Journal of Experimental Botany, Vol. 49, No. 328, pp. 1845–1854, November 1998
Water-content components in bryophytes:
analysis of pressure–volume relationships
Michael C.F. Proctor1,3, Zoltán Nagy2, Zsolt Csintalan2 and Zoltán Takács2
1 Department of Biological Sciences, University of Exeter, Hatherly Laboratories, Prince of Wales Road,
Exeter EX4 4PS, UK
2Department of Botany and Plant Physiology, Agricultural University of Gödöllő, Páter K. u. 1, H-2103 Gödöllő,
Hungary
Received 25 February 1998; Accepted 6 July 1998
Abstract
The water associated with a bryophyte can be divided
into (a) apoplast water held in cell-wall capillary spaces
and by matric forces, (b) osmotic (symplast) water, and
(c) external capillary water. In many bryophytes (c) is
a large and variable component, preventing easy determination of full-turgor water content and of relative
water content (RWC) values physiologically comparable with those for vascular-plant leaves. Pressure–
volume (P–V) curves are presented and water-relations
parameters estimated for bryophytes, including
species with large thin-walled cells (Hookeria lucens
and three marchantialian thalloid liverworts), species
with notably thick cell walls (Neckera crispa), and
species with wettable surfaces and well-developed
external capillary water conduction (Tortula ruralis,
Anomodon viticulosus), and for the lichen Cladonia
convoluta. Full-turgor water content ranged from c.
110% DW. in T. ruralis and Andreaea alpina to 1400%
DW. or more in Dumortiera hirsuta and Conocephalum
conicum. Osmotic potential (Y ) at full turgor was
p
between −1.0 and −2.0 MPa in most species, but
substantially less negative values were found in the
thalloid liverworts (−0.35 to −0.64 MPa). The x-intercept of the P–V curve is not a reliable estimate of
apoplast volume and may give negative values; better
estimates of apoplast volume may be obtained by
vapour equilibration at known low water potentials.
Blotting external water from shoots usually gave fullturgor water content estimates in reasonable agreement with those obtained by analysis of P–V curves,
but for different reasons they could be either higher
or lower than the true value. The importance of know-
ing full-turgor water content for physiological work on
water-stress responses in bryophytes is emphasized.
Key words: Thermocouple psychrometry, apoplast fraction,
relative water content, osmotic potential, poikilohydry.
Introduction
In contrast to vascular plants, bryophytes at full turgor
typically carry substantial and variable amounts of extracellular water. Many mosses have their main pathways
of water movement outside rather than inside the plant
(Buch, 1945, 1947; Proctor, 1979, 1982). This makes
some standard methods for studying the water relations
of vascular plants difficult or impossible to apply to
bryophytes, and uncritical transfer of concepts and techniques from vascular-plant physiology can lead to serious
misunderstanding and error. In particular, ‘relative water
contents’ based on notional ‘saturated’ water contents of
bryophytes are not physiologically comparable to RWC
as generally understood in vascular plant physiology.
Dilks and Proctor (1979) considered the water content
of a bryophyte as divisible into three parts—external
capillary water, symplast water within the cells, and
apoplast water in the cell walls—and emphasized that
the external capillary water is an essential functional
component in the physiology of many bryophytes. The
external water is held at relatively high (near-zero) water
potentials related to the size of the capillary spaces
provided by the morphology of the plant; much of the
physiologically-important part of it is likely to be held
between −0.01 and −0.5 MPa. The symplast water
declines over a range of water potentials related to Y ,
p
3 To whom correspondence should be addressed. Fax: +44 1392 263700. E-mail: [email protected]
© Oxford University Press 1998
1846
Proctor et al.
generally from about −0.5 to −10 MPa. Much apoplast
water remains at lower water potentials than this and is
lost over a range which data for several species suggest
extends to c. −800 MPa, corresponding to tensions in
capillary spaces of near-molecular dimensions. These last
two categories are better called ‘osmotic water’ and
‘microcapillary+matric water’, because a proportion of
the latter will be in the cytoplasm.
Analysis of pressure–volume (P–V ) relationships using
thermocouple psychrometry opens up useful possibilities
for elucidating the water relations of bryophytes, some
of which have been explored by Santarius (1994) and
Beckett (1997). The aim of the present paper is to assess
the magnitude and relationships of the water content
components, and to estimate some of the main waterrelations parameters, in a range of bryophytes of varied
structure and ecological adaptation.
