Tree Physiology 20, 159–168
© 2000 Heron Publishing—Victoria, Canada
Water utilization, plant hydraulic properties and xylem vulnerability
in three contrasting coffee (Coffea arabica) cultivars
1
2
PETER C. TAUSEND, GUILLERMO GOLDSTEIN and FREDERICK C. MEINZER
1
National Tropical Botanical Garden, 3530 Papalina Rd., Kalaheo, Hawaii 96741, USA
2
Department of Botany, University of Hawaii, 3190 Maile Way, Honolulu, Hawaii 96822, USA
3
Hawaii Agriculture Research Center, 99-193 Aiea Heights Drive, Aiea, Hawaii 96701, USA
3
Received March 24, 1999
Summary Water use, hydraulic properties and xylem vulnerability to cavitation were studied in the coffee (Coffea
arabica L.) cultivars San Ramon, Yellow Caturra and Typica
growing in the field under similar environmental conditions.
The cultivars differed in growth habit, crown morphology and
total leaf surface area. Sap flow, stomatal conductance (gs),
crown conductance (gc), apparent hydraulic conductance of the
soil–leaf pathway (Gt ), leaf water potential (ΨL) and xylem
vulnerability to loss of hydraulic conductivity were assessed
under well-watered conditions and during a 21-day period
when irrigation was withheld. Sap flow, gc, and Gt were greatest in Typica both with and without irrigation, lowest in San
Ramon, which was relatively unresponsive to the withholding
of irrigation, and intermediate in Yellow Caturra. The cultivars
had similar gs when well watered, but withholding water decreased gs more in Typica and Yellow Caturra than in San
Ramon. Typica had substantially lower ΨL near the end of the
unirrigated period than the other cultivars (–2.5 versus
–1.8 MPa), consistent with the relatively high sap flow in this
cultivar. Xylem vulnerability curves indicated that Typica was
less susceptible to loss of hydraulic conductivity than the other
cultivars, consistent with the more negative ΨL values of
Typica in the field during the period of low soil water availability. During soil drying, water use declined linearly with relative conductivity loss predicted from vulnerability curves.
However, cultivar-specific relationships between water use
and predicted conductivity loss were not observed because of
pronounced hysteresis during recovery of water use following
soil water recharge. All cultivars shared the same functional relationship between integrated daily sap flow and Gt , but they
had different operating ranges. The three cultivars also shared
common functional relationships between hydraulic architecture and water use despite consistent differences in water use
under irrigated and dry soil conditions. We conclude that hydraulic architectural traits, rate of water use per plant and
crown architecture are important determinants of short- and
long-term variations in the water balance of Coffea arabica.
Keywords: hydraulic architecture, hydraulic conductance,
sap flow, water deficits, water relations.
Introduction
Studies of whole-plant water use have increased with the development of reliable and inexpensive methods for directly
measuring sap flow in intact plants over a wide range of soil
water availabilities (Granier 1987, Breda et al. 1993a,
Gutierrez et al. 1994, Becker 1996, Meinzer et al. 1999). Much
attention has also been paid to the effects of drought on xylem
function, because severe drought can induce cavitation under
experimental conditions, thereby reducing water transport.
Large differences in xylem vulnerability to water-stress-induced cavitation, some relevant to plant performance in nature, have been found among species (Sperry et al. 1988b,
Sperry and Tyree 1990, Tyree et al. 1992, Sperry and
Saliendra 1994, Kavanagh and Zaerr 1997). In contrast to
whole-plant sap flow studies, xylem vulnerability studies usually involve laboratory measurements on excised small-diameter branches. In a few cases, xylem vulnerability to cavitation
and variation in whole-plant hydraulic conductivity have been
examined concurrently (Lloyd et al. 1991, Sperry and
Pockman 1993, Kavanagh and Zaerr 1997). However, in all of
the preceding studies, values of transpiration required to estimate in situ hydraulic conductivity were obtained from
porometry rather than directly from sap flow.
Populations of the same species, and cultivars of the same
crop may exhibit different morphological and physiological
traits, even when grown under similar environmental conditions. Population- or cultivar-specific differences may have a
genetic basis. For example, the large changes in leaf morphology and physiology observed in Metrosideros polymorpha
Gaud., a common tree in Hawaiian ecosystems, along an
altitudinal gradient are caused by both environmental and genetic variations (Cordell et al. 1998). Similarly, closely related
coffee cultivars growing under identical conditions exhibit
large differences in water relations characteristics, including a
30% variation in intrinsic water-use efficiency (Meinzer et al.
1990b), suggesting a genetic basis for the variation observed.
With respect to hydraulic characteristics, there are few studies
of closely related populations or cultivars of the same crop
species that would permit the functional consequences of vari-
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TAUSEND, GOLDSTEIN AND MEINZER
ation in the properties of the water-conducting pathway to be
assessed.
