Plant Growth Regulators – Mode Of Action
Matthew Bywater, Manager, Technical Services, Nuturf

Australian Turfgrass Management Volume 3.3 (June - July 2001)

Pant growth regulators (PGRs) have been used for many years in agriculture for such purposes as ripening of fruit, defoliating of fruit trees and cotton plants and boll opening of cotton plants pre-harvest. In the last decade growth regulators have predominately been used to reduce the clippings produced and subsequent mowing of turfgrass.

Research during the past few years has revealed other physiological benefits apart from growth regulation. These include increases in carbohydrates and chlorophyll, reduced water requirements, reduction in Poa annua infestations and greater stress tolerance.

A PGR is an organic compound, natural or synthetic, that when present or applied in small amounts, results in a change in plant growth and development. For turfgrasses these changes can include increased density, better colour, fewer clippings, reduced seedheads, deeper roots, increased food reserves, and better recuperative potential.

Table 1. Natural Plant Growth Regulators

Hormone

Function

Abscisic acid

Closes stomates, and inhibits germination

Auxins

Apical dominance, cell enlargement, root growth, inhibits axillary buds

Cytokinins

Cell division and enlargement, flowering senescence, inhibits auxin

Ethylene

Stress is stimulated, root growth

Gibberellins

Cell elongation, photoperiod response, and chilling tolerance

A PGR can be either be growth promoting or growth reducing. Gibberellic acids and cytokinins are examples of growth promoters, the former via cell elongation, the later via cell division (refer to table 1). These compounds are naturally produced by the plant and can also be applied as supplements.

The purpose of this paper is to focus on PGR’s that reduce the growth of turfgrass.

Classification and Mode of Action

An understanding of the mode of action is important not only to ascertain the application, but also the limits and extremities of a product.

Plant growth regulators are classified into groups according to their biological mode of action, that is, the activity inside the plant that causes the regulation of growth.

Traditionally there were three groups of classification: Type I or Cell Division Inhibitors; Type II or Gibberellin Biosynthesis Blockers (Murphy, Whitwell, McCarty, Yelverton, 2000); and herbicides that exhibit growth regulating effects. Watschke and DiPaola (1998) have recently proposed that PGRs be classified as A, B, C, D, and E to better differentiate their mode of action. This new system allows for the different modes of actions between gibberellic synthesis blockers along with an additional grouping for the ethylene production enhancement type products.

Class A: Late Gibberellic Acid Synthesis Blockers

            Example – Trinexapac-ethyl (Primo)

Class A PGR interferes with the biosynthesis of gibberellic acid (GA) thus suppressing growth but not inhibiting it. The interference takes place late in the biosynthetic pathway preventing the conversion of GA20 to GA1. It is GA1 that is the final biologically active form of GA in the plant and the primary form that effects cell elongation (Shepard, DiPaola, 2000). Class A PGRs are not known to stop the production of any other of the 100+ gibberellic acids.

Trinexapac-ethyl, the only compound in this class, is foliar absorbed, delivers a reduction in foliage growth (figure 1) but only partial suppression of seedheads.

Class B: Early Gibberellic Acid Synthesis Blocker’s

            Examples – Paclobutrazol (TGR), Flurprimidol

Class B PGRs also interfere with the biosynthesis of gibberellic acid, however this happens early in the biosynthetic pathway. This interference takes place before the production of the first gibberellic acid compound, GA12. GA12 is not only the first GA produced but is also a precursor for all other GAs. This in effect means that total GA production is stopped.

Both compounds in this class are root absorbed, which indicates that post application irrigation/rainfall is important to incorporate the product. Both compounds deliver a reduction in foliage growth, but only paclobutrazol offers at least partial seedhead suppression.

 Class C: Mitotic Inhibitors

             Examples – Maleic Hyrdazide, Mefluidide

These compounds inhibit the division of cells and differentiation in meristematic regions of the plant. The metabolism of cytokinins in the plant is slowed by PGRs in this class. They inhibit both vegetative growth and seedhead development (Murphy, Whitwell, McCarty, Yelverton, 2000).

The compounds in this group are foliar absorbed and offer suppression of foliage and seedheads. However seedhead suppression is dependent upon application before seedhead formation and emergence.