Materials and methods
Plant material
Material was collected in the field either wet or dry. In general,
desiccation-intolerant species were collected wet and maintained
so, in a near-saturated atmosphere, in daylight but out of direct
sun. All of the species remained in good condition for at least
a few days (and in several cases for weeks) in polyethylene
bags. The thalloid liverworts grew well in a glasshouse mistpropagation unit. Desiccation-tolerant species collected dry
were kept dry in polyethylene bags in a refrigerator at c. 5 °C
until needed, and remoistened at least 12 h before measurements.
Material collected wet, if not needed for immediate measurements, was allowed to dry out slowly in subdued light, and
stored dry at c. 5 °C. Sources of material are listed in Table 1.
Methods
Fully-hydrated fresh green shoots were cut, lightly blotted free
of excess water, and samples of about 20–80 mg FW placed in
the psychrometers, where necessary in small HD polyethylene
cups for ease of handling. A succession of measurements were
made on each sample; after weighing, the samples were allowed
to lose up to a few milligrams of water before being resealed in
the psychrometers. Measurements at Exeter were made in
psychrometers with screen-caged thermocouples (Ramsden
Scientific Instrument Co. Ltd, Billericay, Essex, UK ), equilibrated for a minimum of 4 h in an expanded-polystyrene
insulated box, with a Wescor HR-33T dew-point microvoltmeter. The earlier Exeter measurements were made in the
psychrometric mode using a potentiometric chart recorder, but
later measurements used dewpoint readings. Measurements at
Gödöllő were made in a Wescor C-52 chamber or in Wescor
L-51 leaf-clip psychrometers with a small aluminium equilibration chamber replacing the leaf, using a Wescor HR-33T meter
in dewpoint mode. The psychrometers were calibrated after
each run of measurements with standard solutions of NaCl on
filter-paper discs.
For a number of the species measured in Exeter, two low
points on the P–V curve were obtained by equilibrating samples
for 1–2 weeks in air over saturated KNO and KCl solu3
tions, giving approximately Y=−8.9 MPa and −21.5 MPa,
respectively.
Two graphical representations were used in analysing the
results, the linear plot of water content against water potential,
and the P–V curve relating the reciprocal of the water potential
to water content. The former is in effect a Höfler diagram
rotated so that water content becomes the y-axis. Water contents
were initially expressed as percentage of dry weight (measured
after oven drying at 70 °C ), and separate curves were plotted
for each replicate set of measurements (Fig. 1). This leads to
an unfamiliar presentation of the reciprocal P–V curve, but
does not in any way complicate its interpretation, and it relates
water content to the only reproducible datum that is available
a priori. For each replicate, the approximate turgor-loss point
(as % DW ) was read from the P–V curve. The full-turgor point
was estimated by eye from the plot of water content on water
potential, by extrapolation of the next few points above turgor
loss before the curves took a steep upward turn due to the
presence of external water. In vascular-plant cells this part of
the curve, determined by increasing turgor pressure, is roughly
linear or slightly concave towards the water-potential axis.
Linearity was assumed as an approximation unless the data
clearly indicated otherwise. The data were then replotted on a
RWC basis. In some cases the first plot suggested the need for
adjustment of the original estimates of full turgor before
accepting a final figure. Linear regressions were calculated for
the straight-line part of the P–V curve below the turgor-loss
point for each replicate, and hyperbolic fits (RWC=a/Y+b)
were calculated for the corresponding data points in the
RWC/Y curves, using an iterative curve-fitting program. The
y-intercept of the linear regression estimates the reciprocal of
Y at full turgor, and the x-intercept is generally taken as an
p
estimate of the effective osmotic volume of the symplast. The
Table 1. Species and collecting localities
Polytrichum commune Hedw. Wet ground, White Wood, Holne, Dartmoor, Devon, UK
Mnium hornum Hedw. Clay bank, Stoke Woods, Exeter, Devon, UK
Tortula ruralis (Hedw.) Gaertn. et al. Sandy grassland, Kiskunság National Park, Fülöpháza, Hungary
Antitrichia curtipendula (Hedw.) Brid. Part-shaded rock near coast between Osöyro and Lepsöy, S of Bergen, Norway
Neckera crispa Hedw. Shady limestone rock face, Broadridge Wood, Newton Abbot, Devon, UK
Hookeria lucens (Hedw.) Sm. Shady stream gully, Stoke Woods, Exeter, Devon, UK
Anomodon viticulosus (Hedw.) Hook. & Tayl. Shady limestone rock face, Broadridge Wood, Newton Abbot, Devon, UK
Homalothecium lutescens (Hedw.) Robins. Fixed-dune grassland, Braunton Burrows, Devon, UK
Rhytidiadelphus loreus (Hedw.) Warnst. White Wood, Holne, Devon, UK
Andreaea alpina Hedw. Wet mountain rock face, Blåmmen, Bergen, Norway
Frullania tamarisci (L.) Dum. Trunk of oak, Cloutsham, Exmoor, Somerset, UK
Dumortiera hirsuta (Swartz) Nees. Wet shady gully, East Lyn valley, Lynmouth, Devon, UK
Conocephalum conicum (L.) Underw. Weed in glasshouses, University of Exeter, Devon, UK
Marchantia polymorpha L. Wet calcareous fen, Malham Tarn, Yorkshire, UK
Cladonia convoluta (Lam.) Cout. Sandy grassland, Kiskunság National Park, Fülöpháza, Hungary
Water-content components in bryophytes
Fig. 1. Individual water-content–water-potential and pressure–volume
curves, with fitted regressions, for three replicate samples of Polytrichum
commune. Water content is expressed as % of dry weight. TLP=turgorloss point (first estimates).
