Agriculture Reference
In-Depth Information
Roles of calcium in plants and animals
Calcium occurs at a median level of ca. 0.5 % by dry weight in higher plant tissues
while maximum concentrations in excess of 9 % are known in some calcicole species
(Table I.15). Monocotyledonous plants require lesser quantities than dicotyledons. Most
of the calcium in higher plant tissues occurs in exchangeable form associated with the
cell walls or the plasma membrane; intra-cellular calcium is most concentrated in the
mitochondria and leaf vacuoles (Marschner, 1995).
As a Group III element, calcium functions to regulate ion balance and osmotic
potential in plants. In addition to this, it has a major role in stabilising membranes
and maintaining cellular integrity. Calcium also has an important role in stabilising cell
walls and is essential to root elongation. It also has a minor role in activating a number
of enzymes.
Calcium is poorly mobile within the phloem, in cell-to-cell movement and between
tissues. Because of its low mobility in the phloem, the calcium needs of roots must be
largely supplied from the soil solution surrounding the root tip (Marschner, 1995);
this requirement may limit the penetration of the subsoil by plant roots in acid soils.
Little calcium is withdrawn from leaf tissues prior to abscission and forest and other
litters may often have higher concentrations of this element than the foliage from which
they were derived.
In higher animals, calcium comprises a major proportion of the structural tissues
(bone, teeth) and occurs in notable concentrations in the blood plasma. It plays important
enzymic roles in the transmission of nerve impulses and in muscle contraction. It is also
important in the coagulation of blood. In some soil animals ( e.g., Isopoda, Amphipoda,
millipedes), calcium is an important structural component of the exoskeleton; calcium
concentrations strongly influence the distribution of earthworms (Lee, 1985) and other
soil organisms.
Roles of magnesium in plants and animals
Magnesium normally occurs in lower concentrations than calcium in the earth's crust,
soils, higher plants and other terrestrial organisms. Typical concentrations in the earth's
crust are about 1.3 % while non-calcareous soils contain ca. 0.5 %; concentrations in
higher plants are ca. 0.2 % with maximum values up to approximately 1 % of dried weight.
In addition to its important role in controlling intra-cellular osmotic potential,
magnesium is the central atom of the chlorophyll molecule and is therefore critical to
photosynthesis, and in protein synthesis. It is also responsible for activating a wide range
of enzymes and, together with calcium, has a structural role in stabilising the cell wall
(Marschner, 1995). In contrast to calcium, magnesium is highly mobile in the phloem.
In higher animals, magnesium is associated with calcium and phosphorus and ca. 70 %
of the magnesium in the body is found in the skeleton (McDonald et al., 1988). As in
plants, magnesium plays an important role in activating a number of enzymes.
Roles of calcium and magnesium in soils
In contrast to its immobility in plant tissues, calcium is relatively mobile in soils within which
it moves largely by mass flow. Encrustations around plant roots testify to its movement in
the mass flow of water and its contained elements caused by the transpiration stream.
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