Journal of Experimental Botany, Vol. 54, No. 385, pp. 1221±1229, April 2003
DOI: 10.1093/jxb/erg132
RESEARCH PAPER
Root and stem hydraulic conductivity as determinants of
growth potential in grafted trees of apple
(Malus pumila Mill.)
C. J. Atkinson1, M. A. Else, L. Taylor and C. J. Dover
Crop Science Department, Horticulture Research International, East Malling, West Malling, Kent ME19 6BJ, UK
Received 23 August 2002; Accepted 14 January 2003
Abstract
Introduction
The anatomy of the graft tissue between a rootstock
and its shoot (scion) can provide a mechanistic
explanation of the way dwar®ng Malus rootstocks
reduce shoot growth. Considerable xylem tissue disorganization may result in graft tissue having a low
hydraulic conductivity (kh), relative to the scion stem.
The graft may in¯uence the movement of substances
in the xylem such as ions, water and plant-growthregulating hormones. Measurements were made on 3year-old apple trees with a low-pressure ¯ow system
to determine kh of root and scion stem sections
incorporating the graft tissue. A range of rootstocks
was examined, with different abilities of dwar®ng;
both ungrafted and grafted with the same scion shoot
cultivar. The results showed that the hydraulic conductivity (khroot) of roots from dwar®ng rootstocks
was lower compared with semi-vigorous rootstocks,
at least for the size class of root measured (1.5 mm
diameter). Scion hydraulic conductivity (khs) was
linked to leaf area and also to the rootstock on to
which it was grafted, i.e. hydraulic conductivity was
greater for the scion stem on the semi-vigorous rootstock. Expressing conductivities relative to xylem
cross-sectional areas (ks) did not remove these differences suggesting that there were anatomical changes
induced by the rootstock. The calculated hydraulic
conductivity of the graft tissue was found to be lower
for grafted trees on dwar®ng rootstocks compared to
invigorating rootstocks. These observations are discussed in relation to the mechanism(s) by which rootstock in¯uences shoot growth in grafted trees.
Changes in graft anatomy are often evident when stem
tissue (scion, the part of the plant used for grafting on a
stock) of perennial woody species is grafted onto a
rootstock (stem tissue and associated roots from another
plant) that restricts its vegetative growth (Warne and Raby,
1938; Simons, 1986; Soumelidou et al., 1994a). The
mechanism(s) by which Malus rootstocks in¯uence scion
vegetative growth and development, and vice versa, are
not fully understood (Beakbane, 1956; Tubbs, 1973a, b;
Jones, 1986).
Some authors have suggested that the graft tissue
in¯uences vegetative shoot growth by restricting water
¯ow from the root to the shoot or by removing substances,
particularly minerals and plant growth regulators (i.e.
cytokinins), from the transpiration stream (Knight, 1926;
Jones, 1974, 1984). This may not be the case with all
composite plants, for example, some Vitis combinations,
when self-grafted, have lower hydraulic conductivities
compared with on their own roots (Bavaresco and
Lovisolo, 2000). A restriction of water ¯ow is entirely
consistent with the anatomical changes associated with
graft tissues and the different degrees of shoot dwar®sm
shown in grafted plants (Mosse, 1962; Simons, 1986;
Soumelidou et al., 1994a). These anatomical changes may
be due to limitations in polar auxin (IAA) transport across
the graft and its accumulation at the graft (Soumelidou
et al., 1994b; Simons, 1986); IAA is a key leaf-derived
regulator of xylem cell differentiation and division within
the cambial zone and an initiator of vascular redifferentiation across the graft union (Parkinson and Yeoman, 1982;
Hess and Sachs, 1972; Aloni, 1987; Savidge, 1988). A
reduced ¯ow of IAA to roots could provide an explanation
of the observed changes in the phloem-to-xylem ratio in
apple rootstocks (Beakbane, 1956). A similar argument has
Key words: Apple, dwar®ng, graft tissue, growth control,
hydraulic conductivity, Malus, rootstock.
