Botany online 1996-2004. No further update, only historical document of botanical science!

Synthetic Growth Regulators and Growth Inhibitors

The regulation of animal cells is largely based upon macromolecular effectors like peptide hormones, etc. The production of synthetic compounds has not yet proceeded further, though the strengthening of genetic engineering shows possibilities to yield peptide hormones (like somatostatin, growth hormone or insulin) in larger amounts. The situation is different with phytohormones. Their chemical structures shown before are rather easy, and the synthesis of analogous substances is no larger problem for chemists.

The biosynthesis of phytohormones is in many regards interesting. On one hand is the effect of a hormone, its reaction kinetics (dose-effect-curve), its catabolic and anabolic pathways in the plant cell, as well as species-specific and developmentally specific differences of interest. On the other hand is the use of phytohormones or their analogous substances an economic factor that can be outlined by the following headwords:

harvesting aids, and
the synchronization of maturation and its temporal control.

In order to solve the existing problems are either synthetic components simulating the effects of phytohormones used, or inhibitors of the biosynthesis of phytohormones are taken to generate a lack of hormones in the cells. In agriculture and forestry are herbicides used to stop the growth of unwanted plants – like the presence of yield-reducing weeds.

Auxin derivatives like 2,4-dichlorphenoxy acidic acid (2,4-D) or 2,4,5-trichlorphenoxy acidic acid (2,4,5-T) proved to be herbicides effecting selectively dicots.

Although the knowledge about the selectivity and the mode of action of these substances is still very limited, do they belong to the most commonly used herbicides effecting preferably dicots. 2,4-D increases the rate of DNA, RNA and protein synthesis and impedes thus an outbalanced, controlled growth. The plant does actually grow to death. The phenotype of a thus treated plant is characterized by the abnormal growth of the deformed shoots, a breakdown of chlorophyll (bleaching), the dying of the roots and several more features. 2,4,5-T has shown to be especially toxic for perennial wooden plants and is therefore most often used in forestry. Its is less easily degraded than 2,4-D. Auxin, in comparison, is very easily degraded and is consequently of no use as a herbicide.

Although no clear indication for the damage of humans or animals by 2,4-D or 2,4,5-T is known, exists no doubt about the high toxicity of the by-product of industrial synthesis, tetrachlorodibenzoparadioxin (TCDD) also called dioxin. Legal regulations shall secure that the 2,4,5-T sold is free of dioxin, but little control for production exists. Accidents like Seveso (Upper Italy, 1976) or the unsolved disappearance of waste products in Middle Europe (1983) demonstrate the problem. 2,4,5-T has been lavishly used in the Vietnam war for defoliation (‘Agent Orange’) where it caused numerous incidents. Proof exists that the substance used then did not fit the standards of purity valid in Europe, the United States, and other industrial nations.

Gibberellin (GA3) is the only phytohormone that is still applied in horticulture. It is used for the cultivation of seedless grapes in Californian viniculture where it increases the size of the grapes two to three times. No such success could be yielded with seed-containing grapes.

Gibberellin is also applied to some citrus fruits for the improvement of the fruit setting as well as for a delay of maturation (storage advantage).

In practice are also compounds used that set free ethylene after spraying them on the fruits. Best known is chlorethylphosphoric acid sold under the name Ethrel, Ethephon or CEPA. It is required for:

the quickening of tomato maturation where it causes a simultaneous synchronization of maturation,
for easing the cherry harvest (speeds up maturation), synchronization, easier picking because of an enhanced development of the separating walls, and
for the stimulation of latex flow in HeveaHeveaHHhhhHevea (increased flow per cut).

A number of substances like, for example, chlorcholinchlorid (CCC) inhibits elongation even at low concentrations. Spraying of cereal seedlings with CCC causes thickening of the culm and thus an enhanced stability.

It is assumed that it has an effect antagonistic to that of gibberellin (like GA3) since a selective inhibition of their biosynthesis was proven. The quaternary ammonium salt AMO 1618 belongs to the same class of inhibitors. It is used in the cultivation of ornamental plants and causes a bushy shape and a stocky growth of the treated plants.

A further class of growth regulators applied in the cultivation of ornamental plants are the morphactines that do not only inhibit elongation but stop in addition the apical dominance altering the shape of the plant drastically due to the development of numerous lateral shoots that cause a bushy look. Furthermore are geotropism and phototropism influenced as well as the development of carpels and stamina. Morphactines obstruct the mitotic activity of meristematic tissues and do thus change the orientation of the mitotic spindle thus inhibiting the usually strictly followed polarity typical for plants. This disturbance is caused by a far-reaching, morphactine-induced stop of auxin transport. The effects of morophactine are normally irreversible.