Annals of Botany 79 : 145–152, 1997
Pressure–Volume Analysis of a Range of Poikilohydric Plants Implies the
Existence of Negative Turgor in Vegetative Cells
R. P. B E C K E T T
UniŠersity of Natal, Botany Dept, PriŠate Bag X01, ScotsŠille 3209, Republic of South Africa
Received : 5 February 1996
Accepted : 12 August 1996
Pressure–volume (PV) isotherms were determined for a range of poikilohydric plants. The plants included a lichen,
a filmy fern, three bryophytes and two angiosperms. Graphs of turgor potential (ψp) as a function of relative water
content (RWC) derived from the PV curves suggested that most of the cryptogams, but not the angiosperms,
contained significant amounts of intercellular water when fully hydrated. In several species part of the PV curve fell
below the extrapolated linear portion of graph, implying that over this range of RWCs the plant’s cells have negative
turgor ; values as low as ®0±3 MPa were recorded. Negative turgor occurred in those species with a high bulk modulus
of elasticity, implying that it develops only in plants that have cells with rigid walls. Plants that can display negative
turgor will undergo cytorrhysis at lower RWCs than plants in which negative turgor does not occur. The significance
of these findings for the water relations of poikilohydric plants is discussed. # 1997 Annals of Botany Company
Key words : Pressure–volume isotherms, poikilohydric, desiccation, water stress, thermocouple psychrometry,
negative turgor, wall elasticity.
INTRODUCTION
The ‘ pressure–volume ’ (PV) isotherm for a plant is one of
the most widely used tools for characterizing its water
status. Constructing a PV curve involves measuring the
relationship between plant water potential (ψ) and tissue
relative water content (RWC), then plotting (®1}ψ) as a
function of (1®RWC). In the literature, this relationship is
often termed a PV curve (Wenkert, Lemon and Sinclair,
1978 ; Beadle, Ludlow and Honeysett, 1993), although
strictly speaking, it is an inverse pressure–volume graph.
The resulting curve is initially concave, but beyond the
region where turgor is lost (i.e. where turgor no longer
contributes to ψ) the curve becomes linear. Examples of the
parameters that can be obtained from a PV curve include
the osmotic potential at full turgor (ψπs), the apoplastic
water fraction, a graph of turgor potential (ψp) as a function
of RWC, and the tissue bulk modulus of elasticity (εv).
In studies involving higher plants, the pressure chamber
or ‘ pressure bomb ’ is the usual technique for measuring ψ
when determining the PV relationship (Beadle et al., 1993).
However, the pressure bomb technique is unsuitable for
many plants, e.g. bryophytes and lichens and, recently,
several workers have used the thermocouple psychrometer
to measure ψ (Santarius, 1994 ; Beckett, 1995, 1996).
However, very little information is available on the water
relations of ‘ resurrection ’ or poikilohydric plants, whose
metabolic activity can recover after their tissues have been
reduced to very low RWC. The aim of the present
investigation was to construct PV isotherms for a range of
poikilohydric plants using the thermocouple psychrometer,
and to use the information derived from these curves to
increase understanding of how the cells of these plants
0305-7364}97}020145­08 $25.00}0
respond to desiccation. The work was prompted by
preliminary observations suggesting that some poikilohydric
plants display PV relationships which are significantly
different from those usually observed in homiohydric higher
plants.
MATERIALS AND METHODS
Plant material
Roccella hypomecha (Ach.) Bory. was collected from a
quartzitic sandstone rocky shore in the supralittoral zone
near Cape Town, Republic of South Africa. All other plants
were collected from KwaZulu Natal Province, Republic of
South Africa. The thalloid liverwort Dumortiera hirsuta SW
(Arnell) was collected from rocky boulders forming a
waterfall at Ferncliffe, Pietermaritzburg, and the filmy fern
Trichomanes melanotrichum Schlechtend. from adjacent
boulders. The leafy liverwort Porella capensis Mitt. and the
moss Plagiomnium rhynchophorum (Hook.) Kop. were
collected from boulders under a tree canopy in the Royal
Natal National Park. Plants were stored wet or dry (as
collected) for up to 1 week, then fully hydrated by first
storing them at a relative humidity of 100 % (in a desiccator
over distilled water) at 20 °C and a light intensity of
135 µmol m−# s−" for 2 d, then placing them in distilled
water for 1 h. Plants showed very little increase in weight
after this time and were therefore assumed to be fully turgid.
