🌻 Photoperiodism

• The maturity of vegetative growth of the plant proceeds flower initiation. But the flower initiation of flowering depends on the environment especially light period (photo period) and temperature (vernalisation).

Photoperiodism

• Photoperiodism is the physiological response of plants to relative length of light (day) and dark (night) periods.
• Growth of plants towards light is called Phototropism.
• W.W. Garner and H.A. Allard (America, 1915-20) found that a newly developed tobacco mutant, Maryland Mammoth and Soybeans (Glycine Max) had strange seasonal pattern in flowering. Maryland Mammoth did not flower during summer but had good vegetative growth. When it was grown within a green-house during winter, it had excellent flowering and fruiting. Similarly soybean flowered only in the late summer irrespective of the sowing time in the previous spring.

• The term ‘photoperiodism’ was coined by Garner and Allard (1920) to designate the response of organisms to the relative length of the day and night and the `photoperiod’ to designate the favourable length of day for each plant.
• Garner and Allard classified the plants into three groups according to their photoperiods:

Short day plants

• “These plants give flowers on exposure to photoperiod equal or shorter than their critical day length”
• They need a continuous (uninterrupted) dark period for flowering. Thus SDP also called as LNP (Long Night Plants).
• In SDP the dark period is critical and must be continuous. If this dark period is breaks by a brief exposure to red light, then SDP will not flowers.
• If the dark period is less than a critical length, there will be no flowering. Short day plants will not flower if the dark period is interrupted by a flash of light during the continuous dark period but such light interruption is not very effective if it is given near the beginning or the end of the dark period. Such plants don’t flower if short dark and short light periods are provided alternatively.
• Examples of short day plants are Maryland Mammoth tobacco, Soybean, Chrysanthemum, Chenopodium album, Xanthium stumarium (Cocklebur), Rice, Sugarcane, Pea, Generally Kharif Crops and many tropical plants.

Long day Plants

• Such plants which flower only when the day length is longer than a certain critical period. The critical length varies from 4 to over 18 hours.
• They require either a relatively small period of darkness or no darkness at all.
• A flash of light to long day plants during long dark period can induce flowering even during short day period. This is called Night break. Here, darkness has an inhibitory effect on flowering.
• The flowering in long-day plants is inhibited not because of the short light periods but because of too long dark period. That’s why such plants are also called short night plants.
• E.g. Wheat, Barley, Oat, Sugarbeet, Spinach, Radish, Lettuce, Castor, generally rabi crops.

Day neutral plants

• These plants do not need specific light period for the flowering.
• E.g. Maize, Cotton, Tomato, Sunflower, Cucumber, Balsam etc.

Important Terms

• Critical Period: It is the photoperiod required to induce flowering e.g. for M. M. tobacco, critical day period (CDP) is 12 hours and for xanthium it is 15.5 hours.
• Photoperiodic Induction: In short day and long day plants, a continuous favourable photoperiod till blossoming is not essential but a few days exposure to the appropriate photoperiod is enough to induce flowering. This photoperiodic influence persists even when a treated plant is kept in unfavourable photoperiods. This initial important effect on flowering is known as photoperiodic induction or photo-induction.
• Flowering Stimulus: Cajlachjan (Cailakhyan) while working on chrysanthemum and perilla demonstrated that the photoperiodic stimulus was perceived by the leaves of a plant. Garner and Allard found that the stimulus was highly localized or systemic. Even if a single leaf of cocklebur was exposed to short days and the rest of the plant to long days, flowering was occured in the whole plant. It means that this stimulus is systemic.
• Mature leaves are very sensitive to the photoperiodic stimulus while very young and old leaves are generally insensitive.
• Green colour of light spectrum is normally ineffective in inducing flowering whereas blue colour induces poor flowering. It is the red spectrum (wavelength of 580 nm to 680 nm) of light which is the most effective for inducing flowering in both short day and long day plants.
• Cajlachjan (Chailakhyan) used the term Florigen for flowering stimulus hormone. It has not been isolated, thus called as hypothetical hormone.
• Formation of florigen is triggered by phytochrome. Phytochrome mainly located on cell membrane of all type of plants. Phytochrome exists in two interconvertable different forms:
  • Pr (Phytochrome Red) – Red light absorbing form, induce flowering in SDP.
  • Pfr (Phytochrome Far Red) – This is far-red light absorbing form, induce flowering in LDP.

• Dr. S.M. Sircar (Bose Institute) induced flowering of a winter variety (Aman) of rice in 50 days against normal 140 days.

Vernalisation

“Acceleration of ability to produce flower by chilling treatment is called vernalisation.”

• In annual plants the flowering is primarily affected by the photo period but in biennials flowering requires prolonged periods of low temperature.
• The effect of temperature is secondary to light.
• Firstly G. Gassner of Germany (1918) reported the effect of temperature on reproductive development. ‘Vernal’ means spring.
• The term ‘Vernalisation’ was coined by T.D. Lysenko (1920).
• The Russian term ‘Yarovization’ means pre-sowing treatment.
• Gassner demonstrated his experiment on winter Petkus rye (Secale cereale cv. petkus). The low temperature requirement of winter petkus rye was given by chilling treatment. The imbibed seeds (water soaked were stored at 2.5°C for 5-6 weeks and then was sown in the spring season. Flowering was occurred on the same schedule as on spring cultivar of Petkus rye flowered.

Summary of Experiment

• Cold treatment is quantitative or facultative (means low temp. results in faster flowering) but not qualitative or absolute (qualitative means flowering absolutely depends upon cold).
• Vernalization is the cold treatment to a plant bud or seedling in order to fulfil a specific low temperature requirement for accelerating the flowering. In other words the acquisition or hastening of the ability to flower by chilling treatment is called vernalization or yarovization (Russian term).
• Site of Vernalisation: growing point (apical buds)/early stages of germination/embryo undergoing rapid cell division/initiating metabolic process in shoot meristematic zones. It means that dividing cells (embryo tip, shoot apex & leaves) are the site of vernalization.
• Cause: The growth substance formed by low temperature induction was named ‘vernalin’ by G. Melchers (1939). It is hypothetical plant hormone, because not has been isolated till today.
• According to Mikhail Chailakhyan (1968) there were two substances responsible for vernalisation:
  • Gibberellin or gibberell in like substances
  • Anthesin
• Devernalization: Vernalization effect is reversible. If the vernalized seed or plant is kept at high temperature just after vernalization, the effect of the low temp treatment is completely removed. This process is called devernalisation.

Factors of Devernalization

• High temp. 👉🏻 [Just after vernalization]
• Anaerobic condition 👉🏻 [Just after vernalization]

Practical benefits

• To induce earlier flowering and earlier maturity of crop.
• To escape frost, drought and flood.
• To extend cultivation to the region with very low temp. (extensively used in Russia i.e. Siberia where only 2 months are ice free for growing early crops).
• Prevention of precocious reproductive development late in growing season.