Uptake of amino acids by plants from the soil: A comparative study with castor bean seedlings grown under natural and axenic soil conditions

Castor bean seedlings grown in different media (soil, quartz sand, or liquid culture) under natural or axenic conditions take up¹⁴C labelled proline when offered to the rooting medium at concentrations similar to those occuring in the soil. Most of the absorbed proline was transferred through the root into the xylem without metabolic conversion, though some conversion to glutamine and alamine occurred.It is concluded that roots successfully compete with microorganisms for free amino acids in the soil for the following reasons: (a) The initial rate of appearance of radioactivity in the xylem sap was the same in plants grown in natural or in axenic soil, and (b) the specific activity of proline in the xylem sap was approximately the same in plants grown in natural conditions and in axenic soil (even somewhat higher under natural condition).The role of soil microorganisms became evident however in long-term experiments (e.g. 5h), because the soil solution was much more rapidly depleted of labelled amino acids in natural soil than in axenic soil. Therefore after 20 hours roots grown in sterilized soil or quartz sand always contained more¹⁴C label than those grown in natural soil.It is suggested that viable roots use free amino acids from the soil and that the main flux of carbon to the rhizosphere might be in the form of organic acids.