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Electrophysiology of rice roots


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Aramrattana, Pikul 1985 , 'Electrophysiology of rice roots', PhD thesis, University of Tasmania.

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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 .
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.

Item Type: Thesis - PhD
Authors/Creators:Aramrattana, Pikul
Keywords: Electrophysiology of plants, Roots (Botany)
Copyright Holders: The Author
Copyright Information:

Copyright 1985 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1985. Includes bibliography

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