Effect of nitrate on water transfer across roots of nitrogen pre-starved maize seedlings

The addition of 10 mM KNO3 to the solution bathing the roots of young nitrogen-starved seedlings of Zea mays L. enhanced root water transfer within 15 h, compared with 10 mM KCl addition. The free exudation flux was 2.2-3.9 times higher in excised KNO3-treated roots than in KCl-treated ones. Cryo-osmometry data for xylem sap suggested that, compared with chloride, nitrate treatment increased the steady solute flux into the xylem, but did not modify the osmotic concentration of sap. Root growth was not significantly modified by nitrate within 15 h. Root hydraulic conductances were measured by using either hydrostatic-pressure or osmotic-gradient methods. During hydrostatic experiments, the conductance (k(p)), which is thought to refer mainly to the apoplasmic pathway, was 1.6 times larger in KNO3- than in KCl-treated plants. From experiments in which polyethylene glycol (PEG) 8000 was used as external osmolyte, osmotic conductances (k(s)) were found to be smaller by 5-20 times than k(p) for the two kinds of plants. The KCl-treated roots were characterized by a low k(s) which was the same for influx or efflux of water. By contrast, KNO3-treated roots exhibited two distinct conductances k(s1) and k(s2), indicating that in-flux of water was easier than efflux when the water flow was driven by the osmotic pressure gradient. Infiltration of roots with KNO3 solution supported the idea that nitrate might enhance the efficiency of the cell-to-cell pathway. The low k(s) value of KCl-treated roots and the existence of two contrasting k(s) values (k(s1) and k(s2)) for KNO3-treated roots are discussed in terms of reversible closing of water channels.