Membrane Transport in Anoxic Rice Coleoptiles and Storage Tissues of Beetroot

Abstract

Rice coleoptiles and aged storage tissues of beetroot are both tolerant to anoxia. Both types of tissues retain their membrane integrity over several days of anoxia and take up glucose and sucrose from 0.5 mol m^-3 external concentrations at 40-65% of the rate of the aerated tissues. However, these tissues show substantial differences in other membrane properties under anoxia. Anoxic rice coleoptiles had membrane potentials generated by electrogenic ion transport, i.e. these tissues retained, at least partially, their ATP-dependent H⁺ extrusion. This conclusion is based on the following evidence: (1) The membrane potentials under aeration were between -120 and -130 mV and these depolarised to between -75 and -98 mV immediately upon exposure to anoxia, but repolarised to between -109 and -122 mV after 1 h. (2) Addition of carboxy-cyanide m-chlorophenylhydrazone (CCCP) and iodoacetamide during anoxia depolarised the membranes from -120 mV to -68 and -86 mV, respectively. (3) Transient membrane depolarisations following addition of sugars and amino acids, ranging between 10 and 35 mV, were similar during anoxia and aeration. In contrast to the rice coleoptiles, cell membranes of anoxic storage tissues of beetroot were, at most, weakly energised since they depolarised from between -156 and -159 mV in aerated tissues to -97 mV upon transfer to anoxia and this membrane potential was retained for at least 72 h of anoxia (longest time measured). Another difference between the two types of tissues was for Cl^- uptake from 0.25 mol m^-3 external Cl^-. The anoxic rice coleoptiles took up Cl^- at 10-15% of the rates during aeration, while there was no detectable Cl^- uptake by the anoxic storage tissues of beetroot. The persistence of membrane potentials generated by electrogenic ion transport in anoxic rice coleoptiles, but not in anoxic storage tissues of beetroot, may be associated with higher rates of alcohol fermentation, and therefore of glycolysis and ATP synthesis, in the rice coleoptiles than in the storage tissues of beetroot. Extra keywords: rice coleoptiles, beetroot, anoxia, membrane transport, membrane potential.