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Plant Growth And Development

Plant Growth and Development - Complete NEET Guide with Diagrams & Practice Questions

Table of Contents

  1. Introduction
  2. Key Concepts
  3. Important Formulas & Equations
  4. Memory Techniques (Mnemonics)
  5. Previous Year Questions (NEET)
  6. Key Takeaways for Quick Revision

Introduction

The journey of a plant from a single-celled zygote to a complex, mature organism is a highly ordered and magnificent process. This chapter, Plant Growth and Development, delves into the "how" and "why" behind this transformation. For NEET aspirants, this is a high-yield topic, as it integrates concepts of cell biology, anatomy, and physiology. You can consistently expect 1-2 questions from this chapter, focusing heavily on the physiological functions of Plant Growth Regulators (PGRs) and concepts like plasticity and growth curves.

This guide will provide a comprehensive breakdown of plant growth, the dynamic processes of differentiation, the crucial roles of the five major plant hormones, and how plants respond to their environment. By mastering these concepts, you'll be well-equipped to tackle any question from this section with confidence.

Key Concepts

1. Growth: The Irreversible Increase

Growth is defined as an irreversible, permanent increase in the size of an organ, its parts, or an individual cell, typically accompanied by an increase in dry weight. It is a metabolic process that consumes energy.

  • Indeterminate Growth: Plants exhibit a unique ability for unlimited growth throughout their life. This is due to the presence of meristems, which are localized regions of actively dividing cells. This is called an open form of growth.

  • Phases of Growth: Growth occurs in three distinct phases:

    1. Meristematic Phase: Characterized by rapid cell division in the root and shoot apices. Cells are rich in protoplasm with large nuclei.
    2. Elongation Phase: Cells proximal to the meristematic zone undergo enlargement, vacuolation, and new cell wall deposition. This phase accounts for the primary increase in length/size.
    3. Maturation Phase: Cells attain their final form and function. This phase involves major structural changes in the protoplasm and cell walls.

  • Growth Rates: The rate of growth can be expressed mathematically.

    1. Arithmetic Growth: The rate of growth is constant. Following mitosis, one daughter cell continues to divide while the other differentiates. It results in a linear growth curve. Example: A root elongating at a constant rate.
    2. Geometric Growth: The initial growth is slow (lag phase), followed by a rapid exponential increase (log phase), and finally slows down to a stationary phase due to limited resources. This produces a characteristic Sigmoid (S-shaped) growth curve. It is typical for an organism or its parts growing in a natural environment.

Figure: The two primary growth curves. (a) Arithmetic growth showing a linear increase. (b) Geometric growth showing a characteristic Sigmoid (S-shaped) curve with lag, log, and stationary phases.

2. Differentiation, Dedifferentiation, and Redifferentiation

These three processes describe the changes in a cell's structure and function throughout a plant's life.

  • Differentiation: The process by which cells derived from meristems mature to perform specific functions. This involves significant structural changes. For example, to form a tracheary element, a cell loses its protoplasm and develops a strong, lignocellulosic secondary wall.
  • Dedifferentiation: The phenomenon where differentiated, living cells that have lost the capacity to divide regain it under certain conditions. Example: The formation of interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
  • Redifferentiation: The process where dedifferentiated cells divide and produce new cells that once again lose the capacity to divide and mature to perform specific functions. Example: The secondary xylem, secondary phloem, and cork cells formed by the cambium are redifferentiated tissues.

3. Development and Plasticity

Development is the sum of all changes that an organism undergoes during its life cycle, from seed germination to senescence. It is the product of Growth + Differentiation.

  • Plasticity: Plants follow different developmental pathways in response to environmental cues or phases of life, leading to the formation of different kinds of structures.
    • Heterophylly is a prime example of plasticity. It refers to the occurrence of different leaf forms on the same plant.
      • Developmental Heterophylly: Leaves on a juvenile plant are different from those on a mature plant (e.g., in cotton, coriander, larkspur).
      • Environmental Heterophylly: The shape of leaves depends on the environment. In buttercup (Ranunculus), leaves produced in water are finely dissected, while those produced in air are broad and lobed.

Figure: Heterophylly in (a) larkspur, showing different leaf shapes in juvenile vs. adult plants, and (b) buttercup, showing different leaf shapes in terrestrial vs. aquatic habitats.

