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An Appraisal of the da_i/dx Hypothesis¹

ABSTRACT

An attempt was made to subject the da_i/dx hypothesis to a more critical experimental appraisal than has been accomplished previously. A system was devised by the use of synthetic exchanger particles in which the suspension effect, as measured electrometrically, was exceptionally large; the value indicated an apparent difference in a_Rb + in the most extreme parts of the system of about one million. According to the da_i/dx hypothesis, the rate of absorption of Rb⁺ by plant roots should have differed from these two parts of the system by a correspondingly large factor. The observed rates of absorption from all parts of the system, however, were within a factor of 1.2 of being identical. It is concluded that either: (i) electrometric methods do not provide a reliable estimation of ion activities in aqueous systems containing exchanger particles, (ii) the rate of absorption of an ion by plant roots is not a function of the activity gradient extending normal to the absorption surface, or (iii) both of these commonly accepted postulates are incorrect. The data are considered to be consistent with the alternative d_i/dx hypothesis.

¹ Technical contribution no. 956, Department of Chemistry-Station, Montana State University.

² Professor of Chemistry, Montana State University, Bozeman.

Received for publication March 5, 1969. Accepted for publication July 14, 1969.