Growth regulators

In the last 30 years, plant growth regulator (PGR) use has progressed from a marginal or niche application to an integral part of golf course management programmes. In the early 1980s mefluidide (Embark®) was the first plant growth regulator widely used on fine quality turf. Mefluidide was, and still is, primarily used on Poa annua or Poa annua containing fairways for seedhead control in the cool temperate regions. An application timed properly prior to seedhead emergence could provide over 90% seedhead control. It suppresses growth and seedhead formation through inhibition of cell division (mitosis) in the meristematic region of the plant. Due to its mode of action, mefluidide was characterised as a Type I compound. Picture Left (fig 1)

In the mid 1980s two additional PGRs - flurprimidol (Cutless®) and paclobutrazol (Trimmit®) - became available. These were different from mefluidide in their mode of action (Table 2). The newer PGRs, including flurprimidol, paclobutrazol, trinexapac-ethyl and ethephon (an older chemistry but relatively new to turf), work to inhibit or enhance plant hormones (Table 1).

Gibberellic acid (GA) inhibitors like flurprimidol and paclobutrazol are not very effective for seedhead suppression. Due to the lack of seedhead control and root absorption these PGRs were classified as Type II compounds. Initially, both flurprimidol and paclobutrazol were used in creeping bentgrass/Poa annua fairway conversion. The GA inhibitors have a greater growth inhibiting effect on Poa annua than creeping bentgrass. Once applied Poa annua growth was suppressed to a greater extent than the creeping bentgrass. This suppression difference allowed the creeping bentgrass to literally grow "over the top" of the Poa annua.

In the early 1990s the release of trinexapac-ethyl (Primo®) ushered in a new era of PGR use. Trinexapac-ethyl was not a true Type II PGR. It was foliar absorbed and was a late GA inhibitor. Reclassification of the plant growth regulators into Type A through E groups has been proposed. Trinexapac-ethyl catalyzed the use of PGRs from a couple of niche applications a year to multiple applications throughout the growing season. Its use has, in many ways, changed how turfgrass is maintained in the United States. Although the use of trinexapac-ethyl reduced the amount of growth, and thus clippings, combined with mowing, the overall quality of the turf improves (6). Repeated applications of trinexapac-ethyl was found to enhance the summer stress tolerance of the turf.

In the temperate region of the United States we found from surveying 90 golf course superintendents, 84% of the respondents used PGRs for greens management. In the United Kingdom a 4-week application interval is recommended (as with any chemical compound always follow the label directions). In our survey we found weekly (7-day) or biweekly (14-day) applications of PGRs at reduced rates for greens management accounted for 85% of the applications. Given the frequent applications of trinexapac-ethyl on golf greens, we wanted to look at the continual use of trinexapac-ethyl on divot and aerification (coring) hole recovery. Picture right (fig 2)

Divot Recovery

The development of high shoot density creeping bentgrass cultivars and the subsequent use on fairways has raised concerns on how these new cultivars will respond to trinexapac-ethyl treated turf and subsequent divot recovery. The high shoot density creeping bentgrass cultivars tend to produce dense upright shoot growth with little horizontal stem (stolon) growth. We conducted a study on an L93 creeping bentgrass turf maintained at 1.2 cm height of cut during the summer of 2001. Trinexapac-ethyl (Primo MAXX®) was applied at 8ml per 100m2.

Three days after application six divots were made by hitting golf balls in each plot. Divot size, as measured by the length and width, was recorded weekly. Regarding divot recovery as measured by the size of the divot over time, we did not find any significant difference between trinexapac-ethyl treated plots and the untreated. Given these results, the inference may be made that divot recovery on creeping bentgrass maintained at putting green height would not be impacted either positively or negatively by trinexapac-ethyl treatments.

Aerification (Coring) Hole Recovery: In a series of studies looking at both trinexapac-ethyl and paclobutrazol we found no difference in core hole recovery with trinexapac-ethyl applied at 4ml per 100m2 on a 7-day schedule, nor with paclobutrazol applied at 0.14kg per ha and 0.07 kg per ha1 on a 14 day schedule. As a note of caution both studies were conducted from late summer through early fall.

Physiological impacts: Applications of trinexapac-ethyl have also shown that there is a positive impact on colour of turfgrass canopies, and this has occurred across many studies. The dark greening of the grass surface that occurs from repeated applications has found favour with players and golfers alike. The other major angle of research in the use of growth regulators is improving tolerance of shaded turfgrasses. Picture left (fig 3)

Turfgrass in shaded conditions elongates and loses density which leads to loss of wear tolerance. It is a problem for stadium and golf course managers alike. There are some succinct differences though; neutrally shaded sites such as stadiums have loss of light intensity compared to golf courses shaded by mature tree lines which impact light quality. The change in the light has different impacts on plants both considered negative. The loss of light quantity however is of major concern and it has been shown that repeated applications of trinexapac-ethyl have delayed the loss of turfgrass cover in shaded conditions. This has been successful in as high as 80% reduced light on certain species.

Research Implications: From our studies it appears that repeated use of trinexapac-ethyl has no detrimental affects on turfgrass recovery from divoting or coring. The use of the growth regulator has also proved beneficial in managing turf in shaded conditions and it is a tool that is open to all turfgrass managers both in the United States and the United Kingdom.

Table 1: Plant hormones and their function (3).

