Agriculture Reference
In-Depth Information
has been observed in important crops such as corn, sorghum, peanuts, soybeans, common bean,
oats, barley, and upland rice (Fageria et al., 2006), whereas Fe toxicity is mostly restricted to flooded
or lowland rice (Fageria et al., 1990). In general, monocotyledonous species are less Fe efficient than
dicotyledonous species. Plants are classified as Fe efficient if they respond to Fe deficiency stress by
inducing biochemical reactions that make Fe available in a useful form and Fe inefficient if they do
not. These induced reactions or compounds are: release of hydrogen ions from their roots, release
of reducing compounds from their roots, reduction of Fe
3+
to Fe
2+
at their roots, and increases in
organic acids in their roots (Brown, 1978).
Iron is required in higher plants for chlorophyll synthesis. Iron is a component of many enzymes
that catalyze the metabolism of plants. In general, enzymes containing metal are divided into two
groups. One group is known as a metal activator and the other is known as a metalloenzyme. In
the former, Fe acts as a temporary link between the enzyme and the substrate during biochemical
reactions, hence activating a number of oxidases. However, the majority of Fe enzymes belong to
the metalloenzyme group, in which Fe is firmly bound to a protein. Frequently, Fe is chelated by
or attached to a small molecule called a prosthetic group; peroxidase, catalase, and cytochrome
oxidase all contain Fe bound in the heme group. This means that Fe nutrition has a close relation-
ship with these oxidase activities (Okajima et al., 1975). Among the Fe enzymes, the ferredoxins
are of special interest because of their importance in photosynthesis. Ferredoxins are small protein
molecules containing Fe and labile sulfides. They catalyze the phosphorylation of ADP in the pres-
ence of light.
Iron is immobile in plant tissues; therefore, its deficiency first appears in the young leaves as an
interveinal chlorosis. At an advanced stage, the entire leaf blade may become yellow or white. The
leaf veins are the last to lose chlorophyll. In green leaves, approximately 80% of the iron is located
in the chloroplasts (Terry, 1980); thus, iron deficiency affects processes located in the chloroplasts.
In higher plants, one of the most obvious effects of iron deficiency is the development of chlorotic
leaves. Chlorosis is associated with a loss of not only chlorophyll but also all thylakoid constituents.
The reduction in thylakoid membranes during iron deficiency is accompanied by decreases in all
photosynthetic pigments (Terry and Abadia, 1986; Monge et al., 1987). Some of the environmental
factors that can induce iron deficiency in crop plants are (i) low iron content of the soil, (ii) high
level of lime application or lime-induced chlorosis, (iii) poor aeration, (iv) high P concentration,
(v) high levels of Mn, Zn, and Cu, (vi) high light intensity, (vii) high level of nitrate, (viii) high or low
temperatures, (ix) unbalanced cation ratios, (x) addition of organic matter to soil, (xi) virus infection
(Hale and Orcutt, 1987), and (xii) liming acid soils.
Tissue analysis as ordinarily performed does not distinguish between functional iron and that
inactivated by precipitation or complexing. Active iron is that which is extracted by normal hydro-
chloric acid from grounded dry plant tissue (Hale and Orcutt, 1987). Iron uptake reduced signifi-
cantly with increasing soil pH (Figure 6.12). At pH 4.9, the value of Fe uptake by dry bean plants
was 328 mg kg
−1
shoot dry weight, while at pH 7.0, the Fe uptake value dropped to 96 mg Fe kg
−1
.
This means that the decrease at the highest pH value was 242% as compared to the lowest pH value.
Plants have different strategies for solubilization and uptake of iron. Graminaceous plants exude
phytosiderophores or Fe bearers, for example, mugeneic or avenic acids that carry Fe to the roots
(Romheld, 1987). Phytosiderophores have been defined by Takagi et al. (1984) as a group of root
exudates exhibiting strong complexing properties with respect to ferric Fe and identified as nonpro-
teinogenic amino acids, such as mugineic acid and its derivatives. In this respect, they are analogs
of microbial siderophores, which are literally iron bearers (Hinsinger, 1998). The literature on this
topic has been extensively reviewed by Romheld and Marschner (1986), Marschner et al. (1989), and
Romheld (1991). The synthesis and release of phytosiderophores in the rhizosphere are stimulated
by Fe deficiency (Romheld, 1991) and have been described as strategy II for Fe acquisition as devel-
oped exclusively by graminaceous species (Marschner, 1995). Graminaceae species differ widely in
their ability to produce phytosiderophores, both quantitatively and qualitatively. Most remarkably,
among the range of graminaceous species studied by Marschner et al. (1989), the enhancement of
