Calcium physiology and metabolism in fruit trees

Calcium plays an important role in plant growth and development. Its importance as a mineral nutrient has two main purposes: apoplastic and symplastic. Apoplastic calcium has an important structural and functional role in the plant cell walls and membranes. Cell wall is reach in pectinaceous materials that contribute to cell cohesiveness and is believed to constitute an important site of calcium interaction. Integrity of the membranes is important for maintaining its selectivity. In these cases a millimolar calcium concentration is necessary in order to fulfil its requirements as cementing material and as source of calcium readily accessible to membrane surface and subsequently to internal compartments. The symplastic concentration of calcium has to be maintained at the micromolar level to serve as an intracellular "second messenger". This is maintained through the action of different transport mechanisms localised on the membranes which extrude the ion from the cytosol towards the cell wall or other intracellular organelles. Transient increase in cytosolic calcium is the way to activate metabolic reactions in response to extra- or intracellular signals. Calcium alone or its complex with calmodulin activates protein kinases that through covalent modification of other target enzymes will modulate the physiological responses.