First published online July 17, 2003; 10.1104/pp.103.023879
Plant Physiology 132:2166-2173 (2003)
© 2003 American Society of Plant Biologists
WHOLE PLANT AND ECOPHYSIOLOGY
Stomatal Closure during Leaf Dehydration, Correlation with Other Leaf Physiological Traits1
Tim J. Brodribb* and
N. Michele Holbrook
Department of Organismic and Evolutionary Biology, Harvard University, 16
Divinity Avenue, Cambridge, Massachusetts
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ABSTRACT
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The question as to what triggers stomatal closure during leaf desiccation
remains controversial. This paper examines characteristics of the vascular and
photosynthetic functions of the leaf to determine which responds most
similarly to stomata during desiccation. Leaf hydraulic conductance
(Kleaf) was measured from the relaxation kinetics of leaf
water potential ( l), and a novel application of this
technique allowed the response of Kleaf to
l to be determined. These "vulnerability
curves" show that Kleaf is highly sensitive to
l and that the response of stomatal conductance to
l is closely correlated with the response of
Kleaf to l. The turgor loss point
of leaves was also correlated with Kleaf and stomatal
closure, whereas the decline in PSII quantum yield during leaf drying occurred
at a lower l than stomatal closure. These results
indicate that stomatal closure is primarily coordinated with
Kleaf. However, the close proximity of
l at initial stomatal closure and initial loss of
Kleaf suggest that partial loss of
Kleaf might occur regularly, presumably necessitating
repair of embolisms.
Stomata appear in the fossil record approximately 400 million years ago
(Edwards et al., 1998 ) at
approximately the same time as the evolution of an internal water conducting
system in plants. Stomatal evolution is believed to be a response to selective
pressure to optimize the ratio of CO2 uptake to water lost during
photosynthesis (Raven, 2002).
The evolution of internal conduits for water transport added a level of
complexity to optimizing gas exchange during photosynthesis, because of the
dependence of water supply capacity upon the water potential in the plant
(Sperry et al., 2002). This
complexity is evidenced by the variable effects of leaf water potential
(l) and vapor pressure deficit on stomatal movements
among species. Although stomatal aperture responds predictably to guard cell
turgor (Franks et al., 1995),
the relationships between guard cell turgor and either transpiration (E) or
mesophyll turgor are still hypothetical
(Buckley and Mott, 2002). Amid
mechanistic debate as to the process of stomatal closure, the fundamental
question of why stomata close remains unanswered. Given that stomata may
predate the evolution of xylem (Edwards et
al., 1998; Raven,
2002), it is appropriate to question whether it is vascular or
other tissues that provide the trigger for stomatal closure.
We focus here on the question of what sets the point of stomatal closure in
leaves. That is to say which aspect of a plant's physiology is sufficiently
sensitive to decreasing l that it requires stomata to
be closed and photosynthesis sacrificed to protect from loss of function and
damage. A key assumption here is that traits responsible for determining the
stomatal response to leaf desiccation are coordinated with physiological
characters dictating the sensitivity of the metabolic or transport machinery
of the plant to water stress. Candidates for these coordinated traits are
likely be located in or near the leaf, because transduction of signals from
far upstream of the leaves is generally slow relative to the half-time for
stomatal responses to perturbations in leaf water balance
(Tardieu and Davies, 1993).
Additionally, it would be expected that among these traits, adaptation to
sustain lower l would come at a significant cost.
Features such as the vulnerability of leaf xylem to cavitation and the
resistance of leaf cells to collapse fulfill these criteria in that they are
prone to failure (either structural or functional) under conditions of low
water content and are both costly to augment. However, it is clear that
photosynthesis in most species becomes irreversibly depressed when leaf
relative water content (RWC) falls to around 70%
(Lawlor and Cornic, 2002), and
thus the resistance of the photosynthetic apparatus to desiccation is also a
potential trigger for stomatal closure.
In this paper, we examine the vascular and photosynthetic apparatus of the
leaf to test whether stomatal closure is correlated with the water-stress
tolerance of different leaf tissues or functions. This work follows a number
of studies that have demonstrated similarity between the response of both
stomatal conductance (gs) and stem xylem
cavitation to decreasing l
(Salleo et al., 2000;
Hubbard et al., 2001;
Cochard et al., 2002). It is
likely that this correlation between stomatal closure and xylem cavitation
will be most prominent in the leaf, given that leaf minor veins appear more
prone to cavitation than stems (Nardini et
al., 2001), and that leaves represent a large proportion of the
whole plant hydraulic resistance (Nardini,
2001; Brodribb et al.,
2002). Surprisingly there have been few studies that have
quantified the effect of l on leaf hydraulic
conductance (Kleaf) in woody plants
(Nardini et al., 2001),
probably due to technical difficulties in measuring the hydraulic conductance
of the leaf.