Coffea arabica L. originated as an understory plant in Ethiopian tropical forests at elevations of 1600 to 2800 m (Maestri
and Santos Barros 1977). These forests receive around
1800 mm annual rainfall, with a dry season of 3 to 4 months.
Despite being shade adapted, commercial coffee cultivars are
increasingly grown in full sun, and are able to out-yield
shade-grown plants (Kumar and Tieszen 1980). There has
been much interest in water relations of coffee since water
stress was identified as a requirement for breaking floral dormancy (Alvim 1960), leading to efforts to identify water deficit thresholds required to synchronize anthesis (Schuch et al.
1990, Crisosto et al. 1992). Whole-plant sap flow has been
studied in commercial fields of the semi-dwarf arabica
cultivar Yellow Catuai ranging in age from 1 to 5.3 years
(Gutierrez et al. 1994). Coffee cultivars differ greatly in crown
morphology (compact versus open), and total leaf surface area
per plant. Because leaf size and density are related to boundary
layer conductance and aerodynamic roughness, hydraulic architectural traits of coffee cultivars are expected to vary accordingly. In addition, the total amount of leaf surface area
supplied by a given cross-sectional area of conducting tissue
(LA/SA) has important implications for the water relations of
plants (Zimmermann 1978, Whitehead et al. 1984).
In the present study, sap flow, leaf water status, stomatal
conductance, and total vapor phase conductance were measured concurrently in field-grown plants of three coffee
cultivars with contrasting growth habits and crown architectures. Leaf area-specific hydraulic conductivity and vulnerability of xylem to reduced hydraulic conductivity were also
assessed in excised branches in the laboratory. Our objectives
were to characterize patterns of water use over a range of soil
water availabilities, and to determine the relationships between xylem vulnerabilities in excised branches and variation
in the apparent leaf area-specific hydraulic conductance of the
soil–leaf pathway of intact plants in the field. We hypothesized that the cultivar with the most open crown architecture or
the largest relative area of foliage per area of sapwood would
experience lower leaf water potentials, and consequently be
less vulnerable to loss of xylem hydraulic conductivity than
the other cultivars.
Materials and methods
Field site and plant material
The study was conducted from mid-April to mid-June 1996 in
the Hawaii State Coffee Trial site located near Eleele, Kauai,
Hawaii (21°54′ N, 154°33′ W, altitude about 90 m). A total of
19 coffee cultivars were present at the 0.53 ha site. The soil at
the site is of the Makaweli stony silty clay loam series of the
Low Humic Latosol great soil group (Foote et al. 1972). The
soil is relatively free of rocks, and has a pH of 6.0. The
cultivars were planted in paired rows in August 1987, at a
spacing of 1.2 m between plants and 3.7 m between rows.
Each row consisted of eight unpruned plants growing in full
sun. The plants typically had one to three main orthotropic
(vertical) shoots bearing many plagiotropic (horizontal)
branches. During the course of the study, plants were in transition from anthesis to fruit development, with 3 to 4 months remaining at the end of the study until fruit maturation. There
was a moderate amount of developing fruit on the plants at the
site. Three cultivars with contrasting shoot morphologies were
chosen for study. Typica, the tallest of the three cultivars, had
a conical, relatively open crown. San Ramon, the shortest of
the three cultivars, had a narrow conical shape, with a dense
crown. Yellow Caturra was intermediate in height, and had a
flat top, with a dense crown. Additional morphological characteristics of the three cultivars are summarized in Table 1.
The measured plants were located within 40 m of each other.
Each row was supplied with drip irrigation, and received a total of approximately 30 mm water in one to two applications
per week, except for a 21-day period when irrigation was withheld. Total precipitation during the 63-day study period was
54.4 mm, including a total of 8.6 mm of widely scattered precipitation during the period without irrigation.
Microclimate
An automated weather station was installed in an open area
near the midpoint of the study site. Relative humidity and air
temperature were measured with shielded sensors (HMP35C,
Campbell Scientific Corp., Logan, UT) mounted at a height of
2 m. Ambient vapor pressure was calculated based on humidity and temperature data. All of the other sensors used in the
study were mounted at a height of 3 m. Photosynthetic photon
flux density (PPFD) was measured with a quantum sensor
(Li-190SB, Li-Cor Inc., Lincoln, NE), wind speed was measured with a cup anemometer (Model 03101-5, R.M. Young
Company, Traverse City, MI), and precipitation was measured with a tipping bucket rain gauge (Model TE525, Texas
Electronics, Inc., Dallas, TX). Readings from all of the sensors
were recorded at 15-s intervals with a data logger (CR10,
Campbell Scientific), and 10-min averages were stored in a
solid state storage module (SM196, Campbell Scientific).