asymptote (b) of the hyperbolic fit estimates the corresponding
‘apoplast volume’, and an estimate of Y at full turgor is given
p
by a/(1–b). With our data, the linear and hyperbolic regressions
generally gave closely similar estimates of the water-relations
parameters.
In general, it was assumed from the results of Dilks and
Proctor (1979) that, essentially, all water held at Y<−21.5 MPa
is apoplast water. The equilibrium water-contents at this water
potential varied very little between replicates within a species,
and on a dry weight-basis values for different species were
remarkably similar, all lying between 28% and 38% DW. The
vapour-equilibration points for −8.9 MPa generally lined up
well with the straight-line portion of the P–V curve and could
be considered part of it.
Turgor pressures (Y ) were calculated for each replicate at
P
all data points between turgor loss and the point on the P–V
curve at which the effect of external water became apparent,
and plotted against RWC’, i.e. RWC expressed in terms of the
effective osmotic volume defined by the mean x-intercept of the
P–V curve. Polynomials, generally quadratic, were fitted to
each curve; in a few cases a cubic was used where this was
clearly a better fit to the data. The slope of the curve (dy/dx)
at RWC=1.0 was calculated as a measure of the bulk elastic
modulus e . Figure 2, based on the combined data from all
B
replicates for a selection of the species, illustrates the general
form of the curves obtained. The Y /RWC curves provided a
P
further means of locating (or confirming) the turgor-loss point.
1847
Fig. 2. Representative plots of turgor pressure against relative water
content. The graph for each species includes the combined data for all
replicates; each replicate includes a point defined by RWC 1.0, and
Y
. Dashed lines are polynomial regressions calculated from the
p(FT)
points on the graph; those for Polytrichum commune and Neckera crispa
are cubics, the others are quadratics.
Polytrichum commune
Polytrichum is among the bryophyte genera most closely
approaching vascular plants in function. The stems of
this tall wet-ground species have a well-developed internal
conducting system in the stem, and water is also conducted in the capillary spaces within the sheathing leaf
bases. The plant depends on water conduction from the
base for normal metabolism. Water is lost rapidly, and
the thick dark-green leaves fold towards the stem on only
slight loss of turgor. The edges of the lamellae on the
upper leaf surface are densely covered with granular wax
and most surfaces of the young shoots are water-repellent.
The shoots used had been kept in a saturated atmosphere
with their bases in water; the cut upper portions were
visually dry when first sealed in the psychrometer chambers, but the measurements indicate the presence of some
extracellular capillary water, probably mainly within the
leaf bases (Fig. 3).
Mnium hornum
Results
The results are summarized in Table 2. Curves for some
representative species are illustrated in Figs 3–7.
This common woodland species has slightly waterrepellent leaf surfaces, and a well-developed central conducting strand in the stem, but the unistratose leaves dry
1848
Proctor et al.
