Plant Growth Regulator Effects on Foliar Discoloration, Pigment Content, and Photosynthetic Rate of Kentucky Bluegrass

If plant growth regulating compounds (PGRs) are to be accepted for use on moderate‐ to high‐quality turf, shoot discoloration must be reduced to an acceptable level. The first step in minimizing foliar discoloration is to attempt to understand how PGRs induce this problem. Field and greenhouse studies were undertaken to trace the developmental sequence of foliar discoloration in PGR‐treated Kentucky bluegrass ( Poa pratensis L.) and to determine whether a relationship exists between PGR induced shoot discoloration and either photosynthetic rate or pigment content. The growth regulators amidochlor [ N ‐({acetylamino}methyl)‐2‐chloro‐ N ‐(2,6 diethylphenyl)acetamide] and mefluidide [ N ‐(2,4 dimethyl‐5‐{[(trifluoromethyl)sulfonyl]amino}phenyl)acetamide] were applied at 2.8 and 0.56 kg ha ‐1 , respectively, to mature Kentucky bluegrass growing on a Hadley silt loam soil (coarse‐silt}, mixed, nonacid, mesic Typic Udifluvent). Turf quality, pigment content, and apparent photosynthetic rate (APR) were determined for each treatment. Application of amidochlor caused discoloration of the turf following one of six applications, resulting in an average decline of about 1 quality point. A decrease in turf quality resulted from all mefluidide applications. Quality rating declined from 8 to 2.5 following mefluidide application at 100% turf greenup. Application of mefluidide decreases canopy APR 0.07 mg CO 2 mg CO 2 m ‐2 s ‐1 compared with nontreated turf and 0.08 mg CO 2 m ‐2 s ‐1 compared with amidochlor‐treated turf. The lag between onset of shoot discoloration and decreased APR indicates a lack of a cause and effect relationship. Thus, it does not seem likely that either reduced pigment content or APR was responsible for the foliar discoloration induced by amidochlor or mefluidide in these studies.