VARIABLE 24!am CONCENTRATION IN SOIL UPTAKE AND C.R. IN BARLEY PLANTS A. Wallace, R. T. Mueller, and E. M. Romney Laboratory of Nuclear Medicine and Radiation Biology University of California, Los Angeles ABSTRACT Barley plants (Hordeum vulgare L. Atlas 57) were seeded into small pots of Yolo loam soil containing different concentrations of @*!Am (4,000, 10,000, 20,000, and 40,000 dpm/g soil). Four successive harvests were made with the first three harvests each of one-third of the original plants, but of different ages. One set of all concentrations was without and one set with the chelator DTPA. There were four replicates. Plant uptake of 24lam was related to concentration, but the C.R. decreased slightly without DTPA (about 3x) tenfold. as the soil concentration was increased With DTPA, uptake was proportional to concentration (that of concentration of DTPA supplied was constant). The effect of DTPA relative to that of no DTPA increased with increasing concentration of 241 am in soil. The C.R. under these conditions was reasonably constant, but severalfold higher than without DTPA. The last harvest contained less 24lam than did the first, both with and without DTPA. The value of the coefficient "Y" in the equation (Cy/Co)* = uptake ratio was about 50% lower without DTPA compared to with DTPA. With DTPA the value of "Y" was near 1, which implies that 241am in plants was directly proportional to that in soil at all concentrations. INTRODUCTION The C.R. (concentration ratio) defined as the activity per weight of plant part (or any biological organism) divided by activity per weight or unit of substrate (ERDA, 1975) is the most frequently used means of 629