PLANT GROWTH AND DEVELOPMENT - QUICK REVISION.
GROWTH: an irreversible permanent increase in size of an organ, its parts or a cell, accompanied by metabolic processes at the expense of energy. At cellular level it is principally an increase in protoplasm, measured indirectly by fresh weight, dry weight, length, area, volume or cell number. Plant growth is indeterminate (open form) because of meristems that divide and self-perpetuate. Apical meristems (root and shoot) drive PRIMARY growth (elongation); lateral meristems (vascular cambium, cork-cambium) drive SECONDARY growth (increase in girth).
PHASES OF GROWTH (root tip, apex inward): meristematic (dividing cells, dense protoplasm, large nuclei, thin primary walls), elongation (vacuolation, cell enlargement, new wall deposition), maturation (maximal size, wall thickening, protoplasmic modifications).
GROWTH RATES: increased growth per unit time. ARITHMETIC growth - after mitosis only one daughter cell keeps dividing, other matures; linear curve; Lt = L0 + rt (Lt length at time t, L0 length at zero, r = growth rate). GEOMETRIC growth - both progeny cells keep dividing; slow lag phase, rapid log/exponential phase, then stationary phase with limited nutrients; gives a sigmoid (S) curve; W1 = W0 e^rt (W1 final size, W0 initial size, r relative growth rate = efficiency index, e = base of natural logarithms). ABSOLUTE growth rate = total growth per unit time; RELATIVE growth rate = growth per unit time per unit initial parameter. CONDITIONS for growth: water (turgidity, enzyme medium), oxygen (metabolic energy), nutrients, optimum temperature, light and gravity.
DIFFERENTIATION = maturation of cells from meristems/cambium to perform specific functions (e.g. tracheary elements lose protoplasm, gain lignocellulosic secondary walls). DEDIFFERENTIATION = differentiated cells regaining capacity to divide (e.g. interfascicular and cork cambium from parenchyma). REDIFFERENTIATION = these dividing cells again losing the capacity to divide and maturing. Differentiation is OPEN: same meristem gives different structures at maturity depending on position (e.g. root-cap cells vs epidermis).
DEVELOPMENT = all changes from seed germination to senescence; broadly the sum of growth and differentiation. PLASTICITY = ability to follow different developmental pathways by environment or life phase (e.g. heterophylly in cotton, coriander, larkspur; air vs water leaves in buttercup). Control is intrinsic (genetic intracellular + chemical intercellular PGRs) and extrinsic (light, temperature, water, oxygen, nutrition, gravity).
PLANT GROWTH REGULATORS (PGRs): small simple molecules; five major groups. PROMOTERS = auxins, gibberellins, cytokinins; INHIBITOR-type = abscisic acid (ABA); ethylene fits either but is largely an inhibitor. Discovery: auxin (Darwin's phototropism in canary grass coleoptiles; isolated by F.W. Went from oat coleoptile tips); gibberellin (bakanae disease of rice by Gibberella fujikuroi, Kurosawa 1926); cytokinin (kinetin from autoclaved herring sperm DNA, Miller et al. 1955; natural zeatin from corn-kernels and coconut milk); ABA (inhibitor-B, abscission II, dormin - all identical); ethylene (Cousins 1910, ripening oranges).
AUXIN (IAA, IBA natural; NAA, 2,4-D synthetic): rooting of cuttings, apical dominance, prevents early fruit/leaf drop but promotes abscission of older organs, parthenocarpy (tomato), xylem differentiation, herbicide (2,4-D kills dicot weeds not mature monocots), flowering in pineapple. GIBBERELLIN (GA3, acidic, 100+ forms): stem/axis elongation (grape stalks, sugarcane +20 t/acre), bolting in rosette plants, delays senescence, malting in brewing, early seed production in conifers. CYTOKININ (cytokinesis): new leaves, chloroplasts, lateral and adventitious shoots, overcomes apical dominance, nutrient mobilisation delaying leaf senescence. ETHYLENE (gas; ethephon is main source compound): apical hook and horizontal growth in dicot seedlings, fruit ripening (respiratory climactic), senescence and abscission, breaks seed/bud dormancy, internode elongation in deep water rice, root and root-hair growth, flowering in pineapple and mango, female flowers in cucumber. ABA (stress hormone): general growth inhibitor, inhibits seed germination, stomatal closure, stress tolerance, seed development/maturation/dormancy; antagonist to GAs.
PHOTOPERIODISM and VERNALISATION: light (photoperiod) and low temperature (vernalisation), both extrinsic factors, influence the initiation of flowering, often acting via PGRs.