Journal of Experimental Botany, Vol. 49, No. 325, pp. 1421–1430, August 1998
Water relations of citrus exocortis viroid-infected
grapefruit trees in the field
Samuel Moreshet, Shabtai Cohen1, Zamir Assor2 and Moshe Bar-Joseph3
Department of Environmental Physics and Irrigation, Institute of Soils and Water, Agricultural Research
Organization, The Volcani Center, PO Box 6, Bet Dagan 50250, Israel
Received 24 September 1997; Accepted 17 April 1998
Abstract
Citrus viroids (CVds) were used to limit citrus tree size
(‘dwarfing’). It is hypothesized that changes also occur
in the hydraulic properties of the conducting system,
thereby affecting water relations. Grapefruit trees (var.
Star Ruby on Troyer Citrange rootstock) in northern
Israel were infected with citrus exocortis viroid (CEVd).
The orchard was drip irrigated. The infected (I) and
apparently healthy (H) control plots were subdivided
into wet (W) and dry (D) irrigation treatments receiving
100% and 70% of the recommended irrigation quantity
for the region. Soil water content, soil temperature,
water uptake, leaf and stem water potential, and leaf
conductance were measured in addition to climatic
variables. Water uptake response to increasing springtime soil temperatures was greater in healthy than in
infected trees. Leaf conductance in the infected trees
was lower than the non-infected trees only in the sixth
year after inoculation. Stem water potential was
significantly lower in the CEVd-infected trees than in
the control trees. Water uptake was lower and
hydraulic resistance higher in infected than in healthy
trees. It was concluded that CEVd causes a reduction
in the water uptake ability of the root and canopy
system. Possible mechanisms for this are discussed.
Key words: Plant water relations, exocortis viroid, Citrus,
hydraulic resistance.
Introduction
Uptake, transport, and loss of water by citrus trees in
relation to the environment have been investigated extens-
ively in the last two decades (for reviews see Kriedemman
and Barrs, 1981; Shalhevet and Levy, 1990; Syvertsen
and Lloyd, 1994). The studies have included the effects
of soil moisture (Bielorai, 1980; Dasberg, 1995; Moreshet
et al., 1988a, b) and of climate (Cohen, 1991; Cohen and
Fuchs, 1987; Cohen et al., 1997a) on tree water relations.
The shape and size of the canopy affects the water use
efficiency of citrus trees (Cohen and Fuchs, 1987). Leaf
conductance and transpiration under various soil water
contents and wetted soil volumes have also been examined
(Cohen, 1991; Cohen et al., 1987; Moreshet et al., 1983,
1990). Hydraulic resistance of the canopy is comparable
in magnitude to that of the roots (Moreshet et al., 1990).
Pruning of approximately 40% of the roots of mature
citrus trees reduced water uptake by only 20% compared
with trees with intact root systems, so root pruning had
little effect on the hydraulic resistance of trees (Moreshet
et al., 1988b). Thus there is a feedback mechanism that
adjusts the plant water potential to values that enable the
tree to withdraw sufficient water from the soil to satisfy
water loss. Drying the soil did not affect the hydraulic
resistance of the trees, but presumably decreased the
water potential at the soil–root interface for citrus (Cohen
et al., 1983) and cotton (Moreshet et al., 1996a). Changes
of tissue properties may have an effect on the conducting
system, but this has never been reported under field
conditions. Such changes may occur, however, when tree
tissues are invaded by pathogens ( Talboys, 1968).
Bacteria and fungi can cause changes in the conducting
tissues and thus presumably change water transport properties (Dimond, 1970) either directly or indirectly through
the elicitation of plant acclimation such as cell wall
lignification (Rodrigo et al., 1993). Various fungi that
1 To whom correspondence should be addressed. Fax: +972 3 960 4017 E-mail: [email protected]
2 Present address: Fruit Crop Experimental Station of the Hula Valley, Upper Galilee Agricultural Company, Qiryat Shemona 10200, Israel.
