Chapter 18

Plant Growth Regulators for Greenhouse Crops

Plant Hormones

Plant hormones (also known as phytohormones) are 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. Hormones regulate cellular processes in targeted cells locally and when moved to other locations, in other locations of the plant. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Plants, unlike animals, lack glands that produce and secrete hormones, instead each cell is capable of producing hormones. Plant hormones shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves and fruits. They affect which tissues grow upward and which grow downward, leaf formation and stem growth, fruit development and ripening, plant longevity, and even plant death. 

Characteristics of Plant Hormones

Plant hormones are naturally produced within plants, though very similar chemicals are produced by fungi and bacteria that can also effect plant growth. Plant hormones occur naturally; when they’re synthesized chemically they’re known as plant growth regulators or PGRs. 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.

Classes of Plant Hormones

In general, it is accepted that there are five major classes of plant hormones, some of which are made up of many different chemicals that can vary in structure from one plant to the next. The chemicals are each grouped together into one of these classes based on their structural similarities and on their effects on plant physiology. Each class has positive as well as inhibitory functions, and most often work in tandem with each other, with varying ratios of one or more interplaying to affect growth regulation.

Abscisic Acid

Abscisic acid (ABA) is a growth inhibitor. It controls the dormancy of buds and seeds and inhibits shoot growth.

Auxins

Auxins control many plant growth functions including cell enlargement, apical dominance, and stimulation of roots. Indoleacetic add (IAA) is a naturally occurring auxin, but is not used commercially as it breaks down rapidly in light.

Cytokinins

Cytokinins promote cell division. They are thought to be produced in the roots and by young fruit.

Ethylene

Ethylene 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.

Gibberellins

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.

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