1
To whom correspondence should be addressed. Fax: +44 (0)1732 849067. e-mail: [email protected]
Journal of Experimental Botany, Vol. 54, No. 385, ã Society for Experimental Biology 2003; all rights reserved
1222 Atkinson et al.
been made for the differentiation of water-transporting
tracheids in graft tissue of Picea sitchensis (Weatherhead
and Barnett, 1986). There is considerable evidence
(Beakbane and Thompson, 1947; Simons, 1986), with
perennial fruit trees, that the rootstocks that dwarf shoots
have a lower xylem-to-phloem ratio compared with
rootstocks that promote shoot growth (Beakbane and
Thompson, 1947). The consistency of this ratio has
enabled it to be used successfully for seedling selection
in apple rootstock breeding programmes (Beakbane and
Thompson, 1947; Miller et al., 1961).
The objectives of this research were (i) to determine the
relationship between scion stem hydraulic conductivity
(kh) and the ability of rootstocks to restrict shoot growth,
and (ii) to determine whether the kh of graft tissues is a
means by which shoot growth is in¯uenced in grafted trees
of Malus.
Materials and methods
Plant material and experimental design
Clonally produced 1-year-old material (Malus pumila Mill.) of a
dwar®ng (M.27) and a semi-vigorous rootstock type (MM.106) were
planted in mini-rhizotrons, in a standard potting compost mix. Five
replicate trees were used per rootstock. Rhizotrons were used for two
reasons; they allow roots to grow and develop without the restriction
of a conventional plant pot. They also enable individual rootstock
root systems to be sampled repeatedly for root hydraulic conductivity (khroot) measurements with minimal disturbance to the tree or
the rest of the rootstock root system. The rhizotrons were placed in a
randomized order on benching which kept the boxes at an incline of
20° from the vertical with removable access windows on the lower
surface. All boxes were covered with re¯ective foil to reduce heat
build-up and water was supplied automatically through a precision
electronic controller and pressure compensating delivery values, i.e.
`trickle irrigation'. The benching and rhizotrons were orientated
north±south on concrete stands and maintained under ambient
environmental conditions.
Measurements of root khroot were obtained from roots with
diameters around 1.5 mm. From each of ®ve replicate plants used,
per rootstock, six roots of this size were removed for hydraulic
analysis. The 1.5 mm diameter size class of root formed around 25%
of the dry matter allocated to roots on a mass basis, but only 2±3% on
a root length basis (data not shown). Roots of a diameter <1 mm
formed more than 96% of the total root length, irrespective of
rootstock type but were too small and delicate to measure khroot
accurately.
In a second experiment 2-year-old grafted Malus trees were used.
Clonal trees were produced by bud grafting all at the same height (15
cm) on the cultivar Queen Cox (self-fertile clone 18). Grafting was
carried out in the ®eld nursery, in situ, using 1-year-old rooted
rootstocks. Three different types of rootstocks were used to
encompass those that dwarfed scion extension shoots (M.27), those
that were semi-dwar®ng (M.9) and those that were semi-vigorous
and promoted shoot growth (MM.106). After one year's growth in
the nursery, these grafted trees were planted in pots (10 dm3) and
placed on sand/gravel beds and grown for a further year under
ambient conditions at HRI-East Malling. During this period,
watering was carried out automatically twice daily. The soil was
kept moist by inspection and the length of irrigation events adjusted
to avoid excessive run-off. After a further year, trees were repotted
into larger volume pots. For the measurements of hydraulic
conductivity, only trees in 15 dm3 pots were used, while the
measurement of functional xylem area, using aqueous safranin
solution, were carried out on trees, of the same age, in a range of pot
sizes (i.e. 10, 15 and 25 dm3). Different pot sizes were used initially
in order to examine the potential for different rooting volumes to
in¯uence conductivity. However, statistical analysis (ANOVA) for
the different size pots showed that there were no statistically
signi®cant in¯uences on functional xylem area due to pot size, only
those due to rootstock, so all data were pooled. In total 15 replicate
trees were used per treatment (rootstock), ®ve per pot size.
Quanti®cation of functional xylem area using safranin
staining
After the termination of shoot extension, when the leaf canopy was
at full expansion (mid August), trees were carefully removed from
their pots with the root system intact and still within the compost.
