Journal of Experimental Botany, Vol. 46, No. 289, pp. 895-905, August 1995
Journal of
Experimental
Botany
Mineral nutrition and transport in xylem and phloem of
Banksia prionotes (Proteaceae), a tree with dimorphic
root morphology
W. Dieter Jeschke2'3 and John S. Pate1
1
Botany Department, University of Western Australia, Nedlands, WA 6009, Australia
2
Received 21 November 1994; Accepted 13 March 1995
Abstract
Introduction
Xylem sap of proteoid roots, lateral roots, sinker root
and age classes of trunk segments, and phloem sap
of mid-trunks were collected from trees of Banksia
prionotes (Proteaceae) in native habitat on highly
impoverished sands in south-western Australia.
Proteoid roots were major exporters of phosphate, K+
and amino acids during the wet winter season and
showed in vitro nitrate reductase activity during
periods of soil nitrification. Other parts of the root
served as general sources of Na + , Cl~, Mg2 + , Ca 2+ ,
and SO*". Lateral root xylem sap was more concentrated in virtually all solutes than that of sinker roots,
even during the dry summer following senescence of
proteoid roots. Gradients in xylem sap concentration
up the main trunks suggested lateral abstraction and
storage of incoming phosphate in basal stem parts
during winter and a subsequent release to the xylem
in summer. Phloem sap was many times more concentrated in nutrient ions than xylem sap, and, like xylem
sap, showed unusually low K+ and H2PO4^ relative to
Na + , Cl~ and SO*', suggesting a sparing role of the
latter three ions in meeting ionic requirements of
transport. Amino acid analyses showed higher overall
concentrations in phloem than xylem sap, but much
lower proportions of total amino N as glutamine and
asparagine in the former sap. This suggested utilization of xylem-derived amide N by leaves for growth
and synthesis of phloem-mobile amino acids.
Banksia prionotes (Lindley), Proteaceae is a fast growing,
fire-sensitive tree of the highly impoverished deep white
or yellow sands supporting open woodland and scrub
heath ecosystems of the so-called northern sandplains
of south-western Australia (George, 1981). After fire it
regenerates from seeds. In common with many other trees
and shrubs of such mediterranean-type ecosystems (Pate
et ai, 1984a; Bowen and Pate, 1991; Lamont and Bergl,
1991), its root system is highly dimorphic, comprising a
single stout, but strongly attenuating sinker root, which
extends vertically downward to reach the water table, and
a number of very superficial lateral roots, which radiate
out from the base of the sinker root (Pate and Jeschke,
1993). Following the onset of winter rain, dense nearsurface mats of 'proteoid' root clusters form directly from
the laterals or from second order laterals. These proteoid
roots function in nutrient uptake in the winter season
and spring and then senesce as the soil surface dries out
the following early summer (Lamont, 1982). Increase in
shoot dry weight proceeds mainly between August and
February and shoot extension and production of new
leaves between November and February (Bowen, 1991),
i.e. 4-6 months after the period of maximum nutrient
availability. It therefore becomes of interest to evaluate
the relative contributions which proteoid root-bearing
laterals and the sinker root make to the mineral nutrition
of the shoot during uptake in winter and spring, and
examine how and where nutrients are stored prior to
shoot extension during summer. The present paper reports
specifically on the composition of the xylem (tracheal)
sap of sinker and lateral roots and the root pressure
Key words: Banksia prionotes, mineral transport, proteoid
roots, xylem sap, phloem sap.
3
To whom correspondence should be addressed. Fax: +49 931 888 6158.
Oxford University Press 1995
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Julius-von-Sachs-lnstitut fiir Biowissenschaften, Lehrstuhl fur Botanik 1, Universitat Wurzburg,
D-97082 Wurtzburg, Germany
896
Jeschke and Pate
xylem sap of proteoid roots, and compares the data with
that of xylem sap and phloem exudates obtained from
differently-aged segments up the trunk. The study complements recent investigations of root anatomy and hydraulic
architecture of the roots of Banksici prionotes (Pate et al.,
1995). Seasonal patterns of utilization of surface and
ground water by this species will be reported on soon
(Dawson and Pate, in preparation).
portions frozen in situ with dry ice prior to excision to avoid
gravitational loss of sap from their wide vessels.