Table 2. Water-relations parameters of bryophytes and one lichen
In general, entries in the table are mean±s.d. for 3 or 4 replicates. Standard deviations for RWC at −21.5 MPa largely reflect variation in the estimate of RWC at full turgor; the
measurements are closely reproducible when expressed on a dry-weight basis. It is not possible to give rigorous formal error estimates for RWC at turgor loss; the figures are to be regarded
as a guide, and should generally be reliable within ±5%. The estimates of e are slopes (dy/dx) at RWC=1.0 of polynomial fits to plots of Y on RWC (osmotic water basis; q=quadratic,
B
P
c=cubic); the value for Marchantia polymorpha is from a single replicate, and that for Cladonia convoluta is a notional figure which include a P–V component from the second symbiont. The
June 1997 Dumortiera hirsuta measurements were on recently field-collected material, those in January 1998 on material of the same provenance in glasshouse cultivation; both sets of
Hookeria lucens measurements were on material recently brought from the field.
Species
Osmotic potential
at full turgor
(−MPa)
x-intercept of
P–V curve
(RWC)
RWC at
−21.5 MPa
RWC at
turgor loss
Bulk elastic
modulus e at
R
RWC=1.0
(MPa)
Water content at
full turgor
(% DW )
Water content of
blotted material
(% DW )
Polytrichum commune
Mnium hornum
Tortula ruralis
Antitrichia curtipendula
Neckera crispa
Hookeria lucens June 1997
H. lucens January 1998
Anomodon viticulosus
Homalothecium lutescens
Rhytidiadelphus loreus
Andreaea alpina
Dumortiera hirsuta June 1997
D. hirsuta January 1998
Conocephalum conicum
Marchantia polymorpha
Frullania tamarisci
Cladonia convoluta
2.09±0.09
1.21±0.07
1.36±0.18
1.47±0.28
1.27±0.09
0.86±0.18
0.95±0.03
1.65±0.07
2.08±0.08
1.34±0.02
1.59±0.03
0.38±0.04
0.49±0.13
0.54±0.08
0.38±0.02
1.78±0.20
1.52±0.20
0.116±0.023
0.099±0.049
0.266±0.093
0.175±033
0.271±0.092
−0.075±0.071
0.021±0.004
0.230±0.009
0.086±0.054
0.237±0.049
0.265±0.006
0.048±0.009
0.033±0.030
−0.002±0.032
0.052±0.027
0.189±0.017
0.241±0.081
0.173±0.007
0.165±0.005
n.d.
0.190±0.014
0.243±0.004
0.057±0.012
n.d.
0.188±0.052
0.178±0.014
0.212±0.015
0.267±0.014
0.027±0.005
n.d.
0.027±0.003
0.035±0.002
0.242±0.012
n.d.
0.75
0.70
0.75
0.65
0.65
?c. 0.90
0.70
0.65
0.70
0.70
0.70
0.85
0.80
0.45
c. 0.60
0.60
c. 0.6
19.2±0.4 (c)
6.1±1.7 (q)
5.8±1.5 (q)
5.9±0.6 (q)
7.7±1.6 (c)
n.d.
6.2±1.5 (q)
8.5±2.3 (q)
18.8±2.9 (c)
5.9±1.2 (c)
6.8±0.4 (c)
2.5±0.3
4.4±1.6 (q)
2.2±0.8 (c)
(1.5, q)
7.6±0.5 (q)
[2.7±0.6 (q)]
179±6
215±7
108±11
152±11
140±5
633±136
571±42
133±3
193±15
142±10
110±4
1423±238
2070±238
1400±132
1025±35
134±3
125±13
186±11
175±6
n.d.
174±16
150±13
480±44
n.d.
176±10
218±27
180±13
141±9
1686±83
n.d.
1277±108
956±65
216±7
n.d.
Water-content components in bryophytes
Fig. 3. P–V and RWC/Y curves for Polytrichum commune (3 replicates),
with a linear regressions fitted to the combined data for the linear
portions of the P–V curves and a hyperbolic regression fitted to the
corresponding data points from the RWC/Y curves. The dotted lines
show the approximate turgor-loss point. These graphs are plotted from
the same data as Fig. 1. Different symbols and lines distinguish
the replicates.
out quickly and are only fully expanded in moist weather
or for a limited period after rain. The data in Table 2 are
from shoots which initially carried substantial amounts
of external water, mainly on the stems. The results were
similar to those from P. commune, but close to the fullturgor point the RWC/Y curves turned steeply upwards,
water content increasing to around twice that at full
turgor with little further change in water potential.
