Electrophysiology of rice roots
Creators: name: Aramrattana, Pikul
Publisher: name: University Of Tasmania
Year: 2023

description: The electrophysiology of rice (Oryza sativa cv. Calrose) roots has been investigated in both intact and excised tissues. Mathematical procedures for measuring ion fluxes across the plasmalemma, the tonoplast and the xylem has been developed based on the Pitman model of ion transport. The mathematical analysis has been tested by means of the radioactive tracers. The method allows one to estimate ion fluxes in the longitudinal and radial directions and the efflux of ions from the xylem vessels to the symplast. The movements of K+ ions were followed in this study, using 86Rb as well as 42K. A study has been made of primary roots grown under the same salt concentration thoughout their life, using a mature portion of the root where ionic fluxes are changing relatively slowly with time. It was found that ion exchange between cortical cells in the longitudinal direction was not more than 5% of the radial transport into the xylem. This was also true when the megnitude of apoplastic transport was compared to that of symplastic transport, regardless of whether or not the Casparian strip had developed at the endodermal cell walls. It was found that the net absorption into this region of the root from the external medium was small. Hence, most of ions which were transported into the shoot were from reserves in the cell vacuoles. Consequently, there was a gradual loss of K+ ions from the studied portion, suggesting the loss of absorption capability of cortical cells after being fully differentiated. There was also evidence showing the formation of large air spaces at the mature root region. In the transport system, it was found that an appreciable amount of K+ ions in the xylem vessels was re-absorbed by the cells surrounding them. Re-translocation of ions from the shoot via the phloem to the root was also found in this study. Most of this transport was found to be to the tip region rather than to the mature cell region. The application of the Ussing-Teorell equation showed that there were K+ inward pumps locating at the plasmalemma and at the site of delivery of ions into the xylem. These pumps disappeared in excised root segments. In excised root tips, there appeared to be weak K+ pumps at both sites in the . root. This study also showed that 86Rb can only be a suitable tracer for K+ ions as far as transport into the symplast and into the xylem were concerned. There was a discrimination against Rb+ ions at the tonoplast. It was found that accumulation of K+ ions in the vacuole was about 1.8 times greater if 42K was used as a tracer than if 86RB was used. In an attempt to distinguish the effect of changes in the oxygen state of the root medium on transmembrane potentials, it was found that cortical cells depolarized from -132 mV to -122 mV under anaerobic conditions. Moreover, there was a transient hyperpolarization shortly before the depolarization took place, which was similar to the effect of CN- on membrane potentials found by other workers. When DNP was used as an inhibitor, cells depolarized by about 70 mV. This potential change is accounted for by an electrogenic ion pump component. When roots were excised, cortical cell depolarized. This was followed by a hyperpolarization of cells and accompanied with the loss of K+ ions from the excised tissue. No enhancement of uptake of K+ was observed in young roots adapted to anaerobic conditions that were transferred to aerobic conditions. The observations by other workers of enhanced uptake in mature plants was accounted for through the adaptation of a greater secondary root system.