Uptake and Accumulation of the Herbicides Chlorsulfuron and Clopyralid in Excised Pea Root Tissue

Abstract

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.

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

“…This increase in sugars in herbicide-treated leaves has also been reported for imazapyr (21), an imidazolinone-type herbicide known to inhibit ALS (20). On the basis of the mass flow hypothesis of phloem transport, it can be assumed that (6,11). The fact that the chlorsulfuron-induced decrease in assimilate transport also occurs in excised leaves strongly suggests that the herbicide has an effect on the transport of assimilates into the phloem in the source tissue rather than an effect in the sink tissue.…”

“…Devine et al (6) have suggested that on the basis of its physicalchemical properties chlorsulfuron should be translocated more readily than it is in whole plants. Assuming that chlorsulfuron has no direct effect on the loading of herbicide molecules into the phloem tissue, i.e.…”

“…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.…”

Uptake and Accumulation of the Herbicide Bentazon by Cultured Plant Cells

“…Accumulation by plant cells and tissues of herbicides that are weak acids was found to be greater at the pH close to the pKa of each herbicide (6,7,17,23). Natural compounds which are either weak acids (1) or weak bases (1 1) have also been shown to accumulate differentially in response to pH, and ion trapping has been suggested to at least partially explain accumulation of those compounds by plant cells.…”

Journal of Range Management, Volume 55, Number 3 (May 2002)

“…Absorption through the symplast and translocation through the phloem are essential for activity. The uptake of clopyralid has been classified as nonfacilitated diffusion by Devine et al (1987) apacentamiento realizados en Norte America muestran que la disponibilidad de plantas toxicas y las perdidas por muerte de ganado a causa de este tipo de plantas estan estrechamente asociadas con la intensidad de apacentamiento. Los estudios revisados muestran que bajo apacentamiento moderado las perdidas por muerte de ganado debido a plantas toxicas promedian aproximadamente 2.0%, comparado con 4.8% bajo intensidades de apacentamiento fuerte.…”