4. Plant Growth Regulators (PGRs)

PGRs (or phytohormones) are small, simple molecules that act as chemical messengers to regulate plant growth and development.

  • Groups of PGRs:
    1. Growth Promoters: Involved in cell division, enlargement, flowering, etc. (Auxins, Gibberellins, Cytokinins).
    2. Growth Inhibitors: Involved in responses to stress, dormancy, and abscission (Abscisic Acid, Ethylene). Ethylene can be both a promoter and an inhibitor but is largely considered an inhibitor.
Auxins: The Master Growth Hormone
  • Discovery: Charles and Francis Darwin observed that the coleoptile of canary grass bent towards a unilateral light source. F.W. Went later isolated Auxin from the tips of oat coleoptiles.
  • Types: Natural (IAA, IBA) and Synthetic (NAA, 2,4-D).
  • Key Functions:
    1. Apical Dominance: The apical bud inhibits the growth of lateral (axillary) buds. Decapitation (removal of the shoot tip) promotes lateral branching.
    2. Root Initiation: Promotes rooting in stem cuttings for plant propagation.
    3. Flowering: Promotes flowering in plants like pineapples.
    4. Prevents Abscission: Prevents the early drop of fruits and leaves but promotes the abscission of older, mature ones.
    5. Parthenocarpy: Induces fruit development without fertilization (e.g., in tomatoes).
    6. Herbicide: 2,4-D is a selective herbicide used to kill broad-leaved (dicot) weeds in monocot fields.
Gibberellins: The Bolting & Elongation Hormones
  • Discovery: E. Kurosawa identified it from the fungus Gibberella fujikuroi, which caused the "bakanae" (foolish seedling) disease in rice.
  • Key Functions:
    1. Stem Elongation: Increases the length of the axis, famously used to increase the length of grape stalks and the yield of sugarcane (by up to 20 tonnes per acre).
    2. Bolting: Induces internode elongation (bolting) just before flowering in rosette plants like cabbage and beet.
    3. Breaks Dormancy: Overcomes seed and bud dormancy.
    4. Malting Process: GA₃ is used to speed up the malting process in the brewing industry.
    5. Delays Senescence: Delays aging, allowing fruits like apples to be left on the tree longer to extend the market period.
Cytokinins: The Cell Dividers
  • Discovery: F. Skoog and his co-workers discovered kinetin (a modified adenine) from autoclaved herring sperm DNA. The first natural cytokinin, zeatin, was isolated from corn kernels and coconut milk.
  • Key Functions:
    1. Promotes Cell Division (Cytokinesis): Its primary function.
    2. Overcomes Apical Dominance: Promotes the growth of lateral buds, acting antagonistically to auxins.
    3. Delays Senescence (Richmond-Lang effect): Promotes nutrient mobilization, which helps in delaying leaf aging.
    4. Promotes Chloroplast Development and new leaf formation.
Ethylene: The Gaseous Ripener
  • Discovery: H.H. Cousins confirmed that ripe oranges released a volatile substance (ethylene) that hastened the ripening of bananas.
  • Key Functions:
    1. Fruit Ripening: Highly effective in ripening fruits; enhances the respiration rate during ripening (respiratory climactic). Ethephon is a liquid source of ethylene.
    2. Senescence and Abscission: Promotes the senescence and shedding of leaves, flowers, and fruits.
    3. Breaks Dormancy: Breaks seed and bud dormancy and initiates germination (e.g., in peanut seeds).
    4. Promotes Root Growth: Increases absorption surface area by promoting root and root hair formation.
    5. Flowering: Synchronizes fruit-set in pineapples and induces flowering in mango.
    6. Promotes Female Flowers: Increases the yield in cucumbers by promoting female flowers.
Abscisic Acid (ABA): The Stress Hormone
  • Discovery: Discovered independently as inhibitor-B, abscisin II, and dormin.
  • Key Functions:
    1. General Growth Inhibitor: Inhibits plant metabolism and seed germination.
    2. Stomatal Closure: Stimulates the closure of stomata during water stress, helping the plant tolerate desiccation. This is why it's called the stress hormone.
    3. Induces Dormancy: Promotes dormancy in seeds and buds, helping them survive unfavorable conditions.
    4. Promotes Abscission of leaves, flowers, and fruits.
    5. Antagonist to Gibberellins: Acts as an antagonist to GA, particularly in relation to dormancy.

