Abstract
The capability of cucumber (Cucumis sativus L., cv. Serpente cinese), a Strategy I plant and barley (Hordeum vulgaris L., cv. Europa), a Strategy II plant to use Fe complexed by a water-soluble humic fraction (WEHS) extracted from a peat, was studied. Uptake of Fe-59 from Fe-59-WEHS by cucumber plants was higher at pH 6.0 than at pH 7.5. Roots of intact cucumber plants were able to reduce the Fe III-WEHS complex either at pH 6.0 or 7.5, rates being higher in the assay medium buffered at pH 6.0. After supply of Fe-59-WEHS, a large pool of root extraplasmatic Fe-59 was formed, which could be used to a large extent by Fe-deficient plants, particularly under acidic conditions. Uptake of Fe-59 from Fe-59-WEHS by Fe-sufficient and Fe-deficient barley plants was examined during periods of high (morning) and low (evening) PS release. Uptake paralleled the diurnal rhythm of PS release. Furthermore, Fe-59 uptake was strongly enhanced by addition of PS to the uptake solution in both Fe-sufficient and Fe-deficient plants. High amount of root extraplasmatic Fe-59 was formed upon supply of Fe-WEHS, particularly in the evening experiment. Fe-deficient barley plants were able to utilize Fe from the root extraplasmatic pool, conceivably as a result of high rates of PS release. The results of the present work together with previous observations indicate that cucumber plants (Strategy I) utilize Fe complexed to WEHS, presumably via reduction of Fe III-WEHS by the plasma membrane-bound reductase, while barley plants (Strategy II) use an indirect mechanism involving ligand exchange between WEHS and PS.