Effect of some electron donors and acceptors on redox capacity and simultaneous net H+/K+ fluxes by aeroponic sunflower seedling roots: Evidence for a CN−-resistant redox chain accessible to nonpermeative redox compounds

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.Abbreviations cis-Pt(II) cis-platinum(II) diammine dichloride - HCF(II) hexacyanoferrate(II) (or ferrocyanide) potassium salt - HCF(III) hexacyanoferrate(III) (or ferricyanide) potassium salt - HCI(IV) hexachloroiridate(IV) - PMOR plasmalemma oxidoreductase complex     

Source of Ca,Cd, Cu,Fe, K,Mg, Mn,Mo and Zn in grains of sunflower (Helianthus annuus) grown in nutrient solution: root uptake or remobilization from vegetative organs?

Plant and Soil - This study investigated the possible source organs delivering several trace elements to seeds (root uptake versus net remobilization), by studying changes in biomass and element...