the translocation of minerals in trees - Canadian Forest Service

on the fact that the living cell is not in equilibrium with the environment (Steinbach,
1951). Concerning this aspect, the studies of Epstein and Hendricks
(1955) on ion transport in the roots of higher plants have shown that ions move
freely into and out of an "outer" space of the roots. This takes place by diffusion
and exchange, independently of the simultaneous active transport of the same
ions which results in their transfer into an "inner" space where they are no
longer exchangeable with the same or other ions. Epstein (personal communication)
considers that these inner and outer spaces in the root do not represent
distinct tissues but rather parts of cells. The outer space is intracellular and is
identified with the cytoplasm. Since entry into and exit from this space is by
diffusion, it develops that the outer membrane of cytoplasm is quite permeable
to ions, and therefore is not the membrane which operates the active metabolic
mechanism of ion transport. These exchange and diffusion processes are
reversible, non-selective, non-metabolic, and come to equilibrium approximately
within an hour after immersion of the roots in a new solution. Kramer and
Wiebe (1954) noted that the meristematic region of the roots accumulates large
amounts of minerals but very little is translocated from it to other parts of the
plant. They concluded that most of the minerals translocated out of the roots
are absorbed by the root-hair zone, or region of differentiation.
RELATIVE ROLES OF XYLEM AND PHLOEM IN TRANSLOCATION
There has been conflicting evidence as to the relative roles played by xylem
and phloem in the up\vard translocation of minerals. Some physiologists
(Curtis and Clark, 1950) do not consider any difference in the mechanisms of
transport of inorganic and organic substances, except that the latter are more
usually transported and required by different parts of the plant in larger amounts.
Curtis (1925) found removal of the bark (ringing) would interfere with the
upward transfer of solutes. In his experiments, divided stem were used, \\'here
water was supplied to the top by one set of roots and nitrogen by another set.
Results indicated that if the roots supplied with nitrogen were connected to the
tops by xylem only, there was little transfer of nitrogen, while if they were
connected by phloem only, considerable transfer occurred. Diffusion alone
could not account for this rapid movement, and Curtis thought that protoplasmic
streaming within the cells may have been a factor.
Evidence showing that there is no direct relation between water absorption
and salt absorption was given by Muenscher (1922) in his work on the effect of
transpiration on· ab orption of salts by plants. Mason, Maskell, and PhiJIis
(1936) verified this work and mentioned that oxygen deficiency, induced locally,
could reduce transport, but not always, since the transpiration stream may carry
enough oxygen.
It is considered that minerals can move upward through the phloem,
especially if the leaves have not expanded. There are conflicting observations
on the importance of the phloem in upward transport from roots to leaves.
Stout and Hoaglund (1939), using radioactive tracers, found that when the
xylem and phloem were separated, upward transport took place almost entirely
in the xylem. Curtis and Clark (1950) compared these negative results for
phloem transport with the positive results of Biddulph and Markle (1944).
The latter authors allowed their plants to stand overnight to recover from the
shock of having the phloem separated from the xylem and they found phosphorus
movement upward and downward through the phloem.
It is probable that either phloem or xylem alone can transport the required
minerals. Curtis and Clark (1950) summarize mineral transport thus: "that
the xylem is concerned with mineral transport more (1) in herbaceous plants
than in woody; (2) in those plants showing active root pressure than in
those without root pressure; (3) near the base of the plant than in the
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