Phloem sap was induced to bleed from the trunk by making
shallow, oblique incisions with a sharp razor blade through the
bark into the depth of the actively transporting phloem.
Bleeding occurred from trunks sampled at virtually any time of
the year using 2-4-year-old regions of the stem, i.e. the 1988 to
1990 extensions. Each cut typically yielded 10—25 ^il of phloem
exudate over a 2-5 min period of bleeding. Phloem sap samples
were immediately frozen at — 78 °C.
Materials and methods
Analyses of sap samples
Cations were analysed by atomic absorption spectrometry
(FMD 3 Carl Zeiss, Oberkochen) after addition of an
atomization buffer (CsCl + Sr(NO3)2) and appropriate dilution.
Anions were measured by HPLC analysis, using either the
Anionenchromatograph, Eppendorf-Biotronik, Maintal FRG
or Ion 432 Anion HPLC, Millipore (USA). Analyses of sap
samples for amino acids and sugars in phloem sap were
accomplished using the technique described by Pate et al. (1985).
In vivo nitrate reductase assays
These were conducted on freshly harvested healthy sets of
proteoid roots using the assay procedures recently described for
root tissues by Stewart et al. (1993).
Statistical treatment
Harvests were conducted on 21 close-to-monthly intervals over
the period July 1992 to December 1993 and each involved the
.sampling of 2-5 trees. Xylem sap of three or more lateral roots
was sampled from each tree, sinker root sap was collected from
two successive segments from the same sinker. Phloem sap was
collected from the trunks of 3-10 different trees. Standard
errors of means are given for the data at each sampling time.
Collection of xylem and phloem sap
Sap bleecjing under root pressure was obtained in situ from
second order lateral roots (diameter about 2-3 mm) subtending
healthy, fully developed sets of proteoid roots byfittinga sleeve
of silicon rubber tubing to the washed and blotted, cut proximal
end of the root, following partial excavations avoiding damage
to the attached proteoid roots. The first drop to bleed was
discarded to avoid contamination from cut tissues and all
samples were immediately frozen on dry ice. About 100 fA sap
was obtained over a 3 h period. Xylem sap was obtained
concurrently from trunk segments of identified age and then
from segments of excavated lateral roots and the sinker root
using the mild vacuum extraction technique of Bollard (1960).
In brief, the distal cut end of a 20-40 cm long trunk or root
segment was freed from its periderm and cortex, rinsed and
blotted, inserted tightly into a collection unit (see Fig. 1 in Pate
et al., 1994) and sap collected by applying a mild vacuum
(pressure 25-60 kPa) to the collection unit. 1-2 cm segments
were then cut successively from the proximal end of the segment
to facilitate sap displacement from conducting xylem vessels.
Extracted xylem fluid (0.5-10 ml) was collected in Eppendorf
tubes and stored on dry ice to await analyses. All ages of shoot
extensions and three or more laterals were sampled in this
manner from each study plant. Using a lower suction, xylem
fluid was collected similarly from 30-70 cm and 70-100 cm
deep segments of excavated sinker roots, but with their distal
Results
Plant and root morphology
At the time of the investigation the trees were aged 6
(1992) to 7 (1993) years and were 2.5-3.5 m high. Sinker
roots had a proximal width of 20-35 mm at the point of
attachment of the lowest major lateral and then attenuated sharply to 10-12 mm at 70 cm deep and between 5
and 8 mm at the depth of the stony layer (1.8-2.5 m). A
crown of around 10 lateral roots located at 3-10 cm
depth radiated horizontally outwards from the top of the
sinker root, each lateral extending from 1.5-5.5 m (Fig. 1)
and varying in proximal diameter from 4-15 mm. Some
of the study trees had some of their laterals severed by
haustoria of the root hemiparasite Nuytsiafloribunda,in
which case side-branches developed beyond the point of
lesion (Fig. 1). Some of the larger laterals developed
'mini' sinker roots at various points along their length
(Fig. 1). These mini sinkers extended to depths of only
0.5-1.4 m, i.e. failing to reach the water table.