5. Interaction of PGRs and Environmental Factors

Development in plants is controlled by a complex interplay of intrinsic (genetic, PGRs) and extrinsic (light, temperature, water, oxygen) factors. Events like dormancy, flowering, and germination are rarely controlled by a single factor but by synergistic or antagonistic interactions between multiple PGRs and the environment.

  • Photoperiodism: The response of plants to the relative lengths of day and night for flowering.
  • Vernalisation: The induction of flowering by a period of low-temperature treatment. Both photoperiodism and vernalisation are examples of how extrinsic factors (light and temperature) control development, often mediated by PGRs.

Important Formulas & Equations

  • Arithmetic Growth Rate: Lt = L0 + rt Where:

    • Lt = length at time ‘t’
    • L0 = length at time ‘zero’
    • r = growth rate / elongation per unit time

  • Geometric Growth Rate: W₁ = W₀ * e^(rt) Where:

    • W₁ = final size (weight, height, number etc.)
    • W₀ = initial size at the beginning of the period
    • r = relative growth rate (efficiency index)
    • t = time of growth
    • e = base of natural logarithms

Memory Techniques (Mnemonics)

  • PGR Promoters: "All Good Citizens" - Auxin, Gibberellin, Cytokinin.
  • PGR Inhibitors: "Always Exiting" - Abscisic Acid, Ethylene.
  • Auxin Functions: "Apple Prevented Root Fall" - Apical dominance, Parthenocarpy, Root initiation, Flowering in pineapple.
  • Gibberellin Functions: "Sugarcane Bolts, Seed Sprouts" - Stem elongation, Bolting, Seed dormancy breaking.
  • Differentiation Sequence: "Do Differently, Repeat" - Differentiation -> Dedifferentiation -> Redifferentiation.

Previous Year Questions (NEET)

Q1. Name the plant growth regulator which upon spraying on sugarcane crop, increases the length of the stem, thus increasing the yield of the sugarcane crop. (NEET 2020) a) Gibberellin b) Ethylene c) Auxin d) Abscisic acid

Explanation: Gibberellins are renowned for their function in promoting stem and internode elongation. Spraying them on sugarcane directly increases the length of the stem, which is the part where sugar is stored, thus increasing the biomass and yield significantly. Answer: (a) Gibberellin

Q2. The process of growth is maximum during: (NEET 2020) a) Log phase b) Lag phase c) Senescence d) Dormancy

Explanation: In a sigmoid (S-shaped) growth curve, the growth rate is highest during the middle phase, known as the log or exponential phase. This is when resources are abundant and cells are dividing and expanding rapidly. Answer: (a) Log phase

Q3. Which of the following is not an inhibitory substance governing seed dormancy? (NEET 2020) a) Abscisic acid b) Phenolic acid c) Para-ascorbic acid d) Gibberellic acid

Explanation: Abscisic acid, phenolic acid, and para-ascorbic acid are all known growth inhibitors that contribute to seed dormancy. Gibberellic acid, on the other hand, is a potent growth promoter that is well-known for its role in breaking seed dormancy. Answer: (d) Gibberellic acid

Q4. It takes a very long time for pineapple plants to produce flowers. Which of the following combinations of hormones can be applied to artificially induce flowering in pineapple plants throughout the year to increase yield? (NEET 2021) a) Gibberellin and Cytokinin b) Gibberellin and Abscisic acid c) Cytokinin and Abscisic acid d) Auxin and Ethylene

Explanation: Both Auxin and Ethylene are explicitly mentioned in the NCERT text as PGRs that promote and synchronize flowering in pineapple plants. Answer: (d) Auxin and Ethylene

Key Takeaways for Quick Revision

  • Growth is an irreversible increase in size. The Sigmoid (S-curve) is the characteristic growth curve in a natural environment.
  • Development = Growth + Differentiation. Plants show plasticity, like heterophylly, in response to their environment.
  • Auxins promote apical dominance and rooting.
  • Gibberellins are key for stem elongation (sugarcane) and bolting.
  • Cytokinins are primarily responsible for cell division and delaying senescence.
  • Ethylene is a gaseous hormone that ripens fruit and promotes senescence.
  • Abscisic Acid (ABA) is the "stress hormone" that closes stomata and maintains dormancy.
  • The life events of a plant are controlled by complex, often antagonistic or synergistic, interactions between these five PGRs and environmental factors.
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