The poikilohydric angiosperms Myrothamnus flabellifolia
Welw. and Xerophyta Šiscosa Bak. were collected from the
Itala Nature Reserve and the Royal Natal National Park,
respectively. Plants were maintained in the Botanic Gardens
of the University of Natal, Pietermaritzburg until required.
bo960318
# 1997 Annals of Botany Company
146
Beckett—Water Relations of Poikilohydric Plants
Several whole leaves of M. flabellifolia and 1¬4 cm leaf
strips of X. Šiscosa were fully hydrated by incubating them
in deionized distilled water for 12 h.
Determination of ψ
Water potential was determined using a Decagon SC-10A
thermocouple psychrometer linked to a Wescor HR-33T
microvoltmeter. After equilibration for 4 h and measurement of approx. 100 mg of hydrated plant material, the
tissue was allowed to lose 2–3 mg of water and, after 4 h, ψ
was again measured. This was repeated until 18 to 20
measurements had been made on each of five or six samples,
and the tissues had reached a RWC of 0±2–0±4 and a ψ of
®5 to ®10 MPa. Standard solutions of known ψ were
always run with samples, and values of ψ corrected to a
temperature of 20 °C.
Determination of the cellular location of water
Water can occur in the symplast, in the apoplast, i.e. the
pores in the cell wall, and intercellularly, i.e. between the
cells. The symbols Rs, Ra, and Ri indicate the proportion of
water in fully hydrated plants in these three fractions,
respectively. To estimate Ri a PV curve, i.e. (®1}ψ) as a
function of (1®RWC), was drawn. The resulting curve was
initially concave, but beyond the region where turgor is lost
(i.e. where turgor no longer contributes to ψ) the curve
became linear. From the PV isotherm, turgor potential (ψp)
was calculated as the difference between the extrapolated
linear portion of the curve and the actual curve, and ψp
was then plotted as a function of RWC. In most of the
cryptogams, unlike the angiosperms, ψp did not fall with the
initial loss of water. Water lost between 100 % RWC and
the RWC at which turgor started to fall was assumed to be
intercellular, i.e. Ri. The RWCs for all the data were
recalculated to exclude intercellular water, i.e.
RWCc ¯
9
fresh weight®dry weight
turgid
dry
weight of inter®
®
weight
weight
cellular water
: 9
: 9
:
where RWCc is the relative water content corrected to
exclude intercellular water. The PV curve was then replotted and Ra calculated from the x-axis intercept, then
corrected back as a percentage of total thallus water. It
should be noted that this method of estimating Ra assumes
that apoplastic water is constant and occurs in very small
(5–10 nm) pores in the cell wall. Plants will only lose this
water at a low RWC, when ψ is less than 15 MPa (Meidner
and Sheriff, 1976).
1981). Turgor potentials were recalculated, and showed the
expected decline with decreasing RWCc. Tissue elasticity
(εv) was calculated from the relationship between ψp and
RWCc (Stadelmann, 1984).
Check of the estimate of ψπs
As a check on the estimate of ψπs derived from the PV
isotherm, the ψπ of five replicate samples were estimated as
follows. Tissue (100 mg) was rehydrated as described above
and placed in the sample cups, wrapped in at least three
layers of ‘ Parafilm ’ and immersed in liquid nitrogen for
approx. 5 min. The cups were then allowed to warm to
room temperature (approx. 1 h) The Parafilm was then
removed, and the sample cups rapidly transferred back to
the thermocouple psychrometer. After an equilibration time
of 1 h, ψ was determined. Assuming that freezing ruptures
membranes and thus destroys turgor, ψ will equal ψπ. This
method underestimates ψπ because intercellular and apoplastic water dilutes ions and molecules in the symplast. To
correct for this, a modification of the equation of Jones and
Rawson (1979) was used :
ψ πs ¯
ψ πk
1®Ra®Ri
where ψπk is the water potential of water-saturated killed
lichens. The value of ψπs obtained in this way was usually
in good agreement with that derived from the PV curve
(Table 1).
