CANADA
Department of Northern Affairs and National Resources
FORESTRY BRANCH
THE TRANSLOCATION OF MINERALS
IN TREES
by
D. A. Fraser
Forest Research Division
Technical Note No. 47
1956
Published under the authority of
The Minister of Northern Affairs and National Resources
Ottawa, 1956
CONTENTS
PAGE
INTRODUCTION.. .
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ADVANTAGES AND METHODS OF USING RADIOISOTOPES IN TRANSLOCATION
STUDIES........ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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SUMMARY A D CONCLUSIONS...... . ......... . ....... . .. . . . ...... . ...
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REFERENCES. .
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RELATIVE ROLES OF XYLEM AND PHLOEM IN TRANSLOCATION. . . .
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The Translocation of Minerals in Trees!
Project P-377
by
D. A. Fraser�
INTRODUCTION
Green plants manufacture their own food from carbon dioxide and water
through the process of photosynthesis. However, the presence of other elements
in addition to carbon, hydrogen and· oxygen is essential both to the manufacturing
process and to the healthy development of the plant. For almost a century
there ,,,as a general as umption that only seven additional chemical elements­
nitrogen, sulphur, potassium, calcium, magnesium, phosphorus and iron-were
required. In the last decade, in addition to the ten elements which have been
mentioned, several others have been found to be essential.
Carbon and oxygen are obtained chiefly from the air through leaves of the
tree, and the remaining elements are absorbed from the soil by the roots and are
translocated to other parts of the plant.
Trees require these essential elements for optimum growth, but the amounts
and the time of their absorption from the soil depend on the species as well as
on site conditions. Since silviculture is the art and science of cultivating forest
crops, the forester must try to select treatments most appropriate to site con­
ditions. It is true that empirical observations have greatly assisted the forester
in assessing the quality of site as reflected in the volume of merchantable timber.
The study of mineral requirements of tree species as well as the mineral trans­
location within the tree will assist in site evaluation and recognition of diseases
caused by mineral deficiencies. The present contribution reviews the literature
pertaining to the translocation of minerals in trees and includes data from
experiments in progress at the Petawawa Forest Experiment Station.
The treatment of physiology of trees apart from that of plant physiology in
general may be justified by the special research methods necessitated by the size
of trees. Usually instead of the regular laboratory techniques, those of experi­
mental ecology, more suited to open-air conditions, have to be applied. How­
ever, trees are still plants and therefore it is necessary to review translocation of
minerals in plants before giving special consideration to trees.
The first phase in mineral transport involves the absorption of the element
by the root, its movement across the cortex to the stele, and its subsequent
upward translocation. Investigators have focused their attention on this first
phase, and endeavoured to explain movement of minerals through recognized
physical-chemical reactions that could result in a cellular transfer of minerals.
Such processes include diffusion, ion exchange, Donnan equilibria and membrane
potential. Although each of these mechanisms is important and could eventually
in itself lead to the establishment of equilibria, yet one of the characteristics of
ion transport in biological systems is its dependence on active metabolism, and
1 Invitation paper presented at the annual meeting of Canadian plant physiologists, October 3!, 1955, Ottawa, Ontario,
Canada.
'Tree physiologist, Petawawa Forest Experiment Station, Chalk River, Ontario.
5
on the fact that the living cell is not in equilibrium with the environment (Stein­
bach, 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 communi­
cation) 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 protoplas­
mic 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
6
upper part; (4) when there is an excess of the particular element than when
it is deficient; (5) more, especially of nitrogen, when the roots have a low car­
bohydrate content than when it is high; (6) possibly more of the salts like calcium,
zinc, and iron, that are not greatly accumulated in living cells; and (7) possibly
more of the elements that are less likely to be carried in combination with organic
molecules" .
ADVANTAGES AND METHODS OF USING RADIO ISOTOPES IN
TRANSLOCAT ION STUDIES
In older methods of research on translocation, dyes )vere used which were
injected into the tree, and could be detected visually when the tree was cut down.
This technique was applicable more to the study of water movement than to
that of translocation of minerals. Other workers used poisons and they hoped
that the movement of a poison would be accompanied by external symptoms
such as a change in appearance of the foliage. It was not until the radioisotope
became available that investigators could tag an element which would emit
detectable radiation, and thus permit its observation without k
· illing the tree.
There are several kinds of radiations. According to the present atomic
theory, the atom is made of a nucleus about which revolve particles known as
electrons. The mass of the atom is found primarily in the nucleus which, for
present purposes, consists of protons having a positive charge and neutrons
having no charge. The neutrons affect the mass but not the chemical properties
of the element, and atoms which vary in nuclear mass but have the same chemical
behaviour are called isotopes. When these emit radiations which may be
detected by various types of counters, they are referred to as radioisotopes.