Tortula ruralis
T. ruralis is a desiccation-tolerant species, locally abundant in dry sandy grasslands and other drought-prone
habitats. The leaves are readily wettable and strongly
papillose, the spaces between the papillae forming a fine
network of capillary water-conducting channels over most
of the surface of the lamina. The cells of the leaf base are
large, smooth-walled, and are dead and empty at maturity, opening to the exterior by pores. They are thus
potentially both water-storing cells, and points at which
capillary continuity between the papilla systems of adjacent leaves may be broken.
1849
Fig. 4. P–V and RWC/Y curves for Tortula ruralis (7 replicates), with
a linear regressions fitted to the combined data for the linear portions
of the P–V curves and a hyperbolic regression fitted to the corresponding
data points from the RWC/Y curves. Conventions as in Fig. 3.
At low water potentials this species gave typical P–V
curves with a well-defined linear region and turgor-loss
point. At water potentials higher than about −0.5 MPa
the curves became erratic and were clearly not tending to
a consistent full-turgor value ( Fig. 4). Their behaviour
may be seen as analogous to the ‘plateau’ effects found
in tree leaves by Parker and Pallardy (1987), Kubiske
and Abrams (1990) and Abrams and Menges (1992). In
the plot of RWC against Y, all the replicates showed a
substantially linear region between turgor loss and the
full-turgor point, before abruptly turning upwards as
external water increased with only slight and irregular
further change in water potential.
The leaves of T. ruralis shrink and twist strongly as
they dry; presumably much of the volume change on
drying occurs through lateral shrinkage of the cells in the
plane of the leaf.
Antitrichia curtipendula
A. curtipendula grows in well-drained situations on rocks
and cliffs, often near water, and as an epiphyte, generally
within the crowns of trees. It is moderately desiccation
tolerant, but is absent from severely water-stressed hab-
1850
Proctor et al.
is held within the very large thin-walled cells of the
complanately-arranged leaves. Samples of shoots gave
long, straight P–V curves up to the turgor-loss point at
about 0.7 RWC. P–V curves constructed in spring 1997
consistently gave negative x-axis intercepts, but a small
positive intercept was found in January 1998.
Anomodon viticulosus
This is a desiccation-tolerant species of shaded limestone
rocks and other dry calcareous habitats, with wettable,
densely papillose leaf surfaces and cell walls of moderate
thickness. The latter features recall Tortula ruralis; as in
that species the leaves shrink strongly on drying.
Homalothecium lutescens and Rhytidiadelphus loreus
These two species are pleurocarpous mosses from contrasting habitats, H. lutescens in dry calcareous grasslands
and R. loreus in oceanic and montane woods. H. lutescens
shows a higher Y and e at full turgor than R. loreus,
p
B
which may be related to habitat. The lower water content
relative to dry weight in R. loreus probably reflects the
greater bulk of stem material in shoots of this robust
species.
Andreaea alpina
Fig. 5. P–V and RWC/Y curves for Neckera crispa (4 replicates).
Individual linear regressions are fitted to the P–V curves, and individual
rectangular hyperbolas are fitted to the corresponding portions of the
RWC/Y curves. Other conventions as in Fig.3.
itats. The P–V curve and water-potential parameters of
A. curtipendula are unremarkable in the context of the
other species considered in this paper, and provide no
explanation for its rather restricted ecological distribution.
Neckera crispa
This species typically occurs on shaded limestone rocks.
The glossy, undulate complanate leaves have thick cell
walls and small cell lumina. This is reflected in the high
apoplastic water content indicated by the x-intercept of
the P–V curve (Fig. 5), and by the RWC of material
equilibrated at −21.5 MPa. The leaves of N. crispa show
little apparent change in size on drying, so the rather low
bulk modulus of elasticity is surprising. Presumably most
of the volume change is accommodated by change in leaf
thickness. Saturated N. crispa blotted free of apparent
excess moisture carries a considerable amount of external
capillary (or ‘intercellular’) water.
Hookeria lucens
H. lucens is a drought-sensitive species of shaded, moist
habitats. Most of the water associated with the plant
A. alpina is a small bryophyte forming dense blackish
cushions or patches on seasonally-wet acid mountain
rocks (Heegaard, 1997). Like other members of this
taxonomically isolated ‘moss’ family it is very tolerant of
desiccation. Blotted shoots evidently still carried large
amounts of external capillary water, and the P–V curves
were erratic in form at water potentials above about
−0.8 MPa. The estimated water-relations parameters all
fall within the range found for the other desiccationtolerant mosses in Table 2.
