Overview of Plant Growth and Development

  • Organ Development: You already know the organization of a flowering plant from Chapter 5. Have you ever wondered where and how roots, stems, leaves, flowers, fruits, and seeds develop in an orderly sequence? As a plant matures, its height and girth (width) increase, while leaves, flowers, and fruits have limited dimensions and grow in cycles.
  • Vegetative and Flowering Phases: Plants go through a vegetative phase (root, stem, leaves) before flowering. The structure and function of plant organs are linked to their tissues. Plant development follows a precise sequence, starting from a fertilized egg (zygote), which eventually grows into a mature plant with roots, leaves, flowers, fruits, and seeds.

Growth in Plants

  1. Seed Germination: Growth begins when a seed germinates under favorable environmental conditions. In the absence of these conditions, seeds remain dormant until the environment becomes suitable again.
  2. Factors Affecting Growth: Plant growth is controlled by both intrinsic (internal) and extrinsic (external) factors. These factors govern the developmental processes that lead to the formation of plant organs.

Growth Defined

  • Definition: Growth is the permanent, irreversible increase in the size of an organ, part of an organ, or an individual cell. Growth is linked to metabolic processes, which require energy.
  • Plant Growth: Unlike animals, plants can grow continuously throughout their life due to the presence of meristems. These are regions where cells keep dividing to form new cells, leading to the growth of plant organs.

Meristems and Types of Growth

  • Meristems: These are regions where active cell division occurs. The main meristems are the root apical meristem (in roots) and shoot apical meristem (in stems), responsible for the primary growth (growth in length).
  • Lateral Meristems: In dicots and gymnosperms, lateral meristems (vascular cambium and cork cambium) appear later and help in secondary growth, increasing the girth (width) of the plant.

Measuring Growth

  • Growth is often measured by the increase in size or weight (fresh or dry weight), length, area, volume, or cell number. For example, a maize root meristem can generate 17,500 new cells per hour.

Phases of Growth

  1. Meristematic Phase: Cells divide rapidly in meristems (root and shoot tips).
  2. Elongation Phase: Cells enlarge and elongate, increasing the organ’s length.
  3. Maturation Phase: Cells stop dividing and mature, thickening their walls and becoming specialized into various cell types (like xylem or phloem).

Growth Rate

  • Growth Rate: Growth rate refers to the amount of growth per unit time. It can be expressed mathematically, with different types of growth patterns:
    1. Arithmetic Growth: One daughter cell divides, and the other differentiates. Example: Root elongation at a constant rate.
    2. Geometric (Exponential) Growth: Cells divide rapidly at first, then slow down due to resource limitations. This forms a typical sigmoid (S-shaped) growth curve.
  • Growth Curves:
    • Sigmoid Curve: Typical for living organisms, showing slow initial growth, followed by rapid growth, and then a plateau when resources become limited.
    • Mathematical Expression: Exponential growth can be expressed as:
      • W1=W0ertW_1 = W_0 e^{rt}W1​=W0​ert
      • Where W1W_1W1​ is the final size, W0W_0W0​ is the initial size, rrr is the growth rate, and ttt is time.

Types of Growth Rate Measurements

  • Absolute Growth Rate: Total growth per unit time.
  • Relative Growth Rate: Growth compared to the initial size of the plant or part (e.g., leaf area growth in comparison to initial leaf size).

Applications in Competitive Exams:

  • Meristematic Activity: Understanding how meristems function and their role in primary and secondary growth is key in plant biology.
  • Growth Patterns: Be able to identify examples of arithmetic vs. geometric growth, and understand growth phases (meristematic, elongation, and maturation).
  • Growth Measurement: Know how to measure and calculate growth using parameters like weight, size, and cell number.
  • Growth Curves: Be familiar with sigmoid and other growth curves and their characteristics for different organisms, especially plants.

Conditions for Growth in Plants

  1. Water: Essential for cell enlargement and turgidity (water pressure), which supports growth. It also helps enzymes work for growth activities.
  2. Oxygen: Needed for energy release in metabolic activities, which are essential for growth.
  3. Nutrients: Macro and micro nutrients help plants synthesize protoplasm and provide energy.
  4. Temperature: Each plant has an ideal temperature range for growth. Extreme temperatures can harm the plant.
  5. Environmental Signals: Factors like light and gravity influence growth and development.

Differentiation, Dedifferentiation, and Redifferentiation

  • Differentiation: Process where cells mature to perform specific functions, e.g., tracheary cells (for water transport) lose protoplasm and develop strong, elastic cell walls.
  • Dedifferentiation: Mature, differentiated cells regain the ability to divide under certain conditions, like the formation of cambium from parenchyma cells.
  • Redifferentiation: After dedifferentiation, cells mature again to perform specific functions.
  • Example: The root cap cells and epidermis cells in plants are examples of open differentiation, where the location of a cell determines its function.

Development in Plants

  • Development includes all the changes from seed germination to senescence (aging).
  • Plasticity: Plants can form different structures in response to environmental changes. An example is heterophylly, where juvenile leaves are different from mature ones, or leaves change shape depending on whether the plant is in air or water.
  • Growth and Differentiation: Development in plants is a combination of growth (increase in size) and differentiation (maturation of cells into specialized types).

