Chapter-wise Breakdown: Plant Growth & Development

Chapter Section 1: Growth — Definition and Characteristics

Plant growth is a fundamental biological process defined by three criteria: it must be permanent, irreversible, and measurable. This distinguishes true growth from reversible volume changes (e.g., due to hydration). Growth occurs at specialized zones called meristems — the shoot and root apical meristems for primary growth, and vascular cambium/cork cambium for secondary growth.

The three sequential phases within any growing region are: (1) Meristematic — undivided cells actively dividing; (2) Elongation — vacuoles enlarge, cell volume increases dramatically; (3) Maturation — cells differentiate and acquire permanent shape and function. These phases occur in ordered zones along root and shoot axes.

Growth follows two mathematical models. Arithmetic growth (Lt = $L_{0}$ + rt) occurs when one daughter cell differentiates while the other divides — producing linear growth. Geometric growth ($W_{1}$ = $W_{0}$eʳᵗ) occurs when both daughters continue dividing — producing exponential growth. Natural growth combines both phases into an S-shaped (sigmoid) curve: lag phase (slow preparation), log phase (rapid exponential growth), and stationary phase (plateau due to resource limitation).

Chapter Section 2: Differentiation and Dedifferentiation

Cells that emerge from meristems undergo differentiation — the acquisition of permanent structural and functional characteristics. Examples include tracheids losing their protoplast and developing lignified secondary walls, or sieve tube cells losing their nucleus to become transport conduits.

Dedifferentiation is the remarkable reverse process where mature, specialized cells lose their differentiation and regain meristematic capacity. Parenchyma cells forming interfascicular cambium during secondary growth is the textbook example. The subsequently active cambium undergoes redifferentiation — producing new permanent tissues (secondary xylem inward, secondary phloem outward). Together, these three processes (DDR) explain plant regeneration and secondary growth. The totipotency of plant cells (retention of full genetic potential despite differentiation) is the biological basis for tissue culture and clonal propagation.

Chapter Section 3: Plant Growth Regulators (PGRs)

PGRs are small organic molecules produced in trace quantities that regulate plant growth. Five major classes are recognized:

Auxins (indole compounds, natural = IAA): Promote cell elongation, establish apical dominance, guide tropic responses, induce parthenocarpy. Key application: 2,4-D as selective herbicide. Dose-response: low promotes, high inhibits growth.

Gibberellins (terpenoids, $GA_{3}$ most studied): Cause bolting, break dormancy, induce alpha-amylase in barley aleurone layer (commercial malting). Discovered from Gibberella fujikuroi. Antagonist to ABA.

Cytokinins (adenine derivatives, kinetin/zeatin): Promote cell division, delay senescence (Richmond-Lang effect), counteract apical dominance. Discovered by Skoog and Miller.

Ethylene (gaseous hydrocarbon, $C_{2}H_{4}$): Promotes fruit ripening, abscission, triple response (inhibited elongation + radial swelling + horizontal growth). Only gaseous hormone.

ABA (terpenoid): Stress hormone. Promotes stomatal closure via $K^{+}$ efflux from guard cells. Maintains seed dormancy. Antagonizes gibberellin.

Chapter Section 4: Photoperiodism and Vernalization

Photoperiodism: Plants sense the ratio of light to dark in a 24-hour period through a photoreceptor protein called phytochrome (Pr = inactive; Pfr = active). Based on their response, plants are classified as SDP (rice, chrysanthemum; need long nights), LDP (wheat, spinach; need short nights), or DNP (tomato, cucumber; independent). It is the length of the uninterrupted dark period that is physiologically critical. Pfr promotes LDP flowering, inhibits SDP flowering. The florigen (FT protein) produced in photoinduced leaves is transmitted via phloem to trigger meristem transition.

Vernalization: Cold treatment (0–5°C) required for flowering induction. Winter wheat and biennials (carrot) require weeks of cold before they can transition to reproductive growth. Mechanism: cold epigenetically silences FLC (floral repressor) via H3K27me3 marks; this allows florigen expression and subsequent flowering.