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Potassium Uptake by Onion Roots Characterized by Potassium/Rubidium Ratio¹

ABSTRACT

A common belief is that plant roots absorb cations from the solution phase of the soil. Experiments with corn (Zea mays L.) using K/Rb ratio to evaluate the source of K and Rb absorbed indicated the plant roots absorbed these ions in the ratio of exchangeable K and Rb. The objective of this research was to study the source of K and Rb absorbed from soil by onion (Allium cepa) roots since they do not have root hairs and this may influence the uptake mechanism. Onions absorbed K/Rb with a ratio which was intermediate between the ratio of exchangeable K and Rb and the ratio of these cations in solution. In comparison with corn, onions absorbed K at one-third the rate, but absorbed water three times faster so that mass flow contributed a greater proportion of K absorbed by onions than that by corn. This, rather than differences in root hairs may be the reason for the observed differences in K uptake between corn and onions.

¹ Journal Paper no. 6936 Purdue Univ., Agric. Exp. Stn. West Lafayette, IN, 47907. Contribution from the Dep. of Agronomy. This research was supported in part by the U.S. Energy Res. and Develop. Administration under Contract AT (11-1)-1495.

² Post Doctoral Research Associate and Professor of Agronomy, respectively.

Received for publication October 27, 1977. Accepted for publication April 14, 1978.