07 Plant Growth and Mineral Nutrition - part 14 - Photoperiodism

07 Plant Growth and Mineral Nutrition - part 14 - Photoperiodism


Photoperiodism : 
  • Higher plants reproduce sexually by producing special structures called flowers. 
  • Plants exhibit transition from vegetative growth to reproductive growth during which flowers are produced. 
  • Like vegetative growth, reproductive growth is also influenced by several environmental and nutritional factors. 
  • Among the environmental factors - light and temperature exert profound influence on flowering. 
  • The influence of light is known as Photoperiodism and that of temperature, is Vernalization. 
  • Light as an environmental factor influences -  
  1. germination of seed
  2. vegetative growth
  3. photosynthesis, etc.
  •  Light as a factor as three aspects viz - 
  1. Quality, 
  2. Intensity and 
  3. Duration of light. 
  • It is the duration of light that has profound effect on flowering in higher plants. 
  • The term photoperiodism was used by Garner and Allard (1920). 
  • They were studying the flowering behaviour in plants - Soyabean and Meryland mamoth variety of tobacco. 
  • They found that soyabean plant flowers during late summer and tobacco variety during winter, irrespective of their germination and growing season. 
  • They studied effects of different temperatures, nutrition, soil moisture, etc. in respect of flowering. 
  • None of these were found to regulate flowering. 
  • However, experimentally they found that the exposure to specific duration of light (i.e. photoperiod) had profound influence on flowering. 
  • They examined the effect of day length on flowering by using artificial illumination. 
  • Conclusion : the relative length of the day was most crucial in the growth and development of flowers to which they coined the term photoperiodism. 
Classification of Plants [Based on the photoperiodic response.]: - 
  1. Short Day Plants (SDP)
  2. Long Day Plants (LDP) and 
  3. Day Neutral Plants (DNP).

1. Short Day Plants (SDP) : 
  • These plants usually flower during winter and late summer when day length is shorter than the critical photoperiod.
  • Are called long night plants because they require long uninterrupted dark period/ night for flowering. 
  • If dark period is interrupted even by a flash of light, SDP will not flower. 
  • Some of the short day plants are Dahlia, Aster, Tobacco, Chrysanthemum, Soybean (Glycine max), Cocklebur (Xanthium), etc. 
2. Long Day Plants (LDP) : 

  • Plants that flower during summer are called long day plants. 
  • They require longerduration of light than the critical photoperiod, for flowering. 
  • They are called short night plants as they require short dark period. 
  • When long dark period is interrupted by a brief flash of light, LD plants can flower 
  • e.g. pea, radish, sugar beat, cabbage, spinach, wheat, poppy, etc.

3. Day Neutral Plants (DNP) : 
  • These plants flower throughout the year round, independent of duration of light(photoperiod). 
  • They do not require specific photoperiod to flower. Therefore, they are called Day neutral plants 
  • e.g. Cucumber, tomato, cotton, sunflower, maize, balsam, etc.

Phytochrome : 
  • Hendricks and Borthwick (1952) observed
  • Observation : Flowering in SD plants is inhibited, if dark period is interrupted even by a flash of red light of 660 nm. 
  • If it is immidiately followed by far red light (730 nm), then SD plants will flower. 
  • Conclusion :  Some pigment system in plant receives the photoperiodic stimulus. These pigment proteins are called phytochromes. 
  • The leaves produce light-receiving proteinaceous pigment called phytochrome that induces flowering. 
  • It exists in two interconvertible forms viz -  
  1. Red (Pr) and 
  2. Far red (Pfr). 
  • When Pfr absorbs far red light, it is converted into Pr and vice versa. 
  • These are located in the cell membrane of green cells.
  • During day time, Pfr accumulates in the plants. 
  • It inhibits flowering in SDP but initiates flowering in LDP. 
  • During dark period - 
  1. Pfr changes into Pr
  2. it stimulates/ promotes flowering in SDP and 
  3. inhibits flowering in LDP.

Do you know ?
Organs for reception of photoperiodic stimulus - 
  • Leaf is the chief organ for receiving the photoperiodic stimulus as demonstrated by Knoff (1934). 
  • Defoliated plants will not flower even if the plants are exposed to proper duration of light. 
Photoperiodic stimulus - 
  • It is a chemical stimulus transported through phloem and is called florigen which is hormonal in nature. 
  • Photochemical receptors in the leaves are the biloproteins (pigments) located in the cell membrane. These are called Phytochromes.
  • Blue wavelengths of light influences flowering.

Always Remember :
  • Control of morphogenesis by light and phytochrome, is called photomorphogenesis.

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