Annals of Botany 88: 537±543, 2001
doi:10.1006/anbo.2001.1499, available online at http://www.idealibrary.com on
Anomalous Pressure Volume Curves of Resurrection Plants Do Not Suggest
Negative Turgor
C L A R E VA N D E R W IL L IG E N * {{, J I L L M. FA R R A N T { and N . W. PA M M E N T E R }
{Botany Department, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa, {Department of
Molecular and Cellular Biology, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa and }School
of Life and Environmental Sciences, George Campbell Building, University of Natal, Durban, 4041, South Africa
Received: 31 January 2001 Returned for revision: 12 March 2001 Accepted: 1 June 2001 Published electronically: 17 August 2001
Pressure-volume (PV) curves of the desiccation-tolerant angiosperms, Eragrostis nindensis, Craterostigma wilmsii and
Xerophyta humilis, and the desiccation-sensitive species, E. curvula, were compared. The shape of curves for
E. nindensis and C. wilmsii di€ered from the usual curvilinear form. Over the relative water content (RWC) range of
approx. 70 to 25 %, PV curves indicated water potentials higher than directly measured water activity on frozenthawed tissue. Anatomical studies showed considerable cell wall folding and a consequent reduction in cell volume in
these two species; this was not seen in X. humilis or E. curvula which showed normal PV curves. It is suggested that
this wall folding may have prevented the development of negative turgor and physical stress in the cells, and
# 2001 Annals of Botany Company
contributed to desiccation tolerance.
Key words: Negative turgor, Craterostigma, Eragrostis, Xerophyta, psychrometry, water relations, water activity, PV
isotherm, resurrection plants, desiccation tolerance.
I N T RO D U C T I O N
* For correspondence. E-mail [email protected]
0305-7364/01/100537+07 $35.00/00
Plant material
Eragrostis nindensis Ficalho & Hiern and E. curvula
(Schrad.) Nees plants were grown from seed. Whole plants
of Xerophyta humilis (Bak.) Dur. and Schinz and Craterostigma wilmsii Engl. were collected from the ®eld as
described previously (Sherwin and Farrant, 1996). All
Water potential
Mechanical damage associated with loss of turgor is
proposed to be one of the major causes of irreversible
desiccation-induced damage in plants (Iljin, 1957; Vertucci
and Farrant, 1995). However, a few plants are able to
survive dehydration to an air-dry state (Ga€, 1971; Bewley
and Krochko, 1982 inter alia). Desiccation tolerance must
involve repair processes during rehydration (observed in
many lower order desiccation-tolerant plants) and/or
protection mechanisms during dehydration (reviewed by
Ga€, 1989, 1997; Oliver and Bewley, 1997; Farrant, 2000),
including protection against mechanical damage.
Pressure-volume (PV) curves are a valuable tool in
describing plant water relations; the isotherms characterize
the relationship between water potential (measured as the
negative of the applied pressure) and relative water content
(RWC) (Tyree and Hammel, 1972). Previous studies have
revealed that some desiccation-tolerant plants have atypical
PV curves as illustrated in Fig. 1 (Sherwin, 1995; Beckett,
1997). Beckett (1997) proposed that the unusual isotherms
indicate that negative turgor develops during dehydration
of these plants. However, no direct evidence or functional
signi®cance of the phenomenon was given.
In this study the water relations of three desiccationtolerant angiosperms (resurrection plants) and one desiccation-sensitive plant were investigated. Anatomical evidence
and direct measurements of water activity during dehydration might o€er an alternative explanation for the anomolous PV curves.
M AT E R I A L S A N D M E T H O D S
i
ii
Relative water content
F I G . 1. Diagram illustrating the atypical PV curve. Lines i and ii
indicate possible extrapolations from the `linear' part of the curve. i
suggests development and release of negative turgor and ii suggests cell
volume reduction and hence water loss with little attendant change in
water potential.
# 2001 Annals of Botany Company
538
Vander Willigen et al.ÐAnomalous PV Curves in Resurrection Plants
plants were maintained in a glasshouse without additional
lighting.
