Plant Growth & Development

Plant growth is defined as an irreversible permanent increase in size, involving cell division, cell enlargement, and cell differentiation. It proceeds through three distinct phases: the meristematic phase (formative, active cell division at apices), the elongation phase (cells increase in size, vacuoles enlarge), and the maturation phase (cells attain their final shape and function through differentiation).

Growth rate follows two mathematical patterns. Arithmetic growth occurs when only one daughter cell continues to divide (linear progression, expressed as Lt = L₀ + rt, where r is the growth rate). Geometric growth occurs when both daughter cells retain the ability to divide (exponential progression, expressed as W₁ = W₀eʳᵗ). In nature, growth follows a sigmoid (S-shaped) curve with three phases: lag phase (slow initial growth as cells prepare), log/exponential phase (maximum growth rate), and stationary/plateau phase (growth slows and eventually stops due to limiting factors).

Differentiation is the process by which cells develop permanent structural and functional specialization. Dedifferentiation occurs when mature differentiated cells lose their specialization and regain meristematic capacity (e.g., formation of interfascicular cambium from parenchyma). Redifferentiation is when dedifferentiated cells mature again into a new permanent tissue (e.g., secondary xylem and phloem from cambium).

Five major plant growth regulators (PGRs) govern development. Auxins (IAA is natural; 2,4-D is synthetic) promote cell elongation, apical dominance (suppression of lateral buds), phototropism, and induce parthenocarpy (seedless fruits). At low concentrations auxins promote growth; at high concentrations they inhibit it. The synthetic auxin 2,4-D is used as a selective herbicide. Gibberellins (GA₃ is the most studied) cause stem elongation, particularly bolting in rosette plants like cabbage, break seed and bud dormancy, induce parthenocarpy, delay senescence, and stimulate $\alpha$-amylase production in the aleurone layer of barley endosperm during germination. Cytokinins (kinetin, zeatin, BAP) promote cell division (cytokinesis), delay leaf senescence (the Richmond-Lang effect), counteract apical dominance by promoting lateral bud growth, and mobilize nutrients towards sites of application. Ethylene is the only gaseous plant hormone. It promotes fruit ripening (banana, mango), induces abscission of leaves and flowers, breaks seed dormancy, causes the triple response in etiolated seedlings (inhibition of stem elongation, radial swelling, horizontal growth), and promotes feminization of flowers in cucumber. Abscisic acid (ABA) is called the stress hormone — it induces stomatal closure during drought (by promoting K⁺ efflux from guard cells), maintains seed dormancy, inhibits growth, and promotes leaf senescence. ABA is antagonistic to gibberellins (GA promotes germination; ABA inhibits it).

Photoperiodism is the response of plants to the relative duration of light and dark periods. Short-day plants (SDP) flower when the day length is below a critical photoperiod — e.g., chrysanthemum, rice, tobacco, Xanthium. Long-day plants (LDP) flower when the day length exceeds a critical photoperiod — e.g., wheat, radish, spinach, Henbane (Hyoscyamus). Day-neutral plants (DNP) flower regardless of photoperiod — e.g., tomato, cucumber, sunflower. Phytochrome, a photoreceptor protein, mediates the photoperiodic response: Pr (red-absorbing, inactive form) converts to Pfr (far-red-absorbing, active form) in light. Pfr promotes flowering in LDP and inhibits flowering in SDP.

Vernalization is the requirement of a cold treatment period (0-5 degrees C) to induce or accelerate flowering. Winter varieties of wheat and biennial plants like carrot require vernalization before they can flower.

The key testable concept is the identification of the five PGRs with their specific functions, especially that ethylene is the only gaseous hormone, ABA is the stress hormone, and gibberellins induce $\alpha$-amylase in the aleurone layer.