3 Present address: The S. Tolkowsky Laboratory, Department of Virology, Institute of Plant Protection, Agricultural Research Organization, The Volcani
Center, PO Box 6, Bet Dagan, Israel.
© Oxford University Press 1998
1422
Moreshet et al.
invaded the vascular system of American chestnut trees
( Ewers et al., 1989) and of peaches (Hampson and
Sinclair, 1973; Chang et al., 1991) increased their
hydraulic resistance. Young Tree Decline (Blight), a
disease of unknown etiology increased the resistance of
roots ( Young and Garnsey, 1977) and of stems of orange
trees (Cohen, 1979). This disease also reduced leaf size,
leaf water potential and leaf conductance to below that
of healthy trees (Syvertsen et al., 1980).
Graft-transmissible dwarfing complex (GTDC ) consisting of a combination of citrus viroids (CVds), were
used experimentally to dwarf viroid-tolerant citrus
varieties grafted on CVd-tolerant rootstocks (Ashkenazi
and Oren, 1988). CVd dwarfing offers several horticultural advantages especially as a means of sustainable
horticulture (Bar-Joseph, 1993). GTDC source consists
of CVds with various molecular sizes (Duran Vila et al.,
1988; Gillings et al., 1988; Hadas et al., 1989; La Rosa
et al., 1988). Molecular characterization of the different
CVds involved in dwarfing suggested that they will affect
the extent of the dwarfing (Mendel, 1968; Broadbent
et al., 1986; Hadas and Bar-Joseph, 1991; Bar-Joseph,
1993). In practice, plants are either infected in the nursery
before being transplanted to the orchard or are infected
directly in the orchard one year after transplanting. The
inoculation procedure involves grafting a piece of infected
tissue on to the stem. In Citrus medica seedlings the viroid
moves initially to the roots and is redistributed within the
canopy in the course of several months (Gafny et al.,
1995). The effect of CVd infection on water loss and
water relations has not been examined previously, but
decreases in canopy growth implicate changes in water
relations. Some research has shown that viroid infection
is associated with changes in the plasmallema (Sanger,
1982), which may affect the cellular pathway for water
transport in roots (Taiz and Zeiger, 1991). It was
hypothesized that CVd infection changes cell membrane
properties in a way that affects water status and water
transport. Our objective was to determine if water
relations of CVd-infected trees are affected.
Materials and methods
Experimental layout
The experiment was conducted in 1993 to 1995 on grapefruit
trees (Citrus paradisi var. Star Ruby) on Troyer Citrange
(Poncirus trifoliata×Citrus sinensis) rootstock, at the
Horticultural Experiment Station located in the Hula Valley in
northern Israel, 33°10∞ N 35°36∞ E, 70 m above MSL. Although
the soil is a reddish fine clay, it is well drained and classified as
Chromic Haploxerert.
The climate is Mediterranean with winter rainfall occurring
between November and March. The average annual rainfall for
the last 50 years in Kfar Blum, 2 km west of the station, was
516 mm, and for 1993 to 1995 was 644 mm. For the summer
months (June–September) of 1993 to 1995 the average daily
maximum temperature was 36.3 °C, the average daily minimum
temperature was 16.3 °C, the minimum relative humidity 30.6%,
the maximum wind velocity 3.2 m s−1, and the maximum global
radiation 901 W m−2. The mean daily evaporation (based on
the Penman (1948) equation) measured from a 2 m high
meteorological mast 4 km west of the station (by Dr M Meron,
Megal, Israel ), was 9.2, 8.9, 7.8, and 6.8 mm in June, July,
August, and September, respectively.
Orchard tree spacing was 2×5 m, and row azimuth 77° from
N. The orchard was graft-inoculated in the spring of 1989 (one
year after planting) with five different GTDC sources including
GTDC No. 225 T, GTDC No. 225M (Hadas et al., 1989),
citrus bent leaf viroid No. 225 and CEVd isolates No. 225 and
CEVd-M (Ben Shaul et al., 1995). The experimental orchard
included one control treatment which was not inoculated. The
orchard was divided into six 1-row blocks and subdivided into
six 7-tree plots in each row. Each block contained all six
treatments randomly distributed among the plots. The present
water relations study contrasted two of the treatments; the
CEVd-M variant (Ben Shaul et al., 1995) and the control.