Pressure-volume curves
Well-watered potted plants were fully hydrated overnight
in plastic bags to bring leaves to full turgor. Leaf discs from
mature leaves [second outermost whorl (C. wilmsii,
X. humilis) or segments from the second oldest leaf on a
tiller (E. curvula, E. nindensis)] were used. Water potentials
were measured using C52 sample chambers and an HR-33T
microvoltmeter (Wescor, Logan, Utah, USA) in the dew
point mode. Leaf material and instruments were kept at
25 8C for the duration of the experiment. The following
procedure was used to construct PV curves. Leaf tissue was
weighed at full turgor, sealed in the sample chambers, and
water potential was measured after an appropriate equilibration period. The sample holder was then removed from
the chamber and the tissue was allowed to dry slightly,
before being resealed in the chamber; the water potential
was remeasured after equilibration. Tissue was weighed
immediately after measuring water potential (in case slight
water loss occurred during the equilibration period). This
process was repeated until the tissue lost no more weight.
Generally, only short times of bench drying beween
measurements were necessary, although this increased as
the tissue dried. The time required for equilibration varied
among species and increased (up to 6 h) as tissue water
content decreased. When there was no further decrease in
weight, leaf explants were dried for 48 h at 70 8C to
determine dry weight. Between seven and ten measurements
were recorded for each tissue piece, and tissue was taken
from at least four di€erent plants for each species. All data
were combined to draw a single PV curve. To verify the
unusual shape of the curves in E. nindensis and C. wilmsii,
water potentials and RWC of leaves that were excised from
both dehydrating and rehydrating plants were also
measured. These data were combined with the previous
measurements from the bench-dried samples for the PV
curves presented for these two species.
Direct measurement of `osmotic' potential (water activity)
Fully hydrated plants were allowed to dry naturally by
withholding water. Leaf samples were taken at regular
intervals during dehydration. Leaf water potential and
sample weight were recorded as described above. Thereafter, the leaf sample and sample holder, wrapped in at least
four layers of Para®lmTM (American National Can) and
covered with aluminium foil, were plunged ®ve times into
liquid N2 for approx. 30 s over a period of 10 min. After the
sample and cup had warmed to 25 8C, they were uncovered
and replaced in C52 sample chambers. After an equilibration period, water potential and weight were recorded
again. Dry weight of the leaf sample was measured after
oven drying at 70 8C for 48 h. Measurements were taken
from at least 15 di€erent leaf segments from each of at least
four di€erent plants for each species. The water potential
measured after freezing and thawing of tissue is generally
considered to be osmotic potential if apoplastic water is
negligible (Jones and Rawson, 1979). However, at the low
RWCs achieved in this experiment, it is highly likely that
the system deviated from ideal behaviour. Thus the term
`water activity' (measured by vapour phase equilibration) is
used.
Anatomical studies
Leaves were sampled at regular intervals from plants
which were allowed to dry naturally. For each leaf sectioned
RWC was determined on a portion of the leaf immediately
distal to that sampled for microscopy. Water content and
dry weight (oven-dried for 48 h at 70 8C) were determined
gravimetrically and RWC calculated from this and the
mean water content at full turgor. Mean water content at
full turgor was determined from at least 30 replicates (leaves
hydrated on well watered plants sealed in plastic bags
overnight).
Light microscopy
Transverse hand sections of leaf tissue were viewed with
a light microscope (Ascoscope; Zeiss, Hallbergmoos,
Germany) and photographed. Cell wall perimeter and
area of the region enclosed by the walls were measured from
electronic images using an image analysis program
(AnalySiS, Soft Imaging Software). At least ten cells were
measured on each of approx. 20 images from four replicate
leaves from two di€erent plants for each species. Vascular
tissue was not measured.
Care was taken to maintain the leaf tissue at the RWC to
which it had dried. Preliminary experiments were undertaken to ensure that the RWC of the tissue did not change
during sectioning and viewing. Fully hydrated tissues were
cut and viewed in deionized water. Tissues of RWC from
between 80 and 50 % were hand cut and viewed in a
solution of sucrose of equal osmotic potential (measured
psychrometrically) to that of the tissue. Hand sections of
tissues of 40 % RWC and lower were cut dry and viewed in
a solution of PEG 6000 ( polyethylene glycol) of comparable osmotic potential (measured psychrometrically). The
osmotic potential of the sucrose and PEG 6000 solutions
was remeasured after viewing to con®rm that there had
been no change.