Class D: Herbicidal Mode

Examples – Sulfometuron-methyl, Chlorsulfuron, Glyphosate, Ethofumesate (Prograss)

Herbicide growth regulators are compounds possessing post-emergence herbicidal activity that have also been shown to inhibit the growth and development of turfgrasses at sub-lethal rates (Watschke, Prinster, Breuninger, 1992). It is the interruption of amino acid synthesis or fatty acid biosynthesis that causes the regulation.

These compounds are characterised as having an extremely narrow margin of safety and misapplications resulting in overdose can cause severe injury or death to grass stands (Kaufmann, 1986).

Class E: Ethylene Production Enhancement

Example – Ethephon

Class E PGR’s promote the production of ethylene, which is a regulatory hormone that restricts plant growth. Ethephon is hydrolysed in the plant to form ethylene. Ethylene inhibits elongation of stems, roots and leaves (Salisbury, Ross, 1978).

Ethephon is absorbed by the foliage with subsequent suppression of the foliage and partial suppression of seedheads.

Plant Growth Suppression

By inhibiting the formation of Gibberellic acid, products from Classes A and B reduce the growth rate in very much a different manner to the other classes. By actually governing the rate of plant growth they are considered to be true growth regulators and not growth inhibitors (Batten, 1983).

Class C PGRs typically offer quick suppression of growth after approximately 5-7 days. However the residual length of suppression (maximum 3-4 weeks) is generally shorter than Classes A and B (4-8 weeks). Suppression of growth for Class B PGRs starts around the 10-14 day mark while Class A PGRs take effect after 7 days. Ethephon (Class E) is better suited to cool season than warm season grasses and will provide suppression for a period of 4-6 weeks.

Class C PGRs inhibit seedhead suppression through the inhibition of the production of cytokinins and mitosis. Class A and B PGRs inhibit the seedhead stalk length but only partially suppress the formation of seedheads. Class D PGRs suppress seedhead formation while those in Class E only partially suppress seedhead formation.

Turfgrass Quality

Not only do the aforementioned products have different modes of actions, but they also differ substantially in turfgrass safety. Use of products from Class D are generally only recommended on low maintenance turfgrass due to the high potential of phytotoxicity and the poor quality of turfgrass that results.

Class C PGRs are used in low maintenance areas where the primary objective is the control of seedheads. These products are used primarily on low-medium maintenance turfgrass areas because phytotoxicity (yellowing) can be a problem (Murphy, Whitwell, McCarty, Yelverton, 2000).

Classes A and B are the two most common PGRs for turf in Australia due mainly to the length of regulation and turfgrass safety. Class A PGRs are noted as being the safest on all turfgrass species as they inhibit only GA1 (final biologically active GA), this means that all other GAs are produced (refer to Figure 1.). The early blockage of GA exhibited by the Class B products prevents the biosynthesis of further GAs which can lead to injury on environmentally stressed turfgrass.

Conclusion

The use of plant growth regulators is set to increase in ensuing years with pressure to reduce clipping wastes, fossil fuel use and carbon dioxide emissions. Improvements in turf quality and stress management make this an exciting area of development for the turf industry.

References:

Batten, S.M., 1983. Growth Regulators – new tools for the 80’s? USGA Green Section. Vol.21, No.3, May/June, p1-3

Kaufmann, J.E. 1986. Growth Regulators for Turf. Grounds Maintenance. 21(5):72

Murphy, T.R., Whitwell, T., McCarty, B., Yelverton, F.H., 2000. Turfgrass Plant Growth Regulators. Best Golf Course Management Practices. Prentice Hall. pp 552-561

Salisbury, F.B., Ross, C.W., 1978. Plant Physiology. Wadsworth Publishing Company. pp 264-267

Shepard, D., DiPaola, J.M., 2000. Regulate Growth and Improve Turf Quality. Golf Course Management. Volume 68, No.3, pp56-59

Watschke, T.L., DiPaola, J.M. 1998. Evaluation of V-10029 and Trinexapac-ethyl for Annual Bluegrass Seedhead Suppression and Growth Regulation of Five Cool Season Turfgrass Species. Weed Technology, Volume 12, Issue 3, pp 436-440

Watschke, T.L., Prinster, M.G., Breuninger, J.M.,1992. Plant Growth Regulators and Turfgrass Management. Agronomy No.32. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, Publishers Madison, Wisconsin USA. pp 557-588