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 are synthesized chemically, they are known as plant growth regulators, or PGRs. Some PGRs mimic the natural plant hormones, and others 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 preventing branching. 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 used as herbicides (2,4-D and other phenoxy herbicides). In apple production, NAA and NAD are synthetic auxins that can be used to thin fruit, inhibit water sprout and sucker growth, and prevent pre-harvest fruit drop. Carbaryl, while not strictly an auxin, has a similar chemical structure and has similar activity in fruit thinning.
Gibberellins also promote growth. They are produced in very young leaves, developing seeds, fruit, and roots. Gibberellins cause cell elongation during shoot growth, and are involved in regulation of dormancy. Commercially, gibberellins have been used to improve fruit size, prevent fruit russeting, and induce lateral branching. 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). Combinations of benzyladenine and gibberellins (ex. Promalin®) are used to improve fruit shape and to stimulate lateral branching.
Abscisic acid (ABA) is a growth inhibitor. ABA is produced in mature leaves along with many other plant tissues where it controls the dormancy of buds and seeds and inhibits shoot growth. It also appears to be involved in plant response to water stress. Commercial formulations of ABA (ProTone can be used to accelerate color development in grapes, for post-bloom thinning of apples and to accelerate fall defoliation in a number of fruit crops.
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 and is most commonly associated with plant stress. It promotes abscission of leaves and fruits, inhibits shoot elongation, 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 and is used for fruit thinning and for synchronizing fruit ripening and abscission in preparation for mechanical harvest. ACC (Aminocyclopropane carboxylic, trade name Accede) is the natural precursor to ethylene synthesis in the plant and acts similar to ethephon. It is used for thinning of apples and peaches.
AVG (aminoethoxyvinyl glycine, tradename ReTain) interferes with ethylene biosynthesis, allowing fruit to hang on trees longer and lengthens storage life. 1-MCP (1-Methylcyclopropene, trade name SmartFresh™) blocks the receptor for ethylene, preventing ethylene action. Since 1-MCP is a gas, it has been used in storage facilities to slow post-harvest ripening. A sprayable formulation of 1-MCP (Harvista) can be applied to apples and pears before harvest, to slow fruit maturation and extend postharvest storage life. The sprayable formulation can be applied to sweet cherries during bloom to increase fruit set and is used on cultivars such as Regina that tend to have poor fruit set.