REDUCTION OF EXTRACYTOPLASMATIC ACCEPTERS BY ROOTS OF PLANTAGO-LANCEOLATA L - EVIDENCE FOR ENZYME HETEROGENEITY

The capacity of iron-stressed (-Fe plants) and non-stressed (+Fe plants) roots of Plantago lanceolata L. to reduce accepters differing in their midpoint potentials has been characterized. Highest reduction activity in iron sufficient roots was observed with the artificial accepters hexachloroiridate (HCI) and ferricyanide. FeEDTA and ferric citrate (FeCitr) were reduced at equal rates with respect to maximal velocity; marked differences between the two latter reactions have been observed in the affinity for the substrate. Iron starvation increased the reduction of FeEDTA and ferricyanide, the rates of HCI and FeCitr reduction were not significantly affected. With the exception of HCI, the reduction rates of all accepters were diminished by inhibitors of protein synthesis. The inhibition was more pronounced in iron stressed roots compared to +Fe grown plants. Protoplasts isolated from Plantago roots were capable of reducing FeCitr, but failed to reduce FeEDTA. The kinetics (K-m) of FeCitr reduction by root protoplasts resembled the characteristics of intact plant roots. The existence of distinct redox systems in root cells and their physiological significance are discussed with reference to results obtained with material reduced in biological complexity.