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Trees of Banksia prionotes growing in open mixed Banksia
woodland on deep white sand near Yanchep, 50 km north of
Perth, Western Australia were investigated. A uniform population of B. prionotes had recruited following a severe wild fire in
January 1986 (Bowen, 1991) and was in its sixth and seventh
years of growth in the study period, July 1992 to December
1993. Some of the plants at the study site were affected by the
root hemiparasite Nuytsiafloribunda(Loranthaceae). Analyses
of the sand with depth showed essential nutrients to be
especially concentrated in the upper 20 cm, with organic C at
0.45%, K at 20-30 ppm, P at 2-3 ppm, and total N at
5-15 ppm. Ammonium (4-7 ppm N in winter and early spring
and 1-2 ppm N in summer) predominated over nitrate (1-5 ppm
N in winter and early spring, 0-1 ppm in summer) as the
soluble sources of N. Other nutrients (principally Na, Cl, Ca,
and Mg) were more uniformly distributed through the sandy
soil profile. The water table at the site varied between 1.8 m
(mid-winter) and 2.6 m (late summer) and had been reached by
the sinker roots of B. prionotes in the first or second season of
growth (Bowen and Pate, 1991). Older trees all showed
unbranched sinker roots attenuating in diameter through the
sand until reaching a compacted lateritic layer of gravel at
1.6-1.9 m. Each then narrowed further, penetrated and then
branched out into a series of fine roots on entering a second
sandy layer in which the water table was located.
Mineral nutrition and transport in Banksia
897
summer cycles) revealed (Fig. 3) pronounced concentration
increases in lateral root sap during each winter, but not in
the sap of sinker roots. Average phosphate levels in lateral
root xylem sap were 2-fold higher during winter when
phosphate uptake via proteoid roots was likely to be
proceeding than in summer when proteoid roots had died
and surface soil layers completely dried out. Seasonal
concentration differences for amino acids and anions other
than phosphate were smaller and in most cases showed
higher concentrations in summer than winter (Table 1).
Cation concentrations were also little affected by seasons
but tended to be higher in winter than summer (Table 1).
Xylem sap composition of the main stem
previous Nuytsia lesions
O
current haustoria of Nuytsia
:
"mini1 sinkers on lateral root
^
position of main sinker root
Fig. 1. True to scale drawing (surface view) of a typical lateral root
system of a 6-year-old tree of Banksia prionotes from natural habitat at
Yanchep, W. Australia. The drawings were constructed from in situ
observations of length and branching patterns of each fully excavated
segment of the root system. The positioning of mats of proteoid roots
is not shown. Dashed vertical lines denote positions of mini sinkers on
each lateral. Locations of currently active haustoria of the root
hemiparasite Nuytsiafloribundaare shown and lesions on roots caused
by haustoria of previous years are indicated.
Xylem sap composition of proteoid, lateral and the sinker
roots
Mean concentrations of major solutes across all samples
of xylem sap from each class of root in the wet seasons
of 1992 and 1993 were as shown in Fig. 2. The overall
concentrations of all solutes except nitrate, sulphate, Ca2+
and Mg 2+ were substantially higher in proteoid root
xylem exudate than in xylem sap extracted from the
laterals, into which proteoid roots were discharging their
sap. The main cation of proteoid root sap was K + ,
followed closely by Na + and the dominant anion was
Cl~. Nitrate was not detected in proteoid xylem sap
despite being present at variable concentrations in all
other xylem fluids. Phosphate was at 10 times higher
concentration in proteoid root xylem sap than in that of
parent lateral roots. Besides Cl~, malate was present in
substantial concentrations in proteoid root xylem bleeding
sap and similarly in lateral roots.
Xylem sap of sinker roots was 2.5-5 times less concentrated than that of lateral roots in phosphate, malate and
total amino acids, about half as concentrated in K + ,
Na + , Mg 2+ , Ca 2+ , nitrate, and sulphate and showed 66%
of the mean level of chloride of the lateral root sap.