They were then sealed within a polythene bag and completely
immersed in a large tank of water (300 dm3). Sealing the tree roots
and soil within the bag minimized the contamination of the water
with particulates, which could have occluded the xylem vessels
when cutting the stem. The tree stems were cut under water at a point
just above the ®rst root, leaving as much `rootstock stem' as possible
(the `rootstock stem' is tissue below the scion stem/root graft and is
genetically root in origin). The cut rootstock stem was then placed
into a beaker while still under water and transferred, without
exposure to air, to a container of clean water. This was repeated to
ensure no particle contamination had occurred. The tree was
removed from the container with its stump within the beaker of
water and clamped in a vertical position, in full sun, as it would have
been when attached to its roots. Suf®cient safranin dye in powder
form was added to yield a 0.1% (w/v) aqueous solution (Sperry et al.,
1988). The aerial parts of the tree were allowed to transpire freely for
around 6 h. The transpiring trees were examined along with the
solution reservoirs every 30 min to ensure the solution was being
taken up and when necessary the aqueous safranin 0.1% solution was
topped up with freshly prepared solution.
At the end of the experiment, similarly positioned radial sections
of tissue were cut from each tree, within the rootstock stem, the
middle of the graft tissue and within the scion stem (Queen Cox).
This was repeated for each of the 15 replicate trees used per
treatment. Stained scion stem sections were prepared (sanded) and
the amount of tissue stained with safranin was quanti®ed with an
image analyser (Seescan Bioscience, Cambridge, UK). At low
power magni®cation, entire radial stem sections were imaged and
the area stained by the safranin determined relative to the total
amount of xylem tissue. Fifteen trees were measured per rootstock.
Measurement of rootstock stem, graft tissue and scion stem
hydraulic conductivity
Hydraulic conductivity (kh) was measured using the method
described by Sperry et al. (1988). Roots, whole stem segments and
subsequently stem samples were prepared under water (using the
tank described above). Total leaf areas per tree were also determined
after the stem sections had been taken. Initially, a stem section
including rootstock stem, graft tissue and a section of scion was
removed under water. The average sections lengths used were as
follows; for rootstock shank, 50.4, 56.8 and 75.4 mm, the graft tissue,
53.0, 59.9 and 62.9 mm and scion stem, 58.8, 61.2 and 72.6 mm, for
M.27, M.9 and MM.106, respectively. The average xylem vessel and
®bre lengths for Malus are around 0.5 and 1.0 mm, respectively
(Loach, 1960). This would appear to be on the short side, relative to
other diffuse porous species, where the largest proportion of vessels
are between 0±10 cm (Zimmermann and Jeje, 1981). Total
conductivity of this complete section (khrgs) was measured ®rst, the
section was re-cut under water to remove a known sample length of
Xylem hydraulics in Malus grafts 1223
rootstock stem tissue and khr was measured. Finally, a sample of
scion stem tissue from the original section was removed under water
and its hydraulics measured (khs). The cut ends of all stem sections
were cleaned and prepared with a razor blade prior to transfer to the
measurement tank.
The measurements made from these stem sections were used to
determine the hydraulic conductivity of the graft tissue, which could
not be measured directly due to its size and shape. This was achieved
by assuming that the rootstock stem, the graft tissue and the scion
stem sections could be considered as a series of three resistances, as
follows: Rrgs=rootstock stem+graft tissue+scion stem resistance;
Rr=rootstock stem resistance; Rs=scion stem resistance; Rg=graft
tissue resistance (unknown).
The resistance of the segments was determined from measured kh
values knowing that Resistance=1/conductance and conductance
=kh/tissue length.
The unknown graft tissue resistance (Rg) was determined as
follows:
and thus
Rrgs=Rr+Rg+Rs
(1)
Rg=Rrgs±(Rr+Rs)
(2)
substituting into equation (2) Resistance=tissue length/kh
tissue length
tissue length
gˆ
rgs
khg
khrgs
tissue length
tissue length
ÿ
r‡
s
khr
khs
…3†
which rearranges to:
khg ˆ tissue lengthg tissue length rgs
khrgs
tissue length
tissue length
ÿ
r‡
s
khr
khs
…4†
Hydraulic conductivity was measured in a gravity-fed ¯ow
system, in which the pressure across the stem section could be
varied (the measurements were made around 6 kPa). Degassed and
®ltered (0.2 mm mesh Millipore ®lter) oxalic acid solution (10 mol
m±3) was used as the perfusion solution to prevent any long-term
decline, due to microbial growth, in conductivity (Sperry et al.,
1988). The mass of solution ¯owing per unit time through the stem
segment was recorded continuously with an electronic balance, to
®ve decimal places. This value, and the pressure head, the length of
the segment (mm) and the temperatures (approximately 20 °C) were
all used to calculate continuous measurements of kh (¯ow rate
through a given length of sample per unit pressure gradient), i.e.
kh …kg m sÿ1 MPaÿ1 † ˆ
flow rate …kg sÿ1 † sample length …m†
pressure head …MPa†
…5†
Measurements of mass ¯ow were made every 10 s for each sample
until the coef®cient of variation had declined to <2%. At this point
the ®nal three readings were averaged to estimate the hydraulic
conductivity (kh; kg m s±1 MPa±1). Hydraulic calculations were made
using a programme written by Tyree and modi®ed by Cochard with
manual data inputs of temperature and reservoir head height.
