Nitrogen Enhancement of Phosphate Transport in Roots of Zea mays L.

Abstract

The effect of nitrogen status on phosphorous uptake and translocation was examined in 6-day-old dark-grown decapitated maize seedlings exposed to 25 micromolar phosphorous. Transfer to complete solutions containing 1 millimolar ammonium resulted in an increase in phosphorous uptake rate after 6 to 8 hours. The stimulus remained effective for at least 5.5 hours upon subsequent transfer to nitrogen-free solutions. Pretreatments for 16 hours with either nitrate or ammonium resulted in enhanced rates of subsequent phosphorous uptake and in enhanced translocation to the xylem of the exogenously supplied phosphorous. Both processes reached a plateau following pretreatment with 0.1 to 1.0 millimolar concentrations of either nitrogen ion. Further enhancement occurred with 10 millimolar nitrate, but not with 10 millimolar ammonium pretreatment. Although nitrogen pretreatments slightly increased the quantity of exogenous phosphorous retained in the root tissue, most of the extra phosphorous taken up by the nitrogen-pretreated seedlings was translocated to the xylem. The enhanced translocation, however, did not totally account for the increase in uptake implying a specific stimulation of the uptake process. The stimulating effect of nitrogen on phosphorous uptake by plants (9, 19) may be attributed to increased root proliferation resulting from nitrogen application (6, 8), changes in the availability ofsoil phosphorous caused by chemical and acidity changes in the rhizosphere (1, 2, 22), and nitrogen-stimulated physiological changes within the plant itself that influence phosphorous transport (5, 13, 26, 27). The possible importance of the physiological changes in Zea mays L. has been indicated by a series of field and glasshouse experiments. The field experiments have consistently shown progressive increases in leaf phosphorous concentrations at an-thesis from about 0.2% to more than 0.3% with increasing nitrogen supplied as topdressing (EL Anderson, PhD dissertation, NC State University, 1982). That the response was at least partially a consequence of enhanced capacity for phosphorous uptake was revealed by a glasshouse experiment in which two maize genotypes were grown for 3 weeks in a Dothan loamy sand with adequate and high nitrogen treatments. Following thorough washing of the soil from the root systems, measure-'Paper (initially 25 jAM P) revealed the phosphorous uptake rates were increased 23% by the high nitrogen treatment. The data indicate that soil-grown plants may exibit a nitrogen stimulation of phosphorous uptake as previously demonstrated in solution culture. Additional attributes of the nitrogen-stimulated phosphorous uptake response are reported in this and the following paper. Phosphorous uptake by root tissue and translocation from it via the xylem were examined in decapitated, dark-grown maize seedlings such that effects resulting from enhancement in shoot functions were eliminated. Results of experiments defining the onset of the stimulus and the concentration-dependence of the stimulatory effects of pretreatments with either ammonium or nitrate are presented herein. Data from kinetic, inhibitor, and split-root studies are included in the second paper (25). MATERIALS AND METHODS Seed of the corn genotype DeKalb XL 45 was germinated in the dark at 30° C on paper towels kept moist with 0.1 mm CaSO4. On the d 3 after germination, seedlings were selected for uniformity and all roots except the primary axis removed. Each seedling was supported in a small plastic stopper, transferred to 15 L tanks of aerated basal nutrient solution (pH 6.0), and grown in the dark at 300 C and 98% relative humidity for a further 2 d. Seedlings were again selected for uniformity and the shoots excised 1.5 cm above the endosperm. Thin-walled glass tubes (2.25 mm i.d. x 95 mm long) were attached to the stump of the shoots and sealed with high vacuum grease. These assemblies were then supported with the roots in the same aerated basal culture solutions containing the appropriate ammonium or nitrate pretreatment reagents. The experimental unit used during the uptake and transloca-tion measurement phase of the experiments comprised four decapitated seedlings (one culture) in 400 ml of basal medium labeled with 32P. There were four replications of this unit for each treatment in all experiments. Cultures were maintained at 28° C during the uptake measurement phase of the experiments. Two basal nutrient solutions were employed. One is referred to as chloride-medium and contained 1 mm CaSO4, 1 mM KCl, 250