Here, we quantify the relationship between l on
Kleaf by examining the kinetics of
l relaxation in rehydrating leaves. A number of
studies have examined the dynamics of pressure equilibration in leaves to
estimate components of their hydraulic resistance. For example, Cruiziat et
al. (1980) and Tyree et al.
(1981) estimated
Kleaf from the kinetics of water flow into dehydrated
sunflower leaves, whereas Nobel and Jordan
(1983) used the time constant
for water potential equilibration following overpressurization to estimate
leaf mesophyll transfer resistance. In this study, we measured the rate of
relaxation of l during the rehydration of leaves
desiccated to different water potentials, enabling the quantitative
determination of leaf vulnerability to cavitation.
Kleaf was calculated by assuming that the rehydration
of desiccated leaves is equivalent to the charging of a capacitor through a
resistor:
where Vo is the initial potential, Vf
is the potential after charging for t seconds, R is the resistance (=1/K), C
is capacitance (Fig. 1), and t
is a period of recharge. Desiccated leaves are detached underwater from their
subtending branch or stem and allowed to rehydrate for known periods of time,
after which the final l is determined. An important
requirement for the accurate determination of Kleaf is
that the initial (prerehydration) l be measured on
adjacent leaves rather than leaves to be rehydrated. For reasons unknown to
us, pressurization in a pressure chamber substantially alters the ability of
the leaf to rehydrate. Leaves previously measured in a pressure chamber show
little or no tendency to rehydrate through their petiole. Measurement of pre-
and post-rehydration l as well as the time of
rehydration enabled Kleaf to be calculated:
where C is leaf capacitance, o is
l before rehydration, and f
is l after rehydration for t seconds.
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Figure 1. The two-phase function fitted to pressure volume data for five
Gliricidia sepium leaves. Leaf capacitance (Cleaf) was
calculated from the slope of the relationship between leaf RWC and
l (see "Materials and Methods"). Low
Cleaf was found in all species before the turgor loss
point (dotted line). Post turgor loss, Cleaf increased
substantially.
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By examining leaf vulnerability, turgor loss point, and loss of quantum
yield of photosynthesis during leaf desiccation, we were able to determine
which of these characters conformed most closely to the stomatal response to
l. Variation in these relationships was examined
among a group of phenologically diverse species to ascertain whether
correlations between stomatal and leaf physiological parameters were conserved
between species. To maximize the diversity of phenology and physiology of our
sample, two deciduous and two evergreen species were selected from the
seasonally dry forest of northwest Costa Rica. Previous work has illustrated a
diversity of hydraulic and photosynthetic behavior among these species
(Brodribb et al., 2003) making
them ideal for comparative study.
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RESULTS
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Stomatal Closure
A general pattern in the stomatal response to l
was seen in all species, whereby gs was
responsive to l only over a narrow range of
l (Fig.
2). As a result, the transition from 90% to 20% of maximum
gs in each species occurred over a band of
l less than 1 MPa. Despite this rapid transition,
most species exhibited a continuous response of
gs to l, and only
Quercus oleoides developed a plateau where
gs was not sensitive l.
Variation between species was expressed in the initial
l that produced strong decreases in
gs and the range of l to
which stomatal aperture appeared to respond. The point of stomatal closure
(defined here as the l at which
gs fell below 20% of maximum
gs) ranged from -1.65MPa in Simarouba
glauca to -2.95MPa in Q. oleoides. High minimum leaf
gs in Rhedera trinervis appeared to
result from an inability to completely close stomata
(Fig. 2).
Leaf Rehydration
Following detachment underwater, l relaxed (became
less negative) exponentially over time as predicted from the behavior of a
simple resistor/capacitor circuit (Fig.
3). In all species, this exponential increase of
l continued until l reached
around -0.1 to -0.3 MPa, after which it became slower and nonexponential as
l approached zero. The optimal period over which to
measure relaxation in the four species studies was 15 to 30 s, because this
resulted in a large  l without
l rising above -0.3MPa.