Plant Growth Regulators (PGRs)

  1. Auxins: Promote cell growth, flowering, and rooting. They help in apical dominance, where the growth of lateral buds is inhibited by the main shoot tip.
    • Uses: Used in plant propagation, controlling weed growth, and fruit development.
  2. Gibberellins: Promote stem elongation, fruit growth, and delay senescence (aging).
    • Uses: Increase sugarcane yield, promote early seed production in conifers, and improve fruit shape.
  3. Cytokinins: Promote cell division, shoot growth, and leaf production.
    • Uses: Overcome apical dominance and delay leaf senescence.
  4. Ethylene: A gas that regulates fruit ripening, flower abscission (dropping), and leaf senescence.
    • Uses: Used in agriculture for ripening fruits like tomatoes and apples, and stimulating flowering in plants like mangoes.
  5. Abscisic Acid (ABA): Inhibits seed germination and promotes dormancy.
    • Role: Helps plants resist stress, controls stomata closure, and regulates seed dormancy.

Interactions Between PGRs

  • PGRs can act together (synergistically) or against each other (antagonistically). For example, ABA promotes dormancy, while gibberellins promote seed germination.

Growth Phases

  • Lag Phase: Initial phase of slow growth.
  • Log Phase: Rapid growth.
  • Senescent Phase: Slowing down or stopping of growth as the plant ages.

Summary

Plant growth involves the increase in size, cell division, and development, regulated by both internal (genetic, PGRs) and external (light, temperature) factors. Differentiation, dedifferentiation, and redifferentiation help plants adapt and develop new structures. PGRs control many aspects of plant growth and development, and their roles can be synergistic or antagonistic, affecting processes like flowering, fruiting, and stress responses.

These all are the notes of chapter 13. And important questions are below HERE. *#THANKS FOR VISITING, VISIT AGAIN#* 😊

1. What is Plant Growth?

Answer:
Plant growth is the permanent and irreversible increase in size of a plant or its parts (like roots, stems, leaves). This process happens as a result of cell division and metabolism, which requires energy.

2. What is Seed Germination?

Answer:
Seed germination is the process where a seed starts to grow into a new plant. It happens when the seed gets the right conditions like water, oxygen, and temperature. Without these conditions, the seed remains dormant (inactive) until they are met.

3. What Factors Affect Plant Growth?

Answer:
There are two types of factors that affect plant growth:

  • Intrinsic (internal) factors: These are factors like the plant’s genes and hormones.
  • Extrinsic (external) factors: These include environmental factors like light, water, temperature, and nutrients.

4. What are Meristems and How Do They Help Plants Grow?

Answer:
Meristems are special regions in plants where cells are always dividing to form new cells. These regions are found in the tips of roots (root apical meristem) and stems (shoot apical meristem). They are responsible for primary growth (growth in length). Some plants also have lateral meristems that help in secondary growth (growth in thickness).

5. What are the Phases of Growth in Plants?

Answer:
Plant growth occurs in three phases:

  1. Meristematic Phase: Where cells divide rapidly.
  2. Elongation Phase: Where cells enlarge and the plant organ gets longer.
  3. Maturation Phase: Where cells stop dividing and specialize into different types (like xylem or phloem).

6. What is Meant by Growth Rate?

Answer:
Growth rate refers to how much the plant grows over a certain period of time. There are two types of growth patterns:

  • Arithmetic Growth: Growth happens at a constant rate (e.g., root elongation).
  • Geometric (Exponential) Growth: Growth starts slow, speeds up, and then slows down as resources become limited. This forms a typical S-shaped (sigmoid) growth curve.

7. How Do Plants Differentiate Their Cells?

Answer:

  • Differentiation: Cells mature and take on specific functions, such as forming the strong cell walls of tracheary cells for water transport.
  • Dedifferentiation: Mature cells can start dividing again under certain conditions (e.g., parenchyma cells forming cambium).
  • Redifferentiation: After dedifferentiation, cells mature again to perform their specific functions.

8. What are Plant Growth Regulators (PGRs)?

Answer:
Plant Growth Regulators (PGRs) are chemicals that help control and regulate plant growth. There are several types:

  • Auxins: Promote growth, flowering, and rooting. They help the main shoot grow stronger than lateral buds (apical dominance).
  • Gibberellins: Promote stem elongation and fruit growth, and delay aging.
  • Cytokinins: Promote cell division and shoot growth.
  • Ethylene: A gas that helps with fruit ripening, leaf fall, and flower drop.
  • Abscisic Acid (ABA): Helps plants resist stress, promotes dormancy, and controls seed germination.

9. What are the Growth Phases in Plants?

Answer:

  • Lag Phase: This is the slow growth phase at the start.
  • Log Phase: This is the rapid growth phase where the plant grows quickly.
  • Senescent Phase: This is when growth slows down or stops as the plant ages.

10. What is Plasticity in Plants?

Answer:
Plasticity is the ability of plants to change their structure based on environmental conditions. For example, plants may have different shaped leaves when grown in air compared to when they are submerged in water.

11. What is the Role of Differentiation, Dedifferentiation, and Redifferentiation?

Answer:
These processes allow plants to adapt and form new structures:

  • Differentiation: Cells mature to perform specialized functions, like water transport or photosynthesis.
  • Dedifferentiation: Mature cells can become active again and start dividing to form new tissues.
  • Redifferentiation: After dedifferentiation, these cells mature back into their specialized forms to perform specific functions.

12. What are Some Environmental Conditions for Growth in Plants?

Answer:
For healthy growth, plants need:

  • Water: For cell expansion and metabolic activities.
  • Oxygen: For energy release through respiration.
  • Nutrients: Macro and micro nutrients to build plant structures.
  • Temperature: Each plant has an optimal temperature for growth.
  • Light and Gravity: These factors influence plant growth and development.

Additional Knowledge:

  • Growth Measurement: Growth can be measured in terms of size, weight, cell number, or volume. For example, root meristems in maize can generate 17,500 new cells every hour.
  • Growth Curves: Growth in plants often follows an S-shaped or sigmoid curve, showing slow growth initially, followed by rapid growth, and then slowing as resources become limited.