Passive Permeation and Active Transport of Ions in Plant Roots

Abstract

1.00 g. samples of excised roots of barley, var. Atlas 46, were exposed to solutions of K2SO4 labeled (*) with S35, then blotted and transferred to 50 ml water or solution of non-radioactive SO4. A fraction of the labeled sulfate (S*O4) was lost to the non-radio-active media. After 1 hr., no further loss occurred, a fraction of the absorbed S*O4 being retained by the roots. The penetration and loss of the former, labile fraction of S*O4 is due to diffusion of the ions into and out of the "outer" or free" space of the tissue. The volume of the "outer" space of the root tissue is defined as [image] external concn. referring to the concn. of the ion during the initial, or absorption, period. The "outer" space equalled 0.23 ml/g fresh wt. and essentially the same whether measured with S*O4, Se*O4, H2P*O4, or Ca* ions. When measured with S*O4, it was independent of the pH of the measuring solution over the range 4.0-7.7. Roots exposed to S*O4 for 1 hr. were blotted, then maintained in humid air for periods ranging up to 4 hrs. Periodically, samples were assayed for "outer" space (diffusible) S*O4, and for "inner" space S*O4, i.e., non-diffusible S*O4, or S*O4 absorbed by the active ion transport mechanism. During their stay in humid air, the roots steadily gained "inner" space (actively absorbed) ions, at the expense of "outer" space ions, evidence that the solution in the "outer" space of the tissue serves as the immediate substrate for absorption by the active ion transport mechanism of the cells.