Question

Write Essay on Following: 

1. Gibberellin and Cytokinin. 

2. Growth inhibitor substances. 

3. Photoperiodism. 

4. Senescence and Abscission. 5. Vernalization

Answer 1

1. Gibberellins: 

• Gibberellins were discovered in Japan and the discovery is related to a disease in Rice (Paddy) plant.
• In 1890 a disease occurred in the Rice fields in japan where some Rice plants grew extraordinary tall and remained flowerless.
• This disease was named as “Bakanae disease” (In Japanese language Bakanae means foolish) Hori (1898) Studied this disease in detail and found that these plants were infected with a fungus Gibberella fuji-Kuro (perfect state of an imperfect fungus Fusarium moniliform).
• The infected seedlings grew extra tall, and thin and do not bear flowers, fruits and seeds. So these were called “foolish seedling” and the disease as a foolish seedling disease.
• Kurosawa (1926) proved that secretion by the fungus sprayed on the healthy plant results in this disease.
• Yobuta and Hayashi (1939), isolated the substance in pure crystalline form from the culture filtrate of Gibberella fuji-Kuro and named it Gibberellin.
• Brian et. al (1954) isolated pure form of single gibberellin and named if Gibberellic acid.
• About 100 different gibberellins have been obtained from different fungi and higher plants. These are known as GA₁, GA₂, GA₃, G₄……… GA100. Among this GA3 is the first discovered and commonly found gibberellin. 

2. Cytokinins: 

• G Haberland (1913) First observed that in the phloem of some plants, some substances are found which stimulate cell division. 
• J. Van Overbeek (1941) showed that there are certain substances in coconut milk which promote cell division. 
• Skoog and Miller (1955) isolated a very useful substance from DNA of yeast which promotes cell division and named it as kinetin. 
• Letham (1963) named Kinetin as Cytokinin. 
• Letham and Miller (1964) isolated a substance, similar to cytokinin, from the endosperm, of maize and called it Zeatin.
  Zeatin is the first naturally occurring cytokinin. 

3. Growth inhibitor substance: 

• Those hormones or substances which inhibit the rate of growth are called growth inhibitors. 
• These substances are required for controlling and balancing growth and development. 
• Abscisic acid is the most important growth-inhibiting hormone.

4. Abscisic Acid (ABA): 

• This is the most important naturally occurring growth inhibitor found in plants.
• It helps the plant to successfully survive adverse environmental conditions.
• Hence it is also called stress hormone.
• Wareing (1963) isolated one growth-inhibiting substance from the leaves of Acer plant and named it as Dormin.
• Addicott et. al. (1963) isolated one substance from the floral buds of cotton and named it as Abscisin.
• Later on, it was proved that domain and abscision are the same substance and these were named as Abscisic Acid (ABA).

5. Abscission: 

• Shedding of leaves, flowers and fruits from the mother plant by a natural process is called abscission.
• It is a biological process which is initiated due to changes in the cells of the basal portion of the parts which fall off. 
• The middle lamella and the cell wall of the cells in the basal portion are digested by pectinase and cellulase enzymes. 
• As a result of breaking of middle lamella and cell-wall, cells in a definite region begin to separate from each other. 
• The tissues in this area become weak and soft and a layer is formed which is called an abscission layer. 
• The cells situated just below the abscission layer become meristematic and form cork cells.
• This cork forms a protective layer or abscission zone.
• Due to strong wind or rain, the plant part separates from the region of abscission and fall off. 
• Abscission is due to a change in the hormonal balance. Abscisic acid plays a major role in the process of abscission. 

6 Vernalization:

• In the majority of plants, besides photoperiod temperature also has an important role in growth and development. 
• The response of plants towards temperature relating to growth and development was first observed by a Russian scientist Lysenko (1928) while working on winter and spring varieties of rice. 
• In annual plants, light has a major effect on flowering and the effect of temperature is secondary. 
• In biennial plants the situation is different. In these plants, there is only vegetative growth in the first year and flowering is completed in the second year. 
• Before flowering cold temperature exposure of winter is necessary for these plants, and in absence of cold treatment reproductive stage does not begin and they remain in the vegetative stage. 
• The method of inducing flowering by chilling (cold) treatment is called vernalization. 
• According to Choird (1960), vernalization can be defined as “Inducing ability of flowering by chilling treatment artificially is called vernalization”