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

Nutrient Stress
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Liming plays an important role not only in amelioration of agricultural land but
also in reclamation of waste heaps. Liming materials like CaCO 3
, CaO and Ca(OH) 2
provide Ca 2+ and induce an increase in pH due to their alkaline reactions. The heavy
metals which are in higher concentrations in the waste material are phytotoxic under
low pH conditions and the increase in pH with liming cause fixation of these otherwise
toxic heavy metals (Wallace et al., 1966).
7. MAGNESIUM
Magnesium (Mg 2+ ) content varies in different soils. For example, sandy soils contain
0.05% and clay soils have 0.5% Mg 2+ respectively. Magnesium occurs in three different
forms such as exchangeable, non-exchangeable and water soluble forms. Water soluble
and exchangeable Mg 2+ are important sources of Mg 2+ for plants.
The functions of magnesium in plants are related to its ability to interact with
nucleophilic ligands (phosphoryl groups) through ionic bonding. Magnesium is also
involved in regulation of cellular pH and cation-anion balance. Magnesium plays a
significant role as the central atom of chlorophyll (Walker and Weinstein, 1991).
Magnesium is essential for the aggregation of ribosome subunits (Cammarano et al.,
1972) and for the synthesis of ATP (Lin and Nobel, 1971).
A number of enzymes like RNA polymerase, PEP carboxylase and glutathione
synthetase require magnesium (Wedding and Black, 1988). Under Mg2+ deficiency
RUBP-carboxylase activity is shifted in favour of RUBP-oxygenase due to accumulation
of photosynthates in leaves and causing the formation of superoxide radicals (Cakmak
and Marschner, 1992). Magnesium deficient leaves contained low levels of chloroplast
pigments and associated with the accumulation of starch that results in the increase of
dry matter yield (Table 6).
Table 6. Effect of magnesium on chloroplast pigments and
dry weight in Rape leaves*
Mg 2+ level Dry weight (%) Chlorophyll Carotenoids
(mg/gm fresh wt.) (mg/gm fresh wt.)
Adequate 13.6 2.33 0.21
Inadequate 17.7 1.33 0.11
*Adapted from Baszynski et al., (1980)
As Mg 2+ is mobile, the deficiency symptoms first appear in the older leaves
and then moves to young leaves. Interveinal chlorosis occurs in older leaves. Older
leaves may become reddish-purple and the tips and margins become necrotic. Magnesium
toxicity is not common. However, high Mg 2+ content in the leaves are caused due to