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 21 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. Fe III reduction was measured using a similar approach by tagging ferrozine diboronic acid with OH groups of the cell wall. Ferrozine forms an Fe 21 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 III 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 III 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 III reduction in the apoplast, and hence Fe 21 uptake into the cytosol.

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European lacustrine systems are frequently exposed to nitrate (NO3–) pollution causing eutrophication processes. An example of these lakes is Shkodra Lake, a large, shallow lake shared by Albania and Montenegro, in the Balkans Peninsula. Shkodra Lake is a natural sink that collects NO3– from agricultural activities, widely diffused in the surrounding area. The additions of wheat straw and biochar have been suggested to increase soil NO3– retention of agricultural lands. To better understand the role of these two organic soil amendments in mitigating NO3– leaching from arable lands, a pot experiment using a representative sandy loam soil of the Skodra Lake basin was performed. More specifically, a greenhouse experiment with Lolium multiflorum L. and Zea mays L., was carried out for three months, to evaluate the concentrations of NO3–-N in leachate and the cumulative leaching losses of NO3–-N, after wheat straw (10 Mg ha–1) and biochar (10 Mg ha–1) soil addition, under the same rate of NPK fertiliser (300 kg ha–1). The effect of the two organic amendments on nitrate retention, was evaluated according to two methods: i) Soil NO3–-N leaching with distilled water; and ii) Soil NO3–-N extraction with 2M KCl. The leached NO3–-N and the Potentially Leachable NO3–-N (2M KCl extraction) were respectively determined. N uptake by plants, as well as the Nitrogen Use Efficiency were also calculated. A retention effect on nitrate was found in Lolium multiflorum L. and wheat straw treatments compared to control, by reducing leached NO3–-N almost to 35%. In SBFL (soil+biochar+fertiliser+Lolium) treatment, biochar effectively reduced the total amount of nitrate in leachate of 27% and 26% compared to SFL (soil+fertiliser+Lolium) and SSFL (soil+straw+fertiliser+Lolium) treatments, respectively. The potentially leachable NO3–-N was two to four times higher than the leached NO3–-N. The amount of potentially leachable NO3–-N per hectare ranged from 220 in SL (soil+Lolium) treatment, to 500 kg ha–1 in SFL. N plant uptake values ranged from 18.16 mg kg–1 in the non- fertilised treatment to 58.06 mg kg–1 soil in SSFM (soil+straw+fertiliser+maize) treatment. The NUE showed a similar trend (from 0 in the non-fertilised treatment to 47.9 % in SSFM). Results indicated a mitigating action of biochar on leaching of NO3–-N (leached up to 100 kg ha–1), despite the retention effect of the two different amendments applied.

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