Journal of Experimental Botany, Vol. 50, No. 337, pp. 1393–1401, August 1999
Alteration of transpiration rate, by changing air vapour
pressure deficit, influences leaf extension rate
transiently in Miscanthus
J.C. Clifton-Brown1,3 and M.B. Jones2
1Universität Hohenheim, Institut für Pflanzenbau und Grünland (340), Fruwirthstrasse 23, D-70599 Stuttgart,
Germany
2Botany Department, Trinity College Dublin, Dublin 2, Ireland
Received 11 February 1999; Accepted 9 April 1999
Abstract
A controlled environment chamber for whole plants is
described in which vapour pressure deficit (VPD) and
temperature can be controlled independently. Plant
responses to changes in VPD at constant temperature
were measured in terms of leaf extension and plant
transpiration rates. Manipulation of VPD independently
of temperature was shown to be capable of altering the leaf extension rates of the C grass
4
Miscanthus×giganteus grown in hydroponics. The
effects of VPD on leaf extension are attributed to
changes in transpiration rate and hence leaf water
status. It was found that, at a temperature of 20 °C,
the influence of a fixed change in VPD was proportionally less than those observed at temperatures which
are close to the threshold for growth (between 6 and
10 °C). These responses are discussed in relation to
our current understanding of the mechanisms of cell
growth. The fact that the VPD effects on leaf expansion
rates were largely transient suggest that simple
models driven by temperature alone are adequate to
predict leaf expansion within the temperature range
6–20 °C, for this genotype of Miscanthus, in the field.
Key words: Leaf growth, Miscanthus×giganteus, temperature, vapour pressure deficit, C plants.
4
Introduction
Leaf growth and canopy development of crops in temperate climates in the spring is largely dependent on ambient
temperatures. In the field, crops experience both diurnal
and seasonal changes in temperature and humidity and
leaf growth has been demonstrated to be very sensitive
to both these environmental variables (Gallagher and
Biscoe, 1979; Parsons and Robson, 1980; Ben Haj Salah
and Tardieu, 1996). The rate of canopy development in
spring is important in determining the quantity of radiation intercepted by a crop and, consequently, the final
yield (Monteith and Elston, 1972; Van der Werf et al.,
1993). Simple models developed to predict canopy closure
often use an empirical relation between accumulated
temperature above a threshold (e.g. degree days) and leaf
expansion rate when a healthy crop is receiving adequate
supplies of water ( Van der Werf et al., 1995). This
approach assumes a linear relationship between temperature and the rate of leaf expansion, and a lack of
significant influence of other correlated variables such as
humidity and light.
It has recently been shown that there was considerable
variation in the leaf expansion rate of different Miscanthus
genotypes over a range of temperatures between 5 °C and
20 °C and that this could influence final yield of the
genotypes significantly (Clifton-Brown and Jones, 1997).
Miscanthus is a grass genus with C photosynthesis with
4
origins in eastern Asia. The genotype M.×giganteus
(Greef and Deuter, 1993) has been widely used in field
trials in Europe because of its rapid growth even in cooler
climates such as those in Ireland (Clifton-Brown et al.,
1996) and the Netherlands ( Van der Werf et al., 1993).
This genotype produces thick rhizomes allowing it to be
easily propagated by rhizome cuttings.
3 To whom correspondence should be addressed. Fax: +49 711 4592297. E-mail: [email protected]
Abbreviations: Q, photon flux (400–700 nm); VPD, vapour pressure deficit; T , air temperature; e , air saturated vapour pressure; PER, plant extension
a
s(a)
rate; g , boundary layer conductance; g , leaf conductance; E, transpiration.
a
l
© Oxford University Press 1999
1394 Clifton-Brown and Jones
In work reported previously (Clifton-Brown and Jones,
1997) it was not possible to regulate the vapour pressure
deficit ( VPD) in the controlled environment chamber at
a constant level over the whole range of temperatures
from 5–20 °C. This may have introduced an error in the
reported leaf extension rates, but when the effects of VPD
were quantified it was found that although VPD responses
occurred they were transient: lasting at most between
4–5 h. In this paper the influence of changes in VPD on
leaf extension are reported in more detail. The response
of leaf extension to VPD over a range of temperatures
has been measured using a specially modified controlled
environment cabinet. The responses of leaf expansion rate
are discussed within the framework of current theories
on the mechanism which regulates cell expansion.
Materials and methods
Plant culture
Miscanthus×giganteus rhizomes were prepared from overwintering parent plants following normal practice for culture
of this sterile clone (Nielsen, 1987). The rhizome pieces were
sprouted in coarse silica sand. When the shoots were 20–30 cm
tall with from 2 to 3 leaves the plants were transferred to
hydroponic culture in half-strength Hoagland’s solution
(Hewitt, 1966) and grown under constant growth conditions of
20 °C, a Q of 300 mmol m−2 s−1 and VPD of 0.6 kPa. Each
plant was identified to enable a random selection of plants to
be transferred to the controlled environment system in which
they were subjected to the temperature and humidity treatments.
Controlled environment system
A growth cabinet was constructed from a modified deep-freezer
(Derby, F41STSL, Esta, Aalestrup, Denmark) similar to the
design described in previous work (Neighbour et al., 1990;
Clifton-Brown and Jones, 1997). Air temperature was controlled
by regulation of a solenoid valve in a hot gas by-pass by a
temperature process controller ( West 2052, Brighton, UK )
connected to a PT100 resistance thermometer (RS components,
Dublin) which was positioned in the middle of the cabinet.