Leaf temperature was measured with fine-wire (0.08 mm)
copper-constantan thermocouples affixed with thin porous
surgical tape to abaxial leaf surfaces. Four leaves, one from
each compass direction, were monitored on the six plants fitted with sap flow gauges. The four thermocouples on a given
plant were connected in parallel in order to obtain a mean leaf
temperature for each plant. Leaf temperature data were recorded at 15-s intervals with a data logger (CR10, Campbell
Scientific), and means were stored at 10-min intervals. The
vapor pressure difference between the leaf interior and bulk
air (Va) was calculated based on saturation vapor pressure at
leaf temperature and the ambient vapor pressure calculated
from the weather station readings.
Sap flow and transpiration
Sap flow through the basal portion of the largest vertical
branch of each plant was measured by the constant heating
method (Granier 1987). The equipment used allowed simultaneous measurement of branches on two plants of each of the
three cultivars. Two 20-mm long 2-mm diameter probes (UP
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161
Table 1. Morphological characteristics (mean ± 1 SE) of the three cultivars in which sap flow was measured. For each row, means followed by different letters were statistically different (P < 0.05; one-way ANOVA).
Morphological characteristic
n
Typica
Yellow Caturra
San Ramon
Plant height (m)
Lateral branch internode length (mm)
Vertical branch internode length (mm)
Basal canopy area (m2)
Total basal sapwood area per plant (cm2)
Leaf area (cm2)
Total leaf area per plant (m2)
4
30
11
4
4
100
4
3.68 ± 0.04 a
39.4 ± 0.7 a
67.8 ± 2.0 a
6.80 ± 0.04 a
78.4 ± 3.0 a
39.2 ± 0.9 a
22.9 ± 1.7 a
1.98 ± 0.04 b
33.0 ± 0.6 b
47.1 ± 1.9 b
4.61 ± 0.03 b
56.3 ± 3.9 b
42.4 ± 0.8 b
27.5 ± 3.3 a
1.50 ± 0.02 c
17.7 ± 0.3 c
34.1 ± 1.8 c
2.90 ± 0.02 c
27.2 ± 2.5 c
30.9 ± 0.6 c
11.0 ± 1.0 b
GmbH, Munich, Germany) were inserted radially near the
base of each selected branch. Each pair of probes was separated vertically by a distance of 15–20 cm. The upper (downstream) probe was continuously heated by a constant current
power supply (UP GmbH), with the lower (upstream) probe
serving as a temperature reference. The protruding portions of
each pair of probes were insulated with a layer of foam rubber
surrounded by an outer shield of reflective car windshield liner
in order to avoid radiant heating of the stem. Probe temperatures were recorded at 15-s intervals with a data logger (CR10,
Campbell Scientific), and 10-min means were stored in a
solid-state storage module. Sap flow density was calculated
from the temperature difference between the probes based on a
standard empirical relationship (Granier 1987). Mass flow of
sap was obtained by multiplying flow density by sapwood
cross-sectional area, which was determined by injection of
0.1% indigo carmine dye into trunks of similar diameters to
those of the measured plants. Trunks were cross-sectioned 2 h
after dye injection 3–5 cm above the injection points and the
colored sapwood measured to calculate the cross-sectional
area of conducting xylem tissue. Branch diameter in the area
of probe insertion on the measured plants ranged from 35 to
88 mm, and the sapwood thickness of these branches was sufficient to avoid potential errors resulting from sapwood thickness being less than the length of the sap flow probes.
Cross-sectional area of the branches measured represented 48,
42, and 63% of the total basal branch cross-sectional area of
entire plants of Typica, San Ramon and Yellow Caturra, respectively. These percentages were used to calculate total
mass flow per plant. Leaf area distal to the probes was determined by multiplying the total number of leaves distal to the
probes by mean area per leaf. Leaf counts were made at the beginning of the study, and subsequently at 4-week intervals.
Mean area per leaf was calculated from subsamples of 100
leaves of each cultivar measured with a leaf area meter (Model
3000A, Li-Cor). Transpiration (E), expressed on a leaf area
basis, was calculated by dividing mass flow of sap by the leaf
area distal to the sap flow probes. The three data loggers used
in this study were synchronized weekly.
Conductances
Stomatal conductance (gs) of six leaves of each plant fitted
with sap flow gauges was measured with a steady state
porometer (Model LI-1600, Li-Cor). The six leaves were chosen to be representative of the range of light exposure during
each set of measurements. In a typical day, four sets of measurements were taken between 1000 and 1600 h. Daily means
of gs for each cultivar were calculated from a total of 48 readings per day. Crown conductance (gc), the total vapor phase
conductance, was calculated as:
g c = EP Va ,
(1)
where E is transpiration rate, P is atmospheric pressure, and Va
is the vapor pressure difference between the leaf interior and
bulk air. Values of gc are expressed on a unit leaf area basis.
