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Q10(d):
What would be expected to happen if:
(d) you forget to add cytokinin to the culture medium.
Solution :
Theoretical Foundation: The Role of Cytokinins in Tissue Culture
In plant tissue culture, the growth and differentiation of explants are governed primarily by the ratio of two plant growth regulators (PGRs): auxins and cytokinins. According to the foundational Skoog-Miller model, the morphogenetic pathway of undifferentiated callus tissue is determined by the relative concentrations of these hormones.
Step 1: Mechanism of Cytokinin Deficiency
Cytokinins are hormones primarily responsible for promoting cell division (cytokinesis) and stimulating shoot organogenesis. In a standard culture medium, the exogenous application of cytokinin facilitates the development of shoots from callus. If cytokinin is omitted from the medium, the following biochemical and physiological events occur:
- Inhibition of Shoot Bud Formation: Without the trigger of cytokinin, the cell cycle progression associated with shoot apical meristem development is arrested [Per Skoog and Miller’s hypothesis of organogenic control].
- Dominance of Auxin Effects: If the medium still contains auxin, the hormonal balance shifts significantly toward auxin-domination. Auxins, in the absence of opposing cytokinin activity, preferentially stimulate cell elongation and the formation of adventitious roots.
Step 2: Visual Representation of Hormone-Controlled Differentiation
Step 3: Analytical Impact on Culture Development
| Component | Expected Outcome (No Cytokinin) |
|---|---|
| Cell Division | Significantly reduced rate of mitosis. |
| Morphogenesis | Formation of roots only, or undifferentiated, slow-growing callus. |
| Apical Dominance | The tissue fails to initiate lateral shoot buds, often leading to necrosis. |
Step 4: Logical Conclusion
When cytokinin is omitted, the culture medium loses its capacity to induce shoot differentiation. The metabolic priority of the plant tissue shifts from cytokinesis and bud development to longitudinal cell expansion and rhizogenesis (root formation). Consequently, the explant will likely either produce only roots or remain as a stunted, non-differentiating mass of callus tissue, failing to achieve the regeneration of a whole plantlet.
Final Solution: The absence of cytokinin will lead to the failure of shoot morphogenesis; the culture will either exhibit excessive root development due to unchecked auxin activity or remain as undifferentiated callus with inhibited cell division.
More Questions from Class 11 Biology Plant Growth and Development EXERCISES
- Q1: Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
- Q2: Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
- Q3(a): Describe briefly: (a) Arithmetic growth
- Q3(b): Describe briefly: (b) Geometric growth
- Q3(c): Describe briefly: (c) Sigmoid growth curve
- Q3(d): Describe briefly: (d) Absolute and relative growth rates
- Q4: List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.
- Q5: Why is abscisic acid also known as stress hormone?
- Q6: ‘Both growth and differentiation in higher plants are open’. Comment.
- Q7: ‘Both a short day plant and a long day plant can produce can flower simultaneously in a given place’. Explain.
- Q8(a): Which one of the plant growth regulators would you use if you are asked to: (a) induce rooting in a twig
- Q8(b): Which one of the plant growth regulators would you use if you are asked to: (b) quickly ripen a fruit
- Q8(c): Which one of the plant growth regulators would you use if you are asked to: (c) delay leaf senescence
- Q8(d): Which one of the plant growth regulators would you use if you are asked to: (d) induce growth in axillary buds
- Q8(e): Which one of the plant growth regulators would you use if you are asked to: (e) ‘bolt’ a rosette plant
- Q8(f): Which one of the plant growth regulators would you use if you are asked to: (f) induce immediate stomatal closure in leaves.
- Q9: Would a defoliated plant respond to photoperiodic cycle? Why?
- Q10(a): What would be expected to happen if: (a) $GA_3$ is applied to rice seedlings
- Q10(b): What would be expected to happen if: (b) dividing cells stop differentiating
- Q10(c): What would be expected to happen if: (c) a rotten fruit gets mixed with unripe fruits
CBSE Solutions for Class 11 Biology Plant Growth and Development
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Digestion and Absorption
- Breathing and Exchange of Gases
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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