Viroid presence in leaf extract was tested and confirmed twice
during the experimental period by bioassay using seedlings of
the sensitive Citrus medica (‘Etrog’) and by an electophoresisbased CEVd analysis (Hadas et al., 1989).
Each of the seven plots of the two treatments was divided
into Wet (three trees) and Dry (four trees) subplots that were
irrigated by separate dripper laterals. Each tree received water
from two drippers (4 l h−1 per dripper) laid out along the row
bed on two sides of the tree at a distance of 0.5 m from the
trunk. All plots were irrigated twice a week. The Wet treatment
received the full regional recommended quotient (0.55 of the
potential evapotranspiration, computed by means of the Penman
(1948) equation) and the Dry treatment received 70% of the
wet treatment. The 1993 to 1995 actual average yearly totals of
applied water (in mm) were: Infected Wet (IW ), 547; Infected
Dry (ID), 392; Healthy Wet (HW ), 558; Healthy Dry (HD), 396.
Measurements
The circumference of the rootstock, immediately below the
graft, was measured once a year during spring in all trees
(approximately 30 trees per treatment). The cross-sectional area
was calculated assuming the stem was circular and was taken
as an indicator of growth.
All other measurements reported here were made during 1993
to 1995 (unless noted otherwise) on four of the six blocks which
crossed the orchard diagonally from south-east to north-west.
One central tree per plot served as the sample tree (i.e. four
trees per treatment).
Soil water content: Water content in the soil was measured
during 1995 with the time-domain reflectometry (TDR) method
(Topp et al., 1980). Probes were made of three 3 mm diameter
stainless steel rods, 200 mm long, with 40 mm between the
central rod and the outer rods, and the probes connected to
detachable coaxial cables. Three probes per tree were embedded
in the soil at depths of 0–0.2, 0.3–0.5, and 0.6–0.8 m, at a
distance of 0.2 m from the dripper. The apparent length of the
soil-embedded probes was determined with a commercial cable
tester (Model 1502B, Tektronics, Beaverton, OR). Topp et al.’s
(1980) equation was used to compute the volumetric soil
water content.
Climatic variables: Global radiation, air temperature, relative
humidity, and wind velocity were monitored by an automatic
weather station located in the center of the experimental site,
Water relations of viroid infected trees
1423
0.5 m above the canopy. Soil temperature was measured during
1994 and 1995 below the trees with coated copper–constantan
thermocouples at three depths: 0.05, 0.10 and 0.30 m. Each of
the temperature sensors was scanned electronically once a
minute and averaged for each depth every half hour.
the water uptake represents the resistance of the whole tree,
and that between stem WP and water uptake represents the
resistance of the root system. This analysis for partitioning
whole tree and root resistance has been described by Moreshet
et al. (1990).
Plant measurements and computations: leaf area: Leaf area was
measured in 1995 by inversion of canopy transmittance
measurements made with several line quantum probes (Sunlink
and Ceptometer, Decagon Inc. Madison, WI; CI150, CID Inc.,
Pullman, WA). Reference global and diffuse irradiance outside
the orchard were measured in a nearby clearing by means of
two quantum sensors (LI-190S, Li-Cor Inc., Lincoln, NE) and
a standard meteorological shadow band (Drummond, 1956).
All radiation sensors were calibrated within 6 months of the
measurements against an Eppley Precision Solarimeter (Eppley
Laboratories, Hartford, CT ). The inversion method has
been described and validated by Cohen et al. (1997b), and is
approximately 20% accurate. For the measurements below the
canopy, the line sensors were oriented parallel to the row and
placed at 0.1 m intervals from one interrow gap to the next.
