Interaction between nitrate assimilation in shoots and nitrate uptake by roots of soybean (Glycine max) plants: Role of carboxylate

Abstract

The rates of accumulation of K⁺, NO₃ ⁻ and reduced N (N_r) in shoots, relative to that of Ca²⁺, were compared to the ratios K⁺/Ca²⁺, NO₃ ⁻/Ca²⁺ and N_r/Ca²⁺ in the xylem sap of NO₃ ⁻-fed soybean plants in order to estimate the xylem transport of K⁺, NO₃ ⁻ and N_r. Then, phloem transport of K⁺ and N_r could be calculated. The quantification of NO₃ ⁻ flux in the xylem and NO₃ ⁻ accumulation in shoots allowed us to estimate the contribution of roots and shoots in the reduction of NO₃ ⁻. Subtraction of the inorganic anion content from the main cation content estimated the accumulation of carboxylate anions (R⁻). The export of R to roots was obtained by subtracting R accumulation in shoots from the theoretical production associated to NO₃ ⁻ reduction in these organs. The OH⁻ net excretion in the medium was monitored. Alkalinization of the medium was stopped by interruption of phloem conduction to the roots, and was not restored by glucose addition to the medium. Excretion of ¹⁴C by roots after incorporation of ¹⁴CO₂ in leaves was greater than that from urea-fed plants. These results suggest that the OH⁻ equivalents excreted in the medium originated from R produced in shoots. The supply of K-malate to the shoots via the transpiration flow stimulated NO₃ ⁻ uptake. This result indicates that the circulation of R⁻ to the roots could be a mechanism by which NO₃ ⁻ assimilation in shoots controls NO₃ ⁻ uptake by roots.