Photoperiodism and vernalization

Photoperiodism and Vernalization,all about phitoperiodism and vernalisation,classification of plants on the basis of photoperiodism.

1. Photoperiodism and Vernalization PRP

2. PHOTOPERIODISM

3. PHOTOPERIODISM Photoperiodism is the phenomenon of physiological changes that occur in plants in response to relative length of day and night (i.e. photoperiod). Plant in order to flower require a particular day length or light period called photoperiod and response of plants to photoperiod in terms of flowering is called photoperiodism . It influences plant development such as leaf fall , dormancy and tuber formation but its major effect is on control of flowering.

4. Discovery of Photoperiodism It is first discovered by W.W. Garner and H.A. Allard (1920) They observed that “Maryland mammoth” variety of tobacco flowered at different times at different places. After controlling other factors like nutrition, temperature etc, they reported that it was the relative length of the day which affected flowering . They gave the term photoperiod to favourable day length for each plant.

5. CLASSIFICATION OF PLANTS BASED ON PHOTOPERIODIC RESPONSES Garner and Allard classified the plants into 5 types based on their photoperiod response of flowering: Short day plants (SDP) Long day plants (LDP) Day neutral plants (DNP) Short -long- day plants (SLDP) Long- short- day plants (LSDP)

6. 1. Short 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. 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. On the other hand, a brief period of darkness during day time had no effect on flowering. Because of the importance of dark period in flowering, these are also called as Long Night Plants. E.g. Rice, Coffee, Soybean, Tobacco, Chrysanthemum, Xanthium and Morning glory ( Ipomoea purpurea ) 2. Long 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 . In long day plants, light period is critical and a brief exposure of red light in the dark period or the prolongation of light period stimulates flowering in long day plants E.g. Barley (winter barley), Wheat, Henbane ( Hyoscyamus Niger ), Radish, Cabbage, Sugar beet , Spinach, carrot, lettuce and onion

7. 3. Day neutral plants Indeterminate or DNP flowers readily over a wide range of day length from relatively SDL to continuous illumination. E.g. Tomato, Chilly, Potato, Cotton, Sunflower, Cucumber, Peas, Balsam and Maize 4. Short-long day plants These are long day plants but must be exposed to short day during early periods of growth for subsequent flowering. These plants require first short photoperiod and then long photoperiod for flowering. E.g. Winter rye , Candy tuft 5. Long-short day plants These are short day plants but must be exposed to long days during early periods of growth for subsequent flowering. These plants require first long days and then short photoperiod for flowering. These plants do not flower when exposed to either SD or LD and need both LD and SD for its flowering. E.g. Bryophllum , Cestrum Nocturnum (Night blooming Jasmine)

8. Differences between short day and long day plants Short day plant Plants flower when photoperiod is less than the critical day length. Interruption during light period with darkness does not inhibit flowering. Flowering is inhibited if the long dark period is interrupted by a flash of light. Long continuous and uninterrupted dark period is critical for flowering. Examples- Xanthium,coffee , tobacco, Glycine max etc. Long day plant Plants flower when photoperiod is more than the critical day length. Interruption during light period with darkness inhibit flowering. Flowering occurs if the dark period is interrupted by a flash of light. Dark period is not critical for flowering. Examples- Sugarbeet , Spinach, carrot, lettuce

9. Photoperiodic Induction The influence of the length of day and night on the initiation of flowering is called photoperiodic induction or photo induction. Plants may require one or more inductive cycle for flowering. An appropriate photoperiod in 24 hours cycle constitutes one inductive cycle. If a plant which has received sufficient inductive cycle is subsequently placed under unfavourable photoperiod, it will still flower. Flowering will also occur if a plant receives inductive cycles after intervals of unfavourable photoperiods (i.e. discontinuous inductive cycle). This persistence of photoperiodic after effect is called as photoperiodic induction. An increase in the number of inductive cycles results in early flowering of the plant. For instance, xanthium (a short day plant) requires only one inductive cycle and normally flowers after about 64 days. It can be made to flower even after 13 days if it has received 4-8 inductive cycle. In such case number of flowers is also increased.

