Structure, Functions and Uses of Cytokinins MCQ - Practice Questions with Answers

• Cytokinins are plant growth hormones that are basic in nature.
• These are either aminopurines or phenyl urea derivatives that promote cell division (cytokinesis) either alone or in conjugation with auxin.

• The first cytokinin was discovered by Miller, Skoog and Strong (1955) during callus tissue culture of tobacco.
• This cytokinin was formed from degraded autoclaved Herring sperm DNA.
• It is called kinetin (6-furfuryl amino-purine). 
• Kinetin does not occur naturally. It is a synthetic hormone. 
• The first natural cytokinin was obtained from unripe maize grains or kernels by Letham (1964). It is known as zeatin (6-hydroxy 3-methyl trans 2-butenyl amino-purine).
• The term ‘cytokinin’ was proposed by Letham.
• Cytokinins are synthesized in roots as well as endosperm of seeds.
• Cytokinins in coconut milk are called coconut milk factor.
• Cytokinins are transported to the different parts of the plant through xylem elements.
• According to Osborne and Black (1964), the movement of cytokinins is polar and acropetal.

Tobacco Pith Culture:

• Out of two tobacco pith cultures, one is supplied with cytokinin while the other is not.
• Increase in fresh weight of the tissue over the control is a measure of stimulation of cell divisions and hence cytokinin activity. 
• The test can measure cytokinin concentration between 0.001-10 mg/litre. 
• It takes 3-5 weeks. 

Retardation of Leaf Senescence:

• It is a rapid bioassay technique. 
• Leaf discs are taken in two lots. 
• In one lot cytokinin is provided. 
• After 48-72 hours, the leaf discs are compared for chlorophyll content. 
• Cytokinin retards the process of chlorophyll degradation. 
• The test is sensitive in concentration of 1 pg/litre.

Excised Radish Cotyledon Expansion:

• The test was developed by Letham. 
• Excised Radish cotyledons are measured and placed in test solution as well as ordinary water (as control). 
• Enlargement of cotyledons is an indication of cytokinin activity.
   

1. Cell Division:

• Cytokinins are essential for cytokinesis though chromosome dou­bling can occur in their absence. 
• In the presence of auxin, cytokinins bring about division even in permanent cells. 
• Cell division in callus (unorganised, undifferentiated irregular mass of dividing cells in tissue culture) is found to require both the hormones.

2. Cell Elongation and Differentiation:

• Like auxin and gibberellins, cytokinins also cause cell elongation.
• Both auxin and cytokinins are essential for morphogenesis or dif­ferentiation of tissues and organs. 
• Buds develop when cytokinins are in excess while roots are formed when their ratios are reversed.
• Cytokinins induce formation of new leaves, chloroplasts in leaves, lateral shoot formation and adventitious shoot formation. 
• They also bring about lignification and differentiation of interfascicular cambium.

3. Delay in Senescence (Richmond-Lang Effect):

• Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients.
• It was reported by Richmond and Lang (1957) in Xanthium.

4. Counteraction of apical dominance:

• Presence of cytokinin in an area causes preferential movement of nutrients towards it.
• When applied to lateral buds, they help in their growth despite the presence of apical bud. 
• They thus act antagonistically to auxin which promotes apical dominance.
• 5. Breaking of Dormancy:
• Like gibberellins, they overcome seed dormancy of various types, including red light requirement of Lettuce and Tobacco seeds.

6. Sex expression:

• Cytokinins promote female flowers in some plants.

7. Pomalin:

• It is the combination of cytokinins and gibberellins (G4 and G7) that increases the apples size.