The following types of hormones can be found in a plant:
1. Hormones that stimulate plant growth
Auxin hormones are commonly found in the growing regions of the plant such as the tip of the leaves, the tip of the shoot, or the buds. When auxins accumulate in the plant cell, the cells of the plant growing regions elongate, i.e., grow taller.
Gibberellins also cause cell elongation but are usually concentrated on the stem in the areas where new leaves, also known as nodes, are formed. Under the effect of gibberellins, the stem between the nodes elongates, i.e., the internode distance becomes longer. The more gibberellins, the longer the plant.
Gibberellins also influence seed germination. A seed will only start growing when enough gibberellins accumulate.
Gibberellins are present across all growth stages of the plant. Their concentration is highest during periods of rapid growth (Feekes 4‒6, Zadoks 30‒32).
Cytokinins promote cell division. If cytokinins are present in one cell, they divide it into two cells. Cell division means new cells and the emergence of new leaves, fruits, branches (tillers), etc.
2. Hormones that inhibit plant growth
During periods of draught, abscisic acid will accumulate at cell borders to keep the stomata of the plant cells closed. Stomata are small openings in the leaf that allow it to breathe but simultaneously cause it to lose water. Another function of abscisic acid is to inhibit seed germination (e.g., until growing conditions are optimal).
3. Hormones associated with fruit ripening
Ethylene is a gas that, once released, breaks down the cell walls of a fruit, making them softer. Ethylene also helps release sugar, making fruit taste sweet. The most common use of ethylene is on bananas, commonly shipped green from country of origin and given ethylene treatment to ripen shortly before reaching supermarkets. In cereals, ethylene induces ripening and stops further growth in the plant.
How do plant growth regulators work?
Different plant growth regulators (PGRs) interfere with different plant hormones and have different effects on plant growth. Even if two PGRs target the same hormone, their underlying chemistry can interfere differently with the targeted plant hormone.
Mode of action of cereal PGRs registered in the United States
Adjust SL (proprietary chlormequat chloride)||
|Inhibits early stages of gibberellin biosynthesis to reduce plant height and increase stem thickness||Inhibits late stages of gibberellin biosynthesis to reduce plant height and increase stem thickness||Releases ethylene to reduce crop height|
|Long residual activity||Short residual activity||—|
|Has a slow onset of action, creating a more gradual response as CCC acts in the early stages of gibberellin biosynthesis||Has a rapid onset of action as trinexapac-ethyl acts in the late stage of gibberellin synthesis||
|Formulated to work effectively at temperatures as low as 34°F||Harsh field conditions at or following application timing can cause crop damage.||Induces ripening and consequently stops further growth processes in a plant |