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Environmental and Stress Physiology

Nitrogen Enhancement of Phosphate Transport in Roots of Zea mays L

II. Kinetic and Inhibitor Studies

Division of Tropical Crops and Pastures, Commonwealth Scientific and Industrial Research Organization, Cunningham Laboratory, St. Lucia, Queensland 4067, Australia, Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27695-7619

Exposure to 1 millimolar ammonium pretreatments increased V_max for phosphorous uptake in dark-grown decapitated maize seedlings without a statistically measurable change in K_m. Sulfate uptake also was stimulated. The stimulation in phosphorous uptake due to ammonium pretreatment was greater in seedlings grown without phosphorous than in those grown with 25 micromolar phosphorous. The stimulus was not expressed unless the entire root system was pretreated with ammonium, and pretreatment of a part of the root system inhibited phosphorous uptake by the remaining part unless it also had been pretreated. Pretreatment with the amino acid analogs p-fluoro-DL-phenylalanine and L-azetidine-2-carboxylic acid (AZ) restricted phosphorous uptake in seedlings that were pretreated with ammonium and in those that were not, but the effect of ammonium pretreatment was not completely eliminated by the analogs. In general, translocation of the entering phosphorous was affected similarly to uptake by experimental treatments. Enhanced translocation, however, was not sufficient to account quantitatively for the increase in uptake, and an increased uptake was still evident when translocation was completely prevented by 50 micromolar AZ pretreatment.