Determination of the tissue K+ concentration and
contribution of K+ to ψπ
For the cryptogams, tissue K+ concentration was determined by digesting approx. 20 mg of material to dryness
in HNO then dissolving the residues in 1  HNO . K+ was
$
$
determined using an atomic absorption spectrophotometer
an air}acetylene flame. The mean concentration of K+ (mol
l−") in the cell water was calculated as follows :
thallus K concentration
9mean intracellular
:¬1000
(mol g dry mass ")
H O content
#
9mean(gthallus
:¬(1®R ®R )
g dry mass ")
−
−
a
i
The ψπ of a solution of KCl of this concentration was
determined from tables, and expressed as a percentage of
ψπs, estimated as the mean of the values derived from the PV
curve and the freezing methods.
Statistical analysis of data
Determination of osmotic potential at full turgor and the
bulk modulus of elasticity
The osmotic potential at full turgor (ψπs) was calculated
from the y-axis intercept of the linear portion of the PV
curve, i.e. the value of ψπ at 100 % RWCc (Tyree and Jarvis,
For the PV curves, data from all the thermocouple cups
(typically 80–100 determinations) were combined. The best
fit curves were calculated using the ‘ Spline ’ program of
Hunt and Parsons (1974). A linear regression was carried
out on the linear portion of the PV curve. This was
Lichen
Roccella
Bryophytes
Plagiomnium (moss)
Porella (leafy
liverwort)
Dumortiera (thalloid
liverwort)
Fern
Trichomanes
Angiosperms
Myrothamnus (dicot)
Xerophyta (monocot)
®2±62³0±18
®1±70³0±18
®2±10³0±08
®0±67³0±05
®3±06³0±39
n.d.
n.d.
®2±19³0±59
®2±37³0±19
®1±89³0±15
®0±59³0±03
®1±77³0±11
®1±92³0±09
®1±41³0±07
n.d.
n.d.
®2±42
®0±63
®2±04
®2±00
®2±40
0
0
9³2
0
17³2
13³8
33³5
0
7³3
6³1
0
23³3
6³1
23³5
1±71³0±04
0±66³0±13
1±84³0±14
16±7³1±0
1±30³0±06
2±19³0±13
0±79³0±02
21³5
10³5
25³10
34³20
9³5
11³3
5³5
Tissue H O
#
ψπs (MPa)
ψπs (MPa)
Intercellular Apoplastic content (g εv at ψπ ¯
(estimated from (estimated from Mean ψπs H O (%
g dry
1 MPa
H O (%
#
#
PV curve)
freezing)
mass−")
(MPa)
(MPA)
total)
total)
0±90³0±01
0±81³0±02
0±78³0±09
0±94³0±02
0±55³0±09
0±71³0±03
0±42³0±06
RWC at
turgor
loss
®0±303
0
®0±081
®0±188
®0±022
®0±041
0
n.d.
n.d.
154³21
1600³210
97³11
340³19
20³1
n.d.
n.d.
97³9
95³9
117³13
193³14
56³4
n.d.
n.d.
18³2
68³6
26³3
43³3
7³0
Maximum Tissue K+
K
negative
content concentration
turgor
(µmol g
of cell
developed
dry
water
ψπs due to
(MPa)
mass−")
( m)
K (%)
T     1. Characteristics of water relations of Šarious poikilohydric plants. Values are means of fiŠe or six determinations³1 s.d. and n.d. indicates not
determined
Beckett—Water Relations of Poikilohydric Plants
147
148
Beckett—Water Relations of Poikilohydric Plants
extrapolated to the y-axis, and, as outlined above, ψp
estimated as the difference between the PV curve and the
extrapolated linear part of the graph. The Spline program
was then used to derive a graph of ψp as a function of RWC.
For the estimates of the characteristics of water relations of
the plants (e.g. the values of ψπs, Ri and Ra presented in
Table 1), a separate PV curve was constructed for the
material in each thermocouple cop, and the means and
standard deviations of values derived from each of the five
or six curves calculated.
RESULTS AND DISCUSSION
Figures 1, 2 and 3 present the PV isotherms and graphs of
ψp as a function of RWC for the plants examined, and Table
1 presents the characteristics of water relations for all the
species examined. In addition, Table 1 also presents tissue
K+ concentrations and contributions of K+ to ψπs for the
cryptogams.
