Plant Growth and Development

Plant Growth is an irreversible increase in size, mass, or number of cells. It is indeterminate (unlimited) due to the presence of meristems (apical, lateral, intercalary).

Phases of Growth

  • Meristematic: Rapid cell division, rich protoplasm, thin primary walls.
  • Elongation: Cell enlargement, increased vacuolation, and new cell wall deposition.
  • Maturation: Cells reach their maximal size and undergo differentiation for specific functions.

Growth Kinetics

  • Arithmetic Growth: Constant growth rate; one daughter cell continues to divide while the other differentiates (Linear curve: Lt=L0+rtL_t = L_0 + rt).
  • Geometric Growth: Both daughter cells continue to divide; initial growth is slow (lag phase) followed by rapid exponential growth (log phase) and a stationary phase (Sigmoid/S-curve: W1=W0ertW_1 = W_0 e^{rt}).

Cellular Processes

  • Differentiation: Cells mature to perform specific functions.
  • Dedifferentiation: Mature cells regain division capacity (e.g., formation of interfascicular cambium).
  • Redifferentiation: Dedifferentiated cells mature again to form permanent tissues (e.g., cork).
  • Plasticity: The ability of plants to follow different pathways in response to environment or life phases, leading to different structures (e.g., Heterophylly in Cotton, Coriander, and Buttercup).

Plant Growth Regulators (PGRs)

  • Auxins (IAA): Promote apical dominance and rooting; used as weedicides (2,4-D).
  • Gibberellins (GA₃): Promote bolting (internode elongation), seed germination, and increase fruit/stalk size.
  • Cytokinins: Promote cell division, overcome apical dominance, and delay senescence (Richmond–Lang effect).
  • Ethylene: The only gaseous hormone; promotes fruit ripening, senescence, and abscission.
  • Abscisic Acid (ABA): The stress hormone; induces stomatal closure and maintains seed dormancy; antagonist to GAs.

Environmental Responses

  • Photoperiodism: Response to day/night length. Leaves perceive the stimulus via phytochrome. The critical factor is the duration of uninterrupted darkness.
  • Vernalisation: Induction of flowering by low temperature treatment; perceived by the apical meristem.

NEET and IAT Exam Tips: High-Yield Revision

The PGR Antagonism Guide

  • Apical Dominance: Auxin promotes it; Cytokinin overcomes it.
  • Dormancy: ABA maintains/induces it; Gibberellin breaks it.
  • Abscission: Ethylene/ABA promote it; Auxins (at early stages) prevent it.
  • Senescence: Ethylene/ABA accelerate it; Cytokinin delays it.

Plasticity Cheat Sheet

  • Developmental (Age-dependent): Cotton, Coriander, Larkspur.
  • Environmental (Habitat-dependent): Buttercup (Ranunculus).

Photoperiodism Logic

  • Short-Day Plants (SDP): Actually "Long-Night Plants." They require a dark period longer than a critical threshold. A flash of light during the night prevents flowering.
  • Long-Day Plants (LDP): "Short-Night Plants." They require a dark period shorter than a critical threshold.

Differentiation Transitions

  • Dedifferentiation: Maturity →\rightarrow Meristematic (Example: Formation of Cork Cambium from parenchyma).
  • Redifferentiation: Meristematic →\rightarrow Maturity (Example: Formation of Secondary Cortex from Cork Cambium).

Agricultural Applications to Memorize

  • Gibberellins: Used to speed up the malting process in the brewing industry.
  • Auxins: Used to induce parthenocarpy in tomatoes and as weedicides for dicot weeds in monocot lawns.
  • Ethylene: Sources like Ethephon are used to hasten fruit ripening and promote female flowers in cucumbers to increase yield.