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TURFGRASS SCIENCE

Plant Growth Regulators Alter Kentucky Bluegrass Canopy Leaf Area and Carbon Exchange

^a 226 J.C. Miller Hall, Louisiana State Univ., Baton Rouge, LA 70803
^b Dep. of Natural Resources and Environmental Sciences, Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801

^* Corresponding author (bbranham{at}uiuc.edu).

Turf managers apply plant growth regulators (PGRs) throughout the growing season to reduce clipping production, provide a uniform canopy, and increase color. Reduced efficacy of trinexapac-ethyl (TE) [4-(cyclopropyl--hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid ethyl ester] and paclobutrazol (PAC) [(2RS, 3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl) pentan-3-ol] has been reported during warmer summer months. Plant growth regulator dissipation and Kentucky bluegrass (KBG; Poa pratensis L.) clipping production, canopy leaf area, and carbon exchange rates (CERs) were measured following single TE or PAC applications applied at three rates and three application timings. Warmer summer conditions reduced the intensity and duration of TE and PAC growth suppression compared to spring applications. Plant growth regulator applications above the manufacturers label rate provided no additional suppression. Residues of PGRs were analyzed by HPLC-UV. Warmer temperatures increased TE and PAC uptake, but accelerated the rate of PGR dissipation. Carbon exchange rates were measured weekly on ground (CERG) and leaf (CERL) area bases. Treatment with PGRs resulted in decreased leaf area early during inhibition followed by increased rate of leaf area production. Changes in canopy leaf area were greatest in spring and PAC caused the greatest change in leaf area. Changes in CERG reflected changes in leaf area. Paclobutrazol generally increased CERL during periods of clipping suppression, while TE had no effect on CERL.

Abbreviations: CER, carbon exchange rate • CERG, carbon exchange rate per ground area • CERL, carbon exchange rate per leaf area • DAT, days after treatment • KBG, Kentucky bluegrass • LAI, leaf area index • PAC, paclobutrazol • PIGE, post-inhibition growth enhancement • PGR, plant growth regulator • POC, percent of control • TE, trinexapac-ethyl • TA, trinexapac acid • WAT, weeks after treatment.

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