R E S U LT S
Pressure-volume curves of the desiccation-sensitive species,
E. curvula, and the three desiccation-tolerant species,
E. nindensis, C. wilmsii and X. humilis are illustrated in
Figs 2±5 respectively. Apoplastic water, assessed as the
intercept on the RWC axes, showed negative values. Similar
observations have been made previously on woody tissue
using both the pressure chamber and thermocouple
psychrometry. E. curvula (Fig. 2) and X. humilis (Fig. 5)
had typical curvilinear isotherms, with turgor loss points of
ÿ1.25 and ÿ1.05 MPa respectively. However, there was a
clear deviation in the shape of the PV isotherms of
E. nindensis (Fig. 3) and C. wilmsii (Fig. 4) such that it
was not possible to determine a point at which turgor was
Vander Willigen et al.ÐAnomalous PV Curves in Resurrection Plants
80
4
60
3
40
2
1
20
0
100
0
80
60
40
20
0
3
2.5
1.0
100
0.8
60
0.4
40
0.2
0.0
100
20
80
60
40
20
0
20
0.5
120
80
0.6
40
1
80
60
40
20
0
0
Relative water content (%)
0
–1/Water potential (MPa)
0.8
60
1.5
0
100
Cell area (% of max)
–1/Water potential (MPa)
B
80
2
Relative water content (%)
1.0
100
Cell area (% of max)
5
Cell area (% of max)
100
120
A
120
B
100
80
0.6
60
0.4
40
0.2
0.0
100
Cell area (% of max)
A
6
–1/Water potential (MPa)
3.5
120
–1/Water potential (–1/MPa)
7
539
20
80
60
40
20
0
0
Relative water content (%)
Relative water content (%)
F I G . 2. A, Pressure-volume curve of E. curvula (W). Open circles
indicate water activity measured on freeze-thawed tissues. Areas
enclosed by the cell walls indicated by open squares. Error bars
indicate standard deviations. B, Enlargement of A, between 0 and
1 MPa ÿ1 (A). Dotted lines indicate the linear regression and 95 %
con®dence limits for the water activity data. Solid lines indicate the
linear regression and 95 % con®dence limits for the water potential
data (closed circles marked with a white dot).
F I G . 3. Pressure-volume curve of E. nindensis (W). Open circles
indicate water activity measured on freeze-thawed tissues. Areas
enclosed by the cell walls indicated by open squares. Error bars
indicate standard deviations. B, Enlargement of A between 0 and
1 Mpaÿ1 (A). Dotted lines indicate the linear regression and 95 %
con®dence limits for the water activity data. Solid lines indicate the
linear regression and 95 % con®dence limits for the water potential
data (closed circles marked with a white dot).
lost in these species. After approaching loss of turgor there
was a period during which tissue water content decreased
with little change in water potential (70 to 45 % RWC,
E. nindensis and 65 to 25 %, C. wilmsii). Thereafter a linear
relationship between tissue volume and the inverse of water
potential was observed (Figs 3 and 4).
There was a linear relationship between the negative
inverse of the direct measurements of water activity and
RWC in all species (Table 1, Figs 2±5). These data
correlated well with extrapolations of the linear portion of
the PV curves at RWCs below the range where deviation
from the norm was observed. Direct measurements of water
activity were consistently slightly more negative than values
predicted from the respective PV curves (Figs 2±5,
Table 1); this result is not due to dilution e€ects consequent
on membrane rupture as that e€ect would be in the
opposite direction. Ultrastructural examination of tissue
samples on which water activity had been measured
con®rmed that in these samples cellular membranes were
ruptured (data not shown).
