Plant Growth and Development Biology Notes - Growth Phases, Growth Rates, Differentiation, PGRs, Photoperiodism, Vernalisation and Seed Dormancy

• All plant cells are descendants of the zygote (fertilized egg).
• The zygote develops into a mature plant through growth and differentiation, forming roots, leaves, branches, flowers, fruits, and seeds. Eventually, they die.

Growth

• Growth is an irreversible permanent increase in size of an organ, its parts, or an individual cell.
• It involves metabolic processes that consume energy.

Plant Growth Generally is Indeterminate

• Plant growth continues throughout life due to the presence of meristems.
• Meristematic cells have the capacity to divide and self-perpetuate.
• The growth where new cells are always added to the plant body by the meristem is called the open form of growth.

Primary growth:

• It occurs due to root apical meristem and shoot apical meristem.
• It causes the elongation of the plants along the axis.

Secondary growth (in gymnosperms and dicots):

• It occurs due to lateral meristems, vascular cambium, and cork-cambium.
• It causes an increase in the girth of plants.

Growth is Measurable

• At the cellular level, growth occurs due to an increase in the amount of protoplasm.
• Since protoplasm increase is difficult to measure directly, growth is measured by parameters such as fresh weight, dry weight, length, area, volume, and cell number. E.g.,
• Cell number: A maize root apical meristem can produce more than 17,500 new cells per hour.
• Cell size: Cells in a watermelon can increase in size by up to 350,000 times.
• Length: Growth of a pollen tube.
• Surface area: Growth in a dorsiventral leaf.

Phases of Growth

• 3 phases: meristematic, elongation, and maturation.
• Meristematic phase: Occurs in meristems at the root and shoot apexes. Cells have rich protoplasm, large nuclei, and primary, thin, cellulosic walls with abundant plasmodesmata.
• Elongation phase: Occurs in cells proximal to the meristematic zone. Cells exhibit increased vacuolation, size, and new cell wall deposition.
• Maturation phase: Occurs in cells further from the apex, proximal to the elongation phase. Cells attain maximal size with wall thickening and protoplasmic modifications.

Growth Rates

• Growth rate is the increased growth per unit time. which may be arithmetic or geometrical.
• 2 types: arithmetic and geometrical.

Arithmetic growth:

• In arithmetic growth, following mitotic division, only one daughter cell continues to divide while the other differentiates and matures.
• On plotting the length of the organ against time, a linear curve is obtained.
• Mathematically, it is expressed as:

L_t = L₀ + rt

L_t = length at time ‘t’

L₀ = length at time ‘zero’

r = growth rate / elongation per unit time

Geometrical growth:

• Here, both daughter cells continue mitotic division.
• The growth is initially slow (lag phase), then increases rapidly (log or exponential phase).
• If nutrient supply is limited, growth slows, leading to a stationary phase.
• Plotting growth against time yields a sigmoid (S) curve, characteristic of living organisms in a natural environment.
• A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.

• Exponential growth is expressed as:

W₁ = W₀ e^rt

W₁ = final size (weight, height, number, etc.)

W₀ = initial size at the beginning of the period

r = relative growth rate

t = time of growth

e = base of natural logarithms

• Here, r is the relative growth rate, also a measure of the plant’s ability to produce new material (efficiency index). Thus, the final size W₁ depends on the initial size W₀.

• Quantitative comparisons of growth can be made in two ways:

  • Absolute growth rate: Measurement and comparison of total growth per unit time.
  • Relative growth rate: Measurement of growth per unit time expressed on a common basis, e.g., per unit initial parameter.

Conditions (essential elements) for Growth:

• Water: Essential for cell enlargement. Turgidity of cells aids in extension growth. Water provides a medium for enzymatic activities needed for growth.
• Oxygen: Helps release metabolic energy for oxidation.
• Nutrients: Macro and micro elements are needed for protoplas synthesis and as an energy source.
• Temperature: Growth is maximum at optimum temperature. Deviations may harm plants.
• Light and gravity: Affect certain phases/stages of growth.

Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation is the process in which cells in meristems (root apical and shoot-apical) and cambium differentiate and mature to perform specific functions.
• Cell walls and protoplasm undergo major structural changes, and the capacity for cell division is lost.

E.g., Loss of protoplasm to form a tracheary element. These develop strong, elastic, lignocellulosic secondary cell walls to transport water over long distances under extreme tension.

• Under certain conditions, living differentiated cells regain the capacity to divide. This is called dedifferentiation.

E.g., Formation of meristems (interfascicular cambium and cork cambium) from differentiated parenchyma cells.

