Chlorophyll mg g1 fw 208 017 074 025 021 021

Figure 7-2. Recovery of Fe-deficiency symptoms in cucumber plants after addition of different Fe sources (0.2 ^M Fe) to a nutrient solution buffered at pH 7.5 (modified from Pinton et al, 1999a).

59Fe-WEHS could also be used by barley plants (Strategy II) via a mechanism which may involve ligand exchange between phytosiderophores and WEHS (Cesco et al, 2002). In fact, a high rate of 59Fe-WEHS uptake in Fe-deficient plants could be observed during the period of high phytosiderophore release. The very low rate of 59Fe uptake in conditions of low phytosiderophore release would exclude a massive direct utilisation of the 59Fe-WEHS complex by barley plants. Similar mechanisms have been proposed for Fe uptake from microbial siderophores by graminaceous plants (Yehuda et al.,1996).

Based on the above observations, it can be concluded that soluble Fe-humate complexes can be regarded as suitable Fe sources both for Strategy I and Strategy II plants.

Table 7-3. Reduction of Fe3+-WEHS or Fe3+-EDTA (1 ^M Fe) by intact roots of Fe-sufficient (+Fe) and Fe-deficient (-Fe) cucumber plants (data from Pinton et al., 1999a).

Growth condition

+Fe -Fe

nmol Fe2+ g-1FW h-1

Fe3+-EDTA

11.6 ± 0.6 (100) 35.2 ± 1.7 (303)

Fe3+-WEHS

26.1 ± 1.2 (100) 205.4 ± 8.2 (787)

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