Physiology and Growth Responses of Cool Season Turfgrasses Treated With Trinexapac -Ethyl or Paclobutrazol

Abstract

Plant growth regulators (PGRs) such as trinexapac-ethyl (TE) and paclobutrazol (PAC) are often routinely applied to highly managed turfgrasses to suppress clipping production, increase sward density, and enhance color. However, reports of variability in the seasonal responses of PGRs indicate further studies are needed to relate environmental factors to PGR efficacy. A series of studies examined TE and PAC efficacy, defined as the duration and intensity of suppression and the effects of changes in PGR efficacy, on turf growth. In a field study, TE and PAC were applied to a mature Kentucky bluegrass (Poa pratensis L.) sward at 0.5x, 1x, and 1.5x, where x equals the label rate, as single applications in spring, summer, or fall and clippings collected for 10 weeks after application. Carbon exchange rates (CERs) and leaf area were measured weekly and PGR metabolism analyzed using HPLC-UV. Both TE and PAC had greater efficacy in spring and fall with cooler temperatures compared to summer. Reduced application rates were less effective in suppressing clippings compared to the label rate; higher than label rates did not increase PGR efficacy. Warmer temperatures accelerated TE and PAC metabolism compared to cooler conditions with PAC being the more persistent compound. Greater spring TE and PAC efficacy resulted in greater fluctuations in leaf area and CERs on a ground area basis compared to summer. Only PAC affected CERs on a leaf area basis. Changes in leaf area and CERs were short-lived. A pattern of increased shoot growth over root growth was confirmed in a hydroponic study evaluating TE effects on KBG root growth. Plant growth regulators affect photoassimilate partitioning in turf plants. Decreased PGR metabolism and changes in seasonal growth patterns appear to be the leading factors in determining PGR efficacy.