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There is no evidence to suggest that plants are unable to take up ferric
iron (+3)... its just that the solubility is so low. There is also no
evidence to suggest that chelators such as EDTA sequester the reduced
ferrous iron... EDTA can chelate ALL metals, regardless of charge. There
are also abundant natural chelators in water that may enhance iron uptake.
The mechanism for plant uptake of chelated iron is also poorly
understood... though it is interesting to note that while EDTA enhances
the uptake of Fe, it reduces the uptake of Cd and other toxic metals...
likely because the uptake rate of chelated iron is faster than the uptake
of aqueous Fe+3 while the uptake rate of chelated Cd is slower relative
to the uptake of aqueous Cd+2 (even though the uptake of FeEDTA and
CdEDTA are likely identical). This suggests some sort of universal
chelator receptor in plant membranes which is not sensitive to the
chelated metal but rather to the chelator itself.