Stem hydraulic conductivity was subsequently used to calculate
stem speci®c conductivity (kss) by dividing khs by xylem crosssectional area (kss; kg m±1 s±1 MPa±1). Leaf speci®c conductivity (ksl)
was also determined by dividing khs by the `supported leaf area'; ksl
is a measure of xylem hydraulic supply capacity (Zimmermann,
1983). This approach follows that used by Tyree and Ewers (1991).
Eight trees were analysed per rootstock.
Results
Hydraulic conductivity of excised individual roots from
ungrafted rootstocks with different size controlling
capacities
The hydraulic characteristics of roots from a dwar®ng and
a semi-vigorous rootstock when grown in rhizotrons are
shown in Table 1. Comparing roots of the same size class
but from different rootstocks, showed that roots from
dwar®ng rootstocks had signi®cantly lower khroot (by about
50%) and these differences remained when the results were
expressed as ksroot, i.e. relative to root cross-sectional area
(Table 1). These measurements however, only account for
a small part of the root system and may not re¯ect the
properties of the entire root system. Stem sections from the
same trees also showed differences in khs and kss values for
material from the more vigorous rootstocks compared to
that from the dwar®ng rootstocks (Table 2). Due to the
high variability in the MM.106 measurements, the rootstock effect was not signi®cant at the 0.05% level. khs was
also expressed relative to total plant leaf area (ksl) and total
root length per plant (ksr); differences between the two
rootstocks were small and were not statistically signi®cant.
Quanti®cation of functional xylem area using safranin
staining
Xylem staining with aqueous safranin combined with
quantitative image analysis enabled rapid measurements of
the amount of functional xylem tissue. Observations from
stem sections showed that the rootstock stem, irrespective
of vigour, had a high proportion (>60%) of the xylem area
stained with dye (Table 3). The exception was the central
xylem core of the rootstock stem, which was nonfunctional. This tissue corresponds to the original xylem
present (®rst years growth prior to scion grafting) when the
main stem of the rootstock was cut off after the graft
established. Sections taken in the middle of the graft tissue
and in the scion stem showed that the ratio of total xylem
area to stained tissue declined with increasing vigour of the
root type, i.e. the proportion of stained tissue increased
(Table 3). The total area of stained stem xylem, calculated
as a percentage, was signi®cantly greater for the semivigorous rootstock (MM.106, 47%) compared to the
dwar®ng (M.27, 24%). The ratio of stem diameter between
the graft tissue and the rootstock stem and scion stem
showed differences in diametric growth size (Table 3). In
all cases, however, even with these comparatively juvenile
1224 Atkinson et al.
Table 1. The hydraulic transport characteristics of an ungrafted dwar®ng (M.27) and semi-vigorous (MM.106) 1 mm to 2 mm
diameter Malus pumila rootstock roots when grown in the glasshouse with unrestricted root development in mini-rhizotrons
Data are means plus predicted standard error of the mean (n=5 trees, from which six different roots were measured per tree).
Rootstock type
Root hydraulic conductivity
khroot
(kg m s±1 MPa±1310±7)
Root speci®c conductivity
ksroot
(kg m±1 s±1 MPa±1310±1)
Dwar®ng (M.27)
Semi-vigorous (MM.106)
d.f.
F probability
2.3460.34
5.8960.89
58
<0.001
1.1160.15
3.1060.41
58
<0.001
Table 2. The hydraulic characteristics of stems of ungrafted dwar®ng (M.27) and semi-vigorous (MM.106) Malus pumila
rootstocks when grown in the glasshouse with unrestricted root development in mini-rhizotrons
Data are means (n=5 trees, from which three different stem segments were measured per tree).
