Chapter 4.3: Plant Growth and Development

1. Seed Germination

• Definition: The process by which a seed embryo develops into a seedling under favorable conditions. It involves the resumption of metabolic activities by the embryo, leading to the emergence of the radicle and plumule.
• Types of Germination:
  • Hypogeal Germination: The epicotyl (the part of the embryonic shoot above the cotyledons) elongates, and the cotyledons remain below the soil surface. The cotyledons serve as a food source for the growing seedling. Examples: Pea (Pisum sativum), Maize (Zea mays), Rice (Oryza sativa).
  • Epigeal Germination: The hypocotyl (the part of the embryonic shoot below the cotyledons) elongates, and the cotyledons are pushed above the soil surface. The cotyledons may become photosynthetic and act as the first leaves of the seedling. Examples: Bean (Phaseolus vulgaris), Castor (Ricinus communis), Sunflower (Helianthus annuus).
  • Viviparous Germination: Seeds germinate while still attached to the parent plant. This is an adaptation found in some mangrove plants, allowing the seedling to establish itself quickly in the saline and muddy environment. Examples: Rhizophora, Sonneratia.

2. Growth

• Definition: An irreversible and permanent increase in size, mass, or volume of a cell, organ, or whole organism. It is a fundamental characteristic of all living beings.
• Phases of Growth in Meristems:
  • Meristematic Phase (Cell Division): This phase occurs at the apical meristems (root and shoot tips). The cells in this region are isodiametric, have dense protoplasm, a large nucleus, and thin cellulosic walls. They divide continuously to add new cells.
  • Elongation Phase: This phase occurs just behind the meristematic region. The newly formed cells increase in size due to the absorption of water and the deposition of new cell wall material. Vacuolation increases, which helps in cell enlargement.
  • Maturation Phase: This phase is located behind the elongation zone. The enlarged cells differentiate and mature to perform specific functions, forming various tissues and organs.
• Growth Rate: The increase in growth per unit time.
  • Definition: The rate of growth can be expressed as the increase in size or number per unit time.
  • Measurement of Growth:
    • Direct Method: Growth can be measured directly by recording the increase in length, area, volume, fresh weight, or dry weight over a period of time.
    • Auxanometer: An instrument used to measure the rate of growth of plants, specifically the elongation of stems. A pointer attached to the growing tip of the plant moves over a graduated arc, and the movement is magnified, allowing for the measurement of small increments of growth.
• Factors Affecting Growth:
  • External Factors: Light, temperature, water, oxygen, and nutrients.
  • Internal Factors: Genetic factors and plant growth regulators (hormones).

3. Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: The process by which cells derived from meristematic tissues mature and undergo structural and physiological changes to perform specific functions. This leads to the formation of specialized tissues like parenchyma, collenchyma, sclerenchyma, xylem, and phloem.
• Dedifferentiation: The process by which permanent, differentiated cells regain the power of cell division. For example, the formation of interfascicular cambium and cork cambium from mature parenchyma cells during secondary growth.
• Redifferentiation: The process by which dedifferentiated cells lose their ability to divide and mature to form new specialized cells and tissues. For example, the cells of the secondary xylem and secondary phloem are formed from the redifferentiation of the cells of the vascular cambium.

4. Sequence of Developmental Processes in a Plant Cell

The development of a plant cell involves a sequence of processes:

1. Cell Division: The process starts with the division of a meristematic cell.
2. Cell Elongation: The newly formed cell increases in size.
3. Cell Differentiation: The cell matures and becomes specialized for a particular function.
4. Cell Senescence: The cell ages and eventually dies.

5. Plant Hormones (Plant Growth Regulators - PGRs)

• Definition: Small, simple molecules of diverse chemical composition that are produced in one part of the plant and transported to other parts, where they regulate growth and development.

Auxin

• Discovery: Charles Darwin and his son Francis Darwin observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). F.W. Went later isolated auxin from the tips of oat coleoptiles and demonstrated its role in cell elongation.
• Physiological Effects:
  1. Apical Dominance: Promotes the growth of the apical bud while inhibiting the growth of lateral buds.
  2. Cell Elongation: Promotes the elongation of cells in stems and coleoptiles.
  3. Root Initiation: Promotes the formation of adventitious roots in stem cuttings.
  4. Parthenocarpy: Induces the development of fruit without fertilization (e.g., in tomatoes).
• Application: Used as a rooting hormone in horticulture, as a herbicide (2,4-D), and to induce flowering in pineapple.

Gibberellins (GAs)

• Discovery: E. Kurosawa observed that rice seedlings infected with the fungus Gibberella fujikuroi grew abnormally tall. The active substance was later isolated and named gibberellin.
• Physiological Effects:
  1. Stem Elongation: Causes a significant increase in the length of the stem axis, especially in genetically dwarf plants.
  2. Bolting: Promotes the elongation of internodes just before flowering in rosette plants (e.g., cabbage, beet).
  3. Seed Germination: Breaks seed dormancy and promotes germination by stimulating the production of hydrolytic enzymes.
  4. Fruit Development: Delays senescence and improves the shape and size of fruits like apples and grapes.
• Application: Used to increase the length of grape stalks, to promote malting in the brewing industry, and to increase sugarcane yield.

Cytokinins

• Discovery: F. Skoog and his co-workers observed that the callus (an unorganized mass of cells) proliferated only if the nutrient medium was supplemented with extracts of vascular tissues, yeast extract, or coconut milk. The active substance was later isolated from autoclaved herring sperm DNA and named kinetin.
• Physiological Effects:
  1. Cell Division: Promotes cell division (cytokinesis) in combination with auxins.
  2. Overcoming Apical Dominance: Promotes the growth of lateral buds, thus counteracting the effect of auxin.
  3. Delaying Senescence: Delays the aging of leaves and other organs by controlling protein synthesis and mobilization of resources (Richmond-Lang effect).
  4. Chloroplast Development: Promotes the development of chloroplasts in leaves.
• Application: Used in tissue culture to induce cell division and differentiation, and to delay the senescence of flowers and vegetables.

Ethylene

• Discovery: H.H. Cousins confirmed that ripe oranges released a volatile substance that hastened the ripening of stored, unripe bananas. This volatile substance was later identified as ethylene.
• Physiological Effects:
  1. Fruit Ripening: Promotes the ripening of fruits like bananas, apples, and tomatoes.
  2. Senescence and Abscission: Promotes the senescence and abscission (shedding) of leaves, flowers, and fruits.
  3. Horizontal Growth: Causes horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings.
  4. Flowering: Induces flowering in pineapple and mango.
• Application: Used to induce uniform ripening of fruits, to promote flowering in pineapple, and to cause thinning of cotton, cherry, and walnut.

Abscisic Acid (ABA)

• Discovery: In the mid-1960s, three independent researchers purified and characterized three different kinds of inhibitors: inhibitor-B, abscission II, and dormin. They were later found to be chemically identical and were named abscisic acid.
• Physiological Effects:
  1. Stomatal Closure: Causes the closure of stomata during water stress, thus preventing water loss through transpiration.
  2. Seed Dormancy: Induces and maintains seed dormancy, allowing seeds to withstand unfavorable conditions.
  3. Abscission: Promotes the abscission of leaves, flowers, and fruits.
  4. Stress Hormone: Acts as a stress hormone by increasing the tolerance of plants to various kinds of stresses like drought, salinity, and extreme temperatures.
• Application: Used to induce dormancy in seeds for storage and to promote the abscission of fruits for mechanical harvesting.