Apparent leaf area-specific hydraulic conductance of the
soil–leaf pathway (Gt ) for the branches fitted with sap flow
probes was determined as:
G t = E ∆Ψ,
(2)
where ∆Ψ is the difference between soil water potential and
leaf water potential at a given time. Predawn leaf water potential was used to estimate soil water potential. Parameter Gt was
calculated from midday E and ΨL, when E was relatively constant.
Leaf water potential
Predawn and midday leaf water potentials (ΨL ) were measured with a pressure chamber (Model 1000, PMS Inc.,
Corvallis, OR). Measurements were made on a total of six
leaves taken from two adjacent plants of each cultivar. To determine if predawn ΨL had equilibrated with soil Ψ, leaves
were covered with aluminum foil to prevent the possibility of
nocturnal transpiration. Covered and uncovered leaves of San
Ramon and Yellow Caturra showed no difference, whereas
covered leaves of Typica were 0.1 MPa less negative than uncovered leaves. Calculations of Gt taking into account nocturnal transpiration in Typica differed by less than 9% from
estimates of Gt for uncovered leaves. Midday pressure chamber measurements were also made on foil-covered leaves of
well-watered plants to estimate differences between xylem
water potentials and ΨL when transpiration was occurring.
The ΨL values of covered leaves (approximating xylem water
potentials) were 0.6, 0.4, and 0.3 MPa less negative than un-
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TAUSEND, GOLDSTEIN AND MEINZER
covered (transpiring) leaves in Typica, San Ramon, and Yellow Caturra, respectively.
Branch hydraulic conductivity measurements
Hydraulic conductivity of lateral branches was assessed for
the three cultivars as described by Sperry et al. (1988a).
Woody lateral branches from mid-crown portions of
field-grown plants were cut under water to prevent air entering
the xylem. Selected branches had basal diameters ranging
from 4 to 8 mm. In the laboratory, stem segments from harvested branches were recut under water and immediately connected to plastic tubing supplied with degassed, acidified
distilled water under gravitational pressure (around 0.01 MPa)
from an elevated beaker. Flow rates through segments were
measured volumetrically to determine hydraulic conductivity.
Six segments were measured simultaneously in the apparatus.
Xylem area of segments was estimated with the aid of a dissecting microscope to allow the calculation of specific conductivity (ks). Leaf area distal to each segment was measured
with a leaf area meter (Model 3000A, Li-Cor) to allow leaf
area-specific conductivity (kl) to be calculated.
Vulnerability of xylem to loss of hydraulic conductivity was
determined by developing vulnerability curves as described by
Sperry et al. (1988b). Lateral branches were cut under water
from field-grown plants and air dried in the laboratory for up
to 72 h to allow the development of leaf water potentials ranging from –1 to –6 MPa. After the measurement of leaf water
potentials, branches were sealed in plastic bags for 2 h to allow
equilibration between leaf and xylem water potentials before
measuring hydraulic conductivity. Hydraulic conductivity
was measured under gravitational pressure and then again after conductivity was restored by a high-pressure flush of 0.1
MPa. Loss of conductivity was determined from the difference
in conductivity before and after the high-pressure flush, and
was plotted against the water potential developed before measurement.
ranged from 57 mol m –2 on Day 156 to 23 mol m –2 on Day 119
(Figure 1A). For the three cultivars, Va closely followed variations in total PPF (Figure 1B). Mean Va tended to be lowest for
Typica, and San Ramon usually exhibited the highest Va, as a
consequence of higher leaf temperatures (data not shown).
Mean daily sap flow was highest for Typica throughout the
study period (Figure 1C). Typica was able to maintain relatively high total sap flow during the first 16 days after withholding irrigation, but after Day 156, total sap flow declined
abruptly to < 5 kg H2O day –1. After 19.6 mm of rain fell on
Day 162, total sap flows of Typica plants returned to values
comparable with those measured when the plants were previously well watered. San Ramon had the lowest total sap flows,
which remained relatively constant around 4 kg H2O day –1,
except for a brief time at the end of the period without irrigation. Yellow Caturra exhibited intermediate values of total sap
flow, particularly when it was well watered. However, soon
after withholding irrigation, total sap flow declined steadily
from about 6.5 kg H2O day –1 to less than 2 kg H2O day –1 on
Day 161. When sap flow was normalized by sapwood area
(Figure 1D), San Ramon had the highest sap flow rates and
Results
Differences in cultivar height were consistent with differences
in internode length of both lateral and vertical branches (Table 1). Total basal sapwood area of San Ramon was 35 and
48% of that of Yellow Caturra and Typica, respectively. Area
per leaf and total leaf area were greatest in Yellow Caturra, followed by Typica and San Ramon.