Radiations are of three types depending on the characteristics of the radioisotope.
(1) Alpha rays:
are helium nuclei or alpha particles which have a relatively large mass compared with electrons but possess a
very limited penetration power.
(2) Beta rays:
are negative electrons emitted from atomic nuclei; these
have a limited penetration power but give good resolution
in autoradiography.
(3) Gamma rays: are electromagnetic radiation similar to X-rays; these
are very penetrating and may be easily detected by
external monitoring.
The last two types of radiations are most suitable for biological research
and problems pertaining to forestry.
Radioisotopes are useful in biological research in several ways. They are
very good analytical tools which increase the sensitivity and ease of analysis
available through conventional methods. They also permit the measurement
of ion fluxes in a given direction even when there is no net flux of that ion, or
when the net flux is in the opposite direction. In tree investigations the use of
the radioisotope permits detection within the tree by external monitoring.
This has certain advanLages over older methods since the tree does not have to
be sacrificed in order to determine the translocation of the element within the
tree.
The major minerals required by trees, as indeed by plants in general, include,
in addition to carbon, hydrogen and oxygen, thirteen mineral elements-potas­
sium, calcium, magnesium, nitrogen, phosphorus, sulphur, iron, manganese,
zinc, copper, molybdenum, boron and chlorine. Of these, only nitrogen and
boron have no radioisotopes suitable for tracers. Several other elements such
as sodium, cobalt, and rubidium, although not known to be essential, posse s
suitable isotopes and are of physiological interest. Growing plants in nutrient
7
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 pro­
nounced 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.
Broadleaved trees have a maximum absorption of minerals in early summer
except for calcium, which is absorbed to a greater extent in late summer. Since
the spring shoots and leaves are formed almost entirely from reserve materials,
the older parts of the tree are to a great extent impoverished in mineral matter
by the formation of shoots.
Ramann (1912) , in an investigation of mineral content of deciduous trees
in daytime and night, found that the calcium content rises at night and falls
during the day. He concluded from this that calcium takes part in the transport
of a similates from the leaves during the formation of organic substances in the
day and a replacement of the calcium takes place at night. No migrations of
other mineral substances were found leading to a noticeable difference in the
composition of the ash in the daytime and at night.
Biddulph (1953) considers that a re-export of minerals from leaves is required
since the transpiration stream will continually make a supply of minerals available
at the end of the veins in the leaves. These minerals would be lost to other parts
of the plant for future growth if the excess were not transported back into the
plant body. Phosphorus moving out of the leaf downwards in the phloem, if
not incorporated in living tissue, may move up again through the xylem.
Another pha e of the re-export of minerals from leaf tissue often takes place
before death or abscission. Deleano and Andreescu (1932) investigated changes
in amounts of certain minerals in willow (Salix jmgilis), as it occurs throughout
the season. They found little change in calcium content before leaf fall. Iron,
manganese, and silicon behaved similarly. However, magnesium, potassium,
phosphorus, chlorine, and nitrogen were mobile and more than half was removed
before leaf fall.
ome of this export may have resulted from leaching by
rain or dew.
In a study of movement of radioisotopes in yellow birch (Betula lutea Michx.
f.) and white pine (Pinus strobus L.) , Fraser and Mawson (1953) developed a
special portable scintillation counter for measuring radioisotope movement
along tree trunks. These investigator had previously u ed a portable health
monitor (a Geiger-Mueller counter) to detect radiation in field experiments
(Fraser, 1950) . Although this instrument was quite sensitive, it possessed
numerous disadvantages. The instrument was rather heavy and awkward to
carry, and the short lead to the probe made it difficult to obtain readings higher
up in the tree. In addition, the probe had no attachment to ensure uniformity
of position in repeated observations at different levels. Accordingly an instru­
ment was constructed incorporating the desired modifications (Figure 1).
In this apparatus the batteries are encased in a metal box which fits into a
pack ack provIded with houlder straps. The counter, with a visual indicator
for recording activity, is equipped with a strap which holds it at chest level.
On the right side of the counter are two sockets, one for the insertion of the lead
to a scintillation probe, the other for a Geiger-Mueller probe.
By the scintillation method, ionizing radiation is counted by photoelectric
measurement of the fluorescent light pulses emitted by certain organic substances.