Changes in cell volume during dehydration, measured as
a proportion of cell area (area enclosed within the cell wall)
at full turgor, are also illustrated for each species in Figs 2±
5. Examples of sections illustrating shrinkage from the
hydrated to the dehydrated state in the four species are
illustrated in Fig. 6. There were no di€erences in mean cell
wall perimeter at any point during dehydration in any
species (P 4 0.05, n 4 170, data not shown). There was a
slight decrease in cell area in the desiccation-sensitive
species, E. curvula (Fig. 2). In the three resurrection plants,
Vander Willigen et al.ÐAnomalous PV Curves in Resurrection Plants
3.5
100
3
3
80
2.5
60
2
1.5
40
1
20
0.5
0
100
80
60
40
20
0
0
40
0.2
20
20
0
0
Relative water content (%)
–1/Water potential (–1/MPa)
60
0.4
Cell area (% of max)
–1/Water potential (MPa)
80
0.6
40
40
1
20
0.5
80
60
40
20
1.0
100
60
60
1.5
0
100
120
B
80
80
2
0
0
Relative water content (%)
0.8
0.0
100
100
2.5
Relative water content (%)
1.0
120
A
120
B
100
0.8
80
0.6
60
0.4
40
0.2
0.0
100
Cell area (% of max)
3.5
120
Cell area (% of max)
A
–1/Water potential (–1/MPa)
–1/Water potential (1/MPa)
4
Cell area (% of max)
540
20
80
60
40
20
0
0
Relative water content (%)
F I G . 4. Pressure-volume curve of C. wilmsii (W). Open circles indicate
water activity measured on freeze-thawed tissues. Areas enclosed by the
cell walls indicated by open squares. Error bars indicate standard
deviations. B, Enlargement of A between 0 and 1 MPa ÿ1 (A). Dotted
lines indicate the linear regression and 95 % con®dence limits for the
water activity data. Solid lines indicate the linear regression and 95 %
con®dence limits for the water potential data (closed circles marked
with a white dot).
F I G . 5. Pressure-volume curve of X. humilis (W). Open circles indicate
water activity measured on freeze-thawed tissues. Areas enclosed by the
cell walls indicated by open squares. Error bars indicate standard
deviations. B, Enlargement of A, between 0 and 1 MPa ÿ1 (A). Dotted
lines indicate the linear regression and 95 % con®dence limits for the
water activity data. Solid lines indicate the linear regression and 95 %
con®dence limits for the water potential data (closed circles marked
with a white dot).
there was either no change (X. humilis, Figs 5 and 6D) or a
dramatic reduction in the cell area (by 53 % in E. nindensis,
Fig. 3, and by 74 % in C. wilmsii, Fig. 4) on drying. The
change in cell volume in the latter two species occurred over
the range of RWCs during which no or minimal change in
water potential was recordedÐthe region of the PV curves
which deviated from the typical shape. Although there are
numerous errors associated with measuring cell volumes
(Zimmermann et al., 1981; Malone and Tomos, 1990), these
data were used for comparative purposes only.
Transmission electron microscopical images of
E. nindensis leaves prepared by freeze substitution (i.e. no
aqueous ®xation) showed that the thinner walled mesophyll
cells and bulliform cells (associated with leaf curling) folded
extensively on drying, but the thicker walled bundle sheath
and epidermal cells did not fold (data not shown). Limited
cell wall collapse was observed in the mesophyll and
bulliform cells of E. curvula (data not shown). The
ultrastructure of X. humilis (no cell wall folding but
extensively vacuolated) and C. wilmsii (considerable cell
wall folding in all cell types) is reported elsewhere (VicreÂ
et al., 1999; Farrant, 2000).
DISCUSSION
Unusual PV curves similar to those found in this study
(Figs 3 and 4) have been reported previously, using both
pressure chambers (Oertli, 1993) and thermocouple
psychrometry (Sherwin, 1995; Beckett, 1997). Two possible
interpretations of curves of this type are illustrated in Fig. 1.
