Photoperiodsim & vernalization

Photoperiodism is the phenomenon of physiological changes that occur in plants in
response to relative length of day and night (i.e. photoperiod). The response of the plants to
the photoperiod, expressed in the form of flowering is also called as photoperiodism. The
phenomenon of photoperiodism was first discovered by Garner and Allard (1920).Depending
upon the duration of photoperiod, the plants are classified into three categories.
1. Short day plants (SDP)
2. Long day plants (LDP)
3. Day neutral plants (DNP)
Vernalization is the induction of a plant's flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower.The vernalization requirement ensures that plants do not flower in the fall when the environmental conditions are unfavorable for reproduction. The strength of the vernalization requirement can vary within plant species.

1. PHOTOPERIODISM AND VERNALIZATION

2. C O N T E N T Pho t operiodis m - Types phytochrome Significance V ernalis a tio n - Introduction Site requirement mechanism significance

3. P HOTOPERIODISM

4. INTRODUCTION Photoperiodism is the phenomenon of physiological changes that occur in plants in response to relative length of day and night (i.e. photoperiod). The response of the plants to the photoperiod, expressed in the form of flowering is also called as photoperiodism. The phenomenon of photoperiodism was first discovered by Garner and Allard (1920).

5. T YPES OF PHOTOPERIODISM Depending upon the duration of photoperiod, the plants are classified into three categories. 1. Short day plants (SDP) 2. Long day plants (LDP) 3. Day neutral plants (DNP)

6. 1. S HORT DAY PLANTS These plants require a relatively short day light period (usually 8-10 hours) and a continuous dark period of about 14-16 hours for subsequent flowering. These plants are also known as long-night plants. E.g. Rice, coffee, soybean, tobacco and chrysanthemum In short day plants, the dark period is critical and must be continuous. If this dark period is interrupted with a brief exposure of red light (660-665 nm wavelength), the short day plant will not flower.

7. 2. L ONG DAY PLANTS These plants require longer day light period (usually 14-16 hours) in a 24 hours cycle for subsequent flowering. These plants are also called as short night plants. E.g. Wheat, radish, cabbage, sugar beet and spinach. In long day plants, light period is critical. A brief exposure of red light in the dark period or the prolongation of light period stimulates flowering in long day plants.

8. P ATTERNS

9. 3. D AY NEUTRAL PLANTS These plants flower in all photoperiod ranging from 5 hours to 24 hours continuous exposure. E.g. Tomato, cotton, sunflower, cucumber, peas and certain varieties of tobacco.

10. INTERMEDIATE CATEGORIES OF PLANTS i. Long short day plants These are short day plants but must be exposed to long days during early periods of growth for subsequent flowering. E.g. Bryophyllum. ii. Short –long day plants These are long day plants but must be exposed to short day during early periods of growth for subsequent flowering. E.g. certain varieties of wheat and rye.

11. L e a f i s t h e s i t e o f p e r ce p t i o n of the photoperiodic signal The photoperiodic stimulus in both LDP and SDP is perceived by the leaves . Leaf transmits a signal that regulates the transition to flowering at the shoot apex. The photoperiod - regulated processes that occur in the leaves resulting in the transmission of a floral stimulus to the shoot apex are collectively as photoperiodic induction. Extensive grafting studies have shown that the induced leaf is the source of a mobile floral stimulus that is transported to the shoot apical meristem.

12. The name Florigen was proposed by Chailakhyan in 1936, for the unknown chemical stimulus which could act as a floral inducer. The significant evidence of the existence of such a substance comes from interspecific grafting experiment.

13. Graftin g experimen t of florigen existence Chailakhyan removed the leaves from the upper half of Chrysanthemum (SDP) but left the leaves on lower half untouched. He then exposed the lower half to short days- the plants did not flower. He concluded that day length does not have an effect directly on the buds but causes the leaves to manufacture a hormone that moves from the leaves to the buds and induce flowering. This hypothetical hormone has been named florigen and it has never been isolated and therefore we now talk of florigen concept instead of florigen .

14. F l o r i g e n c o n c e p t After the discovery of gibberellins , Chailakhyan modified his florigen concept . He proposed that florigen is 2 hormones rather than 1, a GA and a hypothetical hormone he called anthesin . Florigen = GA + Anthesins (perhaps steroids). He suggested that LDP could produce anthesin under any day length but GA only under long days. He also suggested that SDP produce GA under any day length but anthesin only under short days . he also suggested that day neutral plants could produce both under any day length.