In the lichen Roccella hympomecha, ψp began falling only
when the RWC fell below 0±7 (Fig. 1 B), suggesting that
approx. 30 % of the water associated with the plant was
intercellular. This was presumably because, like other
poikilohydric cryptogams, it lacks a cuticle to impede the
penetration of water into the thallus. In higher plants Ri is
4
small or nonexistent (Oertli, 1989). As the values of ψπ and
εv were low, this lichen lost turgor only at a very low RWC
(0±42). While it is tempting to assume that lichens will
benefit from maintaining turgor down to low RWCs,
Harold, Harold and Money (1995) have recently shown that
positive turgor is not essential for hyphal growth in freeliving fungi. In addition, Scheidegger, Schroeter and Frey
(1995) have shown that, in many lichen species, even
collapsed photobiont cells can display net photosynthesis.
Care is therefore needed before attributing advantages to
turgor maintenance at low RWCs in lichens.
The PV curve of the filmy fern Trichomanes melanotrichum
(Fig. 1 C) resembled those obtained for higher plants (e.g.
Beadle et al., 1993) but careful inspection of the graph of
turgor potential as a function of RWC (Fig. 1 D) suggested
that approx. 9 % of the water associated with the fern was
intercellular. In addition, over the range of RWCs from
0±6–0±75 the PV curve fell slightly below the extrapolated
linear portion of the graph, i.e. ψ was more negative than
expected. As a result, calculation of ψp from the difference
between the extrapolated linear portion of the graph and the
PV curve implied that small negative values of ψp (approx.
®0±08 MPa) developed in the tissue before the cells collapsed
and ψp became zero. The fern was the only species for which
estimating ψπs from the PV curve gave a significantly
1.2
A
B
1.0
0.8
ψP (MPa)
–1/ψ (MPa–1)
3
2
1
0.6
0.4
0.2
0.0
0
0.2
0.4
0.6
0.8
1.0
0.5
0.6
1-RWC
3.0
C
0.7
RWC
0.8
0.9
1.0
D
1.5
1.0
2.0
ψP (MPa)
–1/ψ (MPa–1)
2.5
1.5
0.5
1.0
0.0
0.5
0
0.2
0.4
0.6
1-RWC
0.8
1.0
–0.5
0.6
0.7
0.8
RWC
0.9
1.0
F. 1. Pressure volume isotherms and graphs of ψp as a function of relative water content (uncorrected) for the lichen Roccella hypomecha
(A, B) and the filmy fern Trichomanes melanotrichum (C, D). In this and all subsequent figures points represent fitted values with 95 % confidence
limits calculated using the ‘ Spline ’ program of Hunt and Parsons (1974).
149
Beckett—Water Relations of Poikilohydric Plants
6
A
2.0
B
1.5
4
ψP (MPa)
–1/ψ (MPa–1)
5
3
2
1.0
0.5
1
0.0
0
0.2
0.4
0.6
1-RWC
0.8
0.5
1.0
2.0
C
4
0.6
0.7
0.8
RWC
0.9
1.0
0.9
1.0
D
ψP (MPa)
–1/ψ (MPa–1)
1.5
3
2
1.0
0.5
1
0.0
0
0.2
0.4
0.6
1-RWC
0.5
E
2.0
0.1
1.5
ψP (MPa)
–1/ψ (MPa–1)
1.0
0.8
1.0
0.6
0.7
RWC
0.8
F
0.0
–0.1
0.5
–0.2
0
0.2
0.4
0.6
1-RWC
0.8
1.0
0.4
0.5
0.6
0.7
RWC
0.8
0.9
1.0
F. 2. Pressure–volume isotherms and graphs of ψp as a function of relative water content (uncorrected) for the moss Plagiomnium rhynchophorum
(A, B), the leafy liverwort Porella capensis (C, D), and the thalloid liverwort Dumortiera hirsuta (E, F).
different result from the estimate derived from the freezing
method ; no explanation for this discrepancy can be offered.
Figure 2 presents the PV curves and graphs of ψp as a
function of RWC for the three bryophytes investigated.
Results for the moss Plagiomnium rhynchophorum and the
leafy liverwort Porella capensis resembled those of the filmy
fern. The plants contained moderate amounts of intercellular
water ; as the leaves of these plants are only one cell thick,
this water presumably occurred in the stems. The values of
ψπs recorded were similar, but slightly lower, than those
recorded in members of the Bryidae by Santarius (1994).
