THE PHYSIOLOGICAL RELEVANCE OF NA-COUPLED K+-TRANSPORT()

Plant roots utilize at least two distinct pathways with high and low a ffinities to accumulate K+. The system for high-affinity K+ uptake, wh ich takes place against the electrochemical K+ gradient, requires dire ct energization. Energization of K+ uptake via Na+ coupling has been o bserved in algae and was recently proposed as a mechanism for K+ uptak e in wheat (Triticum aestivum L.). To investigate whether Na+ coupling has general physiological relevance in energizing K+ transport, we sc reened a number of species, including Arabidopsis thaliana L. Heynh. e cotype Columbia, wheat, and barley (Hordeum vulgare L.), for the prese nce of Na+-coupled K+ uptake. Rb+-flux analysis and electrophysiologic al K+-transport assays were performed in the presence and absence of N a+ and provided evidence for a coupling between K+ and Na+ transport i n several aquatic species. However, all investigated terrestrial speci es were able to sustain growth and K+ uptake in the absence of Na+ Fur thermore, the addition of Na+ was either without effect or inhibited K + absorption. The latter characteristic was independent of growth cond itions with respect to Na+ status and pH. Our results suggest that in terrestrial species Na+-coupled K+ transport has no or limited physiol ogical relevance, whereas in certain aquatic angiosperms and algae thi s type of secondary transport energization plays a significant role.