The 0.8 m (probe length) linear-averaged transmittance was
log-averaged across the row, and the leaf angle distribution in
the inversion formula was assumed to be spherical (Cohen and
Fuchs, 1987). Trees were assumed to be 2 m long in the row
direction, even though larger trees tended to encroach on the
space of neighbouring smaller trees. Therefore, this assumption
may result in an overestimation of the leaf area of smaller trees
and an underestimation of that of larger trees.
Leaf conductance: Leaf conductance was measured on selected
days with a steady-state diffusion porometer (Model LI1600,
LICOR, Lincoln, NE, USA). The leaf chamber was acclimatized
for 30 min before measurement. Five sunlit and five non-sunlit
exposed leaves per treatment were sampled at 1–2 h intervals
throughout the day.
Water uptake: Volume flow rate of water in the stem, referred
to as water uptake, was measured with the heat pulse method
(Cohen, 1994). The sensors, 3 mm in diameter and 63 mm long,
contained six thermistors mounted 8 mm apart. The first
thermistor sensed stem temperature 4 mm below the bark and
the last one 44 mm below the bark. For a detailed description
of the system and its calibration see Cohen (1994). The
measurement was made in the scion’s stem between the graft
and the lowest branch. Sixteen trees, four per treatment, each
from a different block, were sampled for these measurements.
Each tree was measured once per hour continuously. Data were
collected by two data loggers (Campbell Scientific 21X, Logan,
UT, USA) through two multiplexers (Model TJB 818, Ariel,
Petah Tiqwa, Israel ), each serving eight trees. The system was
powered by four 12 V car batteries charged by solar panels.
Results
Stem cross-section
Stem cross-sections of the infected and control treatments
are shown in Fig. 1. Differences became significant only
in 1994, five years after inoculation. Although the reduced
growth is only moderately significant (P<0.1 in 1994 and
1995), it is clear from the general trend that growth
differences were progressing with the course of infection.
Water uptake and soil temperature
The increased soil temperature in spring is one of the
signals which initiates the recovery of water uptake by
citrus trees, following the low uptake in winter (Reuther,
1973; Moreshet and Green, 1984). The relationship of
water uptake to soil temperature in the late winter
and springtime of the sixth year after inoculation
(15 December 1994 to 30 April 1995) is shown in Fig. 2.
The slope of the regression for the healthy wet (HW )
treatment was significantly (P<0.01) higher than that for
either of the infected treatments. These results indicate
that the springtime activity in the wet treatment was
delayed as a result of the infection.
Leaf and stem water potential (LWP and stem WP): Water
potential was measured with a pressure chamber (Model
Arimad-2, A.R.I. Water Supply Accessories, Kfar Charuv,
12950 Israel ) on selected days. Three leaf types were sampled:
sunlit, exposed but not sunlit, and shaded leaves sealed with
aluminium foil the previous evening. The latter were taken to
represent water potential of the stem xylem, subsequently
referred to as stem WP. Four to five leaves of each type per
treatment were sampled every 1–2 h throughout the day. Each
leaf was placed in a polyethylene bag at the time of harvesting
and carried immediately to the pressure chamber. Sometimes
all leaves from a treatment were harvested together and kept in
their polyethylene bags in a cool box under damp paper towels.
Hydraulic resistance: The slope obtained by regressing negative
LWP on sap flow represents the hydraulic resistance of the tree.
As both sunlit and shaded leaves are active in gas exchange
(Figs 3, 5), a weighted LWP based on the relative sunlit and
shaded leaf area (Moreshet et al., 1996a) was used in the
regression. The relationship between the weighted LWP and
Fig. 1. The growth in stem cross-sectional area of 3–7-year-old
grapefruit trees (Star Ruby on Troyer Citrange rootstock) during 1991
to 1995. Trees inoculated with citrus exocortis viroid in 1989 are
compared to control trees; each symbol is the mean of ~30 trees±1 SE.
1424
Moreshet et al.