Measurements of seasonal changes in root xylem sap
concentrations of phosphate over the period from July
1992 to December 1993 (i.e. two wet winter and dry
Xylem sap extracted from the base of the stem during
the wet winter season carried solute concentrations mostly
intermediate between those of lateral and sinker roots
(Fig. 2), i.e. as to be expected from mixing of the two
xylem streams before entry into the shoot. However, this
principle did not appear to hold for Na + , Cl~ and Mg 2+ ,
for which mean stem base xylem sap concentrations were
somewhat less than those of sinker roots and considerably
below those of lateral roots. During the summer season
phosphate levels in stem base xylem sap (0.05 mM) were
as high as in lateral roots (Fig. 4) and much higher than
in the sinker, even though these latter served as sole
source for water and ascending sap in this season. This
strongly suggested that, when proteoid roots were no
longer present, phosphate in stem base xylem originated
primarily from the release of reserves of phosphate accumulated in the stem during the previous winter season.
Xylem (tracheal) sap collected sequentially from single
year extensions along the main shoot showed a general
tendency for upward increases in most ions in xylem of
the stem (Fig. 4). Changes in xylem sap concentrations
of chloride and sulphate along the axis were of limited
extent, apart from major increases in sulphate in the
youngest extensions, and were similar for the wet and dry
Table 1. Mean solute concentrations [mM] in lateral root xylem
sap of Banksia prionotes sampled in natural habitat at Yanchep
W. Australia during the wet winter and dry summer seasons of
1992 and 1993
Total amino acids
Potassium
Sodium
Magnesium
Calcium
Phosphate
Nitrate
Chloride
Sulphate
Rainy season"
[mM]
Dry season"
[mM]
0.56 ±0.2
2.65±0.27
3.34 + 0.31
0.95 ±0.13
0.80 + 0.09
0.10±0.01
0.011 ±0.003
4.30±0.3
0.70 + 0.1
1.12 + 0.14
2.36±0.79
2.05 + 0.68
0.60 ±0.2
0.59+0.2
0.054 + 0.015
0.013 + 0.003
7.4 ±3.2
0.81+0.25
"The values are given as means of 13 samples (winter) and 10 for
cations or 6 for anions (summer) ± standard errors of the mean.
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•
898
Jeschke and Pate
PO,
STEM BASE
so.
LATERAL
SINKER
PROTEOID ROOT
1L
02
1.2
0.8
0
0.2
04
06
08
0
0.005
0.01
0 015
0 02
malate
0
2
4
6
8
K
STEM BASE
10
12
Mg
LATERAL
SINKER
PROTEOID ROOT
10
Na
STEM BASE
0
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05
0 75
1
125
Ca
LATERAL
SINKER
PROTEOID ROOT
[mM]
Fig. 2. Mean levels+ SEM of major inorganic and organic solutes present during the wet winter season in the xylem bleeding sap of proteoid roots
and in xylem (tracheal) sap obtained by mild vacuum (pressure 25-60 kPa) extraction of lateral roots, sinker roots and trunk bases of 6-year-old
trees of Banksia prionotes growing in natural habitat in Banksia woodland at Yanchep, W. Australia. Each mean of xylem sap samples refers to
collections throughout two wet seasons (July to beginning of September 1992 and end of April to end of August 1993). Standard errors of the
means of concentrations are given (n = 6 for proteoid roots, n= 12 for xylem sap samples). Note broken scale for phosphate to accommodate much
higher levels in proteoid root xylem sap.
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Cl
amino acid N
Mineral nutrition and transport in Banksia
899
lateral root xylem sap
27.7.92
wet
15.9.92
"
4.11.92 24.12.92
12.2.93
3.4.93
23.5.93
dry
12.7.93
wet
31.8.93
20.10.93
dry
season. Changes in cation concentrations (not shown)
were similar. For phosphate, on the other hand, there
was an upward decrease in concentration in lower stem
extensions during the rainy winter, but a continuous
acropetal increase in the dry summer season. The decrease
during the winter time, when phosphate uptake occurred,
suggested that phosphate abstraction from the xylem
occurred into surrounding stem tissues and that the
accumulated phosphate was stored there. The upward
increase in the dry season without further phosphate
uptake by the root yet with high concurrent demand for
phosphate for growth is consistent with mobilization of
the stored P and release into the xylem.
Phloem sap composition
The mean molar composition of phloem sap harvested
from mid regions of trunks over the study period of more
than a year is compared with that of stem base xylem
sap in Table 2. Sucrose was the only sugar in significant
amounts and malate was the dominant organic acid; the
total amino acid concentration was remarkably low.
