Amitrole Absorption by Bean (Phaseolus vulgaris L. cv `Red Kidney') Roots

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Abstract
The mechanism of transport of the herbicide 3-amino-1,2,4-triazole (amitrole) into P. vulgaris roots appears to be passive, as judged by the effect of temperature (Q10 = 1.3 between 15 and 25.degree. C) and the lack of sensitivity to metabolic inhibition afforded by 2,4-dinitrophenol and NaN3. Amitrole absorption is a linear function of external concentration over several orders of magnitude and, thus, is not facilitated by a carrier mechanism. The absorption of amitrole is sensitive to external pH, being stimulated under acid conditions. This stimulation of amitrole absorption is seen at low (.ltoreq. 1 mM) amitrole concentrations, but not at high (50 mM) amitrole levels. While the apparent octanol-water partition coefficient varies with the pH of the aqueous phase, there is no clear correspondence between absorption and the apparent partition coefficient. Roots do not accumulate amitrole above concentration equilibrium; however, at a time when the net amitrole content of the root tissue begins to saturate, amitrole can be detected in the xylem stream. On a fresh-weight basis, amitrole absorption by roots is equal to that accomplished by trifoliate-leaf tissue. An estimate of the permeability coefficient (according to the analysis of Tyree et al. 1979) suggests that amitrole possesses near-optimal permeability for an ambimobile solute, on the order of 2.12 (.+-. 0.47) .times. 10-9 m/s.