• Dedifferentiated cells divide and produce cells that lose the capacity to divide but mature to perform specific functions. This is called redifferentiation.
• Plant growth is open, meaning it can be indeterminate or determinate. Differentiation in plants is also open, as cells/tissues from the same meristem have different structures at maturity.
• Final structure at maturity of cell/tissue is also determined by the location of the cell.

E.g., Cells positioned away from root apical meristems differentiate as root-cap cells, while those pushed to the periphery mature as epidermis.

Development

• Development is the process encompassing all changes in an organism’s life cycle, from seed germination to senescence. It is the sum of growth and differentiation.

• Plants follow different pathways in response to the environment or phases of life, forming varied structures. This ability is called plasticity. E.g. Heterophylly (presence of different types of leaves at different phases of life or due to different environments).
• Heterophylly due to phases of life: In cotton, coriander, and larkspur, leaves of juvenile and mature plants differ in shape.
• Heterophylly due to environment: Differences in leaf shapes produced in air versus water (e.g., buttercup).

Factors controlling development:

• Intrinsic factors: Include intracellular (genetic) or intercellular factors (e.g., plant growth regulators).
• Extrinsic factors: Include light, temperature, water, oxygen, nutrition, etc.

Plant Growth Regulators (Plant Hormones or Phytohormones)

• Plant growth regulators (PGRs) are small, simple molecules that regulate plant growth.
• Based on their functions, PGRs are divided into two groups:
• Plant growth promoters: Promote activities like cell division, cell enlargement, tropic growth, pattern formation, flowering, fruiting, and seed formation. E.g., auxins, gibberellins, and cytokinins.
• Plant growth inhibitors: Involved in growth-inhibiting activities like dormancy and abscission, and respond to wounds and biotic/abiotic stresses. E.g., abscisic acid and ethylene (ethylene is largely a growth inhibitor but can fit either group).

1. Auxins

• Charles Darwin and his son Francis Darwin observed that coleoptiles of canary grass responded to unilateral illumination by growing toward the light source (phototropism). They concluded that the coleoptile tip caused the bending of the entire coleoptile.

• F.W. Went isolated auxin (Greek ‘auxein’: to grow) from the tips of oat seedling coleoptiles. Auxin was first isolated from human urine. Auxins are produced by growing apices of stems and roots, migrating to regions of action.

Types of Auxins:

• Natural: E.g., Indole-3-acetic acid (IAA) and indole butyric acid (IBA), isolated from plants.
• Synthetic: E.g., NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid.

Functions of Auxins:

• Initiate rooting in stem cuttings for plant propagation.
• Promote flowering (e.g., in pineapples).
• Prevent early fruit and leaf drop.
• Promote abscission of older leaves and fruits.
• Induce parthenocarpy (e.g., in tomatoes).
• Used as herbicides (e.g., 2,4-D kills dicot weeds without affecting mature monocots, used for weed-free lawns).
• Control xylem differentiation and aid cell division.

In higher plants, the growing apical bud inhibits lateral (axillary) bud growth, known as apical dominance. Removing shootanhão t tips (decapitation) promotes lateral bud growth, applied in tea plantations and hedge-making.

2. Gibberellins

• Gibberellins are acidic PGRs. E. Kurosawa treated healthy rice seedlings with sterile filtrates of Gibberella fujikuroi, a fungus causing ‘bakane’ disease (foolish seedling) in rice, resulting in disease symptoms. The active substance was identified as gibberellic acid.

• There are over 100 gibberellins (GA1, GA2, GA3, etc.) in fungi and higher plants. Gibberellic acid (GA3 or Terpenes) is one of the first discovered and most studied.

Functions:

• Increase axis length, used to extend grape stalks.
• Elongate and improve fruit shape (e.g., apples).
• Delay senescence, allowing fruits to stay on trees longer for extended market periods.
• GA3 speeds up the malting process in the brewing industry.
• In sugarcane, gibberellin spraying increases stem length, boosting yield by up to 20 tonnes per acre.
• Spraying juvenile conifers with GAs hastens maturity, leading to early seed production.
• Promote bolting (internode elongation before flowering) in beets, cabbages, and plants with rosette habits.

3. Cytokinins

• F. Skoog and co-workers found that tobacco stem internodal segments’ callus proliferated only when the nutrient medium included vascular tissue extracts, yeast extract, coconut milk, or DNA. Skoog & Miller identified and crystallized the active substance, naming it kinetin.

• Cytokinins were discovered as kinetin (N6-furfurylamino purine, an adenine derivative) from autoclaved herring sperm DNA. Kinetin is not naturally occurring in plants. Zeatin (from corn kernels and coconut milk) is a natural cytokinin. Synthetic compounds also exhibit cell division-promoting activity.

• Natural cytokinins are synthesized in regions of rapid cell division (root apices, shoot buds, young fruits, etc.).

