Plant Growth Regulators: Naturally Occurring Plant Hormones
A plant hormone is commonly defined as an organic substance that is produced in one part of a plant and translocated to another part where, at very low concentrations, it stimulates a physiological response. Plant hormones may promote or inhibit growth, depending on the specific hormone involved, the concentration, the time, and the plant part it is acting on. Plant hormones occur naturally. When they’re synthesized chemically they’re known as plant growth regulators or PGRs. Some PGRs act by blocking either the synthesis or the activity of plant hormones.
Plant hormones can be grouped into five classes of compounds: auxins, gibberellins, cytokinins, abscisic acid, and ethylene, each of which is described briefly below.
These are primarily growth-promoting substances that contribute to the elongation of shoots, but at high concentrations they can inhibit growth of lateral buds. Auxins are generally produced in apical buds, young leaves, and developing seeds. In addition to being used as plant growth regulators, auxins can also be herbicides (2,4-D and other phenoxy herbicides). In apple production, NAA and NAD are synthetic auxins that can be used to thin fruit, to inhibit water sprout and sucker growth, and to prevent pre-harvest fruit drop. Carbaryl, while not strictly an auxin, has a similar chemical structure.
Gibberellins also promote growth. They are produced in very young leaves, developing seeds, fruit, and roots. Gibberellins cause cell elongation, including shoot growth, and are involved in regulation of dormancy. Commercially, gibberellins have been used to improve fruit size and to prevent russeting. Several growth retardants, including Apogee, limit biosynthesis of gibberellins and thus inhibit shoot growth.
Cytokinins promote cell division. They are thought to be produced in the roots and by young fruit. Cytokinins are involved in apical dominance, branching, and stimulating bud initiation. Benzyladenine is a synthetic cytokinin used for fruit thinning (Maxcel).
Abscisic acid (ABA) is a growth inhibitor. It controls the dormancy of buds and seeds and inhibits shoot growth. Exactly how ABA works is not well understood. It may act directly by blocking synthesis of enzymes, or it may operate indirectly by blocking RNA synthesis, thus blocking the formation of enzymes that in turn form the growth promoters. ABA is produced in mature leaves, along with many other plant tissues. It is not currently used as a PGR in tree fruits but is available for promoting fruit color development in table grapes.
This is the only known gaseous plant hormone. Many plant organs synthesize ethylene, and it moves readily in the air surrounding the tree. Usually, ethylene has an inhibitory effect on plants. It promotes abscission of leaves and fruits, inhibits shoot elongation and favors caliper development, and, along with auxin, inhibits lateral bud development. On the other hand, it can break dormancy in buds and seeds and causes rapid ripening of apples. In apples, ethylene is involved in the transition of fruit from being physiologically mature to ripe. Once exposed to ethylene, their storage life is shortened.
Ethephon is a synthetic compound that releases ethylene upon application. ReTain interferes with ethylene biosynthesis, allowing fruit to hang on trees longer and lengthens storage life. 1-Methylcyclopropene (1-MCP or SmartFresh) blocks the receptor for ethylene, preventing ethylene action. Since 1-MCP is a gas, it has been used to slow post-harvest ripening in storage, but has not been used commercially in the orchard.