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

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K. Janardhan Reddy
endoplasmic reticulum and vacuoles. In the cytosol, calcium concentration is very low
and maintained in the range of 0.1-0.2 µM (Evans et al., 1991). Calcium is bound as
calcium pectate in the middle lamella and is essential for strengthening of cell walls and
tissues in plants. Calcium is also required for root elongation. Calcium stabilizes cell
membranes by bridging phosphate and carboxyl groups of phospholipids (Cladwell
and Haug, 1981) and proteins (Legge et al., 1982) in the membrane. When calcium
supply is low, 50% of total calcium can be bound as pectate in tomato (Armstrong and
Kirkby, 1979b). Calcium stimulates α-amylase activity in germinating seeds (Bush et al.,
1986).
In the cytosol, calcium binding proteins known as calcium modulated proteins
like calmodulin (Snedden and Fromm, 2001), calmodulin binding proteins (Reddy et al.,
2002) and calcium dependent but calmodulin independent protein kinases are the targets
of calcium signals (Roberts and Harmon, 1992; Harmon et al., 2001). Calmodulin is an
ubiquitous protein present universally in all eukaryotic cells. It is a polypeptide with
148 amino acids and four binding sites for calcium and it is heat stable and insensitive
to pH changes. Calmodulin activates a number of enzymes like NAD kinase, adenylate
cyclase and membrane bound ATPase.
Calcium requirement is lower in monocots than in dicots. For example 2.5 µM
supply of calcium in rye grass and 100 µM of calcium in tomato are required for maximum
growth (Table 5).
Table 5. Effect of calcium on relative growth rates and
calcium content in shoots of Rye grass and tomato*
Calcium conc. Calcium content (mg/gm dry wt.) Relative growth rate
(µM) Rye grass Tomato Rye grass Tomato
0.8 0.6 2.1 42 03
2.5 0.7 1.3 100 19
10 1.5 3.0 94 52
100 1.7 12.9 94 100
*Adapted from Lonergan and Snowball (1969)
Calcium deficiency is very rare if the pH is maintained properly. The deficiency
symptoms first appear in growing tips and young leaves because of low mobility of
Ca 2+ . The tips and young young leaves become chlorotic followed by necrosis of leaf
margins. Calcium deficiency induces premature shedding of fruit and buds. Excess
calcium in the growth medium interferes with Mg 2+ absorption. High Ca 2+ usually causes
alkaline pH which in turn precipitates many of the micronutrients making them unavailable
to plants.