Fig. 2. The relationship between daily total sap flux and soil temperature at the depth of 0.3 m in grapefruit trees infected by citrus exocortis viroid
during late winter and spring of the sixth year after inoculation. Lines—linear regressions. Each point represents an average of two to four trees.
Regression equations (±standard error of estimate) are given.
Leaf conductance
There were no significant differences in leaf conductance
four and five years after inoculation. A typical summertime diurnal course, measured in the fourth year
(1993, Fig. 3), demonstrates the low conductances of
citrus. The morning conductances were higher than those
of midday, with a slight increase late in the afternoon.
Sunlit leaf conductances were lower than those of shaded
leaves. The higher afternoon conductance in healthy trees
as compared to infected trees was not found on other
days of measurement in the same year.
In the sixth year, 1995, a stress was applied to the dry
treatments by withholding irrigation for 13 d. Climate
data for the days of intensive measurements at the end
of this stress period, 14–17 August, are given in Table 1.
Irrigation of the dry treatments was resumed during the
night of 16–17 August. Soil moisture profiles for
August 14 and 16 are shown in Fig. 4. In the top 0.2 m
layer, water depletion was significant only in the HD
treatment; at 0.3–0.5 m it was significant in both HD and
ID treatments; and at 0.6–0.8 m (not shown), it was
significant in all but the IW treatment. In general, Fig. 4
shows that soil water depletion was greater in the healthy
than in the infected treatments.
Midday leaf conductances (11.00 h to 15.00 h) for the
last 3 d of drought, 14–16 August, and on the day after
irrigation resumption (17 August) are shown in Fig. 5.
The low conductances found on August 14 can be attrib-
Fig. 3. Diurnal trends of leaf conductance in sunlit and shaded leaves
of grapefruit trees infected by citrus exocortis viroid in the fourth year
after inoculation (17 August 1993). Solid squares—healthy treatments,
open squares—infected treatments. Each point represents the average
of four leaves taken from two trees. No significant difference between
treatments.
uted to the large vapour pressure deficit (VPD) on that
day ( Table 1). The highest leaf conductances were found
in the HW treatment and those in the HD and IW
treatments were similar to each other. With resumption
of irrigation on 17 August, conductance in HD recovered
Water relations of viroid infected trees
1425
Table 1. Climate data for the four days of intensive measurements
in August 1995
Day
August
August
August
August
14
15
16
17
Maximum
temperature
(°C )
Minimum
relative
humidity (%)
Ea
0
34.8
34.0
33.3
32.8
41
46
54
45
9.9
8.2
7.0
7.6
aCalculated evaporation for an open water surface based on the
modified Penman equation (Penman, 1948; Doorenbos and Pruitt,
1975) using hourly climate data measured at an automatic weather
station located 5 km west of the experimental orchard.
Fig. 5. Midday average (11.00 h to 15.00 h) leaf conductance in
grapefruit trees infected by citrus exocortis viroid, at the end of a
drying cycle in the sixth year (14, 16 August 1995). Vertical lines
represent standard error.
Fig. 4. The volumetric soil water content at two depths of a grapefruit
orchard with and without citrus exocortis viroid, at the end of a drying
cycle in the sixth year (14, 16 August 1995). Vertical lines indicate
standard error.
considerably, while no recovery was observed for ID. It
may be concluded that at this stage of the experiment,
six years after inoculation, the leaf conductance of infected
trees was reduced, as was the recovery of leaf conductance
following water stress.
