Iron reduction by apple roots

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Abstract
A series of experiments was carried out to determine Fe reduction rate of apple (Malus domestica Borkh) seedling roots under Fe‐stress conditions and factors which may affect the rate of reduction. Root Fe reduction decreased with increasing solution pH over the range of 5.0 to 8.0 for Fe‐stressed seedlings while pH had little effect on unstressed seedlings. Iron‐stressed seedlings responded with higher root reduction rates with increased Fe concentrations (23 to 134 uM Fe) with DTPA as the chelating agent. FeDTPA or FeEDTA both resulted in higher root reduction rates compared to FeEDDHA for both Fe‐stressed and unstressed seedlings. The quantity of FeDTPA reducing activity of plant‐free root exudates was measured and found to be about one tenth the rate of reduction by roots, suggesting that enhanced Fe3+ reduction by Fe‐stressed roots was not due to exudates. Several treatments including respiratory inhibitors, girdling of roots and aerobic versus anaerobic conditions were tested. The data indicated that photosynthetic substrates and O2 are needed for Fe3+ root reduction. The overall results lend support to the theory that Fe3+ is reduced by a plasma membrane‐bound enzyme.