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Physiology and Biochemistry

Effect of Chlorophenoxy Herbicides on Free Amino Acids in Sequentially Senescent Leaves of Poa pratensis and on Pathogenesis by Bipolaris sorokiniana. James P. Madsen, Postdoctoral research fellow, Department of Agronomy, University of Kentucky, Lexington 40546-0091; Clinton F. Hodges, professor of horticulture and of plant pathology, Department of Horticulture, Iowa State University, Ames 50011. Phytopathology 74:1407-1411. Accepted for publication 19 June 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1407.

The herbicides 2-(2-methyl-4-chlorophenoxy) propionic acid (MCPP) and 2-(2,4,5-trichlorophenoxy) propionic acid (2,4,5-TP) were evaluated for effect on the free amino acid content of four sequentially aged leaves of herbicide-tolerant Poa pratensis and on subsequent leaf spot severity after infection by Bipolaris sorokiniana. The content of free amino acids in uninoculated leaves of herbicide-untreated control plants generally declined from the youngest to oldest leaf. The herbicides had no influence on total amino acids in leaves of any age. Most leaves of plants treated with MCPP contained less Pro, His, Lys, Arg, Ala, and Phe than did the control. Most leaves of plants treated with 2,4,5-TP contained less Pro and Lys than did the control. Uninoculated MCPP-treated plants contained more Ser and Glu in older leaves only, whereas uninoculated 2,4,5-TP-treated plants contained more Arg, Thr, Ala, and Val in younger leaves only. Infected leaves of plants treated with either herbicide generally were more severely diseased than leaves of herbicide-untreated control plants, but only MCPP-treated plants had increased leaf spot on the youngest leaf. The results suggest that changes in free amino acid levels in leaves after treatment of P. pratensis with chlorophenoxy herbicides may be a component of physiological changes that are similar to changes during senescence. Changes in amino acid content induced by chlorophenoxy herbicides may promote leaf senescence in P. pratensis and the subsequent enhancement of leaf spot. However, changes in amino acid content independent of other metabolic changes occurring during senescence probably have limited direct influence on leaf spot severity.