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

Nutrient Stress
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Table 16.
Relationship between chloride content in leaves and
growth disorders in coconut plants*
KCl application Leaf content Growth disorders (%)
(Kg/plant)
(% dry wt.)
K Cl Frond fracture Stem cracking
0 1.61 0.07 11.6 27.0
2.25 1.64 0.41 1.7 8.1
4.50 1.66 0.51 1.2 4.5
*Adapted from Von Uexkull (1985).
Chloride toxicity is a worldwide phenomenon and is an important factor limiting
plant growth. The general symptoms are burning of leaf tips or margins, bronzing,
premature yellowing and abscission of leaves. In some cases growth is reduced without
any visual symptoms.
15. NICKEL
Nickel is a recently discovered micronutrient that can form chelate compound and can
replace other heavy metals from physiologically important centres of metabolism. Most
of the soils contain small quantities of nickel (< 100 ppm). Nickel is closely related to
iron and cobalt in chemical and physiological properties. Although nickel may exist in
the oxidation state of Ni(I) and Ni(II) but the most preferred oxidation state in biological
system is Ni(II) (Cammack et al., 1988).
The function of nickel in higher plants was first time shown by Dixon et al.,
(1975) in connection with the urease activity. Eskew et al., (1984) reported that leguminous
plants require nickel irrespective of the form of nitrogen source. Later, the essentiality
of nickel for non-legumes was established (Brown et al., 1987). By these findings
nickel is regarded as an essential element. Nickel is a component of urease, microbial
dehydrogenases, hydrogenases and methyl reductase. It plays a significant role in urea
and ureide metabolism, iron absorption, nitrogen fixation and seed development.
Nickel-deficient cowpea plants accumulated large amount of urea in tip of the
leaf blade. Ureide levels were not affected by nickel supply (Walker et al., 1985). Brown
et al., (1990) found that in barley the critical deficiency level of Ni is about 0.1 µg/g dry
weight and associated with the accumulation of nitrate and amino acids. Nickel deficient
plants show interveinal chlorosis and necrosis in leaves. Normally the nickel
content of many plants is in the range of 1-10 µg/g dry matter. In some plants like lupin,
nickel is preferentially translocated to seeds (Table 17).