Group	Chemical Example	Syntheses location	Description
Auxin	Indole-3-acetic acid (IAA)	Leaf primoridia & young leaves	Promote growth through cell division and elongation. Important in inhibiting leaf senescence and phloem and xylem differentiation.
Gibberellins	Gibberellic Acid, GA3, GA1	Young tissue and developing seeds	Promotes stem elongation through cell division and elongation. These chemicals are classified according to structure (ex. GA3) rather than biological activity.
Cytokinin	Zeaton	Root tips & developing seeds	Promote cell division and delay leaf senescence. Used as a growth promoter in tissue culture.
Ethylene	Gas	Leaves, shoots	Ethylene is a gas that forms in tissue that is undergoing stress. It is important in the fruit ripening process. In turf, with enhanced release decreased cell size occurs.
Abscisic Acid	Single compound	Mature leaves under stress	Generally thought of as an inhibitor of growth. It closes stomata under moisture stress and counteracts some of the effects of auxins and gibberellins.

Table 2. Description of the major Plant Growth Regulators (PGR) used on high quality cool season turf. (note: The only available PGR in the UK is Primo Maxx®.)

Common Name	Trade Name (U.S.)	Absorption	Mode of Action	Comments
Mefluidide	Embark	Foliar	Inhibits cell division (mitosis) and differentiation in meristematic tissue	Primary used to inhibit Poa annua seedhead formation. Application made in spring prior to seedhead inflorescence emergence.
Ethephon	Proxy	Foliar	Enhances the release of ethylene into the plant regulating growth through its influence on cell size.	Used alone or in combination with trinexapac-ethyl for Poa annua seedhead control.
Paclobutrazol	Trimmit	Root	Early gibberellic acid (GA) inhibitor	The GA inhibitors were and still are used in the gradual reduction of Poa annua in creeping bentgrass turf (golf course fairways). Due to the greater inhibitory affect on Poa annua.
Flurprimidol	Cutless	Root	Early gibberellic acid (GA) inhibitor	Originally used in the same fashion as paclobutrazol. With both paclobutrazol and flurprimidol conversion to more bentgrass in fairways is accomplished with spring and fall applications at the higher end of the recommended rate.
Trinexapac-ethyl	Primo MAXX	Foliar	Late gibberellic acid (GA) inhibitor	Most widely used plant growth regulator on fine high quality turf. Implicated in enhancing summer stress tolerance of turf, reduced disease severity (in some cases) and improved turf quality.

Figure 1 (main picture). The majority of plant growth regulator studies have been conducted on USGA constructed greens established to 'Penncross' creeping bentgrass located at the Ohio Turfgrass Foundation Research and Educational Facility, Columbus, Ohio. The greens were mowed at a height of 3.1 mm.

Figure 2 (left). The divot recovery study.

Figure 3 (below). Shade cloth was used to enclose creeping bentgrass turf to look at the impact of neutral shade on turfgrass growth with plant growth regulators.

References

1. Bell, G.E., Danneberger, T.K. and M.J. McMahon. 2000a. Spectral irradiance available for turfgrass growth in sun and shade. Crop Science 40:189-195.

2. Danneberger, Karl. 2005. PGR survey: Greens. Website: http://buckeyeturf.osu.edu/component/option,com_turfnotes/Itemid,84/noteid,750

3. Danneberger, T.K. and J.R. Street. 1990. Turfgrass growth substances. Golf Course Management 58(4):80-88.

4. Ervin, E.H. and Z. Zhang. 2007. Influence of sequential trinexapac-ethyl applications on cytokinin content in creeping bentgrass, Kentucky bluegrass, and Hybrid bermudagrass. Crop Science 47:2145-2151.

5. Goss, R.M., J.H. Baird, S.L. Kelm, and R.N. Calhoun. 2002. Trinexapac-ethyl and nitrogen effects on creeping bentgrass grown under reduced light conditions. Crop Science. 42:472-479.

6. Howieson, M. J.; Christians, N. E. 2001. Golf Course Management. June. 69(6): p. 65-70.

7. Lickfeldt, D.W., D.S. Gardner, B.E. Branham and T.B. Voigt. 2001. Implications of repeated trinexapac ethyl applications on kentucky bluegrass. Agron J. 93:1164-1168.

8. McCann, S.E. and B. Huang. 2007. Effects of trinexapac-ethyl foliar application on creeping bentgrass responses to combined drought and heat stress. Crop Science 47: 2121-2128.

9. McCullough, P.E., H. Liub, L.B. McCarty, T. Whitwell, and J.E. Toler. 2006 Bermudagrass Putting Green Growth, Color, and Nutrient Partitioning Influenced by Nitrogen and Trinexapac-Ethyl. Crop Sci 46:1515-1525.

10. Steinke, K. and J. Stier, 2003. Nitrogen Selection and Growth Regulator Applications for Improving Shaded Turf Performance. Crop Science. 43:1399-1406.

11. Watschke, T.L. and J.M. DiPaola. 1995. Plant growth regulators. Golf Course Management 63(3):59-62.

12. Watschke, T.L., M.G. Prinster, and J.M. Breuninger. 1992. Plant growth regulators for turfgrass management. Turfgrass-Agronomy Monograph. no. 32 (ASA-CSSA-SSSA).

13. Wherley, B.G. and D.S. Gardner. 2005. Growth Response of three turfgrass species to nitrogen and Trinexapac ethyl in Shade. HortScience 40(6):1911-1915.

Growth regulators :-Their impact on golf course management By Professor Kark Danneberger Ph. D, Ohio State University