15. A plant could flower only when both GA and anthesin were present . But again anthesin is just as hypothetical as florigen has been. Antiflorigen has found in some LDPs ex- Nicotiana sylvestris. Florigen may be macromolecule- it is possible that florigen is an RNA or protein molecule that is translocated from the leaf to the apical meristem via the phloem .

16. PHOTORECEPTORS All photoreceptors contain an organic non-protein component known as chromophore that serves as the primary site of photon absorption (Möglich et al. 2010). At present, six classes of photoreceptors are known: PHYs, cryptochromes (CRYs), light– oxygen–voltage (LOV) sensors, blue light sensors utilizing flavin adenine dinucleotide (FAD), rhodopsins, and xanthopsins (Möglich et al. 2010).

17. SIGNIFICANCE hybridization experiments(florigenhormone). Physiological preconditioning. Increase yield Plants remain vegetative for longer period (radish,carrot,etc.) Annuals can be grown twice or thrice a year. Prevention of winter dormancy & autumn leaf fall. Strawberry-Increased stolon formation through long days.

18. V ERNALIZATION

19. I N T R O D U C T I O N It was found by Lysenko (1928), a Russian worker that the cold requiring annual and biennial plants can be made to flower in one growing season by providing low temperature treatment to young plants or moistened seeds. He called the effect of this chilling treatment as vernalization. Vernalization is, therefore, a process of shortening of the juvenile or vegetative phase and hastening flowering by a previous cold treatment. Some plants require exposure to a low temperature , 0 to 10°C ,for few days to a few weeks for flowering. such an induction of flowering by a low temperature is called vernalisation

20. S ITE FOR V ERNALIZATION The stimulus of vernalization is perceived only by the meristematic cells(active apical meristem). e.g.- shoot tip, embryo tips, root apex, developing leaves, etc.

21. R EQUIREMENTS OF V ERNALIZATION (i) Low Temperature: Low temperature required for vernalization is usually 0° — 5°. (ii) Period of Low Temperature Treatment: It varies from a few hours to a few days. (iii) Actively Dividing Cells: (iv) Water: (v) Aerobic Respiration (vi) Proper Nourishment.

22. M ECHANISM OF V ERNALIZATION The stimulus received by the actively dividing cells of shoot or embryo tip travels to all parts of the plant and prepare it to flower. The stimulus has been named as vernalin. It can be passed from one plant to another through grafting in case of Henbane but not in others.

23. Mechanism of Vernalization Two theories 1. Phasic D e v elopme n t Hypothesis 2.Hormo n al Hypothesis.

24. D EVERNALIZATION The positive effect of the low temperature treatment on the vernaliztaion of the plants can be counteracted by subsequent high temperature treatment. This is called as devernalization. The degree of devernalization decreases if the duration of the cold treatment has been longer. However, the devernalized plant can again be vernalized by subsequent low temperature treatment.

25. 1. Water and Oxygen Seed need to imbibe 10-20 % water, dry seed cannot be vernalized. Similarly oxygen is also necessary for vernalization. Oxygen is used during respiration. A germinating seed need energy. This energy is provided by respiration. The inhibition of respiration blocks the process of vernalization. 2. Temperature The seeds are exposed to low temperature during vernalization. Temperature lower than 4°C is not effective. Similarly, temperature above 12 ° C does not cause vernalization,the duration of vernalization varies in different species. 3. Devernalization The exposing of vernalized seed to high temperature (30 -35°C) , lose the effect of vernalization called as Devernalization. FACTORS AFFECTING VERNALIZATION

26. 4.Effect of light The seed of a plant can be vernalized in the presence or absence of light, But many plants like carrot require both vernalization and photoperiodic stimulus. They must be first vernalized. Then are given photoperiodic stimulus for flowering. 5.Effect of hormones Many species like carrot and Hyoscyamus have obligate vernalization requirement. These plants show rapid flowering after vernalization due to GA synthesis,It causes both bolting and flowering.

27. PRACTICAL APPLICATIONS Due to vernalization the vegetative period of the plant is cut short resulting in an early flowering. Vernalization increases the resistance of plants to fungal diseases. It increases the cold resistance of plants. In the biennials ,vernalization induces early flowering and early fruit setting. Flowering can be induced by grafting and this feature is used in horticulture. It also helps in crop improvement.

28. C O N C L U S I O N

29. T H A N K Y O U