This contribution on Avena,fatun L., wild oats, is part of a series which presents biological information on plants that are weedy in Canada. wild oats rate as by far the most serious annual weed of cultivated fields in the prairie provinces of canada.
Both14C-clopyralid (3,6-dichloropicolinic acid) and14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzensulfonamide} were readily absorbed by Canada thistle [Cirsium arvense(L.) Scop. ♯ CIRAR] leaves, with 99 and 75%, respectively, of the applied doses absorbed 144 h after application. Absorbed14C-clopyralid was rapidly exported from the treated leaves, whereas14C-chlorsulfuron was translocated much more slowly. After 144 h, 29% of the applied14C-clopyralid and 5% of the applied14C-chlorsulfuron were recovered in the roots and developing root buds of Canada thistle plants. Smaller amounts of the two herbicides were absorbed and translocated in perennial sowthistle (Sonchus arvensisL. ♯ SONAR) than in Canada thistle. More14C-clopyralid than14C-chlorsulfuron was absorbed and translocated out of treated leaves of perennial sowthistle, but equal amounts, 3 to 4% of the applied doses, were recovered in the roots and root buds 144 h after application. Foliar applications of clopyralid, followed by removal of the treated shoot 24, 72, or 144 h after application, markedly reduced shoot regrowth in both Canada thistle and perennial sowthistle. Similar treatment with chlorsulfuron did not prevent shoot regrowth in either species.
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) 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 METHODSThe 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
The herbicides chlorsulfuron and clopyralid were taken up rapidly by excised pea root tissue and accumulated in the tissue to concentrations ten and four times those in the external medium, respectively. Uptake was related linearly to external herbicide concentration over a wide concentration range, implying that transport across the membrane is by nonfacilitated diffusion. Uptake of both compounds was influenced by pH, with greatest uptake at low pH. The pH dependence of uptake suggests that the herbicides (both of which are weak acids) are transported across the plasma membrane in the undissociated form, and accumulate in the cytoplasm by an ion trap mechanism. Most of the absorbed herbicide effluxed from the tissue when it was transferred to herbicide-free buffer, indicating that the accumulation was not due to irreversible binding. Consequently, both herbicides remain available for transfer to the phloem. These results can explain the high reported phloem mobility of clopyralid in intact plants. The low phloem mobility of chlorsulfuron must be accounted for by factors that override its ability to accumulate in the symplast.There has been considerable interest in recent years in the mechanisms by which plant growth regulating chemicals, both natural and synthetic, move across cell membranes. In the case of naturally occurring compounds this interest has been directed towards determining the way(s) in which concentrations of these compounds, at both the tissue and cellular levels, are regulated by the plant. Results indicate that endogenous plant growthregulating chemicals are transported across membranes by both nonfacilitated and carrier-mediated processes (1, 17). Endogenous control of the latter process presumably plays a role in regulating internal concentrations.Herbicide uptake into plant tissue has been studied primarily in relation to the subsequent translocation of the herbicides. Herbicide entry and retention in the symplast are prerequisites for phloem transport (19,26), and the relationship between phloem transport and physicochemical properties of herbicides has been the focus of much attention recently (6,19 429 Bq nmolt') were prepared in solutions of distilled water:ethanol (9:1, v/v) so that the final herbicide concentration, when 10 Ml of these solutions was added to 3.0 ml buffer, was 1.0 uM. The 14C content of the solutions was measured by taking a 50-Ml aliquot at specified time intervals (including t = 0, immediately after the herbicide was added) and assaying for 14C by LSS. Absorption was calculated by correcting the 14C data for the fraction of total solution assayed, with appropriate adjustment for the change in solution volume, and was expressed as picomoles of herbicide absorbed/100 mg root tissue.
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