Specific hydraulic conductivity and leaf area-specific hydraulic conductivity of excised lateral branches were greatest
in Yellow Caturra, intermediate in Typica, and lowest in San
Ramon (Table 2). Flushing increased conductivity by 20% in
Typica, 32% in Yellow Caturra and 15% in San Ramon. The
ratio of leaf area to sapwood area (LA/SA) of lateral branches
was higher for San Ramon and Yellow Caturra than for
Typica. Maximum crown conductance (gc) under well-watered conditions was approximately 40% higher in Typica
than in the other cultivars, and under dry soil conditions it was
100 and 17% higher than in Yellow Caturra and San Ramon,
respectively.
During the course of the 63-day study, total daily PPF
Figure 1. A, Total daily photosynthetic photon flux (PPF); B, mean
leaf to bulk air vapor pressure deficit (Va ); C, mean total sap flow per
plant; and D, mean total sap flow divided by sapwood area for the
Typica, San Ramon, and Yellow Caturra cultivars for Days 107–169,
1996 (63 total days). Irrigation was stopped on Day 141 and no significant rainfall occurred until Day 162, when 19.6 mm of rain fell.
Values for PPF, Va, and sap flow are from measurements taken between 0600 and 2000 h.
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163
Table 2. Specific hydraulic conductivity (ks ) of flushed and unflushed branches, leaf area-specific hydraulic conductivity (kl ) of flushed branches,
the ratio of leaf area to sapwood area (LA/SA) of excised lateral branches, and maximum crown conductance (gc) of the three cultivars with and
without irrigation. Values are means ± 1 SE. Means of gc with irrigation represent three maximum measurements made on two individuals of each
cultivar at around 0930 h on 6 days with irrigation. Means of gc without irrigation represent three maximum measurements made on two individuals of each cultivar on days of year 159, 160, and 161, preceding the rainfall received on Day 162. For each row, means followed by different letters were statistically different (P < 0.05 different; one-way ANOVA).
Parameter
n
Typica
Yellow Caturra
San Ramon
Unflushed ks (mmol m –1 s –1 MPa –1)
Flushed ks (mmol m –1 s –1 MPa –1)
Flushed kl (× 10 –3) (mmol m –1 s –1 MPa –1)
LA/SA (m 2 cm –2)
Maximum gc irrigated (mmol m –2 s –1)
Maximum gc un-irrigated (mmol m –2 s –1)
18
18
18
18
36
18
103 ± 8 a
124 ± 9 a
17.3 ± 1.6 ab
0.76 ± 0.07 a
70 ± 5 a
41 ± 5 a
126 ± 10 a
165 ± 11 b
25.6 ± 3.7 b
0.87 ± 0.11 a
51 ± 5 b
20 ± 2 b
73 ± 4 b
85 ± 5 c
11.2 ± 1.3 ac
0.94 ± 0.11 a
48 ± 2 b
35 ± 3 a
Typica and Yellow Caturra had similar rates to those before
water was withheld.
Representative daily courses of PPFD, Va, and transpiration
(E) for the three cultivars under well-irrigated conditions (Day
107) and 16 days after withholding irrigation (Day 156) are
presented in Figure 2. Both days were almost cloudless, as
shown by the daily courses of PPFD and Va. On Day 107, before withholding irrigation, maximum Va ranged from 2.5 kPa
in Typica and Yellow Caturra to 3 kPa in San Ramon. On Day
156, mean leaf temperature was about 4 °C higher than on Day
107, causing maximum Va to increase to about 3 kPa in Typica
and to 4 kPa in San Ramon and Yellow Caturra. Transpiration
rates during well-irrigated conditions were highest for Typica,
followed by San Ramon and Yellow Caturra (Figure 2E).
Transpiration rates 16 days after irrigation was discontinued
were substantially reduced in Yellow Caturra, but were reduced much less relative to well-watered conditions in Typica
Figure 2. A and B, Representative daily courses of photosynthetic
photon flux density (PPFD); C and D, mean leaf to bulk air vapor
pressure deficit (Va ); and E and F, mean transpiration (E) for the three
cultivars. Panels A, C, and E show measurements made under
well-watered conditions on Day 107, and Panels B, D, and F show
measurements made under dry soil conditions on Day 156, 16 days after irrigation ceased.
and San Ramon (Figure 2F).
Predawn ΨL of the three cultivars ranged from –0.09 to
–0.7 MPa when well watered (Figure 3A). During the period
without irrigation (Days 141–161), predawn ΨL declined to
–1.8 MPa in Typica and to around –1 MPa in San Ramon and
Yellow Caturra. Midday ΨL ranged between –0.6 and
–1.7 MPa under well-watered conditions (Figure 3B). The
high value of –0.6 MPa occurred on a day with low PPFD after
a rain (Day 164). During the period without irrigation, midday
ΨL declined to –2.5 MPa for Typica and to near –1.8 MPa for
Figure 3. A, Predawn and, B, midday leaf water potential (ΨL ); and
C, stomatal conductance (gs) of the three cultivars measured during
periods of regular irrigation (Days 108–140) and a period without irrigation (Days 141–161). On Day 162, 19.6 mm of rain fell. Symbols
are means ± 1 SE, n = 6 in A and B, and n = 48 in C.