Results suggested that small negative turgor values develop
in these plants. However, the shapes of the PV curve and the
graph of ψp as a function of RWC for the thalloid liverwort
Dumortiera hirsuta were completely different from those of
the other bryophytes. From a RWC of 0±4–0±95 the PV
curve fell below the extrapolated linear part of the graph.
This is good evidence that negative turgor (up to ®0±2 MPa)
occurred in this plant before the cells collapsed and ψp
became zero. This liverwort had a very high water content
(17 g of water g−" dry mass, Table 1). These very high tissue
water contents probably explain why inter- and intracellular
water was not detected in this species, because Ri and Ra
would comprise a very small proportion of total thallus
water. As ψπs and εv were high (Table 1), D. hirsuta lost
turgor at a high RWC (0±94).
One possible alternative explanation for the observed
deviation of the PV isotherm of D. hirsuta from the usual
shape is that the cell wall of this species has pores that
release water as the plant dries. Thus the minimum in the PV
150
Beckett—Water Relations of Poikilohydric Plants
1.5
1.5
A
B
ψP (MPa)
–1/ψ (MPa–1)
1.0
1.0
0.5
0.5
0.0
0.0
0.0
0.2
0.4
0.6
1-RWC
0.8
–0.5
0.75
1.0
0.85
0.90
RWC
0.95
1.0
C
1.5
0.80
1.00
D
ψP (MPa)
–1/ψ (MPa–1)
0.8
1.0
0.5
0.6
0.4
0.2
0.0
0.0
0.0
0.2
0.4
0.6
1-RWC
0.8
1.0
0.80
0.85
0.90
RWC
0.95
1.00
F. 3. Pressure volume isotherms and graphs of ψp as a function of relative water content (uncorrected) for the dicotyledonous angiosperm
Myrothamnus flabellifolia (A, B) and the monocotyledonous angiosperm Xerophyta Šiscosa (C, D).
curve occurred at ®1 MPa (Fig. 2 E), a value of ψ that can
release water from a pore with a diameter of 0±3 µm. This
explanation appears unlikely for several reasons. First, the
pore sizes of plant cell walls are usually very much smaller
than this (5–10 nm, Meidner and Sheriff, 1976). Second, loss
of apoplastic water will reduce the RWC without reducing
ψπ. This will cause a positive rather than a negative
deviation from the PV curve. Finally, D. hirsuta has very
large cells (100¬50 µm), and image analysis revealed that
the cell wall occupied only 10 % at most of the area of a
vertical section through the thallus. This makes it impossible
for the wall to have released enough water to explain the
observed anomalies in the PV isotherm. Thus, although
further work involving direct measurement of ψp with a
pressure probe is required, the evidence from the PV curve
strongly suggests that negative turgor develops in drying
tissues of D. hirsuta.
Total thallus K+ concentrations varied considerably
between the cryptogams (Table 1), although K+ concentrations in the cell water were all within the range 50–200 m.
Plants collected from wetter sites tended to have higher K+
concentrations, as previously found by Brown and Buck
(1979) for bryophytes, and Beckett (1995) for lichens.
However, despite having low K+ concentrations, plants
from xeric habitats also tended to have lower values of ψπs
than those from mesic habitats (Table 1). As a result, the
proportion of ψπs accounted for by K+ was lower in these
plants. Presumably, organic molecules, e.g. sugars, contributed the balance of ψπs, possibly protecting membranes
from the high concentrations of ions that occur in desiccated
tissues (Bewley and Krochko, 1981 ; Gaff, 1989).
Figure 3 presents the results from the two species of
angiosperms examined. These species did not possess
intercellular water, presumably because both have welldeveloped cuticles. Both species had little apoplastic water,
and similar values of ψπs. The PV curve of Xerophyta Šiscosa
resembled those typically obtained for higher plants.
