Regulation of K+ and NO3− fluxes in roots of sunflower (Helianthus annuus) after changes in light intensity

Shoot activity has been reported to affect rates of ion uptake by plant roots in other ways than merely through supply of assimilates. To elucidate the mechanisms by which a signal from the upper part of the plant controls the rate of K+ and NO3− uptake by roots, both uptake of K+ and NO3− and secretion into the xylem of young sunflower plants (Helianthus annuus L.) were measured after changes in light intensity. No close correlation was observed between the uptake of NO3− and that of K+; an increase in light intensity produced a much greater stimulation of NO3− uptake than of K+ uptake. On the other hand, secretion of NO3− into the xylem was tightly coupled to that of K+, and this coupling was strongly disturbed by excision of the root. The results suggest the involvement of the K2‐malate shuttle on the regulation by the shoot of K+ and NO3− secretion in the xylem, which is linked to NO3− uptake, while K+ uptake is independent of this regulation mechanism.