Iron [Fe, 26]
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Fe deficiency may occur at very different levels in plants, and is dependent on various
factors, such as soils, plants, and climates. The common initial sign of its deficiency
is interveinal chlorosis of young leaves. Several fruit trees and cereals, oats and rice
in particular, are very susceptible to Fe chlorosis. The control of the Fe deficiency
is often not sufficient. There are some suggestions that this problem may be partly
resolved by plant breeding and selection of genotypes of cultivated plants.
Interactions between Fe and other metals are described as the most common factors affecting its deficiency:
r Excess of Mn, Ni, and Co decreases Fe mobility in soils and plants.
r Fe–Zn interaction leads to the formation of franklinite, ZnFe 2 O 4 , which
depresses the availability of both metals.
r Fe–P interaction, in both soils and plants, causes the precipitation of
FePO 4 ·2H 2 O. The appropriate P/Fe ratio is fundamental to plan Fe mobility, and thus availability.
r Fe–S interaction causes a low Fe mobility and availability.
r Fe–Se interaction is associated with the immobilization of Se by Fe minerals and compounds.
r Fe interaction with several trace metals causes chlorosis due to the immobilization of Fe.
Excess iron is toxic to most plants. However, some plants are resistant to this, which
is explained as follows:
r Plant resistance to excess Fe is associated with different reactions, such as
oxidation, immobilization, and exclusion of mobile Fe species.
r Plants high in nutrients, especially in Ca and Si, can tolerate high levels of Fe.
r Roots of some plants are able to oxidize Fe and deposit it on the root surface.
r Mycorrhizas have a great capability to bind Fe at root surface or in root
cells.
r Plants adapted to waterlogged conditions are more tolerant to high Fe levels
than plants grown in well-aerated soils.
Appropriate Fe content in plants is essential for plant growth and for nutrient supply
to humans and animals. Natural Fe contents of fodder plants range from 18 to about
1000 mg/kg. The nutritional requirement of grazing animals is usually met at Fe
concentration within the range of 50–100 mg/kg.
The common range of Fe in cereal grains is from 30 to about 100 mg/kg. Higher
Fe contents are very seldom cited. Usually higher Fe concentrations are in barley and
oats grains (up to 200 mg/kg), than in wheat and rye grains (up to 50 mg/kg). Edible
parts of vegetable contain Fe within mean values from 33 to 65 mg/kg, in carrot and
soybean seeds, respectively. Some nuts and almonds may have higher levels of Fe, up
to 67 mg/kg (Jędrzejczak vide Kabata-Pendias and Mukherjee 2007).
The common mushroom, chanterelles (Cantharellus cibarius) grown in the
mountains of Poland, contains higher amounts of Fe, mean 180 mg/kg, than those
grown in the Baltic Sea coast, mean 43 mg/kg (Falandysz et al. 2012).