Growth is irreversible permanent increase in size or mass. Differentiation is maturation of cells to perform specialised functions. Development includes all changes from germination to senescence. Dedifferentiation is regaining division capacity by mature cells. Redifferentiation is maturation of cells produced by dedifferentiated meristems. Determinate growth is growth that stops after a limit. Meristem is tissue whose cells divide actively and self-perpetuate. Growth rate is increased growth per unit time.
These terms describe increase in size, maturation, total life-cycle changes, regained division, renewed maturation, limited growth, dividing tissue and growth per unit time.
Growth may appear as increase in cell number, cell size, length, area, volume, fresh weight or dry weight. Root growth may be measured by length, leaf growth by area, and fruit growth by volume or weight. Therefore, one parameter is not enough for the whole plant life cycle.
No single parameter can measure all forms of plant growth because different organs grow by different measurable changes.
(a) In arithmetic growth, one daughter cell continues dividing while the other differentiates; length increases at a constant rate. (b) In geometric growth, both daughter cells retain division ability, giving exponential growth until resources limit it. (c) A sigmoid curve shows lag, exponential/log and stationary phases and is typical of many living systems. (d) Absolute growth rate measures total growth per unit time; relative growth rate expresses growth per unit initial parameter, allowing comparison of differently sized systems.
Arithmetic growth is linear, geometric growth is exponential, sigmoid growth has lag-log-stationary phases, absolute growth is total increase per time and relative growth is increase per unit initial size.
Auxins were discovered through phototropism experiments by Darwin and Darwin; F.W. Went later isolated auxin from oat coleoptile tips. Natural auxins include IAA and IBA; synthetic auxins include NAA and 2,4-D. Auxins promote cell elongation, rooting in stem cuttings, flowering in pineapple, parthenocarpy in tomato, apical dominance, xylem differentiation and cell division. In agriculture/horticulture, they are used for rooting cuttings, inducing parthenocarpic fruits, preventing early fruit/leaf drop and as herbicides such as 2,4-D against dicot weeds.
The five main groups are auxins, gibberellins, cytokinins, ethylene and abscisic acid. Example note: auxins.
ABA inhibits growth and seed germination, promotes seed maturation and dormancy, and closes stomata during water stress. These effects help plants withstand drought, desiccation and other unfavourable conditions.
ABA is called the stress hormone because it increases plant tolerance to stresses and induces responses such as stomatal closure and dormancy.
Growth is open or indeterminate due to persistent meristems such as root and shoot apical meristems and lateral meristems. Differentiation is also open because cells/tissues arising from the same meristem may become different mature cell types according to their location and developmental cues. Differentiated cells can also dedifferentiate and redifferentiate.
They are open because plants retain meristems and can keep producing new cells, while cells from the same meristem can mature into different structures depending on position and conditions.
Auxins initiate rooting in stem cuttings. Ethylene hastens fruit ripening. Cytokinins delay senescence and help overcome apical dominance by promoting lateral shoot growth. Gibberellins induce bolting in rosette plants. ABA causes stomatal closure, especially during stress.
(a) Auxin. (b) Ethylene. (c) Cytokinin. (d) Cytokinin. (e) Gibberellin. (f) Abscisic acid.
GA3 promotes stem elongation and is linked with bakanae-like elongated rice seedlings. If cells divide without differentiation, organised tissues do not form properly. Rotten/ripening fruits release ethylene, which accelerates ripening of nearby unripe fruits. Cytokinins promote cytokinesis and shoot formation in tissue culture, so omission reduces cell division and organogenesis.
(a) Rice seedlings elongate excessively. (b) Undifferentiated dividing mass/callus-like growth may result. (c) Unripe fruits ripen faster. (d) Cell division and shoot formation in culture are poor or absent.