Anthracene is the organic substance used in this probe. A p.ollimator, with its
outer face so indented a to fit securely against the tree trunk, was fitted over
the end of the probe. A notched projection is attached to the collimator so
that when the probe is in use, the notch may be slipped around the projecting
end of a nail inserted into the tree trunk. The probe is then rotated to a hori­
zontal position, thus ensuring a uniform position of the scintillation probe for
repeated observations.
Movements of rubidium86 and calcium45 were followed with this newly­
developed portable scintillation counter. The isotopes were introduced into
soil or nutrient solutions containing branch roots, and also directly into the tree
trunks. For the latter work, rubidium86 carbonate in a solution of five per cent
9
FIGURE
1. Monitoring movement of rubidium86 in yellow birch with portable scintil­
lation counter.
The operator is holding the probe in his left hand, with
the counter strapped to his waist.
potassium chloride was placed in a water-tight trough constructed around part
of the tree trunk. A i-inch chisel with its face parallel to the vertical axis was
used to make an under-water incision to a depth of one inch. U e of the potas­
sium chloride facilitated the entrance and movement of the isotope in the trans­
location stream. The maximum rate of upward movement of the rubidium86
in the xylem of yellow birch approximated one foot per minute along a narrow
channel spiralling upward (usually dextrally) from the point where the isotope
was first introduced. Movement in decadent yellow birch was very slow with
an: apparent increa e of permeability of the bark tissue as indicated by lateral
diffusion of the isotope. In October no upward movement was discerned in
healthy trees but rather a downward translocation in the phloem which may
have been associated with the removal of certain minerals from the senescent
leaves. Samples of phloem and xylem ,vere removed with an increment borer
for radioactive assay, to facilitate the interpretation of the external monitoring
observations.
10
Calcium45 injected into the white pine trunk had a localized distribution
similar to that of the rubidium86 in yellow birch. In the white pine trunk the
isotope moved upwards from the point of infection, along a relatively narrow
channel. Only two branches, both within six feet of the ground, were found to
contain appreciable activity. Monitoring records on August 22, six weeks
after injection, indicated an accumulation of calcium in the main trunk at a
point where the upper active branch had its origin. From this position in the
trunk, most of the calcium was carried out into the branch. The isotope was
not evenly distributed throughout the branch, as in any given whorl of twigs
some were much more active than others.
The internal distribution of the calcium was investigated by autoradiographs.
In this technique (Belanger and Leblond, 1946) sections of tissue are coated with
a photographic emulsion and left for different periods of exposure ranging from
a few hours to several weeks depending on the degree of localization of the
radioisotope and consequent inten ity of radiation to which the photographic
emulsion in contact with the section is exposed. The autoradiographs of ections
of the stem (Figure 2) showed activity in the sieve cells of the phloem in the
white pine.
During September some of the calcium45 concentrations had materialized
into visible crystals. Figure 3 shows a cross section of crystals in the phloem
cells. By focusing up on the photographic emulsion covering the section of
these cells, the dark dots (Figure 4) represent exposure of the photographic
emulsion indicating that these cells contain calcium45•
Presence of calcium45 was also noted in the buds of yellow birch. Figure 5a
presents photographs of the buds, whereas Figure 5b show autoradiographs of
the same buds indicating the presence of calcium45 in the peripheral parts of
these buds.
Moreland (1950) introduced roots of loblolly pine into jars containing
phosphorus32 solution and noted a maximum translocation rate of more than
four feet per hour.
Yli-Vaakuri (1954) in Finland showed that movement of phosphorus32
would take place from tree to tree and also between trees and stumps through
natural root grafts.
Kuntz and Riker (1955) in Wisconsin studied more extensively the use of
radioisotopes for ascertaining the role of root grafts in the translocation of
minerals between trees. Their experiments with iodine131 and rubidium86
included the insertion of cut branches or roots into a bottle containing a radio­
active solution, or injection of a solution from a cone or trough similar to the
technique used by Fraser and Mawson (1953).
The usual rate of upward movement of the isotope in the translocation
stream of oaks in full sunlight and with low relative humidity was between It
and 3 feet per minute. A limited downward movement into the roots was
noticed. Greater movement occurred in roots naturally grafted onto those of
other trees. Radioactivity was detected within 20 minutes in most branches,
twigs, and leaves of pin oak (Quercus ellipsoidalis E. J. Hill) 35 feet high. Radio­
activity appeared only in narrow vertical streaks which originated at the chisel
cuts, and in certain branches of the bur oak (Quercus macrocarpa Michx.) .
The diffuse movement of the isotopes throughout trunks and the crown of the
northern pin oak was quite different from the limited linear flow in the trunk and
certain branches of the bur oak. In this respect the bur oak behaved like the
yellow birch and white pine in the experiments by Fraser and Mawson (1953) .