Chloride and Na + were present at unusually high concentrations in comparison with phosphate and K + , respectively. Mg2+ and Ca2+ levels were high, the latter possibly
resulting partly from contamination of sap by leakage
from cut tissues.
A comparison of the mean composition of the phloem
and trunk base xylem sap samples showed an overall
average of 19-fold greater concentrations of all solutes
other than sucrose (absent from xylem) in phloem
than in xylem, with somewhat higher concentration
differences for Mg (28-fold), Cl (25-fold) and Na
(21-fold), but noticeably lower ones for K and nitrate
(10-fold), sulphate, amino acids, malate, and phosphate
(13-16-fold).
Table 2. Mean major solute, concentrations fmMJ in mid-trunk
phloem sap and base of trunk xylem (tracheal) sap of Banksia
prionotes sampled in native habitat near Yanchep, Western
Australia over the period July 1992 to December 1993
Sucrose
Total amino acids
Malate
Potassium
Sodium
Magnesium
Calcium
Phosphate
Nitrate
Chloride
Sulphate
Phloem sap"
[mM]
Xylem sap"
[mM]
537 + 68
5.8 + 1.2
9.8 + 1.8
17.1+2.3
30.9 + 3.5
12.2±2.4
6.1 + 1
0.83 + 0.16
0.29 + 0.09
60.1+9.6
8.7±1.6
0
0.42 + 0.07
0.61+0.28
1.65 + 0.19
1.50 + 0.15
0.43 + 0.07
0.46 + 0.1
0.054 + 0.01
0.003 ±0.004
2.40 + 0.9
0.60 + 0.24
° The values are given as means of 19 mid-trunk phloem sap samples
and 16 trunk base xylem sap samples + standard error of the mean.
Amino acid composition of xylem and phloem sap
The amide glutamine was the major nitrogenous solute
in all xylem sap samples, accounting for 66-68% of total
xylem N. Xylem sap of lateral roots was distinguishable
by its relatively high proportion of asparagine relative to
sinker roots (Fig. 5). Phloem sap exhibited a much
broader amino acid spectrum than did xylem sap and
glutamate exceeded aspartate and glutamine on a molar
basis. In contrast to the major role of glutamine in the
xylem this compound accounted for only 22% of the total
N of phloem (Fig. 5). Serine, threonine, cystine, glycine,
and alanine were relatively important minor components
of phloem, but asparagine was at noticeably lower relative
and absolute levels than in the xylem of lateral roots.
The mean amino acid composition of proteoid (cluster)
root bleeding sap is given in Fig. 6. Glutamine, the
dominant compound was present at a concentration
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Fig. 3. Time-courses of changes in phosphate concentrations of xylem sap collected from lateral and sinker roots of 6-year-old trees of Banksia
prionotes growing at the Yanchep study site. Note sudden increase in xylem sap levels in laterals following wetting of soil in April.
900
Jeschke and Pate
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STEM EXTENSIONS
Fig. 4. Gradients in the concentration of major xylem inorganic anions±SEM along the trunk axis of Banksia prionotes during either the winter
(rainy) period (see legend of Fig. 2, n= 12) or during the summer (dry) period (October 1992 to March 1993 and end of September 1993, n = 6).
Dotted lines indicate supposed feeding of ions from either lateral and sinker roots or sinker roots only into the trunk. Note the consistently higher
levels in lateral than sinker root sap despite marked changes in concentration for both sap samples with season of collection. Stem extensions
correspond to age classes as indicated.
approaching that in the phloem sap and greatly exceeding
that in xylem sap of lateral roots. Asparagine was the
second major amino acid and its proportion relative to
glutamine was closely similar to that of lateral roots.
Seasonal changes in amino acid levels in xylem over a
period of a year and a half (Fig. 7) showed concentrations
to be consistently lower in sinker than in lateral roots.
Concentrations remained low during the rainy winter
season (May to August 1993) and then increased slowly
over spring and early summer (September to December
1993). The increase was more apparent in lateral roots
than in the sinker root.
