Trinexapac-ethyl and Paclobutrazol Affect Kentucky Bluegrass Single-Leaf Carbon Exchange Rates and Plant Growth

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] are routinely used to suppress clipping production. Single-leaf turfgrass C exchange rates (CERs) in response to plant growth regulator (PGR) treatment have not been characterized. Individual Kentucky bluegrass (KBG, Poa pratensis L.) plants received label rates of TE or PAC and were placed in growth chambers at 23/18 or 30/25°C. Photosynthetic efficiency and CER measurements were recorded every 4 d for 44 d. Total root length (TRL), root surface area (SA), and average root diameter were measured at the end of the study. Reductions in CERs of TE- or PAC-treated plants were short lived with CERs suppressed 17 to 29% of control (POC) at 4 and 12 days after treatment (DAT), respectively. Plants treated at 23/18°C with PGRs typically had short-lived increases in CERs following CER suppression. A similar pattern of CER response to PGR treatment was observed at the 30/25°C temperature regime. Quantum efficiency was unaffected, but plants treated with PGRs had reduced root growth. PAC caused the greatest reduction in TRL and SA while increasing root diameter. A decline in TRL and SA in conjunction with increased tillering indicates that PGR reduced TRL and SA on a tiller basis. Changes in single-leaf CERs do not fully explain PGR-induced changes in plant growth.