.
ii. In tropical regions, the dormancy of seeds resulting from their impermeable seed coats ensures good chances of survival during water stress.
Disadvantages of seed dormancy:-
i. prevents prompt and uniform emergence of seedlings.
ii. interferes with planting schedules.
iii. contributes to "volunteering" of crops.
iv. causes problems to the seed analyst.
Importance of Seed Dormancy:-
> It follows the storage of seeds for later use by animals and man.
> It helps in the dispersal of the seeds through the unfavourable environment.
> Dormancy induced by the inhibitors present in the seed coats is highly useful to desert plants.
> Allows the seeds to continue to be in suspended animation without any harm during cold or high summer temperature and even under drought conditions.
> Dormancy helps seeds to remain alive in the soil for several years and provides a continuous source of new plants, even when all the mature plants of the area have died down due to natural disasters.
Regulation of seed dormancy and germination:- Plant hormones, mainly abscisic acid (ABA) and gibberellin (GA), are the major endogenous factors that act antagonistically in the control of seed dormancy and germination; ABA positively regulates the induction and maintenance of dormancy, while GA enhances germination.