There was a sudden large increase in amino acid xylem
sap levels of laterals and to a lesser extent of the sinker
root just before the onset of winter rains in the 1993
season (values for April in Fig. 7). Similar increases were
also observed for cations and anions and for phosphate
(see Fig. 3). In each case, except phosphate, the increase
in solute concentration was transient, declining rapidly
within 2 d. These subsequent declines in amino acid and
cation concentrations were accompanied by a surge in
levels of abscisic acid (ABA) (Hartung, Pate and Jeschke,
unpublished), suggesting that the severe drought conditions at the time were likely to have been responsible
for the unusually great increases in major solutes subsequently in ABA concentrations.
Amino acids in the xylem sap of trunks
Amino acid concentrations changed along the trunk axis
during the wet winter and dry summer season as shown
in Fig. 8. Concentrations in the xylem sap in winter and
early spring (Fig. 8A) were generally low, with some
evidence of a decrease in passage through lower trunk
regions followed by an upward rise in concentration as
sap reached upper parts of the trunk. By contrast during
late spring and early summer (Fig. 8B), when shoot
growth and extension were proceeding, sap concentrations
increased to levels far exceeding those observed in winter
and also tended to rise continuously from lower to
uppermost parts of the trunk. Proportions of sap amino
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4
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Fig. 5. Mean concentrations of amino acids in phloem sap (vertical scale on right) and in lateral and sinker root xylem sap (scale on left) of
Banksia prionotes. Full arrows point to acidic amino acids (aspartate and glutamate) which are at much higher relative concentrations in the
transport fluid of phloem than xylem. The dashed arrow marks asparagine, an amide present at relatively higher concentrations in lateral root
xylem sap than in phloem sap or in stem or sinker xylem sap. The data represent means of samples collected between the end of April and
December 1993.
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Fig. 6. Mean amino acid composition of the root pressure xylem exudate of proteoid roots of Banksia prionotes. Data represent average
concentrations of six samples collected in the late winter period when proteoid roots were fully developed and presumably most active in
nutrient uptake.
acids as glutamine were higher in these dry season samples
than in those collected in winter.
Reduction of nitrate in proteoid roots
The absence of nitrate and the presence of high levels of
glutamine and asparagine in xylem sap exuding from
active proteoid roots suggested that such roots were
functioning in nitrate reduction. Indeed proteoid roots
sampled through the 1993 wet season showed appreciable
in vivo NRA activities (3.1 ±2.3 peat g ' fresh wt.) and
this applied particularly in May 1993 (value 8.3 peat g" 1
fresh wt.) following a period of nitrification when levels
of NOf in soil surrounding proteoid roots peaked at
5 ppm. However, free nitrate was detected on most sampling occasions in phloem sap or xylem sap of laterals
and sinker, indicating that it might have been absorbed
elsewhere than in proteoid roots and then passed on to
all parts of root and trunk.
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Fig. 8. Gradients in xylem sap concentrations [mM]±SEM of total amino acids and of glutamine up the trunk axis of Banksia prionotes. (A)
Mean concentrations over the winter period (26.5.-27.7.1993, n-4). (B) Mean concentrations over the late spring and early summer period
(12.10.-9.12.1993, n=4). Note different scale in winter and summer. Dotted lines indicate supposed feeding of ions from either lateral and sinker
roots or sinker roots only into the trunk. Standard errors of the means are given.
Discussion
The root system of Banksia prionotes conforms to the
common deep-rooted Retype rooting morphology
described by Pate (1994), in which one or occasionally
two or more sinker roots supply water from the water
table while a crown of superficial laterals radiate outwardly into the upper soil layer and abstract nutrients
through the agency of the proteoid roots which they
produce during the wet winter season (Lamont and Bergl,
1991). It has been generally observed that the lateral
roots of species of Rj-type rooting morphology are fully
charged with xylem fluid during the dry summer months
when the soil is fully dried out to a depth of at least 1 m.
A study of the origin of the water in the laterals using
natural abundance values for D2O in xylem water confirms that recharge of water from the sinker root into
lateral roots occurs throughout the summer (Pate and
Dawson, unpublished). Accordingly, the xylem sap resident in lateral roots during summer is a more or less
stagnant fluid into which solutes from the sinker and
from lateral root tissues may continue to accumulate, but
it is of little or no relevance in providing nutrients to the
shoot at this time. As shown here (Fig. 1), small sinker
roots grow vertically downwards from the larger laterals,
but since these terminate at depths well above the water
table, they are not able to furnish significant amounts of
water to the shoot during summer. Nevertheless, they
might well serve as a transient major source of water for
the transport of minerals from the laterals during late
spring and early summer when upper, but not mid-layers
of the soil are dried out (Dodd et ai, 1984). In other
Banksia species these secondary sinkers have been found
to reach the water table (Lamont and Bergl, 1991).