Rootstock type
Stem hydraulic
conductivity
khs
(kg m s±1 MPa±1310±5)
Stem speci®c
conductivity (hydraulic
conductivity per unit
stem cross-sectional area)
kss
(kg m±1 s±1 MPa±1310±1)
Leaf speci®c
conductivity (hydraulic
conductivity per unit
leaf area sustained)
ksl
(kg m±1 s±1 MPa±1310±4)
Stem hydraulic
conductivity per total
root length per planta
ksr
(kg m2 s±1 MPa±1310±6)
Dwar®ng (M.27)
Semi-vigorous (MM.106)
d.f.
F probability
SED
3.28
7.94
8
0.100
2.34
4.69
9.95
8
0.015
1.71
2.90
4.70
8
0.303
1.52
1.69
3.58
8
0.095
0.99
a
Total root length was determined by addition of the lengths of all three root diameter classes.
trees (3-year-old) the graft tissue had the greater diameter
compared to the scion stem or rootstock stems.
Quanti®cation of rootstock stem, graft tissue and scion
stem hydraulic conductivity
Scion stem khs increased with the vigour of the rootstock
on which it was grafted. Mean khs values, from at least
eight trees within each of three rootstock types, were
5.6360.28, 7.5760.45 and 15.2460.94 kg m s±1
MPa±1310±4 for M.27, M.9 and MM.106, respectively.
As the total tree leaf area of these grafted trees increased
signi®cantly in relation to the rootstock's vigour, khs was
also expressed in relation to the `supported leaf area', i.e.
the total leaf area above the stem section measured
(Fig. 1A). The highest khs values were evident with stems
supporting the largest leaf areas and the most vigorous
rootstocks, i.e. MM.106, while the lowest khs values
measured were from stems on dwar®ng rootstocks (M.27),
which had the lowest leaf area. kh were also calculated per
unit xylem cross-sectional area (stem speci®c conductivity
kss) to remove the potential in¯uence of differences in
radial diameter of the xylem tissue sections sampled
(Fig. 1B). There were, in 3-year-old trees, only relatively
small differences in stem diameter associated with rootstock vigour. The relationship between stem kss and leaf
area was still positive with kss being higher in scion stems
grafted onto semi-vigorous (MM.106) compared to dwarfing rootstock (M.27).
Stem hydraulics were also calculated independently of
stem section diameter, and expressed as leaf speci®c
hydraulic conductivities (ksl) and are shown in Table 4. ksl
was nearly twice that for scion stem sections grafted on
semi-vigorous (MM.106) compared to that on the dwar®ng
rootstock (M.27). ksl values, calculated for the semivigorous and dwar®ng entire stem sections (rootstock
stem, graft union, scion stem) and rootstock stem showed
an 8±10-fold increase for the semi-vigorous rootstock.
The ef®ciency of xylem tissue in supporting a transpiring leaf surface was determined by relating leaf area to
xylem cross-sectional area. The ability of a unit area of
xylem to supply an amount of leaf area was determined
using xylem cross-sectional areas for the rootstock stem,
the graft tissue and the scion stem (Table 5). The ef®ciency
with which leaf area was supported by rootstock stem was
very similar (around 35 cm2 of leaf area mm±2 of xylem
CSA), irrespective of rootstock type. When considering the
ef®ciency of the graft tissue, 1 mm2 of xylem of dwar®ng
M.27 rootstock supported signi®cantly less leaf, i.e. around
60% of the semi-vigorous MM.106 rootstock. The semidwar®ng M.9 rootstock had a slightly higher value
Xylem hydraulics in Malus grafts 1225
Table 3. The percentage xylem area stained (%), ratio of total xylem area to stained area for entire radial sections of Malus
pumila rootstock stem, scion stem and graft tissue (R1) and the ratio of radial diameters relative to the graft tissue (R2) from
3-year-old Malus pumila
Trees were grafted on to rootstocks differing in their ability to control shoot growth.
Rootstock type
Dwar®ng (M.27)
Semi-dwar®ng (M.9)
Semi-vigorous (MM.106)
d.f.
F probability
d.f.