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Figure 3. Typical rehydration kinetics for S. glauca leaves. Single points
represent l of leaflets during rehydration of a
single compound leaf. All curves are exponential, and the slope is used to
calculate Kleaf.
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As o became more negative, the slope of the
l relaxation curve became shallower in all species,
indicating a decrease in Kleaf
(Fig. 3). At very low water
potentials (less than -4MPa), leaves rehydrated extremely slowly as
Kleaf approached zero.
Leaf Vulnerability
In all species, Kleaf decreased precipitously once
l fell below a threshold value. Mean maximum values
of Kleaf varied between species from a high of 24.1 mmol
m-2 s-1 MPa-1 in S. glauca to a low
of 16.7 mmol m-2 s-1 MPa-1 in R.
trinervis. Variation in maximum Kleaf within a
species was relatively large, with SDs between 15% and 19%, and as
a result only R. trinervis and S. glauca were significantly
different in mean Kleaf. At low
l, Kleaf fell to minimum values
of between 2% and 20% of the mean maximum Kleaf for each
species (Fig. 4).
Differences in the shape of the response of Kleaf to
l were seen in the slope of the transition between
maximum and minimum Kleaf, with the two deciduous species,
Gliricidia sepium and R. trinervis, exhibiting much more
rapid transitions than the two evergreen species. A clear correspondence
between this transition zone and the region of l to
which gs responded was evident
(Fig. 4). The
l at turgor loss was also closely correlated with the
transition from minimum to maximum Kleaf
(r2 =0.86 for l at turgor loss
versus l at 50% loss of Kleaf).
This result occurred despite the fact that leaf capacitance
(Cleaf) was up to nine times greater in leaves after
turgor loss than the same leaf preturgor loss
(Fig. 1). The effect of this
high capacitance post turgor loss would be to yield much higher calculated
values for Kleaf if the slope of
l relaxation remained equivalent to preturgor loss
values. In fact, the relaxation of l in leaves
desiccated below the turgor loss point was extremely slow relative to leaves
at higher l (Fig.
3), and hence, the calculated Kleaf also
declined at around this point.
Photosynthetic Response to l
PSII quantum yield at 1,800 µmol quanta m-2 s-1
decreased from maximum values of 0.35 to 0.45 to minimum values less than 0.1
as RWC and water potential decreased. Quantum yield responded to
l in a similar fashion to
gs and Kleaf, with an initial
nonsensitive phase followed by a decline to a minimum. The initial part of
this decline was reversible, presumably due to increasing non-photochemical
quenching resulting from factors such as falling CO2 concentration
in the leaf. However, the final loss of  PSII did not
appear to be reversible. Minimum values of PSII were
around 0.1, and unlike leaves rehydrated before reaching this low level of
fluorescence, PSII in leaves desiccated to this point
could not be revived by rehydration. In all species except Gliricidia
sepium the decline in PSII occurred at lower
l than either stomatal closure or loss of
Kleaf, such that complete stomatal closure occurred at
water potentials above that which caused depression of
PSII (Fig.
5).
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Figure 5. Decreasing quantum yield of PSII during leaf desiccation of detached
branches. Each point represents the means ± SD of three to
five leaves. Curves are cumulative normal distributions, and dotted lines
indicate the l at 80% and 20% of maximum
gs.
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Relationships between Leaf Traits and Stomatal Closure
Stomatal closure was closely correlated with the decline in
Kleaf during desiccation. Examination of the slopes of
regressions between stomatal, hydraulic, turgor, and photosynthetic responses
to l indicated that stomatal closure corresponded
most closely with the initial loss of Kleaf
(Table I). A relationship with
turgor loss was also evident, but the slope of l at
turgor loss versus l at stomatal closure was less
than 1, indicating that stomata tended to close before the turgor loss point.
The depression of PSII below 0.10 occurred at water
potentials significantly lower than stomatal closure, and the slope of the
relationship between l at stomatal closure, and
l at PSII<0.10 was
significantly different to 1 (P < 0.01).
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DISCUSSION
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Kleaf
Analysis of l relaxation kinetics provides an
efficient means of assessing the hydraulic conductance of leaves as well as
the response of leaf conductance to decreasing l.
