Growth Inhibitors: Metabolism and Agricultural Uses | Botany

Here is an elaborated discussion on growth inhibitors, highlighting:- 1. Introduction to Growth Inhibitors 2. Natural Occurrence of Growth Inhibitors 3. Metabolism 4. Responses 5. Agricultural Uses.

Introduction to Growth Inhibitors:

Most growth substances generally stimulate growth and correlate growth and development in morphogenesis. A diverse group of other substances in­volved in growth correlation generally inhibit growth and are termed growth inhibitors. The most common inhibitors are aromatic compounds, such as phenols and lactones, but alkaloids, certain alcohols, organic and fatty acids, and even metallic ions can act as inhibitors.

For convenience of discussion, growth inhibitors are usually classified into three groups:

1. Phytohormones- Terpenoids, such as ABA. ABA-glucose (glucoside), a bound form, also has ABA activity.

2. Other natural inhibitors- including phenolic and benzoic acid deriva­tives and lactones. Unlike the ABA hormone, these apparently are metabolic by-products usually present in large quantities. They may play important in­hibitory-correlative roles in growth and development, such as seed dormancy in certain species.

3. Synthetics- A large number of synthetic compounds exhibit growth inhibitory activity. Many have been labeled for agricultural uses. The quaternary salts of ammonium and Phosfon-D are growth retardants. Another important synthetic is succinic acid-2, 2-dimethylhydrazide (SADH or daminozide).

Chlormequat chloride (CCC) is available commercially and widely used in flax and other seed crops to reduce lodging and recently to coordinate barley and wheat tiller growth rate. The morphactins (flurecol and chloroflurecol) are recent additions to the list of growth retardants. The chloro-acid is the most active form of the two morphactins.

Natural Occurrence of Growth Inhibitors:

A highly active growth inhibitor was isolated from cotton fruits in the early 1960s and named abscisin II. A similar compound was isolated in England from sycamore leaves and named dormin. The two substances were subsequently found to be chemically and biologically identical; it was agreed to name the substance abscisic acid, or ABA.

The hormone ABA has been isolated from tubers, buds, pollen, fruits, embryos, endosperms, and seed coats of herbaceous and woody annuals and perennials of some 40 to 50 species. It is reasonable to conclude that ABA, like IAA, is ubiquitous in higher plants. ABA is normally located in the chloroplast, but after environmental stress it is released to the other organ­elles and is active in stomata control.

ABA analogs are likewise distributed widely, but they are not as active biologically as ABA. Phaseic acid is present in the seeds of bean (Phaseolus multiflorus), and theospirone is a natural inhibitor and a flavor component in tea leaves.

Secondary substances such as alkaloids, phenolics, and lactones often occur in concentrations sufficient to be stored food reserves (i.e., much greater than hormone level). Juglone, a lactone is present in high concen­trations, especially in the mesocarp (husks) and roots of black walnut (Juglan nigra L.).

Buffalo gourd (Curcurbita foetidissima), a weedy vine adapted to semiarid areas, stores large quantities of carbohydrates in conjunction with an unidentified toxic substance in the fleshy taproots. A water extract containing the chemical was lethal to young tomato and lettuce plants and completely prevented germination of radish and lettuce seeds. It seems probable that this growth inhibitor may reduce competition from other species in the rhizosphere (allelopathy), with resultant survival benefit for buffalo gourd in the ecosystem.

Secondary substances probably are allelopathic in numerous species and are operative in natural plant succession. They may also provide the donor protection from insect and herbivore feeders and perhaps from disease organisms, since many secondary com­pounds are known to inhibit microbes.

Metabolism of Growth Inhibitors:

ABA is a terpenoid, as are GA, cytokinin, chlorophyll, carotene, and xanthophyll. Like these, synthesis is evidently through the mevalonic acid and isoprene pathway. Synthesis was also found to result from oxidation of some xanthophylls, such as violaxanthin. Light enhances the occurrence of the most effective form, the cis-trans form of ABA. The site of synthesis appears to be in the plastids, especially chloroplasts.

Deactivation of ABA may be by:

(1) Enzymatic conversion to 2-trans ABA (non-active form)

(2) Oxidation to phaseic acid

(3) Conjugation with sugars to produce glycosides, primarily glucosides

As with other hormones, bound forms have little or no activity. Free ABA is translocated readily throughout the plant like IAA, but apparently at a greater rate.

Synthesis of phenols was by the shikimic acid pathway using phenyl­alanine or tyrosine. Cinnamic acid is ap­parently a precursor to certain benzoic inhibitors. Coumarin, a lactone, is derived from phenylpropane (1-aminocyclopropane-1-carboxylic acid).

Responses to Growth Inhibitors:

Natural or synthetic inhibitors suppress growth and development, as revealed by the standard straight growth test. They also play important correla­tive roles in morphogenesis and survival. Without dormancy or suspension of active growth, seeds and buds might sprout or resume growth only to be killed by intolerable periods of heat, cold, or desiccation.