10. Critical day length Maryland mammoth tobacco and xanthium are short day plants, but the Maryland mammoth tobacco is induced to flower when the photoperiod is shorter than 12 hours whereas, xanthium is induced to flower when the photoperiod is shorter than 15.5 hours. The photoperiod required to induce flowering is referred to as the critical day length. Hence, the critical day length for Maryland mammoth tobacco and xanthium are 12 and 15.5 hours respectively. A short day plant is one that flowers on photoperiods shorter than the critical day length. Long day plants, on the other land, are induced to flower on photoperiods longer than critical day length. For example, the critical day length for Hyoscyamus niger is 11 hours and it is induced to flower on photoperiods longer than 11 hours.

11. Perception of photoperiodic stimulus and presence of a floral hormone Photoperiodic stimulus is perceived by the leaves and a floral hormone is produced in the leaves which are then translocated to the apical tip (apical meristem), subsequently causing initiation of floral primordia . Photoperiodic stimulus perceived by the leaves can be shown by a simple experiment on chrysanthemum, a short day plant. Chailakhyan (1936) divided chrysanthemum plants into four groups and varied the regime of light and dark cycles in all the four groups by using light proof cases. The four groups of plants are as follows: Group A :Entire plant continuously received long day treatment Group B : Lower leaf portion received short day treatment while upper defoliated portion received long day treatment Group C : Lower leafy portion received long day treatment while upper defoliated portion received short day treatment Group D : Entire plant continuously received short day treatment He observed that flowering occurred in those plants where the leaves received short day treatment (Group B &D) but it failed in those plants where the leaves received long day treatment (Group A &C). On the basis of above observation he concluded that short day stimulus is perceived by the leaves.

12. Flowering stimulus: Florigen The flowering stimulus produced in the photo induced leaves is translocated to the shoot apices where flower formation is initiated. Flowering stimulus is similar in long day plants and short day plants. This can be proved by a grafting experiment and can be translocated from one plant to another. If a leaf from a photo induced plant is removed and then grafted on a non-photo induced plant, then this plant flowers. Apparently a chemical substance is produced during photo inductive cycle, which is transmitted during grafting to non induced plants and induces flowering. Chailakhyan (1937) called this flower inducing chemical substance as Florigen (flowering hormone). Florigen may be a 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.

13. CONTROL OF FLOWERING Florigen hormone

14. Phytochrome Phyrochrome is a blue proteinaceous non-photosynthetic pigment. It is composed of two components: a protein and a chromophore - which give its light absorbing properties. Phytochrome exists in two different forms i.e., red light absorbing form which is designated as Pr and far red light absorbing form which is designated as Pfr . Phytochrome is produced in light grown tissues. These two forms of the pigment are photo chemically inter convertible and it is located within plasma membrane. When Pr form of the pigment absorbs red light (660-665 nm), it is converted into Pfr form during day time. When Pfr form of the pigment absorbs far red light (730-735 nm), it is converted into Pr form. The Pfr form of pigment gradually changes into Pr form in dark.

15. Role of phytochrome in short and long day plants It is considered that during day time, the Pfr form of the pigment is accumulated in the plants which are inhibitory to flowering in short day plants but is stimulatory in long day plants. During critical dark period in short day plants, this form gradually changes into Pr form resulting in flowering. A brief exposure with red light will convert this form again into Pfr form thus inhibiting flowering. Plants measure the ratio of P r / P fr . LDP would flower when the P r / P fr ratio is low. SDP would flower when the P r / P fr ratio is high.

16. A. In short-day plants: Short day plants flower when the night period is long. In day light or red light, phytochrome red (Pr) is converted to phytochrome far red ( Pfr ). The conversion actually only requires a brief exposure to white or red light. In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the conversion. Under short day conditions (long night), at the end of the night period the concentration of Pfr is low. In SDP, low Pfr concentration is the trigger for flowering.

