Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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K. Janardhan Reddy
drought conditions. Small necrotic spots occur in older leaves. If the toxicity is more the
young leaves may exhibit spotted appearance.
Although soils are not deficient in Mg 2+ , but with the high supply of other
fertilizers rich in K + and NH 4+
, restrict the Mg 2+ uptake by plants. Therefore, magnesium
application becomes necessary. Sulphate fertilizers are more effective in this respect
than carbonate fertilizers. In sandy soils, Mg 2+ is applied at 80-160 Kg Mg 2+ /ha which
increase yield of various arable crops.
8. IRON
Iron is present in all types of soils making up about 5% by weight of earth’s crust. The
soluble iron content is very low in soils. Ferric (Fe 3+ ) and ferrous (Fe 2+ ) are soluble
forms.
Iron is a transitional element and can change its oxidative state easily and form
octahedral complexes with various ligands. Iron has two oxidation states – Fe 2+ and
Fe 3+ . Metabolically active form of iron is Fe 2+ , which is incorporated in to biomolecular
structures.
Iron is a component of heme proteins, cytochromes, cytochrome oxidase,
catalase, peroxidase, leghemoglobin and nonheme proteins like ferredoxin and
lipoxygenase. Iron is also essential for the biosynthesis of chlorophyll (Pushnik and
Miller, 1989). It plays an important role in biological redox systems and enzyme activation.
Iron deficiency has significant effect on chloroplasts and protein content. The number
of ribosomes decrease under its deficiency. Starch and sugar contents are low under
iron deficiency. Low chlorophyll content, inhibition of photosynthetic electron transport
with reduced regeneration of RUBP may be responsible for low starch and sugar content
and low CO2 fixation in Fe-deficient plants (Sharma and Sanwal, 1992).
Plants can be categorized as iron efficient species (strategy I – dicots and non
graminaceous monocots) and iron inefficient species (strategy II – Graminaceous
monocots). Strategy I is characterized by increased reduction of Fe (III) to Fe (II) at the
root surface. In addition, there is increased extrusion of protons due to the increased
activity of the plasma membrane bound H + - ATPase. This rhizospere acidification also
associated with the increased transmembrane electrical potential for the increased driving
force for Fe (II) uptake (Brancardo et al., 1995). Strategy II is found in Poaceae members
(grasses) under iron deficiency. These plants acquire Fe by releasing non protein amino
acids which are called phytosiderophores (PS). Phytosiderophores form stable complexes
with Fe III. This strategy also includes another component in the form of highly specific
transport system which effectively transfers the Fe III – PS complexes in to the cytoplasm.
Iron deficiency is a world wide phenomena in crop production on calcareous
soils. Deficiency symptoms first appear on young leaves which show interveinal
chlorosis. In some cases, young leaves may be totally devoid of chlorophyll and are
white in colour. In cereals, the leaves show yellow and green stripes along the length of
the leaf.