F probability
Rootstock stem
%
R1
R2
%
R1
R2
%
R1
R2
%
%
R1
R1
a
64.165.6
1.7460.24b
(0.63)c
76.665.9
1.3360.25
(0.57)
63.765.4
1.9460.23
(0.80)
33
0.269
33
0.224
Graft tissue
Scion stem
29.365.5
6.4160.82
23.865.5
8.5361.28
(0.63)
28.265.3
5.1761.23
(0.57)
46.764.8
2.5061.12
(0.82)
31
0.004
31
0.003
37.665.8
3.1060.83
51.965.3
2.2260.75
33
0.016
33
0.002
a
The percentage of the total xylem area stained (%); means, plus the predicted standard error of the mean are shown (n=15 trees per rootstock3three pot
sizes).
b
The ratio of total xylem area to stained xylem (R1); means, plus the predicted standard error of the mean are shown.
c
Ratio of a section's radial diameter relative to that of the graft tissue value of 1.0 (R2).
Fig. 1. Stem hydraulic conductivity (khs, kg m s±1 MPa±1310±4) relative to leaf area (A), and stem hydraulic conductivity per unit stem crosssectional area (kss, kg m±1 s±1 MPa±1310±6) relative to total plant leaf area (B), for tissue from 3-year-old Malus pumila grafted onto rootstocks
with differing abilities to control scion shoot growth.
intermediate between the dwar®ng and semi-vigorous
rootstock. A unit area of scion xylem, of the semi-dwar®ng
and semi-vigorous rootstocks, supported signi®cantly
more leaf area (around 20%) than that evident for the
dwar®ng rootstock (Table 5).
Measurements of hydraulic conductivity across the
entire grafted stem and rootstock sections (khrgs), which
included the rootstock stem, the graft tissue and a section
of the scion stem, showed that mean khs increased with the
vigour of rootstock used in the grafting (Fig. 2A). These
1226 Atkinson et al.
Table 4. Leaf speci®c conductivity (Ksl) kg m±1 s±1 MPa±1310±4 expressed relative to the amount of leaf area for entire radial
sections of Malus pumila rootstock stems from 3-year-old Malus pumila
Trees were grafted on to rootstocks differing in their ability to control shoot growth.
Rootstock type
Dwar®ng (M.27)
Semi-dwar®ng (M.9)
Semi-vigorous (MM.106)
d.f.
F probability
SED
a
Ksl (kg m±1 s±1 MPa±1310±4)
Rootstock stem
Entire stem sections (includes
rootstock stem, graft union and
scion stem)
Scion stem
0.99a
1.68
8.25
21
<0.001
0.435
0.98
1.99
9.91
21
<0.001
0.479
5.25
5.47
10.32
21
<0.001
0.671
Means are shown for eight replicate trees per treatment.
Table 5. The amount of leaf area (cm2) supported per 1 mm2 of radial xylem tissue measured at the rootstock stem, the graft
tissue or the scion stem tissue from 3-year-old Malus pumila
Trees were grafted on to rootstocks differing in their ability to control shoot growth.
Rootstock type
Dwar®ng (M.27)
Semi-dwar®ng (M.9)
Semi-vigorous (MM.106)
d.f.
F probability
SED
a
Leaf area (cm2) supported by 1 mm2 of stem cross-sectional area
Rootstock stem
Graft tissue
Scion stem
35.1a
35.0
33.8
23
0.903
3.32
10.5
11.4
17.8
23
<0.001
1.13
30.6
39.2
35.3
23
0.011
2.39
Means are shown for eight replicate trees per treatment.
entire stem sections from trees grafted onto dwar®ng
rootstocks (M.27) had khrgs values of 1 kg m s±1 MPa±1 3
10±4 compared to 14.6 kg m s±1 MPa±1310±4 for trees on
semi-vigorous rootstocks (MM.106), while those on the
semi-dwar®ng (M.9) were intermediate at 2.7 kg m s±1
MPa±1310±4. Measurements of a subsequently detached
rootstock stem section, showed that khr values increased in
relation to rootstock vigour and in similar magnitude, as
evident with scion stem section conductivity (khs) (see
below, Fig. 2A). These differences were still evident when
differences in total leaf area were taken into account (data
not shown).
The kss values were reciprocated to express conductivity
as resistivity and are shown in (Fig. 2B). These values were
much greater for all sections taken from the dwar®ng
rootstock (M.27) compared to the semi-vigorous rootstock
(MM.106). Using xylem speci®c conductivities (ks)
enables rootstock anatomical properties to be determined
independently of differences in stem cross-sectional area.
The resistivity of the dwar®ng rootstock, measured over
the complete scion stem section (r+g+s) was of the order of
20 times greater than for the semi-vigorous rootstock.
