Biology: Class 11 — Chapter 13: Plant Growth and Development

Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems at certain locations in their body. The cells of these meristems have the capacity to divide and self-perpetuate.

• Open Form of Growth: A form of growth wherein new cells are always being added to the plant body by the activity of the meristem.
• Primary Growth: Attributed to root apical meristem and shoot apical meristem, contributing to the elongation of plants along their axis.
• Secondary Growth: Attributed to lateral meristems, vascular cambium, and cork-cambium, causing an increase in the girth of the organs.

At a cellular level, growth is principally a consequence of increase in the amount of protoplasm. It is measured by various parameters:

• Increase in fresh weight and dry weight
• Increase in length, area, and volume
• Increase in cell number (e.g., one single maize root apical meristem can give rise to more than 17,500 new cells per hour)
• Increase in cell size (e.g., cells in a watermelon may increase in size by up to 3,500,000 times)

The period of growth is divided into three distinct phases:

1. Meristematic Phase: Cells at the root and shoot apex. They are rich in protoplasm, possess large conspicuous nuclei, and have thin, cellulosic primary cell walls with abundant plasmodesmatal connections.
2. Elongation Phase: Cells proximal to the meristematic zone. Characterised by increased vacuolation, cell enlargement, and new cell wall deposition.
3. Maturation Phase: Cells further away from the apex. These cells attain their maximal size in terms of wall thickening and protoplasmic modifications.

The increased growth per unit time is termed as growth rate, which can be expressed mathematically in two ways:

• Arithmetic Growth: Following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures.

  Equation: $L_t = L_0 + rt$

  Where $L_t$ is length at time $t$, $L_0$ is length at time zero, and $r$ is the growth rate/elongation per unit time.

• Geometric Growth: Both progeny cells retain the ability to divide. Initial growth is slow (lag phase), followed by rapid exponential increase (log or exponential phase), and finally slows down due to limited nutrient supply (stationary phase), forming a sigmoid or S-curve.

  Equation: $W_1 = W_0 e^{rt}$

  Where $W_1$ is final size, $W_0$ is initial size, $r$ is relative growth rate (efficiency index), $t$ is time of growth, and $e$ is the base of natural logarithms.

• Absolute vs. Relative Growth Rates: Absolute growth rate is the measurement and comparison of total growth per unit time. Relative growth rate is the growth of a given system per unit time expressed on a common basis (e.g., per unit initial parameter).

• Water: Essential for cell enlargement (turgidity) and provides the medium for enzymatic activities.
• Oxygen: Helps in releasing metabolic energy essential for growth activities.
• Nutrients: Macro and micro essential elements act as a source of energy and are required for the synthesis of protoplasm.
• Temperature: An optimum temperature range is required; deviations can be detrimental.
• Environmental Signals: Light and gravity affect certain phases/stages of growth.

Plasticity is the ability of plants to follow different pathways in response to environment or phases of life to form different kinds of structures.

• Heterophylly: The occurrence of different types of leaves on the same plant.
• Phase-dependent Heterophylly: Seen in cotton, coriander, and larkspur, where juvenile leaves differ in shape from mature leaves.
• Environment-dependent Heterophylly: Seen in buttercup (Ranunculus), where leaves produced in air differ from those produced in water.

PGRs are small, simple molecules of diverse chemical composition:

• Indole compounds: Indole-3-acetic acid (IAA)
• Adenine derivatives: $N^6$-furfurylamino purine (kinetin)
• Carotenoid derivatives: Abscisic acid (ABA)
• Terpenes: Gibberellic acid ($GA_3$)
• Gases: Ethylene ($C_2H_4$)

They are broadly divided into two functional groups:

1. Plant Growth Promoters: Involved in growth promoting activities like cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation (e.g., auxins, gibberellins, cytokinins).
2. Plant Growth Inhibitors: Involved in growth inhibiting activities such as dormancy and abscission, and responses to wounds and stresses (e.g., abscisic acid). Ethylene fits largely as an inhibitor but can have promoting effects.

• Auxin: Discovered through observations by Charles Darwin and Francis Darwin on phototropism in canary grass coleoptiles. Isolated by F.W. Went from tips of oat seedlings.
• Gibberellins: Identified from the study of 'bakanae' (foolish seedling) disease of rice seedlings caused by the fungal pathogen Gibberella fujikuroi, reported by E. Kurosawa.
• Cytokinins: F. Skoog and co-workers observed tobacco callus proliferation only when auxin was supplemented with vascular tissue extracts, yeast extract, coconut milk, or DNA. Miller et al. later crystallised the active substance, kinetin.
• Abscisic Acid: Discovered during the mid-1960s when three independent researches purified inhibitor-B, abscission II, and dormin, which were chemically identical.
• Ethylene: H.H. Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas.

• Auxins:
  • Initiate rooting in stem cuttings.
  • Promote flowering (e.g., in pineapples).
  • Prevent early fruit and leaf drop, but promote abscission of older mature leaves and fruits.
  • Induce apical dominance (growing apical bud inhibits lateral buds). Decapitation is used in tea plantations and hedge-making.
  • Induce parthenocarpy (e.g., in tomatoes).
  • Used as herbicides (e.g., 2,4-D kills dicotyledonous weeds).
• Gibberellins:
  • Cause elongation of axis (e.g., grape stalks, apple shape improvement).
  • Delay senescence to extend market period.
  • Speed up the malting process in brewing industry.
  • Increase stem length and yield in sugarcane.
  • Hasten maturity in juvenile conifers for early seed production.
  • Promote bolting (internode elongation prior to flowering) in rosette plants like beet and cabbage.
• Cytokinins:
  • Have specific effects on cytokinesis (cell division).
  • Synthesised in regions of rapid cell division (roots, shoot buds, young fruits).
  • Help produce new leaves, chloroplasts, lateral shoot growth, and adventitious shoots.
  • Overcome apical dominance.
  • Delay leaf senescence by promoting nutrient mobilisation.
• Ethylene:
  • Promotes horizontal growth, swelling of axis, and apical hook formation in dicot seedlings.
  • Promotes senescence and abscission of leaves and flowers.
  • Highly effective in fruit ripening; enhances respiration rate (respiratory climacteric).
  • Breaks seed and bud dormancy (e.g., peanut germination, potato sprouting).
  • Promotes rapid internode/petiole elongation in deep water rice.
  • Promotes root growth and root hair formation.
  • Used as ethephon in agriculture to hasten ripening, accelerate abscission (thinning), and promote female flowers in cucumbers.
• Abscisic Acid (ABA):
  • Acts as a general plant growth inhibitor and metabolism inhibitor.
  • Inhibits seed germination and induces seed dormancy to withstand desiccation.
  • Stimulates the closure of stomata.
  • Increases tolerance of plants to various stresses, hence called the stress hormone.
  • Acts as an antagonist to Gibberellins (GAs).