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TAUSEND, GOLDSTEIN AND MEINZER
San Ramon and Yellow Caturra. Readings from paired tensiometers installed to depths of 15 and 30 cm next to the study
plants indicated that San Ramon depleted soil water more
slowly than the two other cultivars (data not shown). The
cultivars had comparable stomatal conductances when well
watered, but withholding water decreased stomatal conductance more in Typica and Yellow Caturra than in San Ramon
(Figure 3C). At the end of the period without irrigation, mean
–2 –1
gs decreased to about 50 mmol m s in San Ramon and 30
–2 –1
mmol m s in the other cultivars.
Total daily transpiration (E) was positively correlated with
apparent hydraulic conductance of the soil–leaf pathway (Gt )
for all three cultivars, but with Typica operating at higher values of Gt (Figure 4). The lowest values of Gt and E occurred
during the period when irrigation was withheld. Variation in
Gt of the three cultivars was negatively correlated with variation in LA/SA of the main vertical branches (Figure 5). The
three cultivars appeared to share a common functional relationship between Gt and LA/SA, with Gt decreasing sharply at
low values of LA/SA and more slowly at values of LA/SA
2
–2
greater than about 0.3 m cm .
Excised branches of the three cultivars showed increasing
loss of hydraulic conductivity as ΨL became more negative
(Figure 6). Relatively small changes in water status at high ΨL
reduced hydraulic conductivity and each cultivar experienced
approximately a 20% loss of conductivity at a water potential
of –0.5 MPa. However, because of the steeper slopes of their
vulnerability curves, branches of San Ramon and Yellow
Caturra were substantially more vulnerable to loss of hydraulic conductivity than Typica.
Figure 7 shows the relationship between observed loss of Gt
in field-grown plants when irrigation was withheld and loss of
hydraulic conductivity predicted from ΨL measured in the
field and the vulnerability curves presented in Figure 6. The
observed loss of Gt in Typica was lower than the predicted loss
of hydraulic conductivity until it reached values near 30%,
when the loss of Gt began to exceed the predicted loss of hydraulic conductivity. Loss of Gt in San Ramon was lower than
the predicted loss of hydraulic conductivity over the entire
Figure 4. Total daily transpiration (E) as a function of apparent hydraulic conductance of the soil–leaf pathway (Gt ). Line is a fitted nonlinear regression: y = 3.50 + 28.96x0.77, P < 0.05.
Figure 5. Apparent hydraulic conductance of the soil–leaf pathway
(Gt ) as a function of LA/SA, the ratio of leaf area to sapwood area of
main branches in which sap flow was measured. Line is a fitted nonlinear regression: y 2 = –0.14 + 0.23/x, P < 0.05.
range of values of Gt observed. The observed loss of Gt in Yellow Caturra was lower than the predicted loss of hydraulic
conductivity until values of around 38% were reached, when
the loss of Gt began to exceed the predicted loss of hydraulic
Figure 6. Loss of hydraulic conductivity in excised branches in relation to leaf water potential (vulnerability curve) for the three
cultivars. Symbols are means ± 1 SE, n = 3 for ΨL, n = 6 for loss of
branch hydraulic conductivity. Lines are fitted nonlinear regressions:
(Typica) y = 0.18 – 0.06x + 0.02/x, P < 0.05; (San Ramon) y = (–0.009
– 0.12x)0.5, P < 0.05; (Yellow Caturra) y = (–0.02 – 0.11x)0.5, P <
0.05.
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WATER USE, HYDRAULIC PROPERTIES AND XYLEM VULNERABILITY IN COFFEE
Figure 7. Observed loss of apparent leaf area-specific total hydraulic
conductance of the soil–leaf pathway (Gt ) as a function of predicted
loss of branch hydraulic conductivity as calculated from vulnerability
curves for the three cultivars based on ΨL of field-grown plants. Solid
lines are fitted nonlinear regressions: (Typica) y = –9.46 + 0.009x 2.5,
P < 0.05; (San Ramon) y = 2.29 + 0.002x 2.5, P < 0.05; (Yellow
Caturra) y = –11.32 + 0.006x 3, P < 0.05. Dashed lines indicate the 1:1
relationship. Solid symbols represent data from the portion of the
study without irrigation.
conductivity.
Relationships between normalized daily sap flow and predicted relative conductivity loss during the soil drying and
rehydration cycle are presented in Figure 8. Total daily transpiration was first normalized by total PPFD, and then normalized by its maximum value. Relative conductivity loss was
calculated by using the daily minimum ΨL measured in the
field in the regression equations from the vulnerability curve
of each cultivar. During the drying portion of the cycle (downward arrows) there was a linear decline in total water use with
increasing predicted conductivity loss. Although recovery of
ΨL was rapid following soil rehydration, E did not fully recover, resulting in hysteresis in the dependence of E on relative conductivity loss.
