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Q8(c):
Which one of the plant growth regulators would you use if you are asked to:
(c) delay leaf senescence
Solution :
Identification of the Plant Growth Regulator
To delay leaf senescence (the aging process of leaves), the plant growth regulator of choice is Cytokinin.
Biological Mechanism of Action
The delay of senescence by cytokinins is primarily mediated through the Richmond-Lang effect. The mechanism follows these precise physiological steps:
- Inhibition of Catabolic Processes: Cytokinins suppress the activity of hydrolytic enzymes such as RNase and protease, which are responsible for the degradation of nucleic acids and proteins during leaf aging [Per the regulatory pathway of hormonal control in senescence].
- Nutrient Mobilization: Cytokinins induce the movement of nutrients from surrounding tissues toward the site of application. This creates a "sink" effect, maintaining the metabolic activity of the leaf tissue [By the principle of polar transport and source-sink relationship].
- Chlorophyll Retention: By maintaining the stability of thylakoid membranes and preventing the breakdown of chlorophyll, cytokinins effectively stall the transition of chloroplasts into gerontoplasts [Based on ultrastructural preservation theories].
Visual Representation of Cytokinin-Mediated Senescence Delay
Comparative Analysis of Regulatory Hormones
| Hormone | Function in Senescence |
|---|---|
| Cytokinin | Delays (Promotes protein synthesis) |
| Abscisic Acid (ABA) | Promotes (Accelerates degradation) |
| Ethylene | Promotes (Triggers leaf abscission) |
Theoretical Justification
The ability of cytokinins to delay senescence is fundamental to their role in promoting cell division and maintaining the physiological integrity of plant organs. By antagonizing the senescence-promoting signals (specifically ethylene and ABA), cytokinin-treated tissues remain metabolically active for significantly longer periods [As evidenced by the inhibition of chlorophyll degradation and protein hydrolysis].
Final Solution: Cytokinin is the plant growth regulator used to delay leaf senescence by promoting nutrient retention and inhibiting the degradation of cellular components.
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(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
- Q10(d): What would be expected to happen if: (d) you forget to add cytokinin to the culture medium.
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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