Calculated values of Kleaf from rehydration were very
similar to conductances measured on some of the same species by different
techniques. Maximum values of Kleaf measured by vacuum
infiltration (Nardini et al.,
2001) and pressure drop during E in R. trinervis, for
example, were 15 and 25mmol m-2 s-1 MPa-1,
respectively (Brodribb and Holbrook,
2003), which compares favorably with the mean
Kleaf of 16.7 mmol m-2 s-1
MPa-1 for R. trinervis measured here.
Becker et al. 1999 found a mean
value of 17.2 mmol m-2 s-1 MPa-1 for the
Kleaf of 10 tropical trees measured by a high-pressure
flowmeter (Tyree et al.,
1995), this value also compares well with the mean value of
Kleaf of 20.4 mmol m-2 s-1
MPa-1 from the four species measured here. The
Kleaf of the tropical species studied here was higher than
values of Kleaf for temperate species, which have been
shown to fall in the range of 5 to 20 mmol m-2 s-1
MPa-1 (Nardini,
2001; Sack et al.,
2002).
The rehydration technique employed here produced values of
Kleaf similar to those measured by other techniques such
as the high pressure flowmeter and vacuum infiltration, both of which
potentially allow water to bypass the mesophyll symplast. Given that the
pathway measured during leaf rehydration includes the transfer resistance from
the apoplast into the mesophyll symplast, this agreement suggests that the
mesophyll transfer component of leaf resistance is low. Several recent studies
support this conclusion, suggesting that the majority of the water potential
drop across the leaf occurs in the venation
(Sack et al., 2002;
Zwieniecki et al., 2002; but
see Tyree et al., 1981).
Kleaf was highly sensitive to desiccation, declining
rapidly as l approached the turgor loss point.
Although it cannot be determined which part of the pathway from petiole to
mesophyll is responsible for this decline in Kleaf, recent
evidence from leaf acoustic emissions and dye infiltration have suggested that
leaf minor veins are susceptible to cavitation
(Salleo et al., 2001). We
assume that losses in Kleaf observed here represent
cavitation for two reasons, firstly because the response of
Kleaf in S. glauca to l
here is very similar to the response of petioles of the same species to
water-stress induced cavitation measured by flushing embolisms from the xylem
(Brodribb et al., 2003).
Second, the precipitous decline in Kleaf observed as
l fell below a critical value is indicative of a
process of rapid conduit blockage, and the most parsimonious explanation of
this is cavitation. The close proximity of the l at
incipient loss of Kleaf and l at
50% stomatal closure was surprising and appears to indicate that leaves
closely approach and even cross the leaf cavitation threshold on an average
day of sunny conditions. This would also suggest that cavitation in leaf veins
might be a regular occurrence, requiring the ability to refill cavitated
conduits to maintain photosynthetic capacity of the leaf. Leaves provide
probably the best environment for refilling of embolized conduits (Salleo et
al., 2000;
2001) due to the relative
abundance of inorganic ions and other osmolytes that could be used to generate
positive pressures (Holbrook and
Zwieniecki, 1999), as well as possessing large amounts of
metabolic energy to drive ion movement. Hence, it is plausible that to
minimize leaf resistance, the leaf xylem is constructed with large pores in
inter-conduit pit membranes enhancing conductivity, but increasing the risk of
air-seeding through pit membranes (Sperry
and Tyree, 1988).
Stomatal Closure
Kleaf and gs both showed
very similar responses to l
(Fig. 4; Table I), whereas leaf turgor
loss occurred midway through stomatal closure
(Fig. 4), and damage to PSII
(as indicated by of PSII) occurred at a substantially
lower l. This supports the idea that stomatal closure
occurs as a protective mechanism against xylem cavitation
(Tyree and Sperry, 1988),
although the safety margin, especially in the two deciduous species was
extremely small. A similar relationship between stomatal closure and stem
cavitation was described in a group of tropical deciduous species
(Brodribb et al., 2003),
although a larger safety margin for the stem xylem meant that stomata were
completely closed before a 50% loss of stem conductivity had occurred.
Given that leaves represent a large resistor in the hydraulic pathway
through the plant, it is surprising that this resistor should also be
susceptible to desiccation-induced decline in conductance. The lack of a
safety margin in these species suggests that either the stomatal response to
l is extremely rapid and feed-forward (enabling
relaxation of l to stem xylem water potential after
sudden increases in evapotranspiration) or, as mentioned above, that
cavitation and refilling occur daily. Considering that these requirements, not
to mention the loss of photosynthesis during stomatal closure, would be costly
to the plant, the other alternative of increasing the cavitation resistance of
the xylem must represent an even greater cost. A close link between leaf
turgor loss and loss of Kleaf shown here indicates that a
higher modulus of elasticity and greater osmotic potential of leaf cells would
be required to support lower l as well as greater
lignification of upstream xylem (Hacke et
al., 2001).