The ®rst interpretation is that removal of water beyond the
turgor loss point leads to the development of negative
Vander Willigen et al.ÐAnomalous PV Curves in Resurrection Plants
T A B L E 1. Linear regressions (ÿ1/WP ˆ m RWC ‡ c)
derived from the linear portion of the PV curve (limited
RWC range) and direct measurement of water activity for the
desiccation-sensitive species, E. curvula, and three desiccation-tolerant angiosperms, E. nindensis, C. wilmsii and
X. humilis
Species
Data source
E. curvula
PV curve (70 %±0 %)
Direct measurement
PV curve (45 %±0 %)
Direct measurement
PV curve (25 %±0 %)
Direct measurement
PV curve (70 %±0 %)
Direct measurement
E. nindensis
C. wilmsii
X. humilis
m
C
R2
7.27
5.40
4.20
2.11
0.33
0.29
0.01
0.01
0.11
0.08
0.19
0.20
0.01
6.38
0.10
ÿ0.08
0.90
0.90
0.78
0.62
0.81
0.82
0.94
0.92
There were no signi®cant di€erences between the slopes of the
regressions within each species (ANCOVA, P 5 0.05, n 4 30).
turgor. Cavitation or cytorrhysis (cell wall collapse) then
releases water which increases the turgor of neighbouring
cells (Oertli, 1989, 1993; Beckett, 1997). Although Tyree
(1976) disputed early studies predicting negative turgor from
osmotic and water potential measurements, this author did
suggest that if negative turgor was possible in living plant
cells, the PV curve would indeed have to deviate from the
usual shape described by Tyree and Hammel (1972).
However, even with advances in the pressure probe
541
technique (reviewed by Tomos and Leigh, 1999), negative
turgor has yet to be measured in living tissues. The second
interpretation is that negative turgor does not develop, and
there are two possible mechanisms by which this could
occur. Firstly, there could be more than one population of
cell types, each having a di€erent turgor loss point, such that
the resultant PV curve is in fact a compilation of numerous
PV curves. Secondly, close to the turgor loss point there is a
reduction in the volume enclosed by the cell wall (wall
folding), such that the tissue is maintained at zero to slightly
positive turgor, and negative turgor does not develop even
when considerable water is lost.
Beckett (1997) proposed that resurrection plants with
unusual PV curves had more rigid cell walls, that negative
turgor developed in the tissues, and that the cells initially
resisted collapse until after a period of negative turgor.
However, unlike the irreparable damage that can be caused
by extensive cytorrhysis, tissues in resurrection plants do
recover from cell wall folding, the regulated process
associated with changes in the biochemical properties of
the cell walls at speci®c stages of desiccation (Hallam and
Lu€, 1980; Goldsworthy and Drennan, 1991; Vicre et al.,
1999; Farrant, 2000). Beckett (1997) did not present actual
data points, only the results of a spline ®t to the data, and
so direct comparison of the data presented here and those
of Beckett (1997) is not possible.
We propose that the unusual PV curves found in many
resurrection plants are not a consequence of cavitation and
water release, i.e. negative turgor does not develop. There
F I G . 6. Light micrographs of mesophyll tissue of E. curvula (A), E. nindensis (B), X. humilis (C) and (D) C. wilmsii photographed in isosmotic
PEG 6000 solutions at 5 % RWC (1380 for all images). Note the extensive cell wall folding in E. nindensis (B) and C. wilmsii (D). Some cell
shrinkage is visible in E. curvula (A) but not in X. humilis (C).
542
Vander Willigen et al.ÐAnomalous PV Curves in Resurrection Plants
are a number of lines of evidence to support this hypothesis:
(1) Even though the solute concentration would be high and
possibly non-ideal at low RWCs, direct measurements of
water activity from frozen-thawed tissue were always below
the measured water potential values. This indicates that
negative turgor does not develop; (2) Extrapolating a
straight line from the linear portion of the PV curve data
(indirect estimate of `osmotic potential') at low relative
water contents (below the RWCs corresponding to the
deviation) yields a line statistically indistinguishable from
that determined by direct measurement of water activity
(Table 1). This indirect estimate of water activity also
suggests that negative turgor does not occur; (3) If the
deviation from the normal PV curve is a consequence of
cavitation or cytorrhysis, extensive membrane damage and
hence electrolyte leakage would be expected. There is, in
fact, very little leakage of electrolytes from dehydrated
leaves of either species with unusual curves (C. wilmsii,
Farrant et al., 1999; E. nindensis, Vander Willigen et al.,
2001), whereas considerable leakage occurs from E. curvula
(Vander Willigen et al., 2001).