Three large thalloid liverworts, Dumortiera hirsuta,
Conocephalum conicum and Marchantia polymorpha
These three species all belong to the Marchantiales, a
group taxonomically isolated from the remaining thalloid
and leafy Hepaticae. All are relatively large plants, with
very high water content at full turgor, and Y at full
p
turgor less than half the mean value for the other species.
In all three, the x-intercept of the linear part of the P–V
curve is close to zero RWC. C. conicum and M. polymorpha occur in a range of moist habitats and are common
glasshouse weeds. The P–V curves of both show a long
curvilinear region between turgor loss and full turgor, so
that the turgor-loss and full-turgor points are difficult to
locate, and the estimated e values are the lowest that
B
were found for any species. In D. hirsuta, a very droughtsensitive liverwort confined to deeply shaded, constantlymoist habitats, the turgor-loss and full-turgor points are
more clearly defined, and e is somewhat higher ( Fig. 6).
B
Water-content components in bryophytes
Fig. 6. P–V and RWC/Y curves of the combined data for Dumortiera
hirsuta (3 replicates). Individual linear regressions are fitted to the
straight-line portions of the three P–V curves in the upper diagram,
and individual rectangular hyperbolas to the corresponding portions of
the RWC/Y curves. The measurements in these graphs are from
material in glasshouse cultivation; for water-relations parameters of
field-collected material of the same provenance see Table 2.
Frullania tamarisci
F. tamarisci, a leafy liverwort, is a solitary example of
the dominant group of Hepaticae, the Jungermanniales.
In all the estimated water-relations parameters this species
more closely resembles the mosses than the thalloid
liverworts examined. The high water content of blotted
samples probably reflects the presence of the sac-like
lobules of the leaves and other external capillary spaces.
1851
Fig. 7. Pressure–volume and water-content–water-potential curves for
the squamulose lichen Cladonia convoluta (6 replicates), with linear
regressions fitted to the linear portions of the P–V curves for each
replicate, and hyperbolic regressions fitted to the corresponding data
points from the RWC/Y curves. Other conventions as in Fig. 3.
points of turgor loss and full turgor provides ample data
points for notional estimates of Y and e for the indiP
B
vidual replicates, but the exact significance of these figures
in relation to the mycobiont and photobiont components
of the lichen is uncertain. The variation in apoplast
fraction implied by the regressions may be due to variations in the proportion of living tissue, and incorporation
of small but variable amounts of substrate material within
the squamules.
A lichen: Cladonia convoluta
This lichen is a prominent component of the same dry
sandy grassland where Tortula ruralis, was collected, and
is included here for comparison with the similarly
poikilohydric and desiccation-tolerant bryophytes. The
RWC/Y and P–V curves are broadly similar to those for
the bryophytes but, like the P–V curves of Beckett (1995,
1996, 1997) for other lichens, show no clear breaks of
slope to define turgor loss or full turgor points ( Fig. 7).
This may reflect the composite nature of the lichen thallus.
The long region of the P–V curve between the apparent
Discussion
The results just considered confirm the value of P–V
curves obtained by thermocouple psychrometry as an
alternative or supplement to other methods for evaluating
water-relations parameters of bryophytes, including some
quantities that cannot be estimated satisfactorily in any
other way. These results are broadly consistent with those
of Santarius (1994) and Beckett (1997), but this study’s
interpretations and theirs all involve assumptions which
need examination.
1852
Proctor et al.
The x-axis intercept and the apoplast fraction
The intercept of the P–V curve on the x-axis is commonly
equated with the apoplast volume (Jones, 1983; Beadle
et al., 1993). However, it has long been observed that
this is only an approximation and that P–V curves
occasionally give negative intercepts ( Wenkert et al., 1978;
Richter et al., 1981); data from this study provide further
instances. It is generally assumed as a working approximation that all water loss within the range of the linear
part of the P–V curve is from the symplast, but this will
never be strictly true. The rise in water content as 1/Y
departs from zero initially reflects primarily an increase
in apoplast water. The data of Dilks and Proctor (1979)
indicate that this dominates the P–V isotherm up to a
water potential in the region of −20 MPa. From some
point above this (commonly around −10 MPa) osmotic
water within the symplast becomes the larger fraction
and dominates the P–V relationship, giving rise to the
‘linear’ portion of the P–V curve. However, apoplast
water will also continue to increase with rising water
potential, as clearly illustrated by Kelsey’s data for wood
shavings reproduced by Slatyer (1967). Dilks and Proctor
found that their bryophyte data and Kelsey’s woodshavings data showed an approximately linear relationship on a graph of water content against log Y. This
plots onto a graph of 1/Y against water content as a
curved line concave to the y-axis. RWC in a measured
P–V curve is the sum of this (curvilinear) apoplast component and the (theoretically linear) symplast component.