The dormancy mechanism allows seeds and buds to delay new growth in a resting state and to resume growth only when ABA levels decline concurrently with favorable conditions for completion of the growth cycle. Deciduous species lose their leaves by abscission, creating a winter dormancy cued by natural short days of fall. Potato buds are dormant at tuber maturity due to presence of ABA and do not sprout even though surrounding soil and climatic conditions are favor­able.

Induction of such dormancy mechanisms is controlled by natural inhibi­tors, principally ABA. Dormancy is generally lost during winter as a result of stratification (cold treatment) or sometimes merely by passage of sufficient time. Production of GA is promoted by stratification, which may be the causative factor for regrowth because the ABA effect is masked by a high GA- ABA ratio. Most crop cultivars have been selected against seed dormancy.

Fruits also develop an abscission layer with injury or age; they fall due to ABA accumulation and induced abscission. Diseased lupine pods had an ABA content 2.5 times that of the controls, which resulted in abscission of diseased fruits. The accumulation of ABA in lateral buds points to ABA involvement in apical dominance. It is also highly in­volved with senescence and dehiscence of fruits such as those of cotton.

Distri­bution of ABA in cotton fruits is biomodal; the first peak occurs soon after fertilization, when it may initiate reduction of fruit load by inducing forma­tion of an abscission layer, and a second peak occurs at senescence (ageing) and dehiscence (splitting). Treatment with GA can overcome many of these ABA effects.

The association of ABA accumulation and stomata closure as leaves are stressed for moisture is currently of interest to physiologists. The formation of ABA during stomata closure seems to validate the theory of ABA as the triggering mechanism in stomatal control. It is released from the chloroplast into the epidermis cells during moisture stress.

At the cellular level, ABA can inhibit proton extrusion and potassium uptake. It strongly inhibits activity of enzymes, such as the hydrolytic enzymes induced by GA in barley endosperm. It also inhibited flowering of a long-day plant given short days.

Another natural inhibitor of common occurrence is chlorogenic acid. This PGR is believed to be an antiseptic to bacteria in plant wounds.

Agricultural Uses of Growth Inhibitors:

A number of synthetic inhibitors including growth retardants have been labeled and are available for commercial use. The major effect is to shorten internode length and plant height and generally to reduce lodging, especially in cereal grain crops and flax.

Leaf area, light interception, and product yield are usually not reduced by treatment. Leaf area of sugar beet plants was reduced by 25 to 40% as a result of smaller leaf size from spraying with a solution of PP333 at a concentration of 4000 µg. ml^-1. Daminozide (SADH), chlormequat (CCC), Phosfon-D, and morphactins are effective retardants.

An unlabeled compound, BTS 44584, reduced height of ‘Williams’ soybean (maturity group III) by 20 cm when applied at 1.1 kg. ha^-1 at the V₄ stage but did not increase yields or decrease lodging. In fact lodging was often aggravated by higher rates of this PGR. Morphactins have an advantage in that high dosage rates are not harmful to the plant and extend the period of dwarfing activity. In addition to dwarfing, they induce axillary bud growth and delay senescence. Growth retardants also cause a dark green leaf coloration, ostensibly by increasing chlorophyll content.

Daminozide, marketed as Kylar, is used on more than 140,000 ha runner-type peanuts annually in the southeastern United States. The objec­tive is not to reduce lodging but to curtail late vegetative growth of the vines in order to channel more assimilate to seeds. Lesser vine also facilitates harvesting. Pod yields of a late, viny-type cultivar ‘Dixie Runner’ were increased signifi­cantly by Kylar application at pod initiation.

Regim-8 (2, 3, 5-triiodobenzoic acid, or TIBA), an inhibitor to IAA trans­port, caused more upright leaves (Christmas tree effect) and presumably a more optically efficient canopy in soybean. Canopy structure was altered and pod and seed yields were increased.

Data of other investigators do not support the canopy improve­ment theory; they have suggested instead improved partitioning to fruits re­sulting from reduced vegetative growth. The commercial use of Regim-8 has been discontinued, probably because of an unfavorable cost-benefit ratio as well as inconsistent yield benefits.

In the United Kingdom, CCC is used extensively on barley and wheat to regulate tiller growth rate. The primary and secondary tillers are arrested temporarily to prevent dominance over higher order tillers, which distributes growth and yield more evenly among tillers and increases total yield.

Defoliants are used as harvest aids in mechanical harvesting of cotton. Endothall is used commercially, and Harvade, labeled recently, is effective. Probably the most successful growth inhibitor is maleic hydrazide (MH), used for sucker (axillary shoot) control in tobacco.

MH is used on essentially all the tobacco hectarage in the United States. If tobacco is not treated after topping or removal of the inflorescence, axillary buds freed from apical dominance rapidly produce shoots that shade and withdraw nutrients from the harvest- able leaves, lowering the market quality of the leaves.