Plant Growth and Development - Growth regulators (Auxin, Gibberellin, Cytokinin, Ethylene, ABA)

Plant Growth Regulators (PGRs) are small, simple molecules of diverse chemical composition, such as indole compounds ($IAA$), adenine derivatives ($Kinetin$), derivatives of carotenoids ($ABA$), terpenes ($GA_3$), or gases ($C_2H_4$).

Auxins (e.g., $IAA$, $IBA$) are produced at the apices of stems and roots. They promote apical dominance, initiate rooting in stem cuttings, and prevent fruit and leaf drop at early stages but promote abscission of older leaves.

Gibberellins ($GA$) are acidic in nature. $GA_3$ was one of the first to be discovered. They cause an increase in the length of the axis (used in grapes), cause fruits like apple to elongate/improve shape, and promote 'bolting' (internode elongation) in rosette plants like cabbage.

Cytokinins have specific effects on cytokinesis. Natural cytokinins are synthesized in regions of rapid cell division (root apices, shoot buds). They help overcome apical dominance and delay leaf senescence by promoting nutrient mobilization (Richmond-Lang effect).

Ethylene ($C_2H_4$) is a gaseous PGR. It is highly effective in fruit ripening, where it enhances the rate of respiration (respiratory climactic). it breaks seed and bud dormancy and initiates germination in peanut seeds.

Abscisic Acid ($ABA$) acts as a general plant growth inhibitor and an inhibitor of plant metabolism. It stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses, hence called the 'stress hormone'.

Growth can be measured using parameters like increase in fresh weight, dry weight, length, area, volume, or cell number. For example, a single maize root apical meristem can give rise to more than $17,500$ new cells per hour.