17. In the short-day plant P R builds up P FR Darkness (slow) Short-day plants Florigen activated Flowering

18. B. In long-day plants: Long day plants flower when the night period is short. In day light (white or red) the Pr is converted to Pfr . During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. There is a higher concentration of Pfr . In LDP, high Pfr concentration is the trigger for flowering.

19. In the long-day plant Sunlight Red light P R P FR builds up Long-day plants Florigen activated Flowering

20. Theories of Flowering Phytochrome theory Bunning’s hypothesis Chailakhyan’s hypothesis 1. Phytochrome Theory: Phytochrome controls flowering in plants. P R promotes flowering in SDPs and suppresses flowering in LDPs. On the other hand P FR suppresses flowering in SDPs and promotes flowering in LDPs . In SDP, if the long dark period is interrupted by red light, then sufficient amount of P R is not formed. Hence decrease flowering in SDPs. Flowering in SDPs depends upon P R / P FR ratio. High P R / P FR ratio stimulates flowering in SDPs and inhibits flowering in LDPs.

21. 2. Bunning’s hypothesis: Bunning (1958) assumed the presence of endogenous rhythms (Oscillator) which consist of two half cycles. The first half cycle occurs in day and is called photophilous phase. During this, anabolic process predominates including flowering in plants. The other half cycle is dark, sensitive and is called skotophilous phase. In this, catabolic process (dehydration of starch) predominates. SD plants have a critical day length of 9 hours. This period falls within the photophilous phase. Light during skotophilous phase will inhibit photo process initiated during photophase . The L.D. plants have a critical day length of 15 hours and some light falls in the skotophilous phase. Under these conditions in L.D. plants will flower. In S.D. plants oscillator is present close to skotophilous phase, while in L.D. plants it is close to photophilous phase.

22. 3. Chilakhyan’s hypothesis: This hypothesis assumes that flowering hormone – florigen is a complex of two types of substances – gibberellin and anthesins . Gibberellin is essential for growth of the plant stems and anthesins are required for flower formation. According to him, flowering in all annual seed plants requires two phases: ( i ) Floral stem formation phase (ii) Flower formation phase First phase involves increased carbohydrate metabolism and respiration with increased content of GA in leaves. Second phase requires intensive nitrogen metabolism, higher content of anthesins in leaves and nucleic acid metabolites in stem buds. Long day conditions favour the first phase while short day conditions favour second phase. In long day plants gibberellins are critical, while anthesins are critical in short day plants.

23. Gibberrlic acid and the flowering response Gibberrlic acid (GA) seems to play an important role in flowering. It may be directly involved in the formation of florigen . GA can substitute for long day requirements and also the cold treatment requirement in several species for its flowering. Brian (1958) stated that GA like hormone is produced in the leaves during photoperiod. High concentration of GA like hormone leads to synthesis of florigen in LDPs and when the level of this GA like hormone becomes low, it leads to synthesis of florigen in SDPs.

24. SIGNIFICANCE OF PHOTOPERIODISM The yield of tubers, corns, bulbs and rhizomes can be increased substantially by increasing or decreasing the duration of day or night. Crops like raddish , carrot, sugarcane can be made to remain vegetative for longer periods. Annuals may be grown twice or thrice in a year. Perennials might flower throughout the year. Winter dormancy & autumnal fall can be prevented by increasing light hours. Increase yield Hybridization experiments can be helpful because different varieties growing in different areas with different durations and flowering at different times are made to grow and flower side by side by artificially controlling their photoperiods. Understanding the concept of photoperiodism has helped to choose photo insensitive varieties and cultivars. These cultivars are best suited in intensive agriculture.

