Due to their ability to maintain Fe in solution, soluble Fe-humate complexes can act as natural Fe-chelates available for plants. Because of their complexing ability, it is generally accepted that water-soluble humic molecules can mobilise Fe from soil particles to the root surface. The quantitative aspects of this process have not yet been elucidated. It is reasonable to think that the importance of the Fe-humate complexes depends on the humic substances/Fe ratio. As proposed by Linsday and Schwab (1982) for the movement of Fe-EDTA chelate from the solid phase of the soil to the root, the following scenario can be hypothesised in the case of soluble humic molecules: at a low humic substances/Fe ratio, the molecules will tend to mobilise Fe from the solid phases to form stable complexes. If the amount of Fe is not sufficient to form humic macromolecules of lower solubility (see previous section) the soluble complex will move by diffusion toward the roots. Evidence supporting this view has been provided by Garcia-Mina et al. (2004) by applying Fe complexed to NaOH-extracted humic acids to three different soils. Cesco et al. (2000) observed that a water extractable humic fraction (WEHS), purified from a water extract of sphagnum peat using XAD-8 amberlite resin, can solubilise Fe present as ferrihydrite and mobilise it in a soil column, making it available for exchange with organic chelating agents such as phytosiderophores released by deficient barley roots. The dynamics of Fe mobilisation by humic substances must, however, take into account the prevailing conditions at the rhizosphere, such as pH and redox potential, and the presence of other type of chelating agents of microbial (siderophores) or plant (organic acids and phytosiderophores) origin. In this context, the different plant strategies in response to limited Fe availability need to be considered. To this respect it is interesting to observe that response mechanisms to Fe-deficiency have been studied almost exclusively using synthetic chelates such as EDTA and EDDHA or, in a few cases, organic acids released by the roots (such as citrate and malate). It is however reasonable to suppose that a mixture of natural chelates is present in the soil and in the rhizosphere (Crowley, 2001). Among natural chelates, humic molecules may play an important role in the mechanisms involved in Fe uptake. Several studies have presented evidence confirming this assumption (Lobartini and Orioli, 1988; Pinton et a!., 1998; Chen et a!., 2001). Clear evidence that soluble Fe-humate complexes can be used as a source of Fe has been provided by Pinton et a!. (1999a). A more rapid or better recovery from symptoms of chlorosis was observed in cucumber plants treated with the Fe-WEHS complex than those treated with other Fe sources (Fe-EDTA, Fe-citrate and FeCl3). Recovery from Fe deficiency after addition of Fe-humate also occurred when plants were grown in nutrient solution at alkaline pH (Figure 7-2) and in the presence of CaCO3 (Mohamed et a!., 1998). Furthermore, Cesco et a!. (2002) showed that Fe-sufficient and deficient cucumber plants were able to absorb and translocate to the shoot 59Fe supplied to the nutrient solution as 59Fe-WEHS; uptake and translocation of 59Fe could also be measured at pH 7.5, suggesting that Fe-WEHS could contribute to Fe nutrition even at pH values which are generally associated with low Fe availability in the soil. The Fe3+-WEHS complex (1 ^M Fe) could also be reduced by intact roots of cucumber plants either at pH 6.0 or pH 7.5, supporting the view that soluble Fe-humate can act as one of the naturally occurring substrates for the inducible Fe3-chelate reductase. The Fe3-WEHS complex (1 ^M Fe) was reduced at higher rates as compared to 1 ^M Fe3 -EDTA (Pinton et a!., 1999a), irrespective of the Fe nutritional status of the plants (Table 7-3). Furthermore, the relative increase in Fe3+-chelate reducing activity induced by Fe deprivation, was considerably different between Fe3-EDTA (3-fold) and Fe3+-WEHS (8-fold). This result can be particularly interesting in the view of what was observed by Lucena (2003) who showed that the rate of reduction of Fe3-EDTA by roots of mild-chlorotic cucumber plants was higher than that of other synthetic chelates. However, the higher efficiency of Fe-WEHS as Fe source with respect to synthetic chelates can not be solely ascribed to a higher Fe3+ reduction by roots but also to the capacity of the humic fraction to stimulate proton extrusion (Pinton et al., 1999a), a component of the Fe-deficiency response in Strategy I plants.
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