Hence, real P–V curves should, in general, be slightly
convex to the x-axis, the more so the larger the fraction
of apoplast water. An ‘apoplast fraction’ only has meaning at a defined water content, e.g. at full turgor. A linear
regression through any segment of the curvilinear P–V
isotherm just described will always give an x-intercept
less than the full-turgor apoplast fraction. This will not
account for negative intercepts, which must arise from
other causes. Some causes of non-linearity in P–V curves
are discussed by Tyree and Richter (1981, 1982); these
assume that the symplast behaves as a perfect osmometer.
It is more surprising that P–V curves accord so well with
simple theory than that occasional anomalies appear. The
samples equilibrated at Y=−21.5 MPa give a more direct
estimate of ‘apoplast’ water (some of which is actually in
symplast material ) than the x-intercept of the P–V curve,
though obviously neither will estimate the apoplast fraction at full turgor.
Determination of the full-turgor point, external water and
‘relative water content’
Beckett (1997) approached the problem of locating the
full-turgor point by plotting his data in terms of a notional
‘RWC’ relative to saturation, and then using a fitted
spline curve to estimate Y . He regarded water between
P
100% saturation and the point at which this calculated
Y began to fall as ‘intercellular’. He then replotted his
P
data excluding external water. The present study took a
different approach, similar to that of Kubiske and Abrams
(1990) and Abrams and Menges (1992), based on the
observation that the relation of Y to water content in
P
the Höfler diagram is approximately linear (the departure
of this curve from linearity will lead to slight, but generally
only slight, overestimation of the full-turgor point). The
‘plateau’ effect (Parker and Pallardy, 1987; Kubiske and
Abrams, 1990) which is an occasional nuisance in vascular-plant studies is the norm in bryophytes. The relevant
part of the curve to the y-axis was extrapolated by eye,
correcting the resulting estimates where necessary by
successive approximation. It is believed that the figures
so obtained are essentially sound, and provide the best
basis at present available from which to calculate true
RWC values for bryophytes comparable with those of
higher plants. For many purposes, an acceptable approximation to full-turgor water content is provided by blotting
saturated material free of superficial water. However,
too-light blotting will fail to remove all external water
(especially in species with inaccessible concavities), and
too-heavy blotting may have the effect of a pressure
chamber in expressing water from the leaf cells, leaving
them below full turgor when weighed ( Table 2). As
Santarius (1994) shows, blotting gives more generally
reliable estimates of full-turgor water content than
centrifuging.
While some of the irregularity at near-zero water
potentials seen in the data from such species as Tortula
ruralis and Andreaea alpina may be attributed to measurement error, the contrast between different species and
between the measurements above and below critical water
potentials within a species left little doubt that some at
least must be a property of the plant material. Xylem
cavitation can irregularly reverse the monotonic relation
of Y to RWC in vascular plants (Oertli, 1993). Similarly,
but at higher water potentials, drainage of external capillary cavities through narrow ‘bottlenecks’ as water evaporates may also produce irregular reversals in the
progressive fall of Y with declining water content; a
process of this kind is described in Sphagnum by Clymo
and Hayward (1982).
Is turgor pressure ever negative?
Beckett (1997) obtained a P–V curve for Dumortiera
hirsuta markedly concave to the x-axis, which he interpreted as demonstrating substantial negative turgor (to
−0.188 MPa) as the thallus dried beyond the turgor-loss
point (from c. 0.9 to 0.5 RWC ). No indication of this
was found in many replicate sets of measurements on D.
hirsuta, either freshly collected from the field, or cultivated, but that may reflect the different provenance of our
Water-content components in bryophytes
1853
material of this widely distributed and variable species.
However, a slight (but not statistically significant) tendency to a similarly sigmoid P–V curve was apparent in a
few of our sets of measurements. Oertli (1993) has pointed
out that cell collapse on drying must be accompanied by
at least a modest level of negative turgor, perhaps a few
tens of kPa. The question needs further study.
Osmotic potentials
In general, the estimates of Y are somewhat, and in
p
some cases much, less negative than the osmotic potential
figures obtained for the same or similar species by
earlier authors using the plasmolysis method (which by
definition must give values below the turgor-loss point).
