69.8 W m Ϫ2 (10 Ϫ5 kcal cm Ϫ2 leaf min Ϫ1 ). Other PGRs have also been shown to inhibit respiration. 2,4-D inhib-The plant growth regulators (PGRs) 2,4-D [(2,4-dichlorophenoxy) ited respiration of oat (Avena sativa L.) coleoptiles and acetic acid] and malic hydrazide (1,2-dyhydro-3,6-pyridazinedione) have been shown to reduce plant respiration. The effect of other PGRs pea (Pisum sativum L.) stems at concentrations Ͼ450 such as trinexapac-ethyl [(4-cyclopropyl-␣-hydroxy-methylene)-3,5-M (Kelly and Avery, 1949). French and Beevers (1953) dioxocyclohexanecarboxylic acid methyl ester] on respiration is unshowed a 40% decrease in respiration of corn (Zea mays known. Experiments were conducted to evaluate the effects of tri-L.) mitochondria by 2,4-D at 400 M concentrations. nexapac-ethyl and 2-oxoglutarate on the respiration of mitochondria Switzer (1957) observed nearly a complete inhibition of isolated from wheat (Triticum aestivum 'Arapahoe') seedlings. respiration in soybean [Glycine max (L.) Merr.] mito-Trinexapac-ethyl applied at increasing concentrations inhibited nicochondria by 2,4-D at concentrations of 200 M. Another tinamide adenine dinucleotide (NADH)-dependent O uptake, while PGR, malic hydrazide, has also been shown to inhibit 2-oxoglutarate had no significant effect. This is different than other respiration of isolated soybean mitochondria by 25% at reports that trinexapac-ethyl may compete with 2-oxoglutarate for 20 mM levels (Switzer, 1957). In addition, dinitroaniline binding sites. Different regions of the mitochondria were tested to determine the site of inhibition caused by trinexapac-ethyl. Nicotin-herbicides also have been shown to inhibit mitochonamide adenine dinucleotide dehydrogenase activity, duroquinol-depen-drial O uptake. For example, Trifluralin [2,6-dinitro-N, dent O uptake, and cytochrome bc 1 activity were all reduced by N-dipropyl-4-(trifluoromethyl)benzenamine] inhibited ≈30% in the presence of 10 mM trinexapac-ethyl. Succinate-dependent respiration of mitochondria isolated from sorghum [Sor-O uptake, alternative oxidase, and cytochrome oxidase were not reghum bicolor (L.) Moench], corn, and soybean by Ͼ23% duced by any trinexapac-ethyl concentration. This revealed possible at 400 M concentrations (Negi et al., 1967). Simiinterference of trinexapac-ethyl with ubiquinone binding sites. The lar results were found upon exposing isolated mung reduced form of trinexapac-ethyl was observed to inhibit the electron bean (Vigna radiata L.) mitochondria to other substitransport chain greater than the oxidized form. Reduction in respiratuted 2,6-dinitroaniline herbicides (Moreland et al., tion from trinexapac-ethyl may result in greater stress tolerance in 1972; Moreland and Huber, 1979).treated plants. Hanson (1977) procedure. Seventy-five grams of fresh shoots and Horticulture, Univ. of Nebraska, Lincoln, NE 68583-0724; T.E. were ground using a mortar and pestle and 250 mL of grinding Elthon,
Heat accumulation during storage of sod may reach lethal temperatures within 4 days, decreasing sod quality. Treatment with trinexapac-ethyl reduces heat accumulation during sod storage. However, heat tolerance of grasses treated with trinexapacethyl has not been documented. Our objectives were to: 1) determine the lethal temperatures for Kentucky bluegrass (Poa pratensis L.); and 2) identify the effect of a single application of trinexapac-ethyl on heat tolerance. Experimental design was a randomized complete block with three replications and a two (trinexapac-ethyl vs. control) × × × × × two (cultivars) factorial arrangement of treatments. Ten days after chemical treatment, Kentucky bluegrass sprigs were exposed to heat stress for 4 days in a temperature gradient block under low vapor pressure deficit. Treatment with trinexapac-ethyl at 0.23 kg·ha -1 reduced heat tolerance. Temperature needed to kill 50% of the population was 35.5 °C for treated vs. 36.1 °C for nontreated grass. Trinexapac-ethyl is in the same chemical family as the cyclohexanedione herbicides that interfere with lipid syntheses in grasses. This may be a reason for the slight decrease in heat tolerance. The practical value of trinexapac-ethyl treatment in reducing heat accumulation during storage of sod may be partially negated by a decrease in heat tolerance. Chemical name used: [(4cyclopropyl-α α α α α-hydroxy-methylene)-3,5-dioxocyclohexanecarboxylic acid methyl ester] (trinexapac-ethyl).
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