When kh was used to determine the actual conductivity of
the graft tissue, as described in the materials and methods
section (derived from equation 4), the dwar®ng rootstock
(M.27) had a mean calculated conductivity of 2.18 kg m s±1
MPa±1310±4 (Table 6). This was less than half (4.36 kg m
s±1 MPa±1310±4) that of the semi-dwar®ng rootstock
(M.9). The graft tissue of the semi-vigorous rootstock,
however, (MM.106) had a conductivity that was 4 and 9
times greater, respectively, than the semi-dwar®ng and
dwar®ng rootstocks. These calculations do not take into
account the differences in xylem cross-sectional area as
reported in Table 3, but enable conductivity comparisons
to be made of the different component parts of the grafted
tree.
Discussion
The hydraulic conductivity of roots from dwar®ng rootstocks was lower than those measured from invigorating
rootstocks. This con®rms what has been implied from
early anatomical observations of roots (Beakbane and
Thompson, 1947). This study's measurements of khroot
Xylem hydraulics in Malus grafts 1227
Fig. 2. Hydraulic conductivity (kh, kg m s±1 MPa±1310±4) for rootstock stem (r), rootstock stem+graft tissue+scion stem (r+g+s) and scion stem
(s) (A), hydraulic conductivity per unit stem cross-sectional area expressed as a resistivity (1/kss, kg m±1 s±1 MPa) for rootstock stem + graft tissue
+ scion stem and scion stem relative to stem (B), for tissue from 3-year-old Malus pumila grafted onto rootstocks with differing abilities to control
scion shoot growth.
Table 6. The graft tissue conductivity calculated from equation
(4) in the Materials and methods section using kh data from
the rootstock stem, graft tissue and scion stem tissue from
3-year-old Malus pumila
Rootstock type
Conductivity of the graft
tissue calculated from equation (4)
(kg m s±1 MPa±1310±4)
Dwar®ng (M.27)
Semi-dwar®ng (M.9)
Semi-vigorous (MM.106)
d.f.
F probability
SED
2.18
4.36
19.80
14
<0.001
1.28
values were similar to those quoted for excised roots, for a
range of species (Rieger and Litvin, 1999). Even when
expressed relative to differences in root cross-sectional
area, ksroot values were still lower in roots from the
dwar®ng compared to the semi-vigorous rootstocks. This
is similar for root kh of citrus rootstocks which decreases
with the capacity to dwarf grafted stems (Syvertsen and
Graham, 1985). These observations are consistent with the
view that clonally produced dwar®ng rootstocks possess
innate factors, such as lower xylem to phloem ratios and
changes in xylem vessel anatomy (Beakbane and
Thompson, 1947), which might explain how they in¯uence
shoot behaviour when used in grafted plants. However,
some caution should be used because these measurements
are a re¯ection of a size class of root, which re¯ects only a
relatively small part (25%) of the entire root system, at
least with respect to measurements of root length. Ninetysix per cent of the remaining root, determined by length,
was <1 mm in diameter and would probably have had a
greater in¯uence on total root system hydraulics. Recent
data suggests that entire dwar®ng root systems of Malus,
have lower hydraulic conductivities than more vigorous
ones but such differences are lost when root mass is
accounted for (MA Else et al., unpublished data). It should
also be noted that for the expression of root hydraulic
conductivity relative to cross-sectional area, it has been
assumed that the xylem to total root cross-sectional area
was the same for each rootstock, evidence suggests they
may not be (Beakbane and Thompson, 1947).
The velocity of basipetal auxin transport is lower in
the dwar®ng M.9 rootstock compared to the more
vigorous MM.111 (Soumelidou et al., 1994b). A trend
for the concentration of IAA in bark tissue to decline in
more dwar®ng apple rootstocks has also been found
(Kamboj et al., 1999). IAA concentrations were also
lower in shoot tips of dwarf Malus mutants (Jindal et al.,
1974). The physiological differences in kh measured for
rootstock roots agrees with the earlier growth studies of
Tubbs (1973a, b) where it was argued that the rootstock
in¯uence on scion stem growth was independent of the
scion and greater than that of the scion on rootstock
growth.