Patterson (1946) found rather little variation among
70 mosses and leafy liverworts he examined, with Y
p
(mean±s.d.)−2.22±0.41 MPa for 19 species of ‘xeric’
habitats, −1.80±0.42 MPa for 35 ‘mesic’ species and
−1.91±0.42 MPa for 16 ‘hydric’ species; for 9 thalloid
liverworts he found −0.99±0.37 MPa. The values given
by Hosokawa and Kubota (1957), many of which fall
below −3.5 MPa, seem certain to be much too negative.
Estimates of osmotic potentials at full turgor (Y
),
p(FT)
which reflect the normal conditions of cell function, can
only be obtained by analysis of P–V curves. These results
are in the same general range as those of Santarius (1994)
and Beckett (1997). Y
is conspicuously lower in the
p(FT)
three thalloid liverworts than in the remaining species, as
Patterson observed in his material; otherwise the data
show little obvious pattern.
Cell-wall elasticity
The bulk elastic modulus, e (VdY /dV ) can in general
B
P
vary continuously between the turgor-loss point and full
turgor (Tyree and Karamanos, 1981; Roberts et al., 1981),
so it can only be defined precisely for a stated Y . Beckett
P
took values of Y and e from his fitted curves, calculating
P
B
e at an arbitrary Y of 1.0 MPa assuming the exponential
B
P
model of Stadelmann (1984). Values have been given at
full turgor, again arbitrary, but a replicable basis for
comparisons between species. The bulk modulus of elasticity is potentially of considerable ecophysiological interest,
especially in plants liable to periodic water stress. These
results indicate generally low values ([1.5–] 5.8–8.5–19.2)
for bryophytes compared with most vascular plants. The
lower figures are in the same range as Beckett (1995)
found for lichens. The highest values were in Polytrichum
commune, a species with a well-developed internal conducting system in which normal expansion of the leafy
shoots depends critically on full turgor, and the drygrassland pleurocarpous moss Homalothecium lutescens.
Concluding comments
A few broad trends of variation emerge from the results,
illustrated by the principal components analysis of Fig. 8.
Fig. 8. Principal components analysis of the bryophyte water-relations
data from Table 2, excluding the last column. Species of with large cells
and high full-turgor water contents, occupy the right-hand part of the
component-scores diagram. The species in the upper left-hand part of
the diagram may be seen as ‘typical’ bryophytes, with small cells, rather
large apoplast fractions, substantial desiccation tolerance, and lower
values of e than typical vascular plants, which are most nearly
B
approached by Polytrichum commune near the lower left-hand corner
of the diagram.
The first axis largely reflects the contrast between the
large-celled bryophytes of permanently moist sites—
Hookeria lucens and the three thalloid liverworts—and
the remaining species, which may be seen as the bryophyte
mainstream, with small cells and generally tolerant of
desiccation. The second axis runs from species with
extensible cells and often a high apoplast fraction
(probably adapted to intermittent water availability),
to Polytrichum commune which, with its rigid cell walls
and efficient internal conduction, comes closest to vascular plants in its water-relations; the position of
1854
Proctor et al.
Homalothecium lutescens here reflects its strongly negative
Y
and high e . The most desiccation-tolerant species
p(FT)
B
occupy the top left-hand part of the diagram, and those
most confined to moist sites the bottom right.
A final caution is worth reiterating. In bryophytes, the
full turgor water content is a parameter requiring some
trouble to estimate (in relation to the only repeatable
datum, dry weight), and which can generally only be
attained approximately in practice. Nonetheless, knowledge of the water content at full turgor is an absolutely
essential starting point for any physiological work on
effects of water stress on bryophyte metabolism. This
cannot be emphasized too strongly, if data are to be
obtained that are meaningful, repeatable, and comparable
with results from vascular plants. In particular, ‘relative
water content’ figures based on the ‘saturated’ water
content are valueless, because a substantial part of the
water present is extracellular and can be lost without in
any way affecting the water status of the cells. For some
purposes, and in some species, careful blotting of wellmoistened material may give an adequate approximation
to full-turgor water content; in other cases more precise
measurement may needed, and thermocouple psychrometry provides a means to that end.
Acknowledgements
We are grateful for financial support to the British–Hungarian
Science and Technology Programme (GB-38/95). MCFP
warmly thanks Ørjan Totland and Einar Heegaard for their
guidance on field excursions near Bergen.
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