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physiology

1. Topic : “ Role of Photoperiodism in Flowering”

2. Photoperiodism Definition “ Photoperiodism  can also be defined as the reproductive response of plants to the relative length of light and dark periods.” Response of a plant to relative length of light and darkness within 24 hours is known as photoperiodism . Garner and Allard established the concept of photoperiodism by working on Maryland mammoth. Photoperiodism was first studied in relation to flowering. Plants can be described in relation to their photo-period responses as short-day, long-day, day-neutral plants.

3. Photoperiodic induction Condition required for flowering is called inductive phase. Two main for flowering. Photoperiod Vernalization (Low temperature) On the basis of photoperiod (duration of light) there are three categories of plants. Importance of dark period Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours. If dark period is interrupted by a flash, it does not flower; darkness amidst day cycle has no effect.. Therefore, length of the dark period controls flowering, not length of the light period.

4. Photoperiodism and Flowering

5. Short day Plants They flower when the day length is less than a certain critical length say 12 hrs. If these plants are exposed to day lengths in excess of this critical point, they continue growing vegetatively . Pfr inhibits growth of flowers. After long nights, every little Pfr remains, so flowering can still happens. These plants normally flower in winter or early spring. Examples: Strawberry,  Oryza sativa , Chenopodium album , cocklebur.

6. Long day Plants Long day plants flower when the day length is greater than a certain critical length, that is 12-16 hours. P fr required for flowering. Enough P fr is left at the endof short nights. Binds to genes and initiates transcription of gene related to flowering. Normally flower in late spring or early summer. Examples: Poppies, Lectuca sativa and potatoes

7. Critical photoperiod The length of the daily period of light to which a plant is exposed is called photoperiod and the number of hours of light that determines its flowering response is said to be critical photoperiod. For short day plants, the critical photoperiod is the photoperiod at or below which it flowers.  For long day plants, the critical photoperiod is the photoperiod at or above which it flowers

8. Critical period for SDP’s & LDP’s

9. Difference b/w SDP’s & LDP’s

10. Day Neutral Plants These plants flower regardless of number of hours of darkness as long as there is enough photosynthesis to support growth. These plants do not have any specific requirement of day length to flower, i.e. they are not affected by day lengths and flower around the year. Examples: Lycopersicum esculentum,  corn, cucumber

11. Other Classification of plants Short-Long Day Plants ( Trifolium ) They are long day plants but must be exposed to short days during early periods of growth for flowering. Long-Short Day Plants ( Bryophyllum ) They are short day plants but must be exposed to long days during early periods of growth for flowering.

12. Examples

13. Phytochromes Phytochrome  is a photo-sensitive, Photo-pigment molecule. Blue-green pigment and protein in nature. Tetra pyrol ring, dimer is formed. Work as internal clock. In 1952, Borthwick and Hendricks proposed the name phytochrome for a pigment  that receives light. Phytochrome exists in two inter-convertible forms. 1. Pr --- light absorption peak in 660nm         2. Pfr --- light absorption peak in 730nm

14. Structure of Phytochrome

15. Pigment phytochrome is involved in photoperiodic perception and flowering. During a 24-hour period, there is a shift in ratio of these two pigments. Direct sunlight contains more red than far-red light; Pfr  is present in plant leaves during the day. Shade and sunsets have more far-red than red light; Pfr  is converted to Pr as night approaches. There is a slow metabolic replacement of Pfr  by Pr during night

16. Phytochromes are interconvertable

17. Flowering Hormones Florigen Scientist Cajlachjan in 1935. He gave that hypothetical hormone , not isolated yet. Responsible for flowering.  Florigen  is produced in the leaves. Reaches floral buds and acts in the shoot apical meristem of buds and growing tips.

18. Activation of Florigen in leaves

19. Experiment to prove that photoperiodic induction is perceived by leaves 

20. Cannabis sativus (SDP)

21. Thank you