Functions:

• Play a role in cytokinesis.
• Promote new leaves, chloroplasts in leaves, lateral shoot growth, and adventitious shoot formation.
• Overcome apical dominance.
• Promote nutrient mobilization, delaying leaf senescence.

4. Ethylene (C₂H₄)

• Cousins confirmed that ripened oranges released a volatile substance hastening banana ripening. This substance was identified as ethylene, a simple gaseous PGR.

• Ethylene is synthesized in large amounts by tissues undergoing senescence and ripening fruits.

Functions:

• Influences horizontal seedling growth, axis swelling, and apical hook formation in dicot seedlings.
• Promotes senescence and abscission of leaves and flowers.
• Promotes fruit ripening, enhancing respiration rate during ripening (respiratory climactic).
• Breaks seed and bud dormancy, initiates germination in peanut seeds, and promotes potato tuber sprouting.
• Promotes rapid internode/petiole elongation in deep-water rice, keeping leaves/shoots above water.
• Promotes root growth and root hair formation, increasing absorption surface.
• Initiates flowering and synchronizes fruit-set in pineapples; induces flowering in mango.
• Widely used in agriculture.

The most widely used ethylene source is ethephon. Ethephon, absorbed in aqueous solution and transported within the plant, releases ethylene slowly. It hastens fruit ripening in tomatoes and apples, accelerates abscission in flowers and fruits (thinning cotton, cherry, walnut), and promotes female flowers in cucumbers, increasing yield.

5. Abscisic Acid (ABA)

• In the mid-1960s, three inhibitors—inhibitor-B, abscisin II, and dormin—were found to be chemically identical, now known as abscisic acid (ABA).

• ABA, a derivative of carotenoids, regulates abscission and dormancy.

Functions:

• Inhibits plant growth and metabolism.
• Inhibits seed germination.
• Stimulates stomatal closure in the epidermis.
• Increases plant tolerance to various stresses, earning it the name stress hormone.
• Supports seed development, maturation, and dormancy, aiding resistance to desiccation and unfavorable conditions.

Interactions of PGRs

• PGRs play individualistic or synergistic roles, which may be complementary or antagonistic. They interact to affect dormancy, abscission, flowering, senescence, vernalization, apical dominance, seed germination, and plant movements. In most cases, ABA acts as an antagonist to GAs.

Factors Influencing the Action of PGRs

• Intrinsic factor: Genomic control.
• Extrinsic factors: Light and temperature.

1. Photoperiodism

• Photoperiodism is the response of plants to periods of day/night.
• Some plants require specific light durations to induce flowering.
• Based on light duration, plants are classified into three groups:
• Long day plants: Require exposure to light exceeding a well-defined critical duration to flower.
• Short day plants: Require exposure to light less than the critical duration to initiate flowering.
• Day-neutral plants: Flowering is not correlated with light duration.

• While shoot apices modify into flowering apices, they cannot perceive photoperiods directly. The site of light/dark duration perception is the leaves. It is hypothesized that a flowering hormone(s) is produced in leaves under appropriate photoperiods and migrates to shoot apices to induce flowering.

2. Vernalisation

• It is the phenomenon where some plants require exposure to low temperatures, either quantitatively or qualitatively, to flower.
• It prevents precocious reproductive development late in the growing season, allowing sufficient time for plants to reach maturity.

Examples of Vernalisation:

1. Some food plants, such as wheat, barley, and rye, have two varieties:

• Spring varieties: Planted in spring, they flower and produce grain before the growing season ends.
• Winter varieties: Planted in autumn, they germinate, overwinter as small seedlings, resume growth in spring, and are harvested around mid-summer. If planted in spring, they typically fail to flower or produce mature grain within the season.

2. Vernalisation in biennial plants:

• Biennials are monocarpic plants that flower and die in their second season.
• E.g., sugar beet, cabbages, and carrots. Cold treatment stimulates a subsequent photoperiodic flowering response in these plants.

Seed Dormancy

• Certain seeds fail to germinate even under favorable external conditions due to dormancy. Dormancy is caused by endogenous conditions within the seed, such as:

• Hard seed coat.
• Chemical inhibitors (e.g., ABA, phenolic acids, para-ascorbic acid).
• Immature embryos.

• Dormancy can be overcome naturally or artificially, for example:

• Breaking seed coat barrier: Through mechanical abrasions (e.g., using knives, sandpaper, or vigorous shaking). In nature, abrasions occur via microbial action or passage through animals’ digestive tracts.
• Removing inhibitory substances: By chilling seeds or applying chemicals like gibberellic acid and nitrates.
• Changing environmental conditions: Adjusting light and temperature.