Consequently, there has to be an alternative explanation
for the unusual curves which does not suggest negative
turgor. Unfortunately, it was not possible to identify clearly
the point of turgor loss during the anatomical analysis of
the tissue at the various water contents, and thus the
possibility that numerous PV curves have been superimposed to give the anomalous shape cannot be excluded.
However, this would be unlikely as the cells of C. wilmsii,
one of the species with an unusual curve, are fairly uniform
in their appearance and physical properties. We thus
propose that the unusual curves are the result of a reduction
in the volume enclosed by the walls. Firstly, the extensive
cell wall folding and hence reduction in cell volume
measured in this study occurred over the range of RWCs
in which the PV isotherms deviate from the norm (Figs 3
and 4). Secondly, the shape of the PV curves could imply
considerable changes in the elastic properties of the tissue.
No attempt was made to measure elastic moduli, however,
changes in cell wall chemistry during dehydration of
C. wilmsii have been observed. Such changes, which are
likely to a€ect physical properties of cell walls, are
coincident with wall folding (Vicre et al., 1999).
Tissues that are tolerant of desiccation must prevent
mechanical damage associated with the shrinkage consequent upon the removal of water. One way by which this
might be achieved is via accumulation of insoluble material
that e€ectively replaces the lost water. This phenomenon
has been shown to occur in X. humilis: during drying the
vacuoles become packed with insoluble material (Farrant,
2000). This species yields normal PV curves (Fig. 5). An
alternative possibility is a reduction in cell volume by cell
wall folding. We suggest that the wall folding observed in a
number of resurrection plants (which is associated with
atypical PV curves) is a phenomenon that reduces physical
damage on drying and contributes to the desiccation
tolerance of these plants. The fact that there are changes
in cell wall chemistry in C. wilmsii (Vicre et al., 1999)
suggests that this is a regulated phenomenon.
The development of single cell sampling and direct
measurements of turgor using the pressure probe (reviewed
by Tomos and Leigh, 1999) provide an opportunity to
analyse the unusual water relations of resurrection plants in
more detail. As yet these techniques are limited to peripheral cell layers, and since individual cell volume changes
cannot be quanti®ed accurately during dehydration,
measurements of elastic properties and PV curves of
individual cells within complex tissues are not yet possible.
However, measurements of turgor from both pressure
probes and psychrometric techniques do agree (Nonami
et al., 1987), thus whole tissue investigations will continue
to provide holistic interpretations of a system.
AC K N OW L E D G E M E N T S
The authors thank Mel Tyree (USDA Forest Service, Aiken
Forestry Sciences Laboratory, Burlington, VT, USA) for
comments on an early version of the manuscript, and for
helpful discussions on PV curves, and Stuart Thompson
(Division of Biological Science, Lancaster University, UK)
for useful discussions on pressure probes and cell wall
elasticity. Thanks are due to the Pilansberg and Borakalo
National Parks and Bu€elskloof Private Nature Reserve
and Dr Roger Ellis of Agricultural Reseach Council,
Pretoria, South Africa for plant and seed collection.
L I T E R AT U R E C I T E D
Beckett RP. 1997. Pressure-volume analysis of a range of poikilohydric
plants implies the existence of negative turgor in vegetative cells.
Annals of Botany 79: 145±152.
Bewley JD, Krochko JE. 1982. Desiccation-tolerance. In: Lange OL,
Novel PS, Osmond CB, Ziegler H, eds. Encyclopedia of plant
physiology, Vol 12B. Physiological ecology II. Berlin: SpringerVerlag, 325±378.
Farrant JM. 2000. A comparison of mechanisms of desiccation
tolerance among three angiosperm resurrection plant species.
Plant Ecology 151: 29±39.
Farrant JM, Cooper K, Kruger LA, Sherwin HW. 1999. The e€ect of
drying rate on three desiccation-tolerant angiosperms. Annals of
Botany 84: 371±379.
Ga€ DF. 1971. Desiccation-tolerant ¯owering plants of Southern
Africa. Science 174: 1033±1034.
