Proliferation of Maize (Zea maysL.) Roots in Response to Localized Supply of Nitrate

Abstract

Maize (Zea mays L.) plants with two primary nodal root axes were grown for 8 d in flowing nutrient culture with each axis independently supplied with $NO3-$. Dry matter accumulation by roots was similar whether 1.0 mol m⁻³$NO3-$ was supplied to on( or both axes. When $NO3-$ was supplied to only one axis, however, accumulation of dry matter within the root system was significantly greater in the axis supplied with $NO3-$. The increased dry matter accumulation by the +N-treated axis was attributable entirely to increased density and growth of lateral branches and not to a difference in growth of the primary axis. Proliferation of lateral branches for the + N axis was associated with the capacity for in situ reduction and utilization of a portion of the absorbed $NO3-$, especially in the apical region where lateral primordia are initiated. Although reduced nitrogen was translocated to the −N axis, concentrations in the −N axis remained significantly lower than in the +N axis. The concentratio of reduced nitrogen, as well as in vitro$NO3-$ reductase activity, was greater in apical than in more basal regions of the +N axis. The enhanced proliferation of lateral branches in the + N axis was accompanied by an increase in total respiration rate of the axis. Part of the increased respiration was attributable to increased mass of roots. The specific respiration rate (umol CO₂ exolved per hour per gram root dry weight) was also greater for the +N than for the −N axis. If respiration rate is taken as representative of sink demand, stimulation of initiation and growth of laterals by in situ utilization of a localized exogenous supply of $NO3-$ establishes an increased sink demand through enhanced metabolic activity and the increased partitioning of assimilates to the + N axis responds to the difference in sink demand between +N and −N axes.