The movement of calcium in plants

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Abstract
Calcium movement in plants is unidirectional, moving up from the roots and generally routed to meristematic zones and young tissue. Once deposited in leaf tissue it is not recycled, even under calcium stress conditions. The upward movement of calcium takes place in the transpiration stream through the xylem. The calcium ions do not move by mass flow but by a series of exchange reactions along negatively charged sites on the walls of the xylem vessels. Movement can be promoted by the presence of divalent cations and by chelation of the calcium ion. There is considerable lateral leakage of calcium from the xylem. There is overwhelming evidence to show that calcium is not translocated in the sieve tubes of the phloem despite the presence of small amounts in phloem exudates. Calcium appears to be selectively excluded from the sieve tube, possibly to prevent interference with the translocation processes. Foliar applied calcium is normally immobile, but can be induced to translocate by the saturation of adsorption sites in the leaf with divalent cations or by chelation. Best evidence shows that the calcium moves in a reverse xylem flow down water‐potential gradients. The import of calcium into fruit diminishes with development and virtually stops with the onset of the rapid intake of photosynthates via the phloem. As a consequence, fruit are inherently low in calcium. Buried storage organs acquire most of their calcium directly from the substrate and not from the plant. In woody plants, the upward movement of calcium is relatively slow with much accumulation in the bark and little in the wood. In the herbaceous plant, calcium is rapidly immobilized, whereas in the apple, calcium continues to move up the tree over a number of seasons. This secondarily translocated calcium comes from reserves in the root and lower shoot, and is a major source of calcium for the new growth at the beginning of the season.