Another possibility is that the leaf vascular system rather than being a
weak link in the hydraulic pathway requiring protection, has evolved to
cavitate early as a means of sensitizing the stomata to changes in
evaporation. In this role, the leaf vascular system could amplify the effect
of increasing E on the water potential of guard cells. The only danger in such
an augmentation of the rate of response of l could
might be that rapid decreases in l are known to
induce a transient opening of stomata due to loss of subsidiary cell turgor
(Tardieu and Davies, 1993).
What is required to verify such speculation is a clearer understanding of the
response of guard cells to l, and whether guard cell
movements are controlled by a passive loss of turgor in concert with the
surrounding cells, or by an activated ion pump from the subsidiary cells.
This paper provides the first coordinated examination of how the stomatal,
photosynthetic, and hydraulic systems in the leaf respond to changes in
l. The data presented here showed a remarkably
consistent proximity between the point of initial leaf cavitation and stomatal
closure. By contrast, stomatal closure did not appear to be closely linked to
the water potential at which irreversible damage to photosynthetic apparatus
(PSII < 0.1) occurred. Although turgor loss was
also closely associated with stomatal closure, the physiological impact of
turgor loss is unclear given that photosynthesis was not irreversibly damaged
until water potential fell substantially below the turgor loss point. These
data point to vulnerability of the xylem in leaf veins as a primary trigger
for stomatal closure, although the mechanism for this trigger remains
unknown.
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MATERIALS AND METHODS
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Study Site
This investigation was undertaken in the Santa Rosa National Park, located
on the northern Pacific coast of Costa Rica (10° 52'N, 85°
34'W, 285 m above sea level). Mean annual rainfall in the park is 1,528
mm, however, more than 90% of this falls between the months of May and
December, resulting in a pronounced dry season. The dry season is accompanied
by strong trade winds, low relative humidity, and high irradiance, all of
which contribute to generate a high evaporative demand. Diurnal and seasonal
temperature ranges are relatively small, with a mean annual temperature of
28°C.
We chose four species: two deciduous, Gliricidia sepium (Fabaceae)
and Rhedera trinervis (Verbenaceae), and two evergreen, Simarouba
glauca (Simaroubaceae) and Quercus oleoides (Fagaceae). All are
tree-forming species, with Gliricidia sp. and Simarouba sp.
both producing compound leaves approximately 20 to 30 cm in length and
Rhedera sp. and Quercus sp. both with simple leaves 10 to 20
cm in length. Leaf age was monitored on tagged branches, and only leaves 4 to
6 months old were selected for experiments. All data were collected during the
mid-late wet season from July to September.
Kleaf
Measurement of Kleaf was made under non-steady-state
conditions using the rate of relaxation of l in
leaves detached from the stem under water to calculate the leaf conductance
from Equation 2 (see above). This calculation requires knowledge of
Cleaf, mass of water per unit leaf area, and leaf dry mass
per unit area for each species.
Relaxation of leaf
To determine the time course of leaf relaxation, a
number of small branches bearing eight to 10 leaves in a tight cluster were
cut from single trees and allowed to slowly desiccate in the laboratory. Using
data for the vessel length of each of the four species (T. J. Brodribb,
unpublished data), branches were cut of sufficient length that emboli did not
extend in to the petioles of sample leaves. Once a branch had reached
approximately -1 MPa, the branch was placed in a plastic bag in the dark for
approximately 1 h to minimize variation in water potential between leaves. Two
leaves were then harvested as an estimate of the initial
l. If these leaves differed in
l by more 0.10 MPa, the branch was discarded. Leaves
were rehydrated by submerging their subtending branch in filtered tap water
such that the petioles of the target leaves could be cut simultaneously
underwater using a razor blade. Leaf laminas were maintained dry to avoid
possible uptake of water through the epidermis or stomata. Leaves were allowed
to absorb water for a predetermined period of time after which their petioles
were dabbed dry on paper towel, and the leaves placed in plastic bags to
prevent water loss. l was immediately measured using
a Scholander pressure chamber (PMS, Corvallis, OR).
