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

Abstract

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 (42)K 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 (42)K exchange; (ii) a greatly increased ratio of efflux to influx; (iii) increased (42)K loss during the removal of water from roots in preweighing centrifugation or blotting protocols; and (iv) increased (42)K 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.