Thefindingthat concentrations of most solutes in trunk
base xylem sap are intermediate between those of lateral
and sinker roots during the season of active nutrient
uptake (Fig. 2) indicates that the shoot is then supplied
by a mixed xylem fluid originating from both sinker and
lateral roots. However, the proportion of sinker and
lateral root xylem fluid streams needed to meet the
measured concentration of each nutrient in the stem base
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Fig. 7. Seasonal changes in the total amino acid concentration in xylem (tracheal) sap collected from lateral and sinker roots of 6- to 7-year-old
trees of Banksia prionotes growing in natural habitat near Yanchep, W. Australia.
Mineral nutrition and transport in Banksia 903
roots regularly comes from nitrate uptake. The remainder
would be expected to arise from uptake and metabolism
of NH^, the source of N normally dominating in the
acid sandy soils (pH 4.5-5) of habitats in which Banksia
prionotes grows.
The present data clearly suggest that proteoid roots are
primary sources of supply of phosphate during winter
and early spring, when the topsoil is wet, litter is decomposing and organic sources of P such as phytate are
generated. Decreasing xylem sap gradients between the
laterals and up the trunk at this time (Fig. 4) suggest that
phosphate is being withdrawn from the xylem and stored
in older stem tissues (1986-90 extensions). Since the
period of main nutrient demand for shoot growth (August
to February) and formation of new shoot extensions
(November to February, Bowen, 1991) is out of phase
with nutrient uptake during winter, substantial mobilization of these and other stored reserves is clearly required
from old to young shoot parts. The finding of considerably higher phosphate concentrations in stem than sinker
root xylem sap and upwardly increasing phosphate concentrations in stem xylem sap during the dry summer
season (Fig. 4) are consistent with release of phosphate
previously acquired by older stem tissues occurring via
the xylem.
A situation similar to that for phosphate is apparent
for amino acids (Fig. 8), which also appear to be withdrawn into lower parts of the trunk in winter and then
released into the xylem sap the following summer.
Consistent with this, concentrations of amino acid in
stem xylem sap are considerably greater than those in
sinker root sap. Since higher proportions of glutamine
are present in stem xylem sap during summer than in
winter, it is likely that it is this compound which is
predominantly released from sites of storage.
Nutrients are, of course, also likely to become available
to shoot growth by phloem mobilization from older leaves
while these and younger leaves are also the suppliers of
photosynthate via phloem.
Phloem sap composition in Banksia prionotes proves
to be different in several respects from that of other
species (Ziegler, 1975; Hall and Baker, 1972; Pate et al.,
1974; Jeschke and Wolf, 1988).
Firstly, concentrations of amino acids (5.8 mM corresponding to 6.95 mM amino acid-N, Table 2) and of
phosphate (0.83 mM) are relatively low, presumably
reflecting the limited availability of N and P in the
impoverished soil to which Banksia prionotes is native.
Levels of amino acids in phloem sap of herbaceous species
are generally recorded as being much higher than this,
but still very dependent on the Nsupply (Vigna 40 mM,
Pate et al., 19846; Lupinus 55 mM, Jeschke et al., 1986;
Ricinus 35-115 mM, Hall and Baker, 1972, Jeschke and
Wolf, 1988; Triticum 262 mM, Hayashi and Chino, 1986).
However, low concentrations have also been reported in
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xylem sap is not the same for each nutrient, suggesting
that, prior to or after mixing of the sap streams, differential lateral abstraction or binding of different solutes may
occur from the xylem stream (see for example, Jeschke
and Pate, 1991, for sodium in Ricinus).
