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Q10(a):
What would be expected to happen if:
(a) $GA_3$ is applied to rice seedlings
Solution :
Initial Biological Premise: Gibberellins ($GA_3$)
Gibberellins are a class of phytohormones known for their regulatory role in plant developmental processes, most notably the elongation of plant internodes. When exogenous $GA_3$ (Gibberellic Acid) is applied to rice seedlings, the physiological response is governed by the hormone's ability to trigger cell division and cell wall expansion [Per the Acid Growth Hypothesis and the upregulation of xyloglucan endotransglycosylase].
Step 1: Analyzing the Mechanism of Action
Upon application of $GA_3$ to rice seedlings, the following biochemical cascades are initiated:
- Hydrolytic Enzyme Induction: $GA_3$ stimulates the synthesis of enzymes such as $\alpha$-amylase, which hydrolyze stored starch into glucose, providing the necessary energy substrate for rapid growth.
- Wall Loosening: The hormone induces the activity of expansins, which disrupt hydrogen bonds between cellulose microfibrils in the cell wall, increasing cell wall plasticity.
- Cell Elongation: The osmotic pressure increases within the vacuole, leading to water uptake and subsequent turgor-driven cell elongation.
Step 2: Predicted Morphological Outcome
In rice seedlings, the most pronounced effect is a dramatic increase in the length of the internodes. This phenomenon is scientifically termed as "Bolting" or hyper-elongation of the stem axis.
Step 3: Justification and Historical Context
The study of $GA_3$ on rice originated from observations of the Bakanae (foolish seedling) disease in Japan, caused by the fungus Gibberella fujikuroi. Researchers noted that infected seedlings grew excessively tall, thin, and pale before collapsing [Based on the discovery of the 'Gibberella' factor]. Application of purified $GA_3$ replicates this exact phenotype, confirming that the hormone is the primary driver of stem elongation in monocot seedlings.
| Parameter | Without $GA_3$ | With $GA_3$ |
|---|---|---|
| Internode Length | Normal/Compact | Significantly Increased |
| Growth Rate | Baseline | Accelerated |
| Structural Integrity | Stable | Reduced (often prone to lodging) |
Final Solution: The application of $GA_3$ to rice seedlings induces internodal elongation, resulting in an abnormally tall, thin, and "spindly" growth pattern, frequently referred to as hyper-elongation or bolting.
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(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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