Growth Regulators were initially christened 'plant hormones' but this term has fallen from favour, and both 'plant growth substances' and 'plant growth regulators' have been adopted into modern usage (although it can be argued that each of these three descriptions is inappropriate).
There are five classes of compounds in the premier
division of endogenous PGRs. These are auxins, gibberellins (GAs), cytokinins (CKs),
abscisic acid (ABA) and ethylene. Broadly speaking, the auxins and GAs have been
classified as regulators of cell elongation, the CKs as regulators of cell
division, ABA as a general inhibiting influence and ethylene as a volatile with
a finger in numerous 'developmental pies'.
Description such as these are clearly simplistic, and represent vain attempts to
dispense PGRs into neat compartments on the basis of their effects on plant
tissues. This common practice is misguided, not only because it implies that a
PGR has a similar impact on the behaviour on any plant cell, but also, and more
significantly, because it assumes that the application of a PGR must mimic its
effects in situ. The balance of evidence indicates that neither of these
assumptions is correct. In addition is naive to believe that the five groups of
PGRs described above may be the only molecules that can influence plant growth
and development; an increasing number of other compounds such as the polyamines
and the brassinosteroids are being reported to have potent effects on plant
tissues.
Typical plant growth retardants are B-Nine, Cycocel, A-Rest, Bonzi, Florel and Sumagic. These CGRs reduce plant height by inhibiting the production of gibberellins, the primary plant hormones responsible for cell elongation. Therefore, their effects are primarily on stem, petiole and flower stalk tissues. Lesser effects are seen in reductions of leaf expansion, resulting in thicker leaves with darker green color.
Other benefits of using these CGRs in plant production include improved plant appearance by maintaining plant size and shape in proportion with the pot. Plant growth retardants also increase the stress tolerance of plants during shipping and handling and retail marketing, thereby improving shelf-life and extending plant marketability.