The anticipated commercialization of TIBA as one of the first growth regulators for use on a major field crop in the USA, created a need for information on its absorption and movement in the soybean plant under field conditions. Such information would be of assistance in metabolite and residue studies. TIBA tagged with I‐131 was applied at early bloom in two field experiments and in a third study it was applied at the beginning of the pod filling stage of growth. Whole plants were sampled periodically, divided into their morphological parts and their dry weights and I‐131 activities were determined. Approximately 45 percent of the total activity measured at “zero time” was lost from the plants within 48 hours. Biological half lifes of 40.3 and 42.0 days were found for the I‐131 in the early spray experiments. The rapid absorption and movement into the vascular stream was indicated by the detection of I‐131 activity in the roots of the “zero time” samples. Sites of active TIBA I‐131 activity were regions of high meristematic activity. Such sites of accumulation loss I‐131 activity to other areas when their relative metabolic activity declined. Ultraviolet light was shown to cause small but significant deiodozation of TIBA. Sixty days after TIBA was applied the plant parts were found to contain the following percentages of the applied radioactivity: roots 4%, leaves that were sprayed with TIBA 15%, leaves that emmerged after TIBA application 1.5%, petioles that received application of TIBA 2.5%, stems 3.0%, pods including seeds 7%. When the TIBA was applied at the beginning of the pod filling period, the seeds at physiological maturity contained 0.56 ppp TIBA equivalent or 2.5% of the originally applied activity.

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