the translocation of minerals in trees - Canadian Forest Service

solutions and determining the mineral content of plant tissues by ashing are two
important methods for determining mineral requirements and plant reactions
to deficiencies.
Biddulph (1953) considers that reactions involving the precipitation of
certain mineral nutrients in the xylem extremities may be respon ible for the
failure of delivery of some minerals to the leaf mesophyll. Such elements as
zinc and iron form insoluble precipitates with phosphorus. Rediske (1950)
has shown deposition of radioiron in the veins of bean leaves by creating particular
pH and phosphorus levels within the nutrient solution, namely pH 7·0, 0·0001 M
phosphorus and 1 ppm iron. At higher phosphorus levels, iron was largely
precipitated in the roots and little entered the aerial parts. The nutrient
conditions effecting this were pH 7·0, 0·001 M phosphorus and 1 ppm iron.
With nutrient solution at pH 4·00 but other conditions as first mentioned, the
leaves were a healthy green indicating a normal upply of iron.
Certain elements have a characteristic distribution in plants. Some, like
phosphorus, are freely mobile within the plant so that a supply ab orbed early
in the season may be utilized elsewhere in the plant when none is available to
the roots. Sulphur behaves similarly. Calcium is different from phosphorus
and sulphur in that it i usually deposited and the part which is absorbed earlier
is of no value for later new growth. Iron is like calcium, although a very pronounced
absorption in the roots may be released later to newly growing part .
Movement of minerals from leaves to other parts of the plant may be followed
by tracing a radioisotope which was applied to a leaf. The conditions which
control movement of minerals from mature leaves vary with the element studied.
Phosphorus and sulphur move out freely and are little affected by the nutrient
conditions of the plant prior to, or during, the experimental period. Iron,
calcium, and zinc ordinarily move from the leaf in very small amounts and can
be further immobilized by a high concentration of pho phorus in the tissues of
plants grown at pH 7·0 or above.
Biddulph (1951) introduced water made with tritium in which phosphorus32
was dissolved, directly into the leaves, and then isolated the two radioactive
components in the stem and root tissues below the point of introduction. No
marked movement of the water took place in the phloem tis ue of the stem, but
the phosphorus32 introduced with the tritiated water moved freely. He considers
the mechanism of translocation of minerals in the phloem not simple diffusion
ince it is activated in some manner to attain speeds approaching three feet
per hour.
The roots require minerals for their growth and consequently compete for
elements which are to be translocated to the aerial parts. Biddulph (1953)
investigated this aspect of retention and transport by the root for phosphorus
and rubidium. This study extended over a 24-hour period so that diurnal
variations in both retention and translocation were evident. The translocation
cycle wa found to be light-sensitive as well as to possess an inherent cyclic
phase independent of light.
Bu gen and Munch (1929) state that trees obtain their supply of minerals
at different periods in the growing season. Larch and pine take up most of their
potassium from mid-June to mid-September, while spruce supplies itself with
this element from mid-May to mid-June. The larch shows light absorption
of phosphorus in summer and a great absorption in autumn, while pine absorbs
phosphorus exclusively in late summer, and spruce only in the spring and early
ummer. Ramann and Bauer (1912) showed a corre ponding behaviour for
nitrogen in these species and considered it a reason for the success of mixedwoods
as compared to pure stands.