While the previously described transport processes through the soil to the central
cylinder are
generally referred to as short-distance transport, the movement of nutrients in the xylem and
phloem is considered long-distance transport.
Xylem Transport:
- The process of nutrient uptake is
completed by their secretion into the xylem.
- Xylem sap contains inorganic ions,
as well as amino acids, amides, organic acids, sugars,
and phytohormones.
- Xylem transport is driven by transpiration,
with velocities averaging 1–100 m/h (mass flow).
- Distribution within the leaves occurs
via the apoplast or symplast.
- The
nutrient elements Ca and B are transported almost exclusively in the xylem.
Phloem Transport:
- The phloem primarily transports photosynthates
and amino acids (which are essentially
"collected" by the fine leaf veins and released into the vascular bundles by the sieve cells
of
larger veins).
- Loading of the phloem is metabolically
dependent and active.
- Transport velocity in the phloem
is approximately 1 m/h.
- The
following elements are not phloem-mobile: B, Mo, N (as nitrate), and Ca is only very
limitedly mobile
