Journal of Experimental Botany, Vol 49, 1977-1985, Copyright © 1998 by Oxford University Press
M van der Westhuizen and M Cramer
Respiratory CO2 and O2 flux were measured in hydroponically grown
Lycopersicon esculentum (L.) Mill. cv. F144 plants at
either low (O
ARTICLES
The influence of elevated rhizosphere dissolved inorganic carbon concentrations on respiratory O2 and CO2 flux in tomato roots
Botany Department, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa; Corresponding author e-mail: mdc@land.sun.ac.za
mol
mol-1) or elevated CO2 concentrations (>2000
mol mol-1) supplied to
the roots. In NO3- fed plants the consumption of O2
and the engagement of the alternative pathway were increased by elevated
dissolved inorganic carbon (DIC = CO2 + HCO3-)
concentrations. This was ascribed to the influence of organic acids on the
TCA cycle and electron transport pathways. Inhibition of O2 consumption by
elevated DIC in NH4--fed plants may be due to the
reduction requirements of anaplerotic carbon entering the TCA cycle or the
removal of carbon from the TCA cycle for amino acid synthesis. In both
NO3- and NH4+-fed plants
elevated DIC inhibited CO2 release due to high rates of DIC incorporation
by phosphoenolpyruvate carboxylase (PEPc) activity. Transient net CO2
consumption due to the inhibition of respiration by salicylhydroxamic acid
and KCN, together with high respiratory quotients after the addition of
inhibitors of carbonic anhydrase (CA) activity, were also ascribed to high
rates of DIC incorporation at elevated DIC concentrations. Ethoxyzolamide,
an inhibitor of CA activity, inhibited both DI14C
incorporation into organic products and NO3- uptake
by 81% and 40%, respectively. This, together with a 32% increase in
DI14C accumulation and inhibition of
NO3- uptake upon inhibition of anion transport by
diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS) may indicate the
exchange of HCO3- for NO3-
across the root plasmalemma. It was concluded that dark incorporation of
HCO3- by PEPc increased at elevated rhizosphere DIC
concentrations and that the products of DIC incorporation may stimulate
respiratory electron transport. Additional reducing energy and carbon
skeletons from the tricarboxylic acid (TCA) cycle would therefore be
available for respiration and the reduction and incorporation of
NO3- into amino acids.Key
words: Tomato, PEPc, respiration, carbon dioxide nitrate.
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