However, in Myrothamnus flabellifolia the part of the PV
curve from 0±8–0±9 RWC fell well below the extrapolated
linear portion of the graph. Calculation of ψp from the
difference of the extrapolated linear portion of the graph
and the PV isotherm therefore implied that negative values
of ψp up to ®0±3 MPa developed in the tissue. The leaf cells
of X. Šiscosa were long and thin, typical of many
monocotyledons. Low temperature scanning electron microscopy suggested that as the leaves dry, the cell walls offer
little resistance to collapse. Conversely, the cells of M.
flabellifolia appeared to resist collapse until they eventually
imploded and cytorrhysis occurred, the almost cubic cells
becoming grossly distorted. This difference in behaviour of
the cell walls was reflected in the much higher value of εv (i.e.
more rigid cell walls) found in M. flabellifolia (Table 1).
Perhaps the most interesting finding of this survey was
that some poikilohydric plants display anomalous PV
isotherms. In these species, the PV curve falls below the
extrapolated linear portion of the graph, i.e. ψ is more
negative than would be predicted from the usual relationship
between RWC and ψ. The simplest explanation for these
Beckett—Water Relations of Poikilohydric Plants
results is that, as the plant tissues desiccate, negative turgor
develops. Oertli (1989, 1993) has shown that, on theoretical
grounds, plasmolysis is very unlikely to occur in air-dried
plant cells ; the plasma membrane must remain firmly
attached to the wall as the cell loses water. As a result, if the
RWC drops further after a cell has reached the turgor loss
point, negative turgor will develop until either cell wall
collapse (cytorrhysis) or cavitation, i.e. intracellular gas
bubble formation, occurs. Both events will raise cell ψ.
Irrespective of how negative turgor is eventually released,
the PV curve will become linear below the RWC at which
most cells have collapsed or cavitated. Reductions in ψπ will
then be solely responsible for further reductions in ψ. As
cells will not all collapse or cavitate at the same RWC, the
ψp of the tissue will rise gradually, rather than suddenly,
to zero as illustrated in Figs 2 F and 3 B. Development of
negative turgor, followed by its release at lower RWCs,
seems the most likely explanation of the anomalous PV
curves found in the present study.
The PV curves displayed by most plants do not show any
indication of negative turgor, because their cell walls are
not strong enough to resist collapse. Using a technique
involving high molecular weight osmotica, Oertli, Lips and
Agami (1990) showed that the cell walls of most mesic
plants offer less that 0±1 MPa of resistance to collapse.
However, in sclerophyllous desert plants the wall offers
considerably more resistance (up to 1±6 MPa). The amount
of negative turgor that can develop in a cell will depend on
the mechanical properties of the wall. In the present study,
the bulk modulus of elasticity, εv, was reasonably correlated
(P ! 0±1) with the magnitude of negative turgor (Table 1).
In a spherical cell with isotropic walls εv depends on cell wall
thickness (δ), cell radius (r) and Young’s modulus of
elasticity of the wall material (ε*) as follows (Tyree and
Jarvis, 1981) :
2δε*
εv ¯
3r
Preliminary observations showed that no obvious
relationships existed between cell wall thicknesses, cell radii
and εv. This suggests that variations in the values of Young’s
modulus of elasticity of the wall material may have caused
the variations in εv found in this study, thus determining the
extent of negative turgor which developed.
Preliminary low temperature scanning electron microscopy suggested that cell wall collapse, rather than cavitation, occurred in the species examined. However the risk
of cavitation increases as ψp becomes more negative (Oertli,
1993). Interestingly, Honegger (1995) and Scheidegger et al.
(1995) have recently published electron micrographs
suggesting that ascomycetous lichen mycobionts form a
large intracellular gas bubble when desiccated. Surveying a
wide range of lichens, Beckett (1995) found that the values
of εv were very low in all species. This would suggest that the
cells would tend to collapse before pressures low enough to
cause cavitation develop. However, more work is needed
to determine whether cavitation occurs in poikilohydric
species.
Although based on a limited number of species, the
present study does suggest that negative turgor may be more
151
common in poikilo- than homiohydric plants. Presumably a
reduction in the RWC at which cytorrhysis occurs is
beneficial for plants. However, some poikilohydric plants
do not appear to develop negative turgor, and the extent to
which cell collapse and cavitation are harmful to plants
needs further investigation.
A C K N O W L E D G E M E N TS
I gratefully acknowledge the Foundation for Research
Development and the University of Natal Research Fund
for financial support, Drs P. M. Drennan and H. Sherwin
and Profs N. Pammenter and M. Savage for many useful
discussions, and an anonymous reviewer of an earlier draft
of this work for useful comments.
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