Relatively high levels of chloride, Na + , Mg 2+ , Ca 2+ ,
and K + in sinker root xylem sap (Fig. 2) indicate that
these ions are available throughout the year from the
ground water. Alternatively, a fraction of some of these
ions may originate from cycling of phloem-derived solutes. Such cycling is least likely to be operative for Ca2+
in view of its sparing mobility in the phloem (Ziegler,
1975; MacRobbie, 1971; Jeschke and Pate, 1991). Conversely phosphate, malate, and amino acids which are
relatively low in xylem sap of sinker roots may originate
largely from transfer back to xylem of phloem-imported
compounds. Cycling of amino acids through roots is well
documented for other species (Pate et al., 1979) and
phosphate probably behaves similarly since it is readily
phloem-mobile (Ziegler, 1975).
Extending comparisons to the composition of proteoid
root xylem exudate with that of parent lateral roots
(Fig. 2), the former roots emerge as principal sites of
xylem export of phosphate, malate, amino acids, K + ,
Na + , and Cl~ during the wet season. Proteoid (cluster)
roots of other species are well known to be highly efficient
in acquiring phosphorus (Lamont, 1982; Gardner et al.,
1982). Marked efficiency in this respect in Banksia prionotes is suggested from the fact that levels of phosphate
are as high as 1 mM in bleeding sap of proteoid roots
where soil levels of total P are only 2-3 ppm. This level
of P almost equals that recorded for roots of Lupinus
albus grown in the presence of 0.5 mM phosphate
(1.25 mM xylem P, Jeschke et al., 1986).
By contrast, Mg 2+ , Ca 2+ and sulphate are all at lower
concentrations in proteoid root bleeding sap than in
parent lateral root xylem sap (Fig. 2), implying that these
ions are also taken up by lateral roots at sites other than
the root clusters, for instance the mini sinker roots. Since
active secretion of these ions into xylem vessels may be
restricted (Marschner, 1986), concentrations should be
low in proteoid root bleeding sap. In other species secretion of potassium and nitrate is suggested to drive xylem
sap bleeding (Triplett et al, 1980) while, if recently
absorbed nitrate is being reduced in a root, amino acids
and malate contribute substantially as osmotica of xylem
sap (Jeschke et al., 1995). It is especially interesting that
nitrate is virtually lacking from proteoid root bleeding
sap whereas amino acid levels are fairly high (up to
7.5 mM) and may even comprise 10% of its osmolality.
Evidence is presented that at times of high availability of
nitrate in the upper soil horizon, proteoid roots exhibit
appreciable in vivo nitrate reductase activity (NRA), while
lower but still measurable NRA at other times suggest
that at least part of the amino acid N exported from such
904 Jeschke and Pate
Comparisons of the amino acid balance of the three
classes of xylem and phloem saps allow some conclusions
to be drawn on the origin and fate of specific nitrogenous
compounds. Firstly, as proteoid roots are likely to be the
sites of uptake of ammonium and nitrate from the soil
solution during winter, one may conclude that assimilation of these sources is responsible for bulk net synthesis
of glutamine, asparagine and, possibly, other minor compounds recorded in the xylem sap of proteoid roots and
their parent lateral roots. Secondly, asparagine is at low
levels in other xylem streams and in phloem, suggesting
it is quickly absorbed or metabolized before or immediately after leaving the laterals. Thirdly, glutamine remains
as a principal component of xylem sap of sinker root and
the trunk, indicating a general function in transport and
cycling of N throughout the winter season and possibly
as a vehicle for transport of remobilized N during stem
growth in summer. Finally, the stem phloem sap of
B. prionotes contains relatively less glutamine and relatively more of a number of other non-amide amino
compounds than in any of the xylem streams of shoot or
root. This indicates that on its arrival into leaves xylemborne glutamine is broken down and its nitrogen is used
to synthesize protein and establish pools of free amino
acids. Some of the latter, e.g. aspartate, threonine, serine,
glutamate, glycine, alanine, and cystine, together with
some unmetabolized glutamine are then selectively
re-exported from leaves in the phloem together with
sucrose and a range of other solutes. In view of the much
higher concentration in phloem compared with xylem
these translocated amino acids are likely to contribute
substantially to the N requirements of the seasons trunk
extension and its sets of new leaves.
Acknowledgements
This research was supported by grants from the Sonderforschungsbereich 251 of the Deutsche Forschungsgemeinschaft and the Australian Research Council. The skilful
technical assistance of Andrea Hilpert, Edwin Raisins and
Carlos Raphael is gratefully acknowledged. Thanks are extended
to Eva Wirth for anion analyses.
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