Movement of the isotopes was reduced by very low light intensities, free moisture
on the leaves, and absence of functional leaves. Little movement occurred
during the dormant period. When dominant and suppressed trees were con­
nected by root grafts, isotopes moved both ways but most commonly from the
11
FIGURE 2. Autoradiograph of longitudinal section of wood and bark of
white pine stem showing localization of calcium" in phloem.
FIGURE 3. Crystals in phloem of white pine (dark
rectangular shapes in centre of picture).
FIGURE 4. Autoradiograph of section shown in
Figure 3. The black areas indicate the presence
of calcium" in the crystals.
12
FIGURE 5a. Longitudinal sections of yellow birch buds
in
natura.
FIGURE 5b. Autoradiograph of yellow birch buds shown in Figure 5a, indicating presence of calcium".
dominant to the suppressed tree. The failure of the isotopes to move in trees
with oak wilt led to the discovery that xylem vessels in the aerial parts were
plugged with tyloses and gum.
Tukey et al (1955) applied cotton gauze which had been dipped into solutions
of potassium42 carbonate and ortho phosphoric32 acid, around branches of apple
trees and peach trees. Within 24 hours of application during February and
March, even with freezing temperatures, radioactivity was detected within the
branches 18 to 24 inches above and below the points of application. Similar
applications made just. as the buds were beginning to swell indicated that the
activity moved through the bark, up through the branches and concentrated
near the buds, presumably to be available for the flush of new growth. The
work of these investigators indicated that nitrogen, phosphorus, potassium, and
rubidium applied to leaves are transported both acropetally and basipetally.
Calcium, strontium, and barium do not move from the absorbing plant part, and
basipetal transport is negligible.
Ferrel and Hubert ( 1952), in a study of the pole blight condition in western
white pine (Pine monticola Dougl.) in Idaho, found that phosphorus and calcium
had unusual distribution in pole-blighted trees. In diseased trees, phosphorus
content was found to be higher than in normal trees and the calcium content
lower. This greater accumulation of phosphorus was considered related to the
more rapid radioisotope movement up the pole-blighted trees and associated
with the greater transpiration. Farrar ( 1953) also noted greater movement of
phosphorus32 into older needles of red pine as compared with newly formed
needles. Tamm ( 1951) reported that the composition of rain water samples
collected beneath trees, as compared with samples from an open field, contained
considerable amounts of calcium, potassium and sodium, together with smaller
amounts of nitrogen and phosphorus.
13
SUMMARY AND CONCLUSIONS
It appears that either xylem or phloem may transport upwards the elements
required, but that phloem may be more important for mineral translocation in
trees. This seemed evident in the localization of calcium45 in the phloem sieve
cells of white pine. The unusual distribution of calcium in pole-blighted western
white pine along with a very high phosphorus content parallels the experiments
where radioactive iron ,vas deposited in the roots or leaf veins and was un­
available to the leaf mesophyll when nutrient conditions had a high pH and an
abundance of phosphorus. The important influence of the soil solution pH
on the uptake and final distribution of iron thus indicates the effect of the
environment on the translocation of minerals within a plant. The use of the
radioisotope has greatly facilitated the study of mineral movement in trees and
led to the development of a new portable scintillation counter especially suited
for tree studies.
The linear flow of minerals in white pine, yellow birch and bur oak was in
contrast to the diffuse movement in northern pin oak. Minerals moved between
trees through natural root grafts. The failure of isotopes to move in trees with
oak wilt led to the di covery that xylem vessels in the aerial parts were plugged
with tyloses and gum.
The availability for tree growth of minerals in the soil and their subsequent
uninterrupted translocation to all parts of the growing tree are essential for
maximum size. Information on the variability of species as to their ability to
utilize minerals, both at different times of the year and to different extents, should
assist in assessing optimum silvicultural methods for various sites and species.
Acknowledgments
The author wishes to thank Dr. Erika Gaertner for assistance in monitoring
radioactive trees. Dr. C. A. Mawson and N. Vincent, Atomic Energy of Canada
Limited, Chalk River, Ontario, provided invaluable assistance in the preparation
of the isotopes and autoradiographs. Colleagues at the Petawawa Forest
Experiment Station provided constructive criticism in the preparation of the
manuscript. Photograph for Figure 1 was taken by Dr. Gaertner; those for Figures
2, 3 and 4 with the assistance of D. C. Anderson, Forest Biology Laboratory,
Sault Ste. Marie, Ontal:io.
14
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