Ga€ DF. 1989. Responses of desiccation tolerant `resurrection'
plants to water stress. In: Kreeb KH, Richter H, Hinckley TM,
eds. Structural and functional responses to environmental
stress: water shortage. The Hague: SPB Academic Publishers,
255±268.
Ga€ DF. 1997. Mechanisms of desiccation tolerance in resurrection
vascular plants. In: Basra AS, Basra RJ, eds. Mechanisms of
environmental stress resistance in plants. Netherlands: Harward
Academic Publishers, 43±58.
Goldsworthy D, Drennan PM. 1991. Anhydrous ®xation of desiccated
leaves of Myryothamnus ¯abellifolius Welw. Proceedings of the
Electron Microscopy Society of Southern Africa 21: 105±106.
Hallam ND, Lu€ SE. 1980. Fine structural changes in the leaves of the
desiccation-tolerant plant Talbotia elegans during extreme water
stress. Botanical Gazette 14: 180±187.
Iljin WS. 1957. Drought resistance in plants and physiological
processes. Annual Review of Plant Physiology 3: 341±363.
Jones MM, Rawson HM. 1979. In¯uence of rate of development of leaf
water de®cits upon photosynthesis, leaf conductance, water use
eciency, and osmotic potential in Sorghum. Physiologia Plantarum 45: 103±111.
Vander Willigen et al.ÐAnomalous PV Curves in Resurrection Plants
Malone M, Tomos AD. 1990. A simple pressure-probe method for the
determination of volume in higher plants. Planta 182: 199±203.
Nonami H, Boyer JS, Steudle E. 1987. Pressure probe and isopiestic
psychrometer measure similar turgor. Plant Physiology 83:
592±595.
Oertli JJ. 1989. The plant's cell response to consequences of
negative turgor pressure. In: Kreeb KH, Richter H, Hinckley
TM, eds. Structural and functional responses to environmental
stress: water shortage. The Hague: SPB Academic Publishers,
73±78.
Oertli JJ. 1993. E€ects of cavitation on the water status of water in
plants. In: Borgetti M, Grace J, Raschi I, eds. Water transport in
plants under climatic stress. Cambridge: Cambridge University
Press, 27±40.
Oliver MJ, Bewley JD. 1997. Desiccation tolerance of plant tissues: a
mechanistic overview. Horticultural Reviews 18: 171±213.
Sherwin HW. 1995. Desiccation tolerance and sensitivity of vegetative
plant tissues. PhD Thesis, University of Natal, Durban, South
Africa.
Sherwin HW, Farrant JM. 1996. Di€erence in rehydration of three
desiccation-tolerant angiosperm species. Annals of Botany 78:
703±710.
543
Tomos AD, Leigh RA. 1999. The pressure probe: a versatile tool in
plant physiology. Annual Review of Plant Physiology and
Molecular Biology 50: 447±472.
Tyree MT. 1976. Negative turgor pressure in plant cells: fact or fallacy?.
Canadian Journal of Botany 54: 2738±2746.
Tyree MT, Hammel HT. 1972. The measurement of the turgor pressure
and water relations of plants by the pressure-bomb technique.
Journal of Experimental Botany 23: 267±282.
Vander Willigen C, Pammenter NW, Mundree SG, Farrant JM. 2001.
Some physiological comparisons between the resurrection grass
Eragrostis nindensis and the related desiccation-sensitive species
E. curvula. Plant Growth Regulation (in press).
Vertucci CW, Farrant JM. 1995. Acquisition and loss of desiccation tolerance. In: Kigel J, Galili G, eds. Seed development
and germination. New York: Marcel Dekker Inc, 237±271.
Vicre M, Sherwin HW, Driouich A, Ja€er MA, Farrant JM. 1999.
Cell wall characteristics and structure of hydrated and dry
leaves of the resurrection plant Craterostigma wilmsii, a
microscopical study. Journal of Plant Physiology 155: 719±726.
Zimmermann U, Benz R, Koch H. 1981. A new electrical method for
the determination of the cell membrane area in plant cells. Planta
152: 352±355.