Leaf and stem water potential (LWP and stem WP)
Typical diurnal courses of sunit and shaded LWP and
stem WP, measured 1 d after irrigation during the fifth
year (10 August 1994) are presented in Fig. 6. The vertical
lines are the least significant difference (LSD) between
treatments at P=0.05, where relevant. Differences
between treatments for sunlit and shaded leaves were
significant in very few cases. Significant differences
between treatments for stem WP were found during the
whole day, but these were mostly between HW and dry
treatments. Analysis of variance of the grouped afternoon
measurements of shaded LWP and stem WP showed that
the wet treatments had significantly higher water potential
than the dry treatments (P=0.05), even though they were
Fig. 6. Diurnal trends of water potentials of sunlit and shaded leaves,
and stems of grapefruit trees infected by citrus exocortis viroid, 1 d
after irrigation during summer of the fifth year (10 August 1994). Solid
squares—healthy wet, solid triangles—infected wet, open squares—
healthy dry, open triangles—infected dry. Vertical lines indicate least
significant difference where relevant. Each point represents two leaves
from two trees.
1426
Moreshet et al.
Fig. 7. Diurnal trends of stem water potential of grapefruit trees
infected by citrus exocortis viroid at the end of a drying cycle in the
sixth year (open symbols—14 August, solid symbols—16 August 1995.
squares—dry treatments, triangles—wet treatments). Vertical lines
represent 2 SE. Each point is an average of four leaves from two trees.
measured 1 d after irrigation. The infected trees were not
significantly different from the healthy trees. No differences in sunlit LWP were found between treatments.
Measurements of stem WP during the drying cycle in
the sixth year, 1995, are shown in Fig. 7. August 14 was
the day with the highest evaporative demand, and
16 August was the day before resumption of irrigation.
Mean LWP from 10.00 h to 16.00 h of all the HW leaves
were significantly higher (P<0.01) than those of all other
treatments on both days. Water potentials of the infected
treatments were significantly lower than those of the
healthy ones. The shaded leaves and stem of the ID
treatment had significantly higher WP on 16 August than
on 14 August, the day with the higher VPD (P<0.01).
Water uptake
Water uptake by the trees used for the above LWP
measurements is presented in Fig. 8, as measured on
14 August (highest VPD), 16 August (1 d before
re-irrigation), and 17 August (following resumption of
irrigation). The average daily water uptake by all healthy
trees for 13–16 August (±SD) was 526 (±98) g m−2 leaf
area. The average daily water uptake by all infected trees
was 320 (±80) g m−2 leaf area. A t-test for the average
water uptake in all trees (n=16) showed that uptake in
the infected trees was lower (P<0.01). The higher water
uptake per unit leaf area in HD than in HW treatment
on all days is assumed to be a result of the limited
sampling of leaf area and water uptake (i.e. four trees
per treatment).
Peak water uptake in both dry treatments on 14 August
(highest VPD), was reached earlier than in the wet
treatments. This correlates well with the low midday leaf
conductance measured in the dry treatments (Fig. 5), and
indicates that the response of leaf conductance to
atmospheric dryness may be influenced by the soil water
content.
After resumption of irrigation, daily water uptake did
not change significantly, nor was there a difference
between dry and wet treatments. This result indicates that
the variation in soil water content during the 2 week
drying cycle (see Fig. 4) was within the range that does
not influence citrus daily water uptake, even though it
induced changes in leaf conductance behaviour ( Fig. 4).
The insensitivity of citrus water uptake to a relatively
large range of soil water potentials has been noted previously (Green and Moreshet, 1979; Cohen et al., 1983).
Hydraulic resistance
The relationship between LWP and water uptake per tree
from 1 d after irrigation in the fifth year (10 August 1994;
leaf area was not measured in that year) is shown in
Fig. 9. The total tree resistance (i.e. slope of the relationship between exposed LWP and water uptake) and the
root resistance (i.e. slope of the relationship between stem
WP and water uptake) of the HW treatment were
Fig. 8. Diurnal trends of water uptake per unit leaf area of grapefruit trees infected by citrus exocortis viroid, at the end of a drying cycle in the
sixth year (14, 16 August 1995), and following irrigation (17 August). Each point is an average for four trees.
Water relations of viroid infected trees
1427
Fig. 10. Stem water potential as related to water uptake at the end of
a drying cycle in the sixth year (14, 16 August 1995), and following
irrigation (17 August). The slopes of the relationship ( Table 2) indicate
hydraulic resistance of the roots (see text). Solid lines—best fit, dashed
lines—freehand, drawn between points. Numbers near symbols—serial
number of trees. Solid symbols—wet, open symbols—dry.