To test the applicability of the one-compartment rehydration model
(charging of a single capacitor through a resistor), we rehydrated leaves (all
with the same initial water potential) for varying lengths of time. A least
squares exponential regression was then fitted to the plot of final water
potential versus rehydration time. According to Equation 1, the exponent from
this regression is equal to -Kleaf
t/Cleaf.
Cleaf
Cleaf was measured from the slope of the
pressure-volume relationship for each species. The relationship between
l and water volume in the leaf was quantified using
the bench drying technique (Tyree and
Hammel, 1972). Branches were cut underwater in the morning and
rehydrated until l was  0.05 MPa, after which six
leaves per species were detached for PV determination. Leaf weight and
l were measured periodically during slow desiccation
of sample leaves in the laboratory. Desiccation of leaves continued until
ls stopped falling or began to rise due to cell
damage. Due to the elasticity of the cell walls, Cleaf
pre- and post-turgor loss are quite different. It was found that the
relationship between l and leaf RWC could be closely
approximated by a two-phase linear equation intersecting at the turgor loss
point (e.g. Fig. 1). The
capacitance function was defined by measuring the turgor loss point from the
inflection point of the graph of 1/l versus RWC, and
then using this value as the intersection of linear regressions fitted through
data either side of the turgor loss point. Slopes of these curves yielded the
Cleaf function in terms of RWC.
Calculation of Kleaf (mmol m-2
s-1 MPa-1) requires that Cleaf as
determined by the pressure volume curve
( RWC/l, MPa-1) be expressed
in absolute terms and normalized by leaf area. To do this, the capacitance
calculated from the PV curve must be multiplied by the saturated mass of water
in the leaf and then divided by leaf area
(Koide et al., 1991). In
practice, the ratios of (leaf dry weight:leaf area) and (saturated mass of
water:leaf dry weight) were determined for each species and used to calculate
the leaf area normalized absolute capacitance:
where DW is leaf dry weight (g), LA is leaf area (m2), WW is mass
of leaf water at 100% RWC (g), and M is molar mass of water (g
mol-1).
Response of Kleaf to Desiccation
"Vulnerability curves" of each species were constructed by
measuring Kleaf in leaves rehydrated from a range of
initial water potentials. Branches were cut early in the morning while
l was high, and most leaves were removed except for
terminal clusters of four to eight leaves. These branches were then allowed to
desiccate very slowly ensuring all leaves remained at similar
l. Periodically, branches were bagged and placed in
the dark for 30 min to ensure stomata were closed and
l was homogenous among leaves. Two leaves were then
removed to gauge the l of the leaves remaining on the
branch, after which two further leaves were detached with their petioles
underwater and allowed to rehydrate as described above. The standard
rehydration period was between 15 and 30 s. For each sample
Kleaf was calculated using Equation 2, and the mean of the
two samples was used as the Kleaf for the branch at the
specified l. Branches were progressively desiccated
during the course of a single day, and Kleaf was monitored
as l dropped. In a few cases (<5) rehydration
spanned the l at turgor loss. Because
Cleaf differs pre- and post turgor loss, in these
circumstances, the value of Cleaf was apportioned
depending on the relative distances of o and
f from the turgor loss point. This approximation
averages the capacitance during the relaxation period rather than more
correctly applying two separate decay curves to either side of the turgor loss
point. However, because of the short rehydration period, the loss of accuracy
was very small relative to maximum values of Kleaf.
Vulnerability curves were generated by plotting Kleaf
against l. The distribution of vulnerabilities of
conductive elements in the leaf was assumed to be normal, and hence, a
cumulative normal probability curve was fitted to the data.
Response of Photosynthetic Capacity to Desiccation
Chlorophyll fluorescence of PSII was used to measure the sensitivity of
photosynthesis to l during desiccation. Branches were
collected early in the morning and allowed to desiccate under uniform
partially shaded conditions (photosynthetic photon flux density of
1,000–1,500 µmol quanta m-2 s-1). Leaves were
measured in the light to quantify depression of photosynthesis under
conditions experienced in the field. Periodically, leaves were removed and
placed in the leaf clip of a MiniPam (Walz, Effeltrich, Germany) where they
were exposed to an actinic light intensity of 1,800 µmol quanta
m-2 s-1 for 90 s, and the quantum yield of PSII
(PSII) was measured with a single saturating flash to
the middle of the adaxial surface of the leaf (avoiding veins). Leaf
temperature remained between 25°C and 28°C during measurement.
