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Apoplastic pH and FeIII reduction in young sunflower (Helianthus annuus) roots

The relationship between the apoplastic pH in young sunflower roots (Helianthus annuus L.) and the plasmalemma ferric chelate reductase (FC-R; EC 1.16.1.7) activity in roots was investigated. The hypothesis was tested that a high apoplastic pH depresses FC-R activity, thereby restricting the uptake of Fe 2 + into the cytosol. Until recently, little has been known about this relationship, because pH and redox reaction measurements are difficult to perform within the confines of the root apoplast. We recorded the apoplastic pH by means of the fluorescence ratio in conjunction with video microscopy by covalently tagging fluorescein boronic acid to OH groups of the root cell wall. Fern reduction was measured using a similar approach by tagging ferrozine diboronic acid with OH groups of the cell wall. Ferrozine forms an Fe 2 + complex, thus indicating the reduction of ferric iron. In roots bathing in buffered outer solutions of different pH, a high pH sensitivity of apoplastic Fe I I I reduction was found, with the highest ferric iron reduction rates at an apoplastic pH of 4.9; above an apoplastic pH of 5.3, no reduction was observed. Nitrate in the bathing solution increased the apoplastic pH and hence depressed the Fe I I I reduction; ammonium had the reverse effect. Nitrate together with HCO 3 - , a combination which is typical of calcareous soils, had the strongest depressing effect. From the results, it can be concluded that the main reason for the frequently occurring iron deficiency chlorosis of plants grown on calcareous soils is the inhibition of Fe I I I reduction in the apoplast, and hence Fe 2 + uptake into the cytosol.