Discussion
Of the three cultivars studied, Typica exhibited the highest
rates of sap flow, the lowest ΨL, and the least vulnerability to
loss of xylem hydraulic conductivity. The more open crown
architecture of Typica resulted in greater leaf–air coupling
(Tausend et al. 2000), as reflected in greater maximum values
of gc than in the other cultivars, despite similar values of maxi-
165
Figure 8. Normalized daily sap flow as a function of relative conductivity loss predicted from vulnerability curves for the three cultivars,
based on ΨL of intact plants during the soil drying–rehydration cycle.
Downward arrows represent the drying portion of the study; upward
arrows represent values for one individual during the rehydration portion of the study.
mum gs among cultivars. The combined traits of high sap flow
and intermediate values of lateral branch hydraulic conductivity probably contributed to the more negative ΨL consistently
measured in Typica.
Daily sap flow per plant ranged from 1 to almost 13 kg H2O,
with Yellow Caturra exhibiting the most pronounced sensitivity of sap flow to diminished water supply. Daily sap flow of
20 tree species in heath and dipterocarp forests in Borneo remained relatively constant during a 10-day period without
rain, presumably because of the presence of deep roots
(Becker 1996). Rooting patterns have been described for
Coffea arabica in a range of environments. Under moist soil
conditions in Puerto Rico, it was reported that 94% of the total
roots were in the upper 0.3 m of soil (Wellman 1961). It is
likely that a high proportion of shallow roots would be found
at our site, because the plants were drip-irrigated at regular intervals. However, no soil excavation was carried out to determine if the root distributions of the three cultivars differed.
Assuming that nocturnal stomatal conductance was negligible, permitting equilibration between leaf and soil water po-
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166
TAUSEND, GOLDSTEIN AND MEINZER
tential, predawn ΨL should have reflected the mean soil water
potential at the rooting depth of the plants. The substantially
lower predawn ΨL in Typica than in the other cultivars during
the drought period suggests that Typica either had a shallower
root system, or its relatively high sap flow rates depleted accessible soil water more rapidly, or a combination of both.
Leaf water potentials of the three cultivars after 20 days without irrigation were substantially more negative than those
found after 30 days without irrigation in a previous study that
included the same cultivars (Meinzer et al. 1990b). The difference in plant size between the two studies probably accounts
for these differences in minimum ΨL. In the previous study
(Meinzer et al. 1990b), 0.5-year-old plants ranging from 0.3 to
0.9 m in height were used, whereas the plants used in the present study were 9 years old and ranged in height from 1.5 to
3.7 m.
Meinzer et al. (1990a) suggested that stomatal conductance
of coffee plants changes in a coordinated manner with the hydraulic conductance of the soil–leaf pathway during soil drying. The association between E and Gt observed in the current
study is consistent with this suggestion (Figure 4). There is
also evidence that gs and E are positively correlated with the
hydraulic conductance of the soil–leaf pathway in a wide
range of plant species and growth forms (Aston and Lawlor
1979, Küppers 1984, Meinzer et al. 1988, Meinzer and Grantz
1990, Breda et al. 1993b, Sperry and Pockman 1993, Meinzer
et al. 1995). Such coordination would dampen variation in ΨL
with variation in water availability, and may help to avoid
nonstomatal inhibition of photosynthesis and xylem cavitation
(Wullschleger et al. 1998). In the absence of soil drying, variation in hydraulic architecture also exerted a strong influence
on Gt (Figure 5), and therefore on gs and E (Meinzer et al.
1997, Andrade et al. 1998).
The relatively gradual slopes of the xylem vulnerability
curves determined in our study differ from the sigmoidal
shaped curves reported for many other species that exhibit a
distinct water potential threshold for rapidly decreasing hydraulic conductivity (Tyree and Sperry 1988, Sperry and
Pockman 1993, Kavanagh and Zaerr 1997). Non-sigmoidal
vulnerability curves have been reported for the tropical moist
forest tree Cassipourea elliptica (Sw.) Poit. (Sperry et al.
