Effects on Enzymes


Historical Information
  Determination of Essentiality
Functions in Plants
  Effects on Membranes
  Role in Cell Walls
  Effects on Enzymes
  Interactions with Phytohormones
  Other Effects
Diagnosis of Calcium Status in Plants
  Symptoms of Deficiency and Excess
  Concentrations of Calcium in Plants
    - Forms of Calcium Compounds
    - Distribution of Calcium in Plants
    - Calcicole and Calcifuge Species
    - Critical Concentrations of Calcium
    - Tabulated Data of Concentrations by Crops
Assessment of Calcium Status in Soils
  Forms of Calcium in Soil
  Soil Tests
  Tabulated Data on Calcium Contents in Soils
Fertilizers for Calcium
  Kinds of Fertilizer
  Application of Calcium Fertilizers

Unlike K+ and Mg2+, Ca2+ does not activate many enzymes (19), and its concentration in the cytoplasm is kept low. This calcium homeostasis is achieved by the action of membrane-bound, calcium- dependent ATPases that actively pump Ca2+ ions from the cytoplasm and into the vacuoles, the endoplasmic reticulum (ER), and the mitochondria (20). This process prevents the ion from competing with Mg2+, thereby lowering activity of some enzymes; the action prevents Ca2+ from inhibiting cytoplasmic or chloroplastic enzymes such as phosphoenol pyruvate (PEP) carboxylase (21) and prevents Ca2+ from precipitating inorganic phosphate (22).

Calcium can be released from storage, particularly in the vacuole, into the cytoplasm. Such flux is fast (23) as it occurs by means of channels from millimolar concentrations in the vacuole to nanomolar concentrations in the cytoplasm of resting cells (24). The calcium could inhibit cytoplasmic enzymes directly, or by competition with Mg2+. Calcium can also react with the calciumbinding protein calmodulin (CaM). Up to four Ca2+ ions may reversibly bind to each molecule of calmodulin, and this binding exposes two hydrophobic areas on the protein that enables it to bind to hydrophobic regions on a large number of key enzymes and to activate them (25). The Ca2+–calmodulin complex also may stimulate the activity of the calcium-dependent ATPases (26), thus removing the calcium from the cytoplasm again and priming the whole system for further stimulation if calcium concentrations in the cytoplasm rise again.

Other sensors of calcium concentration are in the cytoplasm, for example, Ca2+-dependent (CaM-independent) protein kinases (25). The rapid increases in cytoplasmic Ca2+ concentration that occur when the channels open and let calcium out of the vacuolar store and the magnitude, duration, and precise location of these increases give a series of calcium signatures that are part of the responses of a plant to a range of environmental signals. These responses enable the plant to respond to drought, salinity, cold shock, mechanical stress, ozone and blue light, ultraviolet radiation, and other stresses (24).