Absorption and efflux of chloro‐s‐triazines by Setaria roots*

Summary: The absorption and loss of four chloro‐s‐triazines was investigated in excised roots of four Setaria taxa. Different taxa absorbed the various triazines at different rates. In general, triazine absorption was greater at 2°C than at 22.5°C, and absorption rates were linear functions of external concentrations. Efflux studies showed marked differences in the rate of loss of 14C‐atrazine, 14C‐simazine, and 14C‐propazine from root sections of robust white foxtail (Setaria viridis var. robustaalba Schreiber). The roots lost 14C‐atrazine very quickly, and the loss was similar in either water or 12C‐atrazine. Atrazine appears to be restricted to the apoplast of the root. 14C‐atrazine was lost more rapidly than either 14C‐simazine or 14C‐propazine to water or to solutions containing the unlabelled herbicide. Efflux of 14C‐simazine was greater than that of 14C‐propazine to solutions of CaCl2. From the pattern of efflux, it was concluded that 14C‐simazine and 14C‐propazine accumulated in the root symplast. Furthermore, the decreases in chloro‐s‐triazine absorption in the presence of metabolic inhibitors (dinitrophenol, sodium arsenite) may suggest that 14C‐simazine and 14C‐propazine entered the symplast by an energy‐dependent process.