Plant Growth And Development Class 11 Notes

Key Definitions

Growth

An irreversible permanent increase in size of an organ or its parts or even of an individual cell.

Differentiation

The process by which cells originating from the meristem undergo changes in structure and function to form specific cell types or tissues.

Dedifferentiation

The process where already differentiated cells lose their specialization and regain the capacity to divide, e.g., formation of interfascicular cambium.

Redifferentiation

The process where dedifferentiated cells again mature and differentiate to perform specific functions.

Plant Growth Regulators (PGRs)

Small, simple molecules of diverse chemical composition that regulate plant physiological processes. Also called plant hormones or phytohormones.

Photoperiodism

The physiological response of plants to the relative lengths of day and night (photoperiod), especially in relation to flowering.

Vernalisation

The process by which flowering is promoted by a period of cold treatment.

Senescence

The process of aging in plants, leading to the eventual death of organs or the entire plant.

Dormancy

A state of metabolic arrest or suspended growth in seeds or buds, allowing them to survive unfavorable conditions.

Plant Growth Regulators (PGRs): An Overview

Plant Growth Regulators (PGRs), also known as phytohormones, are crucial chemical messengers that orchestrate virtually every aspect of plant growth and development. They are naturally produced by plants in very minute quantities and act as signaling molecules. PGRs can be broadly classified into two groups based on their primary functions: Plant Growth Promoters and Plant Growth Inhibitors.

Growth Promoters include Auxins, Gibberellins, and Cytokinins. These hormones are involved in cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation.

• Auxins (e.g., IAA) primarily promote cell elongation, apical dominance, root initiation, and prevent premature fruit/leaf drop. They are synthesized at shoot apices.
• Gibberellins (e.g., GA3) promote stem elongation (bolting), seed germination, and overcome dormancy. They are found in higher concentrations in germinating seeds and young leaves.
• Cytokinins (e.g., kinetin, zeatin) promote cell division (cytokinesis), chloroplast development, and delay senescence. They are synthesized in regions of rapid cell division, like root apices.

Growth Inhibitors include Abscisic Acid and Ethylene (though Ethylene can also have promoter effects). These hormones play roles in plant responses to stress, dormancy, and abscission.

• Abscisic Acid (ABA) acts as a general plant growth inhibitor. It promotes dormancy in seeds and buds, causes stomatal closure during water stress, and plays a role in abscission.
• Ethylene is a gaseous hormone primarily involved in fruit ripening, enhancing the senescence of leaves and flowers, and promoting abscission. It can also break seed and bud dormancy in some cases. Its diverse effects often lead to it being categorized with both promoters and inhibitors, but its primary role in stress responses and senescence often places it with inhibitors.

Comparison of Major Plant Growth Regulators (PGRs)

Mechanism of Photoperiodism

1. —
2. —
3. —
4. —
5. —
6. —
7. —

Practical Examples of PGR Actions

• {"title":"Auxin's Role in Apical Dominance","description":"In many plants, the apical bud grows preferentially, inhibiting the growth of lateral (axillary) buds. This phenomenon is apical dominance, primarily due to auxins produced by the apical bud. If the apical bud is removed (decapitation), the lateral buds start to grow, demonstrating the inhibitory effect of apical auxin."}
• {"title":"Gibberellin's Effect on Bolting","description":"Some plants, like cabbage and beet, exhibit rosette habit (short plant with a cluster of leaves) in their early stages. Before flowering, their internodes elongate dramatically, a process called bolting. Gibberellins can induce this bolting in such plants, especially in genetically dwarf varieties or those requiring vernalisation."}
• {"title":"Ethylene and Fruit Ripening","description":"Ethylene is the gaseous hormone responsible for fruit ripening. For example, placing unripe bananas in a paper bag with a ripe apple will accelerate the ripening of bananas because the ripe apple releases ethylene, which then triggers the ripening process in the unripe bananas."}

Key Takeaways for Revision

• Growth is quantitative (increase in size/mass) and qualitative (differentiation/development).
• Growth is measurable: increase in fresh weight, dry weight, length, area, volume, or cell number.
• Phases of growth: Meristematic (cell division), Elongation (cell enlargement), Maturation (cell differentiation).
• Differentiation, dedifferentiation, and redifferentiation are crucial for plant tissue culture and healing.
• PGRs act in very low concentrations and often exhibit synergistic or antagonistic effects.
• Auxins: Apical dominance, rooting, parthenocarpy. Gibberellins: Bolting, seed germination, fruit enlargement. Cytokinins: Cell division, delay senescence.
• Ethylene: Fruit ripening, abscission, senescence. Abscisic Acid (ABA): Stress hormone, dormancy, stomatal closure.
• Photoperiodism classifies plants into Short-Day Plants (SDP), Long-Day Plants (LDP), and Day-Neutral Plants (DNP) based on their critical photoperiod.
• Vernalisation is crucial for flowering in many temperate plants by exposing them to cold temperatures.
• Seed dormancy is a survival mechanism, broken by environmental cues or specific hormone treatments (gibberellins, chilling, light).

Exam Strategy for Plant Growth and Development

When answering questions on Plant Growth and Development, always focus on the specific function and one good example for each Plant Growth Regulator (PGR). Distinguish clearly between growth promoters and inhibitors. For photoperiodism and vernalisation, understand the conditions required and the plant's response. Diagrams illustrating plant responses (e.g., apical dominance, bolting) can earn extra marks. Practice explaining the mechanism of action for key processes like flowering induction and seed dormancy breaking. Pay attention to how different PGRs interact (synergism vs. antagonism).

Practice Questions with Solutions

• Q: Name two commercial applications of auxins. A: Rooting of stem cuttings and preventing premature fruit drop.
• Q: What is the primary role of Abscisic Acid (ABA) during water stress? A: ABA causes stomatal closure to reduce water loss through transpiration.
• Q: Differentiate between Short-Day Plants (SDP) and Long-Day Plants (LDP) in terms of critical photoperiod. A: SDP flower when the light period is shorter than a critical duration, while LDP flower when the light period is longer than a critical duration.
• Q: How do Gibberellins help in breaking seed dormancy? A: Gibberellins promote the synthesis of hydrolytic enzymes (like amylase) that mobilize stored food reserves, initiating germination.

Frequently Asked Questions