25. Vernalization

26. VERNALIZATION The cold treatment given to plant buds, seeds or seedlings for promoting early flowering is known as Vernalization . In short, the chilling treatment for induction of early flowering is called as Vernalization . Due to low temperature treatment the period of vegetative growth of the plant is reduced resulting into early flowering. The term vernalization was coined by T.D. Lysenko in 1920s. Many plants grown in temperate climates require vernalization and must experience a period of low winter temperature to initiate or accelerate the flowering process. This ensures that reproductive development and seed production occurs in spring and winters, rather than in autumn. The suitable temperature for vernalization ranges between 0 to 5 ̊c . At higher temperature from 7 ̊c onwards response of the plant is decreased. A temperature of about 12 to 14 ̊c is most ineffective in vernalizing the plant.

27. Site of vernalization Apical meristem , embryos and actively dividing cells are the potential sites of vernalization . The common feature for vernalization is the presence of actively dividing apical meristems . Perception of cold stimulus and presence of floral hormone The cold stimulus is perceived by the apical meristems . The perception of the cold stimulus results in the formation of a hormone Vernalin which is transmitted to other parts of the plant. In certain cases, the cold stimulus may even be transmitted to another plant across a graft union. Eg . Henbane. Here, if a vernalized plant is grafted to an non-vernalized plant the later also shows flowering.

28. REQUIREMENTS OF VERNALIZATION Low Temperature : Low temperature required for vernalization is usually 0°- 5°C. Period of Low Temperature Treatment : It varies from a few hours to a few days. Actively Dividing Cells Oxygen : The vernalization is an aerobic process and requires metabolic energy. In the absence of O 2 , cold treatment becomes completely ineffective. Water : Sufficient amount of water is also essential for vernalization . Vernalization of the dry seed is not possible. Proper Nourishment

29. Mechanism of Vernalization There are two main theories to explain the mechanism of vernalisation . Phasic developmental Theory Hormonal Theory 1. Phasic developmental Theory This theory was proposed by Lysenko (1934). According to this theory, the process of the development of an annual seed plant consists of a series of phases which must occur in some predetermined sequence. Commencement of any of these phases will take place only when the preceding phase has been completed. The phases require different external conditions for the completion such as light and temperature. Vernalization accelerates: the thermo phase i.e. that phase of development which is dependent upon temperature. Thus, in winter wheat, low temperature is required for the completion of first thermo phase. After this, the next phase i.e., photo phase starts which is dependent upon light.

30. 2. Hormonal theory It has already been described that vernalization probably involves the formation of a floral hormone called as vernalin . Based on this fact, many hypothetical schemes have been proposed by different workers from time to time. The first hormonal theory proposed by Long and Melchers (1947) is schematically shown below. cold treatment normal temp. A B C Flowering Precursor Thermolabile Vernalin compound Higher temp. D (doesn’t flower) Lang and Melcher’s (1947) suggested that a precursor A is converted into a thermolabile compound B during cold treatment. Under normal conditions B changes into a stable product (C Vernalin ) which causes flowering. But at higher temperature B is converted into D and flowering does not take place due to devernalization . Further experiments have shown that vernalin acts in association with the flower inducing stimulus, florigen in order to induce flowering.

31. Vernalization and Gibberellins The gibberellins are known to replace the low temperature requirement in certain biennial plants. E.g. Henbane. This plant normally remains vegetative and retains its rosette habit during the first growing season and after passing through the winter period, it flowers in the next season. The gibberellins cause such plants to flower even during the first year. DEVERNALIZATION The positive effect of the low temperature treatment on the vernaliztaion of the plants can be counteracted by subsequent high temperature treatment. This phenomenon 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.

32. Importance of vernalization Vernalization shortens the vegetative period of the plant. It increases cold resistance of the plants. Vernalization increases the resistance of plants to fungal diseases. In biennials, vernalization induces early flowering and early fruit setting. A non vernalized shoot apex can be induced to flower by grafting the plant with a vernalized plant. Winter varieties of crop plants can be converted into spring varieties by vernalization .