Translocation of phloem-mobile herbicides was inhibited by their phytotoxic action on processes that maintain assimilate translocation. Glyphosate lowered import into developing sink leaves soon after it was applied to exporting sugarbeet leaves. Later, photosynthesis slowed down and starch accumulation stopped, but export of both assimilate and glyphosate continued until it was limited by starch availability at night Experiments with field pennycress and Tartary buckwheat indicated that self-limitation of chlorsulfuron translocation probably occurred and that it resulted from lowered assimilate entry into phloem rather than from inhibition of photosynthesis or carbon allocation. Leakage of chlorsulfuron from the phloem when export was slowed down also may have contributed to its reduced translocation.

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.

Three wash techniques, each with 1, 10, or 95% (v/v) ethanol:water were used to measure foliar absorption of14C-glyphosate [N-(phosphonomethyl)glycine],14C-3,6-dichloropicolinic acid, and14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} in Tartary buckwheat [Fagopyrum tataricum(L.) Gaertn. ♯3FAGTA], Canada thistle [Cirsium arvense(L.) Scop. ♯ CIRAR], and barley (Hordeum vulgareL. ‘Galt’). For the herbicides and species tested, the most suitable common procedure for determining absorption consisted of a double or triple rinse with or immersion in 10% ethanol. Wiping the treated leaves with cotton balls moistened with the solvent was much less effective. Efficiency of herbicide removal by a given solvent was not related consistently to solubility of the herbicide in the solvent.

Foliar-applied clopyralid was translocated much more readily than chlorsulfuron in the phloem of Tartary buckwheat plants. This result was not due to greater penetration of clopyralid into the treated leaf or to greater retention of chlorsulfuron in the cuticle. Experiments with excised leaf disks indicated that chlorsulfuron was taken up more readily by the leaf tissue and accumulated in the tissue to a higher concentration than clopyralid. Both herbicides effluxed readily from the tissue after transfer to herbicide-free medium, indicating that the accumulation was not due to irreversible binding within the tissue. Chlorsulfuron (2.8 nmol) applied with14C-sucrose reduced14C export from the treated leaf. Chlorsulfuron also reduced export of14C following exposure of the treated leaf to14CO2at 6, 12, or 24 h after herbicide application. This effect of chlorsulfuron could be partially reversed by pretreating the plants with a combination of 1 mM valine, leucine, and isoleucine. In similar experiments clopyralid had no effect on assimilate transport. It is concluded that phloem translocation of chlorsulfuron in sensitive species is limited by a rapid, indirect effect on phloem transport that reduces both its own translocation and that of assimilate.