Gibberellins and cytokinins

Gibberellins are growth promoters. More than 100 have been reported from organisms as different as fungi and higher plants, named GA₁, GA₂, GA₃ and so on. Gibberellic acid (GA₃) was one of the first found and is the most intensively studied. All GAs are acidic.

Because they lengthen the axis, gibberellins are used to lengthen grape stalks. They make fruits such as apple elongate and improve their shape.

They delay senescence, so fruit can stay on the tree longer and the market period is extended. GA₃ speeds up malting in the brewing industry.

Sugarcane stores its carbohydrate as sugar in the stem. A gibberellin spray lengthens the stem and can lift the harvest by up to 20 tonnes per acre.

Spraying juvenile conifers with GAs hastens maturity and so gives early seed production. GAs also promote bolting, the elongation of internodes just before flowering, in beet, cabbage and many plants with a rosette habit.

Cytokinins act specifically on cytokinesis. The first one found, kinetin, is a modified adenine (a purine) obtained from autoclaved herring sperm DNA. Plants themselves do not make kinetin.

Zeatin, a natural cytokinin, was isolated from corn kernels and coconut milk; several natural and some synthetic cytokinins are now known.

Natural cytokinins are made where cells divide rapidly: root apices, developing shoot buds, young fruits.

Cytokinins help form new leaves, chloroplasts in leaves, lateral shoots and adventitious shoots. They help overcome apical dominance, and by drawing nutrients into a leaf they delay its senescence.