Plant Growth & Development

Cheatsheet Content

### Growth: Definition & Characteristics - **Definition:** An irreversible, permanent increase in the size of an organ, its parts, or an individual cell, accompanied by metabolic processes (anabolic and catabolic) requiring energy. - **Key Characteristics:** - **Indeterminate:** Plants retain the capacity for unlimited growth due to meristems (e.g., root and shoot apical meristems, vascular cambium). - **Measurable:** Quantified by parameters like fresh/dry weight, length, area, volume, and cell number. - **Phases of Growth:** Meristematic, Elongation, and Maturation. - **Meristematic:** Constantly dividing cells, rich in protoplasm, large nuclei, primary cellulosic cell walls, abundant plasmodesmatal connections. - **Elongation:** Increased vacuolation, cell enlargement, new cell wall deposition. - **Maturation:** Cells attain maximal size, wall thickening, protoplasmic modifications. ### Growth Rates - **Arithmetic Growth:** - One daughter cell continues to divide, the other differentiates. - Linear curve when plotting length vs. time. - Equation: $L_t = L_0 + rt$ - $L_t$: length at time 't' - $L_0$: length at time 'zero' - $r$: growth rate / elongation per unit time - **Geometric Growth (Sigmoid Curve):** - Both progeny cells retain division capacity. - Initial slow growth (lag phase), followed by rapid increase (exponential phase), then slowing down (stationary phase) due to limited nutrients. - Typical for living organisms in natural environments. - Equation: $W_1 = W_0 e^{rt}$ - $W_1$: final size - $W_0$: initial size - $e$: base of natural logarithms - $r$: relative growth rate (efficiency index) - $t$: time of growth - **Absolute Growth Rate:** Total growth per unit time. - **Relative Growth Rate:** Growth per unit initial parameter per unit time. ### Conditions for Growth - **Water:** Essential for cell enlargement (turgidity), enzymatic activities. - **Oxygen:** For metabolic energy release. - **Nutrients:** Macro and micro elements for protoplasm synthesis and energy. - **Temperature:** Optimal range required; deviations are detrimental. - **Environmental Signals:** Light and gravity influence specific growth phases. ### Differentiation, Dedifferentiation, and Redifferentiation - **Differentiation:** Cells from meristems mature to perform specific functions, involving structural changes (e.g., tracheary elements lose protoplasm, develop strong lignocellulosic walls). - **Dedifferentiation:** Differentiated cells regain the capacity to divide under certain conditions (e.g., parenchyma cells forming interfascicular cambium). - **Redifferentiation:** Dedifferentiated cells lose division capacity again and mature to perform specific functions. - **Open Differentiation:** Plants show open differentiation; the final structure of a cell/tissue is determined by its location (e.g., root-cap cells vs. epidermis). ### Development - **Definition:** Sum of all changes an organism undergoes from germination to senescence. - **Plasticity:** Ability of plants to form different structures in response to environment or life phases (e.g., heterophylly in cotton, coriander, larkspur, buttercup). - **Control:** Regulated by intrinsic (intracellular factors like PGRs, genetic) and extrinsic (light, temperature, water, oxygen, nutrition) factors. ### Plant Growth Regulators (PGRs) - **Definition:** Small, simple molecules of diverse chemical composition that influence plant growth and development. Also called plant hormones or phytohormones. - **Groups:** - **Plant Growth Promoters:** Involved in cell division, enlargement, pattern formation, tropic growth, flowering, fruiting, seed formation. - Auxins - Gibberellins - Cytokinins - **Plant Growth Inhibitors:** Role in plant responses to wounds, stresses, dormancy, abscission. - Abscisic Acid (ABA) - **Ethylene:** Can fit both groups, but is largely an inhibitor. #### Discovery of PGRs - **Auxins:** Darwin & son observed phototropism in canary grass coleoptiles; F.W. Went isolated auxin from oat coleoptile tips. - **Gibberellins:** Kurosawa (1926) found gibberellic acid (from *Gibberella fujikuroi*) caused 'bakanae' disease in rice. - **Cytokinins:** Skoog & coworkers observed callus proliferation with vascular tissue extracts, yeast extract, coconut milk, or DNA; Miller et al. (1955) identified kinetin. Zeatin was later isolated from corn kernels. - **Abscisic Acid (ABA):** Mid-1960s, inhibitor-B, abscission II, and dormin were purified and found to be chemically identical. - **Ethylene:** Cousins (1910) confirmed volatile substance from ripened oranges hastened ripening of bananas. #### Physiological Effects of PGRs ##### Auxins - **Examples:** Indole-3-acetic acid (IAA), Indole butyric acid (IBA), Naphthalene acetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D). - **Functions:** - Promote rooting in stem cuttings. - Promote flowering (e.g., pineapples). - Prevent early fruit/leaf drop, but promote abscission of older leaves/fruits. - **Apical Dominance:** Apical bud inhibits lateral bud growth (removed by decapitation in tea plantations, hedges). - Induce parthenocarpy (e.g., tomatoes). - Herbicides (2,4-D for dicot weeds). - Control xylem differentiation, aid cell division. ##### Gibberellins (GAs) - **Examples:** Over 100 types (GA₁, GA₂, GA₃, etc.), GA₃ is most studied. All GAs are acidic. - **Functions:** - Increase axis length (e.g., grape stalks). - Elongate and improve fruit shape (e.g., apple). - Delay senescence (extend market period). - Speed up malting process (brewing industry). - Increase sugarcane stem length (yield by 20 tonnes/acre). - Hasten maturity in juvenile conifers (early seed production). - Promote bolting (internode elongation before flowering) in rosette plants (e.g., beet, cabbage). ##### Cytokinins - **Examples:** Kinetin (synthetic), Zeatin (natural), natural cytokinins from corn-kernels, coconut milk. - **Functions:** - Specific effects on cytokinesis (cell division). - Produce new leaves, chloroplasts. - Promote lateral shoot growth and adventitious shoot formation. - Overcome apical dominance. - Promote nutrient mobilisation, delay leaf senescence. ##### Ethylene ($C_2H_4$) - **Nature:** Simple gaseous PGR, synthesized in senescing tissues and ripening fruits. - **Functions:** - Horizontal growth of seedlings, swelling of axis, apical hook formation in dicot seedlings. - Promote senescence and abscission of plant organs (leaves, flowers). - Highly effective in fruit ripening (enhances respiratory climactic). - Break seed and bud dormancy, initiate peanut seed germination, potato tuber sprouting. - Promote rapid internode/petiole elongation in deep water rice plants. - Promote root growth and root hair formation (increases absorption surface). - Initiate flowering and synchronize fruit-set in pineapples, induce flowering in mango. - **Ethephon:** Most widely used compound; releases ethylene slowly, hastens fruit ripening, accelerates abscission (thinning cotton, cherry, walnut), promotes female flowers in cucumbers. ##### Abscisic Acid (ABA) - **Nature:** Plant growth inhibitor, stress hormone. - **Functions:** - Regulates abscission and dormancy. - Acts as general plant growth inhibitor and inhibitor of plant metabolism. - Inhibits seed germination. - Stimulates stomatal closure. - Increases plant tolerance to various stresses. - Plays role in seed development, maturation, and dormancy (helps seeds withstand desiccation). - Acts antagonistically to Gibberellins.