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JXB Advance Access originally published online on August 7, 2006
Journal of Experimental Botany 2006 57(12):3293-3300; doi:10.1093/jxb/erl088
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© The Author [2006]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

RESEARCH PAPER

The face value of ion fluxes: the challenge of determining influx in the low-affinity transport range

Mark W. Szczerba, Dev T. Britto and Herbert J. Kronzucker*

Department of Life Sciences, University of Toronto, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4

*To whom correspondence should be addressed. E-mail: herbertk{at}utsc.utoronto.ca

The existence of distinct high- and low-affinity transport systems (HATS and LATS) is well established for major nutrient ions. However, influx mediated by these systems is usually estimated using uniformly simple tracer protocols. Two 42K radiotracer methods to measure potassium influxes in the HATS and LATS ranges in intact barley (Hordeum vulgare L.) roots are compared here: a direct influx (DI) method, and an integrated flux analysis (IFA), which is designed to account for tracer efflux from labelled roots and differential tracer accumulation along the plant axis. Methods showed only minor discrepancies for influx values in the HATS range, but large discrepancies in the LATS range, revealing striking distinctions in the cellular exchange properties dominated by the operation of the two transport systems. It is shown that accepted DI protocols are associated with very large errors in the high-conductance LATS range, underestimating influx at least 6-fold due to four characteristics of this transport mode: (i) accelerated cellular 42K exchange; (ii) a greatly increased ratio of efflux to influx; (iii) increased 42K loss during the removal of water from roots in preweighing centrifugation or blotting protocols; and (iv) increased 42K retention at the root–shoot interface, a region of the plant frequently disregarded in DI determinations. The findings warrant a re-evaluation of a large body of literature reporting influx in the LATS range, and are of fundamental importance to ion flux experimentation in plant physiology.

Key words: Barley, cellular ion exchange, efflux, high-affinity transport, influx, low-affinity transport, potassium


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