Some plant species can increase the mass flow of water from the soil to the root surface in response to the appearance of
nitrate in the rhizosphere by increasing root hydraulic conductivity. Such behavior can be seen as a powerful strategy to facilitate the uptake of
nitrate in the patchy and dynamically changing soil environment. Despite the significance of such behavior, little is known about the dynamics and mechanism of this phenomenon. Here we examine root hydraulic response of
nitrate starved Zea mays (L.) plants after a sudden exposure to 5 mM NO(3)(-)
solution. In all cases the treatment resulted in a significant increase in pressure-induced (pressure gradient approximately 0.2 MPa) flow across the root system by approximately 50% within 4 h. Changes in osmotic gradient across the root were approximately 0.016 MPa (or 8.5%) and thus the results could only be explained by a true change in root hydraulic conductance.
Anoxia treatment significantly reduced the effect of
nitrate on xylem root hydraulic conductivity indicating an important role for
aquaporins in this process. Despite a 1 h delay in the hydraulic response to
nitrate treatment, we did not detect any change in the expression of six ZmPIP1 and seven ZmPIP2 genes, strongly suggesting that NO(3)(-)
ions regulate root hydraulics at the
protein level. Treatments with
sodium tungstate (
nitrate reductase inhibitor) aimed at resolving the information pathway regulating root hydraulic properties resulted in unexpected findings. Although this treatment blocked
nitrate reductase activity and eliminated the
nitrate-induced hydraulic response, it also produced changes in gene expression and
nitrate uptake levels, precluding us from suggesting that
nitrate acts on root hydraulic properties via the products of
nitrate reductase.