Agriculture Reference
In-Depth Information
in the tangential walls of its cells. Calcium ions cannot move through it to
the stele by mass flow, diffusion or ion exchange as they do through the free
space, but must pass through at least two membranes. In general it seems that
calcium movement through the symplast, i.e. the cytoplasm and connecting
plasmodesmata, is much slower than that through the apoplast. Root tips and,
temporarily, the sites of branch root formation where the development of the
Casparian strip appears to lag behind endodermal cell division, may therefore
be particularly effective in Ca uptake (Bangerth,
). As
a consequence Ca uptake may depend largely on current root growth and
branching.
There are, however, other plants in which the endodermis does not act as a
barrier for Ca (Chino,
; Ferguson,
). For apple and pear the traditional view has been
that the endodermis provides a major barrier and that most Ca uptake is by
white, unsuberized, roots. Atkinson and Wilson (
), however, found
Ca to
be taken up to a similar extent by white and woody roots. They considered that
the failure of the phellogen of woody roots to act as a barrier to Ca (and water)
movement could be related to the deposition of the suberin on the inside of
the cell walls rather than within these, so that the apoplastic pathway remains
viable.
There is also evidence of involvement of processes dependent on metabolic
energy in Ca uptake. Reduction in photosynthesis by use of photosynthetic
inhibitors, or prevention of translocation of assimilates to the roots by ringing,
decreases Ca uptake by apple root systems and is reversible by supplying
sucrose (Faust,
).
Upward movement
Upward movement of Ca from the roots is at least primarily in the xylem. It is
influenced by the transpirational fluxbut is not simply a matter of mass flow.
Ferguson and Bollard (
) found that movement of
Ca through excised
pieces of apple stem was much slower than that of phosphate supplied at the
same time. It appeared to be by exchange processes rather than mass flow.
Some of the Ca entering the xylem becomes more firmly bound and some
moves into phloem tissues. Bradfield (
% of the Ca
in the xylem sap of apple shoots was in the ionic form. The remainder was
present as complexes with citric and malic acids. He concluded that the mobi-
lity of Ca in the xylem might be influenced by the supply of organic acids in
the sap which could reduce the degree of adsorption at the negatively charged
exchange sites in the xylem vessels. In the intact plant under conditions of
steady water and Ca supply, Ca is likely to move with the mass flow of the
transpiration stream in the larger conduits, particularly when exchange sites
are saturated or in equilibrium with the Ca in the vessel volume. In smaller
) found that about
