Regulation of nitrate uptake in maize seedlings by accompanying cations

Nitrate uptake by 9-day old maize seedlings and the potential difference (PD) along the root were studied in response to different accompanying cations (K+, Na+, Ca2+, Mg2+). A computer-based system was used to measure the depletion of ions from the uptake solution. Stimulation of NO3- uptake by cations followed the sequence K+ > Mg2+ > Na+ greater than or equal to Ca2+. A positive correlation between NO3- and cation uptake rates was observed. The uptake rate analysis indicated higher a V-max value but the same K-m for nitrate uptake from solutions with K+ compared with solutions with Na+, Mg2+, Ca2+. Treatment of nitrate-induced maize roots with KNO3 caused a rapid and steady decrease in PD value. The electrical response of roots to NaNO3, Ca(NO3)(2) and Mg(NO3)(2) was characterized by initial rapid decreasing of PD followed by a slower increasing of PD. The pattern and magnitude of the electrical response were dependent both on ion concentration and the resting potential. The importance of the cation-dependent membrane potential changes in regulation of nett nitrate uptake is discussed. (C) 1998 Elsevier Science Ireland Ltd.