After reading this article you will learn about the role of calcium in plant growth, nutrition and uptake.
Most plants usually contain Ca in adequate amounts and that may be in range of 5-30 mg g-1 Ca on dry weight basis. Such high amounts of Ca content in plants may be due to relatively its higher concentration in the soil solution rather than from the efficiency of the Ca-uptake by the root.
In contrast to other macro-nutrients, a high proportion of the total Ca in tile plant tissue is often located in the cell walls (apoplasm). Such unique distribution is the result of abundance of binding sites for calcium in the cell walls as well as of the restricted translocation of Ca into the cytoplasm.
With an increase in Ca supply, the proportion of Ca-oxalate increases in many plants. High Ca concentrations are found in the middle lamella of the cell wall, at the exterior surface of the plasma membrane, in the endoplasmic reticulum (ER), and in the vacuole.
However, the distribution of Ca in plant cells is shown below:
Calcium bound as pectate in the middle lamella is essential for strengthening of the cell walls and of plant tissues. This function of Ca is clearly reflected in the close positive correlation between cation exchange capacity of cell walls and Ca content in plant tissues required for optimal growth.
The degradation of pectates is facilitated by polygalacturonase, which is drastically inhibited by high Ca concentrations. Increasing Ca content of fruits, leads to an increase in the firmness of the fruits and delyas fruit ripening. Calcium provides cell wall rigidity by cross-linking the pectic chains of the middle lamella.
Callose formation is also influenced by the supply of Ca. In addition, callose deposition in the stigma in response to incompatible pollination appears to be a calcium dependent process. Calcium stabilizes cell membranes by bridging phosphate and carboxylate groups of phospholipids and proteins. Calcium also protects the damage of plant tissues caused by freezing-thawing stress.
In Ca-deficient plants, the impairment of membrane integrity leads to increased respiration rates which cause an enhancement of leakage of respiratory substrates from the vacuole to the respiratory enzymes in the cytoplasm. Calcium helps to maintain cation-anion balance and also plays an important role for the osmoregulation of plant cells.
The most important function of cytoplasmic Ca is related to calmodulin, a polypeptide compound consisting of 148 amino acids that are heat tolerant and insensitive to pH changes. Calmodulin is able to bind four Ca ions forming a compact structure by change of conformation and displacing a hydrophobic part of the polypeptide chain.
This hydrophobic (water repellant) portion of calmodulin complex can be adsorbed by enzyme proteins and can activate enzymes by allosteric induction. However, a number of enzymes such as cyclic nucleotide phosphodiesterase, adenylate cyclase, membrane bound Ca-ATPase, NAD-kinase are activated in this way.
Calcium occurs in plant tissues as free Ca2+ ion, as Ca2+ adsorbed to in diffusible ions like carboxylic, phosphorylic and phenolic hydroxyl groups. It is also present as Ca-oxalates, carbonates and phosphates, and these compounds are deposited in vacuoles.
In seeds, Ca accumulates as the salt of inositol hexaphosphoric acid (phytic acid). It is evident that the uptake of Ca and its translocation within the plant takes place mainly through passive process.
Calcium in the xylem sap is trans-located in an upward direction along the transpiration stream and therefore, the upward movement or translocation within the plant is controlled by the rate of transpiration. On the other hand, the rate of downward translocation of Ca2+ is very low because of very low amount of transportation of Ca2+ in the phloem.
Calcium content of plants varies between 0.1 and > 5.0% on dry weight basis depending upon growing conditions of plants. Calcium requirement for optimum growth is much lower in monocotyledons than in dicotyledons. Calcium requirement of plants also affected by the concentrations of other cations in the soil solution.
An increase in the concentration of Ca2+ in the soil solution leads to an increase in the Ca content in the leaves. Plants have developed mechanisms for restricting the transport of Ca to different organs of plants by maintaining low concentration of Ca in the phloem sap or by precipitation as Ca-oxalate in the sieve tubes or seed coats.
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