1228 Atkinson et al.
Quanti®cation of the area of functional xylem in stem
sections by staining with aqueous safranin solution proved
to be very informative. The staining and apparent movement of safranin solution in the rootstock stem sections
was con®ned to functionally active xylem. This suggests
that within the rootstock stem there was little if any radial
xylem ¯ow under these experimental conditions. More
importantly, however, was the measured reduction in
stained area of the scion stems grafted on to dwar®ng
rootstocks (M.27) relative to those on semi-vigorous
rootstocks, indicating a reduction in the functional area
of xylem above the graft union. This effect could explain
why there is a decline in kh across the graft tissue.
Quantitative measurements of stem kh were similar to
those obtained for Acer saccharum trees with comparable
stem diameters (Yang and Tyree, 1994). It has also been
shown that the vigour of the rootstock directly in¯uenced
the stem's hydraulic conductivity. This difference in kss
occurred in stems independently of changes in tissue stem
diameter or the in¯uence of differences in leaf area.
Calculated values of ksl were greater for the semi-vigorous
rootstock (MM.106) compared to the dwar®ng rootstock
(M.27), re¯ecting a lower pressure gradient (dP/dx, MPa
m±1) supplying water to the leaves of the former. The
calculation of Huber-type (the amount of leaf area
supported by area of stem tissue) values also indicated
that a unit of scion xylem, on a semi-vigorous rootstock,
was able to support a greater leaf area than that on a
dwar®ng rootstock (M.27). This difference in leaf area
supported was evident when expressed relative to graft
tissue or scion stem tissue size (CSA), but not rootstock
stem.
Measurements of stem hydraulics across an entire stem
section of a combined resistance series, from the rootstock
stem, through the graft tissue, to the scion stem, showed
that conductivity was related to rootstock vigour. This
agrees with studies quantifying the movement of aqueous
safranin solution across the graft tissue and those suggested by Warne and Raby (1938). Trees with semivigorous rootstocks had the highest conductivity. When
the graft tissue conductivities were calculated, by difference, the largest factor contributing to the variation in kh
between rootstocks for the grafted stems was the graft
tissue itself.
The differences in kh measured here support suggestions
made from observations of rootstock graft tissue anatomy,
where abnormal xylogenesis was very apparent with M.9,
but not with MM.106 (Soumelidou et al., 1994a). These
data also show that the hydraulic conductivity of graft
tissue from a semi-vigorous rootstock (MM.106) was
much greater than that of a dwar®ng rootstock (M.27). The
hydraulic conductivity of the semi-dwar®ng rootstock
(M.9) was intermediate between the other rootstocks.
It is likely that differences in intact stem conductivity
and, therefore, sap ¯ow in the xylem between rootstocks,
may be less evident as the tree ages. In such cases the graft
tissue cross-sectional area frequently increases with
dwar®ng rootstocks. An increase in the diametric growth
of graft tissue may be a mechanism by which the scion
stem (or more likely the amount of transpiring leaf area) on
a dwar®ng rootstock could overcome the hydraulic limitations imposed by the graft tissue and its abnormal xylem
anatomy.
This research has shown that the hydraulic conductivity
of the measurable roots from a rootstock may vary slightly,
but it is dif®cult to determine the impact of this on the
entire root system. Despite this, rootstocks have been
shown capable of in¯uencing scion hydraulics independently of differences in leaf area. Observations using
staining to determine the amount of functional xylem,
show that there was little innate difference with respect to
rootstock vigour. But the amount of functional xylem
tissue in the graft and scion increased with rootstock
vigour. This was re¯ected in the measurements of scion
stem hydraulics even when differences in xylem crosssectional area were taken into account. This suggests that
such changes were due to anatomical features associated
with xylem anatomy and not simply due to differences in
stem cross-sectional area.
These observations show that graft tissue of a dwar®ng
rootstock has a lower hydraulic conductivity compared to a
more vigorous rootstock. In order to overcome the
anatomical disfunction associated with the xylem anatomy
of graft tissue, the cross-sectional area of the union
increases. This is a typical auxin response, which is
probably due to an imbalance caused by greater basipetal
transport of auxin in the scion, relative to the transport in
dwar®ng rootstock (Soumelidou et al., 1994b). This leads
to the accumulation of auxin in the graft tissue, which
explains the increased graft xylogenesis.
Acknowledgements
This work was funded by the Ministry of Agriculture, Fisheries and
Food. We are extremely grateful for the comments of Drs Mike
Fordham, Tony Webster and David Dunstan on an earlier draft of
this manuscript and Gail Kingswell for the statistical analysis.
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