ALBERT

Notes: Summary Excised roots from aeroponic axenically 48 h dark-grown sunflower (Helianthus annuus L.) seedlings showed redox activities, being able to oxidize/reduce all the exogenously added electron donors/acceptors, that affected the H+/K+ net fluxes simultaneously measured in the medium. Trials were performed with in vivo and CN−-poisoned roots; these showed null+/K+ net flux activity but still oxidized/reduced all the e− donors/acceptors tested except NADH. NADH enhanced the rate of H+ efflux by in vivo roots, otherwise not changing any of the normal flux kinetic characteristics, suggesting that NADH donates e− and H+ to the exocellular NADH oxidoreductase activity of a CN−-sensitive redox chain in the plasmalemma of the root cells. K+ influx was not affected, probably because the NADH concentration was not very high. The e− donor HFC(hexacyanoferrate)(II) activated the H+ efflux in a very different way: maximum H+ efflux rate was maintained, but both the maximum rate plateau and the optimal pH range were extended, and hence the total H+ efflux was significantly enhanced. At the same time, the K+ influx was doubled. The different H+-efflux kinetics, together with the small but significant HCF(II) oxidation by CN−-poisoned roots, were taken as evidence that, besides the CN−-sensitive redox chain, an alternative CN−-resistant redox chain in the plasmalemma was involved in HCF(II) oxidation. The effect of the oxidized form HCF(III) on H+ and K+ fluxes was the opposite to that described for HCF(II), but the other H+ efflux kinetic characteristics were similar (the maximum rate plateau was extended so that total H+ efflux equaled that of the controls). It is proposed that HCF(III) accepts e− only from the alternative CN−-resistant redox chain. We could not measure the effect of HCI(hexachloroiridate)(IV) on H+ efflux, as the pH electrodes alone quickly reduced the compound. HCI(IV) promoted a rapid transitory K+ efflux, followed by recovery of K+ influx. The HCI(IV) reduction by in vivo or CN−-poisoned roots was extremely rapid, following similar kinetics. Thus, only the CN−-resistant redox chain was involved in both cases. The redox chain inhibitor cis-platinum(II) annulled ion fluxes in the presence of both NADH and HCF(III), and later even inverted them (a small H+ influx down the gradient would induce K+ efflux). Cis-platinum(II) did not affect HCF(III) reduction by in vivo roots, and only slightly depressed that by CN−-poisoned roots. Overall, the effects of the exogenously added e− donors/acceptors tested were consistent with the existence of a CN−-resistant redox chain in the plasmalemma of the root cells which would donate/accept e− even when the H+ and K+ fluxes were annulled by CN− or even inverted by cis-platinum(II) treatments. Thus, in the plasmalemma of in vivo roots this chain would compete for electrons with the normal CN−-sensitive one, as in plant mitochondria. The effects on the K+ flux were consistent with the current hypothesis that this contributes to counteracting the changes in membrane potential caused by redox activities and the H+ flux induced by the different redox compounds tested.