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Users Drive Research Into New Growth Regulator Applications
April 1, 2002 By: Dennis Shepard TurfGrass TrendsNever apply a product to an entire golf course, lawn or athletic field without first trying it on a practice green, sideline or small side yard.
Plant growth regulators (PGRs) were initially developed to slow turf growth and suppress seedheads. Their use on high-quality turf was limited because of potential phytotoxicity, unpredictable turf response and differences in activity on cool- and warm-season turf.
Trinexapac-ethyl (Primo MAXX) was the first PGR to provide growth suppression and improved turf quality in all the major turfgrasses. Research projects, combined with suggestions from Primo MAXX users, helped develop uses that had not been successful with other PGRs. These include use during overseeding, tank mixing with fungicides to enhance activity, pre-stress turf conditioning, use on greens, sod production and others.
PGR mode of action
PGR products work in different ways to affect turf growth and development. They either slow cell division in meristematic areas, inhibit cell elongation by slowing the production of gibberellic acid, or act by enhancing ethylene gas release, which affects flowering.Mefluidide (Embark, Embark Lite), developed in the 1970s, is absorbed through the leaves, slows cell division. Paclobutrazol (Trimmit 2SC, Turf Enhancer 2SC) and flurprimidol (Cutless), developed in the 1980s, are root-absorbed and slow cell elongation by stopping formation of the more than 120 forms of gibberellic acid early in the biosynthetic pathway.
 Figure 1: Gibberellic Acid Biosythesis |
Trinexapac-ethyl (TE) has foliar uptake and slows cell elongation by stopping the conversion of one gibberellic acid (GA20) to another [GA1], which is the final step in gibberellic acid production. This leads to slower plant growth.
Ethephon (Proxy) was registered for use on turf in 1998. It affects turfgrass growth and development by enhancing the release of ethylene gas.
PGR development and use
In high-quality, cool-season turf areas, mefluidide is primarily used to reduce Poa annua seedheads. When applied at the correct time, it does an excellent job. Superintendents should be aware that turf plants use much of their stored energy during seed production and flowering, and there is potential for phytotoxicity from any PGR during this period because of the weakened state of the turfgrass plant.Paclobutrazol is primarily used on cool-season turfgrasses to slow growth and reduce Poa annua in bentgrass golf courses. Application rates range from 6.4 ounces to 24 ounces of product per acre (greens vs. fairway use). It will suppress turfgrass growth for a six- to eight-week period.
Poa annua appears to be more sensitive to paclobutrazol than creeping bentgrass. Superintendents should evaluate the amount of Poa annua they have before using paclobutrazol, and determine if they want to keep it or reduce it. The turf response depends on the regulator rate. Higher rates can be used for two to three applications in the spring and fall when the risk of phytotoxicity to the creeping bentgrass is lower. As temperatures grow warmer, superintendents should reduce the rate of paclobutrazol, not use a PGR or switch to TE.
Ethephon was labeled for use on turfgrass in 1998, and research has been conducted to determine rates and turf response. It is primarily used on cool-season species. A key area of research has been on suppressing Poa annua seedheads. Research in California has shown a single application of Proxy (21.7 percent ethephon) at rates of 5 or 10 ounces/1,000 square feet, reduced Poa annua seedheads 80 percent to 90 percent.
Proxy has little effect on seedheads that are present at the time of application (Gelernter). Research is continuing in other areas of the country.
Trinexapac-ethyl (TE) has been the most widely researched PGR to date, with hundreds of research projects at land-grant universities in the United States. Growth reduction and improved quality of warm- and cool-season turf, successful use on greens and fairways, a predictable response and lack of phytotoxicity aided acceptance by turf managers and inspired new research.
Growth management with trinexapac-ethyl
Initial research into TE in the late 1980s and early 1990s investigated rates and response on highway roadside turf, home and commercial lawns, and golf course fairways. It was a new class of chemistry (cyclohexadione) that exhibited different growth responses from other PGRs.It was challenging to determine where it could be the most benefit to turf management. Rates were determined for 50 percent growth suppression for species maintained at fairway heights. Research has also been conducted on the effect of multiple TE applications to turfgrass growth and quality (Fagerness, Lickfeldt).
Along with suppressing turf vertical growth, TE affects a number of other turf morphological characteristics. Turf density normally increases with repeat applications due to enhanced lateral growth of stolons and rhizomes, and increased tillering.
Turf treated with TE normally turns darker green. Research has determined that chlorophyll content increases, and there is a higher concentration in the smaller, more compact leaves (Ervin, Heckman). Scalping is reduced when mowings are missed due to rainfall or other problems.
Superintendents with kikuyugrass fairways have found TE dramatically reduces scalping throughout the growing season (Gelernter).
Mowing equipment performance and lifespan may be enhanced because of less mowing and less force required to mow the turf. Catching and dragging of clippings can often be eliminated, which saves time and labor. In addition, tasks such as trimming and edging are reduced.
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