Fig. 9. Water potential of leaves (LWP) and stem as related to water
uptake, 1 d after irrigation during a summer day in the fifth year
(10 August 1994). Hydraulic resistance is the slope of LWP regressed
on water uptake. Numbers in each graph are regression coefficients
where ‘resistance=slope, y =intercept, Total=regression between
0
water uptake and exposed leaves, Roots=regression between water
uptake and stem. Solid lines—best fit, dashed lines—freehand, drawn
between points.
significantly lower (P<0.05) than those of the HD treatment, which could be a result of the difference in tree size
and hence in water uptake. The total resistance of the
HW trees was significantly lower than that of the IW, as
a result of differences both in tree size (and hence in
water uptake) and in LWP. LWP of IW was lower than
that of HW ( Fig. 9), but root resistance was not significantly different. Total tree resistances of the HD and ID
treatments were not significantly different. The relationship between stem WP and water uptake of both dry
treatments deviated from linearity in the afternoon
( Fig. 9). As shown by Cohen et al. (1983) storage of
water and capacitance in citrus trees is negligible, hence
it is assumed that the deviation from linearity was not a
result of stem water storage and capacitance. A similar
hysteresis has been observed in cotton in dry soil
(Moreshet et al., 1996a) and like Cohen et al. (1983) they
attributed it to depletion of water near the soil–root
interface.
The percentage of tree resistance not attributed to the
roots ( Fig. 9) was greater in both infected treatments
(49% and 36% in IW and ID) than in the healthy
treatments (30% and 26% in HW and HD). This result
indicates that there was an increase in hydraulic resistance
of the above-ground portions of the infected trees relative
to that of the root system.
In order to eliminate the effect of differences between
treatments in tree size, stem WP was related to water
uptake per unit leaf area in August 1995. Figure 10
presents the resistance of the roots, in which infection
was expected to cause differences. Since LWP varied
between trees in the dry treatments, with minor differences
within a single tree, the relationship is presented separately
for each of the two trees examined per treatment. As
observed in the previous year (see above), the stem WPwater uptake relationship of the dry treatments usually
deviated from linearity in the afternoon. Table 2
shows the correlation coefficients of all linear relationships
1428
Moreshet et al.
Table 2. Regression coefficients of the relationship between water uptake and stem water potential (Fig. 10) in single grapefruit trees
infected by citrus exocortis viroid at the end of an extra drying cycle in the sixth year (14 August 1995)
Treatment on
14 August 1995
Slope (s)
(MPa m2 s mg−1)
Standard error
(s)
Intercept (I )
(MPa)
Standard error (I )
R2
IW, tree 1
IW, tree 12
ID, tree 2
ID tree 8
HW, tree 4
HW, tree 6
HD, tree 11
HD, tree 16
0.073
0.088
0.137
0.088
0.030
0.020
0.051
0.040
0.015
0.017
0.032
0.020
0.006
0.009
0.001
0.009
−0.71
−0.61
−1.08
−1.21
−0.59
−0.50
−0.81
−0.53
0.162
0.148
0.163
0.183
0.081
0.146
0.009
0.060
0.823
0.847
0.791
0.833
0.820
0.499
1.000
0.950
(excluding the non-linear afternoon data, where relevant)
of the single trees for August 14. Healthy treatment
resistances were significantly lower than those of infected
treatments. The decreased resistance in wet as compared
to dry treatments was not significant.
Discussion
Five years following inoculation with CEVd-M, the grapefruit trees grafted on Troyer Citrange showed more than
10% reduction in stem width. The growth reduction effect
of GTDC depends greatly on the combination of CVds
( Hadas et al., 1989; Semancik et al., 1997) and the
sensitivity of the rootstocks. The variability in visible
symptoms in the field was great; some of the 30 trees in
each treatment showed very severe symptoms (e.g. stunting and stem scaling) while others showed less pronounced
symptoms. This variability is currently under investigation. Yet the variability in physiological symptoms is
apparently less, since with the limited sampling used in
this study the observed influences of viroid infection on
plant water relations were significant and conclusive.
