MOVEMENT OF CALCIUM IN PLANTS

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. © 1979, Taylor & Francis Group, LLC. All rights reserved.