Herbicide Chlorsulfuron Decreases Assimilate Transport Out of Treated Leaves of Field Pennycress ( Thiaspi arvense L . ) Seedlings 1 Hank

Treatment of field pennycress (Thiaspi arvense L.) leaves with the herbicide chlorsulfuron resulted in a decrease in the export of assimilate. Twelve hours after a spot application of 1 microgram, assimilate translocation was 70% of that in control leaves. In excised leaves treated with chlorsulfuron the total amounts of sugars and free amino acids were 150 and 170%, respectively, of the amounts in control leaves, 30 hours after herbicide treatment. The amount of sucrose was 247% of that in control leaves. The increase in the concentration of sucrose in the chlorsulfurontreated leaves, combined with the absence of an effect of chlorsulfuron on carbon dioxide fixation, suggests that the decrease in assimilate transport is not due to an effect on the synthesis of assimilates, but rather to an effect on their movement out of the leaves. Supplying branched-chain amino acids to the field pennycress seedlings prior to the application of chlorsulfuron prevented the occurrence of the effects described. dose was translocated out of treated leaves of several species in a 24-h period. Export of less than 5% of the absorbed chlorsulfuron in 24 h has been reported for Canada thistle and perennial sow thistle (Sonchus arvensis) (8) and for Tartary buckwheat (Fagopyrum tataricum [L.] Gaertn.) (2). The limited phloem mobility of chlorsulfuron cannot be, explained in terms of the ability of plant tissue to accumulate the herbicide (6, 7) but, instead, is attributed to an effect on assimilate translocation. The objective of the research described in this paper was to understand the effect of chlorsulfuron on the translocation of assimilates out of treated leaf tissue of field pennycress (Thlaspi arvense L.) seedlings. In addition, the rates of uptake and of translocation of the herbicide and the extent of its metabolism were determined. MATERIALS AND METHODS The herbicidal action of a chemical arises from its ability to interact with a plant in such a manner as to inhibit or disturb its growth. This interaction usually involves the inhibition of a process essential to growth. The sulfonylurea herbicide chlorsulfuron4 inhibits the growth of susceptible plants by inhibiting the enzyme ALS, an enzyme common to the biosynthesis of the branched-chain amino acids L-valine, L-leucine, L-isoleucine (4). In order for a postemergence herbicide to provide successful control of a deep-rooted perennial weed, for example, Canada thistle (Cirsium arvense [L.] Scop.), it must be translocated from the foliage to the roots following spray application. Sweetser et al. (23) have reported that 2 to 28% ofthe applied 'Research funded by the Natural Sciences and Engineering Research Council ofCanada. H. D. B. was the recipient ofa Dissertation Fellowship of the University of Alberta. 2 Present address: The King's College, 10766-97 Street, Edmonton, Alberta, Canada. T5H 2M1. 'Department of Crop Science and Plant Ecology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N OWO. 4 Abbreviations: chlorsulfuron, 2-chloro-N-[[(4-methoxy-6-methyl1 ,3,5-triazin-2-yl)amino]-carbonyl]benzenesulfonamide; ALS, acetolactate synthase (EC 4.1.3.18); LSS, liquid scintillation spectrometry; MCW, methanol:chloroform:water 12:5:3 (v/v/v); imazapyr, (±)-2[4,5-dihydroxy-4-methyl-4-( l-methylethyl)-5-oxo-1H-imidazol-2-yl]3-pyridinecarboxylic acid. 1441 Plant Material Field pennycress (Thlaspi arvense L.) seedlings were grown from seed in 175-mL styrofoam cups filled with horticulturalgrade vermiculite. The cups were subirrigated with halfstrength Hoagland solution (15) modified to contain 1.5 4g/ mL iron. The plants were grown in a growth cabinet at 23°C/ 19°C day/night temperatures and with an 18-h photoperiod. Fluorescent lights supplied a photosynthetic photon flux density of 800 ,uE *m-2 * s-'. The RH was 50%. In experiments that included the branched-chain amino acids L-valine, L-leucine, and L-isoleucine, these amino acids were supplied to the seedlings via the roots. Six to 8 h before herbicide treatment the vermiculite was washed off the roots and the seedlings were mounted with styrofoam plugs in holes in a sheet of PVC (6 mm thick) in such a manner that the roots were hanging in half-strength Hoagland solution, with or without 1-mM concentrations of each of the three amino acids. Herbicide Application Chlorsulfuron, '4C-labeled (phenyl-'4C [U]; specific activity 152.1 Bq.nmol-'; radiochemical purity 98.9%) or technical product (95% pure), was applied in 8 to 10 droplets (total volume 10 1iL) of application solution consisting of 10 mM Na2HPO4-citric acid buffer (pH 8.0), with 10% (v/v) tetrahydrofuran and 0.1% (v/v) Citowett Plus surfactant. The droplets were placed on the third true leaf of seedlings that had five to seven true leaves. Control treatments consisted of 8 to www.plantphysiol.org on December 31, 2017 Published by Downloaded from Copyright © 1990 American Society of Plant Biologists. All rights reserved. Plant Physiol. Vol. 93, 1990