Hydraulic resistance and water uptake of citrus and
other trees are soil temperature dependent ( Kaufmann,
1975). Hence, the low water uptake in winter is a result
not only of low potential evapotranspiration, but also of
low soil temperatures. Uptake recovers during spring,
when the soil temperature increases, and uptake may be
directly related to temperature (Moreshet and Green,
1984). Five years after inoculation, the slope of the
relationship of uptake to late winter and spring soil
temperatures was lower than that of healthy trees, as
shown in Fig. 2. Differences between infected and healthy
trees were also found during the summer in specific water
uptake (uptake per unit leaf area), leaf conductance and
hydraulic resistance. When irrigation was withheld, leaf
conductance was further reduced and after resumption of
irrigation, recovery of leaf conductance of the infected
treatment was reduced.
The present study demonstrates the link between the
plant’s ability to draw water from the soil and the leaf
conductance, exemplified by the reductions in water
potential that did not result in increased water uptake
( Figs 8, 10), and coincided with afternoon reductions in
leaf conductance (not shown). This is considered to be
evidence of impairment of the tree’s water uptake capability caused by the CEVd-M infection. The decreases in
leaf conductance were apparently a result of increased
resistance in the hydraulic system.
The changes observed six years after CEVd-M inoculation may be summarized as reduced leaf conductance,
reduced water uptake per unit leaf area, reductions in
stem WP, increased root hydraulic resistance, and a lower
response of sap flow to increasing soil temperature in the
spring. The fact that no changes in LWPs were observed
is an indication that the plant’s basic water transport
system acts to prevent xylem dysfunction (Cochard et al.,
1996; Sperry and Tyree, 1988; Tyree and Sperry, 1988).
This maintains the integrity of the conducting elements
and stomatal response is regulated to prevent excessive
water deficits. The lowered stem WP and the reduced
response to increasing spring soil temperatures indicate
decreases in water uptake, which could be caused by
changes in the hydraulic system of the whole tree ( Fig. 9)
or of the roots (Fig. 10).
Radial resistance of roots is related to cell membrane
properties (Moreshet et al., 1996b). CVd infection was
found to cause the accumulation of pathogenesis-related
proteins in tomato (Lycopersicon esculentum) leaves that
may have a role in lowering LWP (Rodrigo et al., 1993).
Viruses and presumably viroids move from cell to cell
through the plasmodesmata (Nono-Womdim et al., 1993)
and viroid infection is associated with changes in the
plasmalemma (Sanger, 1982). However, the effect of
viroids on membrane permeability to water is not known.
Our findings that in the fifth year hydraulic resistance of
the above-ground system may have increased, and that
in the sixth year stem WP and leaf conductance decreased,
and hydraulic resistance of the root system increased
indicate that the viroid had a detrimental influence on
water transport. This could be explained by a decrease in
membrane permeability.
In conclusion, the present study found evidence that
CVd infection of grapefruit trees grafted on Troyer
Water relations of viroid infected trees
Citrange decreased the ability of the tree to transport
water within the canopy and the ability of the roots to
take up water. Whether the effect is on the cell membrane
properties, on the extent of the root system, or on the
structure of the conducting xylem elements needs to be
examined.
Acknowledgements
We thank Mr Oded Ratner, the previous director of the
Orchard Research Station, and his staff for managing the
orchard and helping with the measurements. Thanks are also
due to Tatiana Milner, Dr Moises Cohen, Yefet Cohen, Suguru
Sato, and Nir Mogilner for their skillful assistance, and to three
anonymous reviewers for their constructive comments.
This paper is contribution No. E-2233, 1997 series, from the
Agricultural Research Organization, The Volcani Center, Bet
Dagan, Israel.
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