Plant growth regulators are becoming important tools in the management of turfgrass. PGRs are used in all turf venues, from the high quality grass of golf greens to the very lowest quality areas like railroad and power line right-of-ways. A PGR is an organic compound, natural or synthetic, that alters the growth and development of the plant (DiPaola 1988).

In the high end turf industry PGRs are growing in popularity mainly because of new chemicals that are more predicable and cause less injury to the plant. The reduction of mowing costs is also a supporting factor in the increasing popularity of PGR's. A well planned program can save on labor costs, machinery costs, fuel costs, and clipping disposal costs (Spokas 1991). Growth regulators have been in use for many years in the turf industry on lower quality areas. Mainly because of the fear that the new chemicals would injure and lower the quality of turf, in situations where turf quality is of utmost importance. PGRs have also been used in the golf industry primarily on areas that are not under high management, to reduce the amount of labor associated with those areas (Spokas 1991). PGRs have a long history of being used on areas that are dangerous or difficult to mow. But now the newer chemistry has significantly reduced the phytotoxicity of the products (Branham 1994). This has enabled PGRs to be used on high quality turf areas such as fairways, tees and even greens. The latest product that has allowed PGR use on high quality areas is (Trinexapac-ethyl) Primo ® from Ciba-Geigy Corporation. This product can be used on all turf areas with limited amounts of injury to even the finest grass (Branham 1994). Trinexapac-ethyl retards the growth of the grass plant by slowing cell elongation rather than altering the growth.

Since a plant growth regulator is any compound that has the ability to alter the amount of dry matter production and the development of the plant (DiPaola 1988), PGRs can be either growth promoting or growth retarding. Growth promoters are usually plant hormones. The two most dominant hormones involved in plant growth are gibberellic acid (GA) and cytokinins. Giberberellins are primarily responsible for cell elongation in the plant (Taiz 1991). Cytokinins are active in the plant and control cell division (Taiz 1991). There are two major actions of plant growth regulators. They can either increase plant growth or retard the growth which actually decreases the amount of overall vegetative material produced.

Growth retardants are divided into two classes, Type I and Type II. The metabolism of cytokinin is slowed mainly by Type I PGRs. Because of the cytokinin inhibition Type I PGRs are excellent suppressors of seedhead production (Elkins 1977). When using Type I PGRs seedhead reduction is usually the preferred effect. The application must be made before the start of seedhead formation (Branham 1994). Amidoochlor a Type I PGR also has GA inhibiting action in addition to cytokinin inhibition (Johnson 1993). Type II PGRs inhibit cell elongation. The overall physiology of the plant is not affected. All of the reproductive and vegetative structures form and are productive (Johnson 1994). It has been shown in early studies of gibbberillins that GA applied to dwarf plants would grow to the height of the normal plant and past. Also, it was shown that GA applied to the rosette form of a plant was induced to grow to normal size and produce reproductive structures. Under normal conditions the rosette form of the plant would not have grown tall or possessed reproductive structures (Taiz 1991). Therefore, the use of Type II PGRs especially Trinexapac-ethyl to suppress seedhead production is ineffective (Johnson 1994).

PGRs applied to turfgrass initiate a number of effects. There is a small amount of injury, even under ideal conditions, that occurs about a week after application (Branham 1994). This is shown by discoloration of the grass and lasts for one to two weeks. After the period of discoloration the turf takes on a deep green color this is associated with growth regulation (Branham 1994). The increased color quality will last until the effects of the PGR wear off. After the period of reduced growth there may be a period of increased shoot elongation and color quality (Branham 1994). Applications of a PGR should be made under ideal turf conditions. They should never be applied when the turf is under high stress or periods of stress are anticipated. PGRs should only be applied to healthy turf with a good fertility program in place. It has been shown that with a mefluidide application to Kentucky Bluegrass during seedhead development there was a substantial reduction in both the leaf color and the photosynthetic rate of the leaf (Spokas 1991). Many times turf that has had PGR applications also has higher quality ratings. Turf stands with PGR applications are composed of mainly mature and senescing plants (Watschke 1985). PGR's affect the composition and development of the turf system. PGRs have various effects on the turf system including; rhizome growth, thinning, reduced tillering, and increased turf density (Christians 1984, Field 1984, Dernoden 1984). These effects cause changes to the turf canopy and structure which disturbs the microenvironment of the leaves of the plant (Spokas 1991). Whether the change in microenvironment is induced by various chemical applications, or in the proportion of mature plants; alterations in these chacteristics affect the photosynthetic rates of individual plants (Nelson 1977, Morgan 1983). "Increased mean leaf age and greater canopy development have been shown to reduce individual leaf photosynthetic rate (Nelson 1977, Morgan 1983)." In the study by Spokas and Cooper it was found that the photosynthetic rate of mefluidide treated turf was lower than the untreated mowed turf (Spokas 1983).

Gibberellic acid is the plant hormone that is responsible for plant tallness. Gregor Mendel's tall and dwarf alleles in peas are expressed in genes that control gibberellin metabolism (Taiz 1991). Gibberellins are terpenoids consisting of 20 carbons, which is made from four isoprenoid units. Each isoprenoid unit has five carbon groups that are joined head to tail (Taiz 1991). GA1 is the active hormone involved with cell elongation. All GA s are formed from a long pathway beginning at photosynthesis. Carbohydrates from photosynthesis fuel glycolysis that produces phosphoenol pyruvic acid and acetyl CoA. Mevalonic acid is formed from acetyl CoA. Mevalonic acid is the starting compound for all terpenoid biosynthesis. Mevalonic acid, six carbon compound, is decarboxylated to form the first isoprenoid compound in the pathway (Taiz 1991). The isoprene units are added together, resulting in geranylgeranyl pyrophosphate that is a compound containing twenty carbon atoms. Geranylgeranyl pyrophosphate is then cyclized or made into ring structures by closing the gaps between the isoprene units. The compound resulting from these closures is called ent-karurene (Taiz 1991). One of the methyl groups on ent-karurene is oxidized to carboxylic acid. Then one of the six C rings contracts to become a five C ring that gives GA12-aldehyde. This compound is the first gibberellin formed in all plants and is the precursor of all the other gibberillins (Sponsel 1987). Plant growth regulators work by inhibiting GA production. Paclobutrazol and Flurprimidol are both Type II PGRs and inhibit GA production early in the GA synthesis pathway. They are effective in the step between ent-kaurene and ent-kaurenol, stopping GA production and therefore, cell elongation. Trinexapac-ethyl inhibits GA synthesis late in the pathway. GAs are formed but are not active (Taiz 1991). The inhibition occurs between GA20 and GA1, resulting in reduced cell elongation.

Plant growth regulators are being used by turf professionals in many different types of turf areas. This has been made possible by increasing availability and reliability of PGRs. The older Type I PGRs have been used in the turf industry for many years. However, the amount injury induced by the Type I PGR's was unacceptable in high management areas. Through research many outstanding improvements have been made in the chemistry and the use of PGRs. As shown, these are powerful tools used to reduce many factors in the budget of the turfgrass management industry. This is especially important now because of the many environmental concerns. The use of PGRs reduces fuel consumption, and clipping disposal. Both of which reduce the amount of harmful gasses released into the atmosphere. PGRs have not been widely used on fine turf areas in any of the high input turfgrass industries, because of the unpredictable effects initiated by the chemical. Also many potential users may have steered away from using PGRs because of the initial high cost associated with the application.