Trans-root electric potential and current in rice

The root is a complex organ playing many functions in plants; among these, the absorption of solutes from the rhizosphere is pivotal for mineral nutrition. Nutrients are transported from root to shoot, through the xylem vessels, in order to support the growth of stem and leaves. The movement of nutrients toward the tracheary elements of the root involves different types of tissues and cells – such as root hairs, cortical parenchyma, endodermis, xylem parenchyma cells – and combines both symplastic and apoplastic pathways. In particular, the movement of charged solutes involves the activity of metabolic dependent transport entities, transporters and channels at cell membrane level and thus results in a complex system in which active, diffusive and equilibrium potentials, and currents are connected both in series and in parallel. The final result is a complex electric network in which generators of potentials and/or currents, resistances, capacitances and diodes are involved. The resolution of the single components of this circuit is very difficult, mainly because of the complex morphological structure of the root. Nevertheless, the measurement of trans-root potentials and currents can provide potentially interesting information about the movement of charged solutes through the root. In rice (Oryza sativa cv Baldo) the electric potential difference measured between the external medium and the stele is about -120 mV. Current measurement shows that the reversion potential approximates the basal potential, suggesting that the net current is very low in this condition. It is possible to determine that – if the potential in the stele is connected with the external medium through a resistance of 250000 Ohm – root is able to generate a current ranging in the order of 0.1 mA, positive outward, corresponding to an electric power of about 3 mW. When the trans-root electric potential is negatively increased, a positive inward net current increases; the contrary happens when the change in the electric potential imposed is positive. Roots treated with 2 mM NaCN depolarize their root potential which becomes 60 mV positive, suggesting that the potential measured in the control is metabolism dependent. Finally, in the presence of NaCN, negative increases in trans-root electric potential induce very high inward positive currents. The presence of 25 mM NaCl in the medium induces a positive trans-root electric potential (+30 mV). Also in this case, it is possible to determine that – if the potential in the stele is connected with the external medium through a resistance of 250000 Ohm – root is able to generate an inward positive current ranging in the order of 0.4 mA. These preliminary results suggest that trans-root electric potential measurements can potentially give some interesting information on the movement of charged solutes through the particularly complex root of rice.