l of the sample leaf was then immediately measured
giving a single PSII and l
per leaf. A minimum of five branches per species were measured, resulting in
at least three measurements per 0.1 MPa from -0.5 MPa until
PSII fell below 0.1. As with the vulnerability data,
cumulative normal probability plots were fitted to the data, and the point of
nonreversible photosynthetic damage was defined as the
l at which PSII fell below
0.1. Leaves with yields below 0.1 did not recover maximum dark-adapted quantum
yield after rehydration (T. J. Brodribb, unpublished data), in approximate
agreement with the general rule indicating 70% RWC as the mean threshold for
photosynthetic damage (Lawlor and Cornic,
2002). Hence PSII = 0.1 was considered to
be the initial damage point for PSII.
Stomatal Closure
Stomatal response to l was measured in all species
under natural conditions as well as using excised branched to determine the
behavior of stomata under extreme drought. All species were surveyed during
the months of August and September 2002. Measurements were made on six trees
of each species and under conditions of full sun.
gs was measured using a porometer (1600, LI-COR,
Lincoln, NE) at different times of the day between 9 AM and 2
PM to include a maximum range of ls.
gs was recorded from a series of marked leaves
that were subsequently removed and bagged for later determination of
l. The relationship between
l and gs was plotted,
and curves were fitted assuming a cumulative normal probability distribution.
We defined the response zone of gs as the region
of l where the fitted curve for
gs fell from 90% to 20% of maximum.
Statistical Analysis
To test which of the three measured leaf parameters
(Kleaf vulnerability, turgor loss point, and
PSII sensitivity) exhibited a relationship to
l most similar to that of
gs, cardinal points in the response functions of
each of these relationships were compared. Slopes of the regressions between
l at early (20%) and mid (50%) stomatal closure, and
l responsible for early (20%) loss of
Kleaf, turgor loss, and decline of
PSII below 0.10 were compared by analysis of variance
with regressions forced through the origin. Using the SE for the
slopes of these regressions, a t test was used to determine whether
slopes were significantly different from 1.
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ACKNOWLEDGMENTS
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We acknowledge the help and support of Maria Marta Chavarria and Rojer
Blanco of Parque Nacional Santa Rosa.
Received April 24, 2003;
returned for revision May 18, 2003;
accepted May 18, 2003.
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FOOTNOTES
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1 This work was supported by the National Science Foundation (grant no. IBN
0212792) and by the Andrew Mellon Foundation. 
*
Corresponding author; e-mail
brodribb{at}fas.harvard.edu;
fax 617–496–5854.
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LITERATURE CITED
|
|---|
Becker P, Tyree MJ, Tsuda M (1999) Hydraulic
conductance of angiosperms versus conifer species: similar transport
sufficiency at the whole-plant level. Tree Phys
19: 445-452
Brodribb TJ, Holbrook NM (2003) Changes in leaf
hydraulic conductance during leaf shedding in seasonally dry tropical forest.
New Phytol 158:
295-303[CrossRef]
Brodribb TJ, Holbrook NM, Edwards EJ, Gutiérrez MV
(2003) Relations between stomatal closure, leaf turgor and xylem
vulnerability in eight tropical dry forest trees. Plant Cell
Environ 26:
443-450
Brodribb TJ, Holbrook NM, Gutiérrez MV
(2002) Hydraulic and photosynthetic co-ordination in seasonally
dry tropical forest trees. Plant Cell Environ
25:
1435-1444[CrossRef]
Buckley TN, Mott KA (2002) Dynamics of stomatal
water relations during the humidity response: implications of two hypothetical
mechanisms. Plant Cell Environ
25: 407-419[CrossRef]
Cochard H, Coll L, Le Roux X, Ameglio T (2002)
Unraveling the effects of plant hydraulics on stomatal closure during water
stress in walnut. Plant Physiol
128:
282-290[Abstract/Free Full Text]
Cruiziat P, Tyree MT, Bodet C, Logullo MA
(1980) Kinetics of rehydration of detached sunflower leaves
following substantial water-loss. New Phytol
84: 293-306
Edwards D, Kerp H, Hass H (1998) Stomata in
early land plants: an anatomical and ecophysiological approach. J Exp
Bot 49:
255-278[Abstract]
Franks PJ, Cowan IR, Tyerman SD, Cleary AI, Lloyd J, Farquhar
GD (1995) Guard-cell pressure aperture characteristics
measured with the pressure probe. Plant Cell Environ
18: 795-800[CrossRef]
Hacke UG, Stiller V, Sperry JS, Pittermann J, McCulloh KA
(2001) Cavitation fatigue: Embolism and refilling cycles can
weaken the cavitation resistance of xylem. Plant Physiol
125:
779-786[Abstract/Free Full Text]
Holbrook NM, Zwieniecki MA (1999) Xylem
refilling under tension: Do we need a miracle? Plant Physiol
120: 7-10[Free Full Text]
Hubbard RM, Ryan MG, Stiller V, Sperry JS
(2001) Stomatal conductance and photosynthesis vary linearly with
plant hydraulic conductance in ponderosa pine. Plant Cell
Environ 24:
113-121
Koide RT, Robichaux RH, Morse SR, Smith CM
(1991) Plant water status, hydraulic resistance and capacitance.
