Application of Streaming Potential Method in Zeta Potential Measurement of Rice Roots and the Interaction of the Roots with Ions and Fe/Al Hydroxides.

Plant roots normally carry negative charge due to dissociation of functional groups on the roots, which may affect the interactions of the roots with various ions and charged soil particles and subsequently the uptakes of nutrients and toxic elements by plants. Zeta potential was usually used to express the charge properties of charged particles and surfaces. However, there were no reports on zeta potential of integrated plant roots due to the lack of appropriate methods. In present study, the streaming potential method was developed to measure zeta potential of rice roots. Results indicated that zeta potential of rice roots obtained with the streaming potential method was highly correlated with the zeta potential of root cell wall of rice measured with electrophoresis, which confirmed that the zeta potentials of rice roots measured with streaming potential method were reasonable.

The zeta potentials of an Al-tolerant cultivar (japonica) and an Al-sensitive cultivar (indica) of rice were measured and compared. It was found that the zeta potential of the former was more negative than the latter. This meant that the roots of Al-sensitive cultivar carried more negative charge than the Al-tolerant one, which led to more Al ions adsorbed and accumulated on the roots of the Al-sensitive cultivar. This was one of reasons for greater vulnerability of Al-sensitive cultivar to Al toxicity.

Adhesion of Fe and Al hydroxides to rice roots made the zeta potential of the roots moved to positive value direction, suggesting that Fe/Al hydroxides made the negative charge on rice roots less negative due to charge neutralization between positively charged Fe/Al hydroxides and negatively charged roots. Al hydroxide showed greater effect on negative charge on rice roots than Fe hydroxides due to more positive charge on Al hydroxide. Fe/Al hydroxides adhesion also decreased K⁺ adsorption by rice roots, which implied that the interaction of plant roots with positively charged particles in variable charge soils may inhibit adsorption and uptake of cations by the roots.