1988b) and for Quercus rubra L. (Cochard et al. 1992). In general, xylem vulnerability of a species decreases with the minimum water potential it experiences in nature (Tyree and
Sperry 1989). The lowest value of ΨL experienced by Typica
without irrigation was around –2.5 MPa. After adjusting for
the less negative xylem water potentials determined for
foil-covered leaves, our vulnerability curves predicted that
branch hydraulic conductivity in vivo would have been reduced by 29, 48, and 50% for Typica, Yellow Caturra, and San
Ramon, respectively (Figure 6). This loss of hydraulic conductivity at relatively high values of ΨL, which could be interpreted as a low tolerance to drought, may relate to the
relatively mild conditions in the understory, mid-elevation environment in which Coffea arabica is believed to have
evolved. On the other hand, even at the lowest value of ΨL
measured in excised branches (about –6.5 MPa), substantial
residual hydraulic conductivity ranging from 20 to 40% re-
mained in all cultivars. Furthermore, the rapid recovery of Gt
following a rainfall event suggests that loss of xylem hydraulic conductivity at low ΨL may have been largely reversible
(Zwieniecki and Holbrook 1998). Rainfall after a drought
could also increase Gt by increasing soil–root hydraulic conductance. Recent work with the xylem pressure probe has
demonstrated that the pressure chamber technique overestimates the magnitude of xylem tension in transpiring leaves
(Melcher et al. 1998). This finding indicates that caution
should be used when predicting conductivity loss in vivo
based on vulnerability curves and balancing pressures obtained from previously transpiring leaves.
Few studies have linked changes in whole-plant sap flow
and hydraulic properties in the field with xylem vulnerability
curves generated in the laboratory as presented in Figures 7
and 8. Close correspondence between measured loss of hydraulic conductivity of petioles dehydrated in the laboratory
and dried in situ on field plants subject to drought was found in
a study of Quercus petraea L. ex Liebl. and Quercus robur L.
(Bréda et al. 1993b). These authors reported that an 80% loss
of hydraulic conductivity measured in petioles during drought
was associated with a reduction in total sap flow of only 50%.
The discrepancy led Bréda et al. (1993b) to suggest that the
main resistance to liquid water flow from roots to leaves is
probably located between the root–soil interface and branches. From this explanation it follows that large increases in a
relatively minor resistance such as petioles would have only
limited consequences on the total resistance to water flow.
The lack of correspondence between loss of Gt and loss of
branch hydraulic conductivity in the present study has several
possible causes. Loss of Gt might always exceed loss of
branch hydraulic conductivity, because Gt encompasses several hydraulic resistance components, such as the root–soil interface, stem resistance, and resistances of minor branches and
leaves arranged in a series-parallel network. The vulnerability
curves in this study were determined on excised, relatively
small diameter branches that represented only one resistance
component. Differences between xylem water potential and
measured ΨL were not factored in for the prediction of loss of
branch hydraulic conductivity (x axis) in Figure 7. Even if
they had been, there would still be discrepancies between predicted and observed losses, especially during the period when
irrigation was withheld.
Deviation below the 1:1 relationship in Figure 7 could have
resulted from repair of xylem embolism in vivo. Assuming
stomata were closed, the residual evening sap flow observed
in each cultivar (Figures 2E and 2F) may have replaced water
lost from conducting elements through cavitation (Tyree and
Ewers 1991). Alternatively, evening sap flow could have refilled capacitances of living cells. Deviation above the 1:1 relationship was seen for Typica and Yellow Caturra during the
period without irrigation (Figure 7; closed symbols). It is
probable that low soil water content increased resistance to
water flow at the soil–root interface to the point where this resistance became dominant, causing pronounced reductions in
Gt. During increasingly dry soil conditions, the assumption
that predawn ΨL equals soil water potential may not be valid,
because of the possibility of significantly reduced hydraulic
TREE PHYSIOLOGY VOLUME 20, 2000
WATER USE, HYDRAULIC PROPERTIES AND XYLEM VULNERABILITY IN COFFEE
conductance at the root–soil interface. Also, ability to repair
embolism would be reduced under these circumstances, which
would contribute to decreased shoot hydraulic conductance.
The relationship between observed loss of Gt and predicted
loss of branch hydraulic conductivity in San Ramon suggests
that resistance at the soil–root interface did not become dominant, and that repair of xylem embolism may have occurred
during the 21-day period without irrigation. Consistent with
this suggestion was the maintenance of soil Ψ above –0.1 MPa
as measured by tensiometers placed between San Ramon
plants and the breakage of water columns in tensiometers placed between Typica and Yellow Caturra plants. These findings
indicate that relatively low sap flow in San Ramon did not deplete soil water to the same extent as in the other two cultivars.
A longer period without irrigation would have probably
caused observed loss of Gt to exceed predicted loss of branch
hydraulic conductivity in San Ramon.
The discrepancies between conductivity losses in vivo and
in vitro (Figure 7) and the hysteresis seen in the relationship
between sap flow and relative conductivity loss during the
drying–re-irrigation cycle (Figure 8) indicate that caution
should be exercised in extrapolating results obtained with excised branches to behavior of intact plants in the field. A given
value of ΨL in field-grown plants will be associated with differing degrees of limitation of transpiration depending on the
point at which ΨL is measured during a drying–re-irrigation
cycle.
Acknowledgments
The authors thank Kauai Coffee Company for providing access to the
research site.
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