In RW Pearcy, J Ehleringer, HA Mooney, PW Rundel, eds, Plant
Physiological Ecology. Chapman and Hall, New York, pp
161-183
Lawlor DW, Cornic G (2002) Photosynthetic
carbon assimilation and associated metabolism in relation to water deficits in
higher plants. Plant Cell Environ
25: 275-294[CrossRef][Medline]
Nardini A (2001) Are sclerophylls and
malacophylls hydraulically different? Biol Plant
44: 239-245[CrossRef]
Nardini A, Tyree MT, Salleo S (2001) Xylem
cavitation in the leaf of Prunus laurocerasus L. and its impact on
leaf hydraulics. Plant Physiol
125:
1700-1709[Abstract/Free Full Text]
Nobel PS, Jordan PW (1983) Transpiration stream
of desert species. Resistances and capacitances for a C-3, a C-4 and a cam
plant. J Exp Bot 34:
1379-1391[Abstract/Free Full Text]
Raven JA (2002) Selection pressures on stomatal
evolution. New Phytol 153:
371-386[CrossRef]
Sack L, Melcher PJ, Zwieniecki MA, Holbrook NM
(2002) The hydraulic conductance of the angiosperm leaf lamina: a
comparison of three measurement methods. J Exp Bot
53:
2177-2184[Abstract/Free Full Text]
Salleo S, Lo Gullo MA, Raimondo F, Nardini A
(2001) Vulnerability to cavitation of leaf minor veins: any
impact on leaf gas exchange? Plant Cell Environ
24: 851-859[CrossRef]
Salleo S, Nardini A, Pitt F, Lo Gullo MA (2000)
Xylem cavitation and hydraulic control of stomatal conductance in laurel
(Laurus nobilis L.). Plant Cell Environ
23: 71-79
Sperry JS, Hacke UG, Oren R, Comstock JP (2002)
Water deficits and hydraulic limits to leaf water supply. Plant Cell
Environ 25:
251-263[CrossRef][Medline]
Sperry JS, Tyree MT (1988) Mechanism of water
stress-induced xylem embolism. Plant Physiol
88: 581-587[Abstract/Free Full Text]
Tardieu F, Davies WJ (1993) Integration of
hydraulic and chemical signalling in the control of stomatal conductance and
water status of droughted plants. Plant Cell Environ
16: 341-349[CrossRef]
Tyree MT, Cruiziat P, Benis M, Lo Gullo MA, Salleo S
(1981) The kinetics of rehydration of detached sunflower leaves
from different initial water deficits. Plant Cell Environ
4: 309-317
Tyree MT, Hammel HT (1972) The measurement of
the turgor pressure and the water relations of plants by the pressure-bomb
technique. J Exp Bot 23:
267-282[Abstract/Free Full Text]
Tyree MT, Patiño S, Bennink J, Alexander J
(1995) Dynamic measurements of root hydraulic conductance using a
high-pressure flowmeter in the laboratory and field. J Exp Bot
46: 83-94[Abstract/Free Full Text]
Tyree MT, Sperry JS (1988) Do woody plants
operate near the point of catastrophic xylem dysfunction caused by dynamic
water stress? Plant Physiol 88:
574-580[Abstract/Free Full Text]
Zwieniecki MA, Melcher PJ, Boyce CK, Sack L, Holbrook NM
(2002) Hydraulic architecture of leaf venation in Laurus
nobilis L. Plant Cell Environ
25:
1445-1450[CrossRef]
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TOP
ABSTRACT
RESULTS
PSII, and the turgor loss
point