Lighting was provided by a single 400 W metal halide lamp
(Osram (HQI-T )). At the top of the plants, Q in the cabinet
was approximately 300 mmol m−2 s−1. In the region of cell
expansion, at the base of the expanding leaf, Q was approximately 100 mmol m−2 s−1. Humidity regulation within the
cabinet, independent of temperature, was achieved with steam
injection to increase vapour pressure and the use of anhydrous
calcium chloride (CaCl ) to decrease vapour pressure. Figure 1
2
illustrates the humidity regulation system within the cabinet.
The limits between which vapour pressure can be regulated
within the cabinet were influenced by a number of factors.
Firstly, as air temperature (T ) increases the saturated vapour
a
pressure (e ) also increases which, in turn, increases the range
s(a)
of possible VPDs. Here e
is calculated using the following
s(a)
equation (Postl and Bolhar-Nordenkampf, 1993):
=6.13753exp[T (18.564–T /254.4))/(T +255.57)] (1)
s(a)
a
a
a
Secondly, in the cabinet used here, the absolute humidity was
influenced by the temperature set-point. For the temperature of
the air to be regulated at a set-point when the cabinet lighting
is on, the temperature of the cooling surface must be
e
approximately 5 °C lower than that of the set-point. This causes
condensation on the evaporator which results in a ‘natural’
relative humidity of about 75% between 6 °C and 20 °C for this
cabinet. Consequently, the minimum VPD at a given air
temperature is controlled by the dew-point characteristics of
the cabinet. Thirdly, maximum obtainable VPDs were limited
by the drying power of the anhydrous CaCl at a given
2
temperature. The 2 kg of anhydrous CaCl used in the cabinet
2
was capable of drying the air to about 60% of the theoretical
maximum VPD between 6 °C and 20 °C.
Transpiration measurements
Whole plant transpiration loss was measured every 5 min using
an electronic balance (Model PG5002, Mettler Toledo AG,
Greifensee, Switzerland) with a computer interface enabling
automated recording. This balance had a range from 0–5 kg
and a sensitivity of 0.01 g, although the sensitivity was
considerably less (c. 0.05 g) when placed inside the controlled
environment where the balance was exposed to vibrations from
the fans circulating the air. Leaf area of the plants was estimated
from measurements, using a ruler, of leaf lamina length and
width midway along the length, and the following relationship
(Clifton-Brown, 1997)
Leaf area=0.75×length×width
(2)
Auxanometers could not be used on the same plants because
the balance and auxanometers interfered with each other. The
rate of transpiration (E) was calculated from a moving point
average over a period of 1 h. Leaf conductance (g ), which
l
consists of the stomatal and boundary layer conductance in
series, was calculated using the following equation:
g =E/[0.622r /P) (e (T )–e )]
(3)
l
a
s s a
where E is the transpiration rate in kg m−2 s−1, r is the density
a
of air, P is air pressure, e (T ) the saturation vapour pressure
s s
at leaf or wick temperature (equation 1), and e the water
a
vapour pressure in bulk air (Jones, 1992). Equation 3 results
in units of mm s−1, which were converted to mmol m−2 s−1
(Jones, 1992).
To calculate the boundary layer conductance (g ) of leaves
a
within the controlled environment system, a model of a plant
with a freely evaporating leaf-like surface was placed on the
balance to determine E. The artificial ‘leaves’ were constructed
from wick material stretched over a flat plastic strip, so that
the ‘leaves’ were 76 mm long and 13 mm wide, giving a surface
area of 19.8 cm2. A thermocouple (0.25 mm cross-sectional
diameter) was inserted into the wick material to measure the
temperature of the artificial ‘leaf ’ (T ). g was calculated using
s a
equation 3.
Experimental monitoring and data recording
Air temperature was monitored by a shielded copper-constantan
thermocouple, with a 0.15 mm cross-sectional diameter (Omega
Engineering Inc., Stanford, USA) located in the centre of the
cabinet beside the PT100 probe for air temperature control.
Leaf temperature was measured with a similar thermocouple
held in place by a spring clip. Vapour pressure was monitored
by a wet/dry bulb psychrometer (Type WVU, Delta-T Devices,
Cambridge).
Plant extension rates were measured using auxanometers
made from linearly variable displacement transformers (LVDTs,
Schlumberger, Bognor Regis, Type DC/50 and AC/50). These
were mounted on a retort scaffold within the controlled
environment cabinet. The LVDTs were attached with a hair
clip to the youngest visible expanding leaf of a M.×giganteus
VPD effects on leaf extension 1395
Fig. 1. Diagram of the controlled environment cabinet and apparatus for measuring plant extension and transpiration rate. Humidity levels were
raised within the cabinet by generating steam and were reduced by drawing air through anhydrous calcium chloride. Plant transpiration and
extension rate were monitored by a balance and an LVDT system, respectively.
plant via a Kevlar Type 989, 440 Dtex (DuPont, Derry, N.
Ireland) thread which ran over a Teflon pulley. The hair clip
had neoprene-faced jaws and gripped the leaf firmly. The
upward tension on the leaf was 12 g. Auxanometer displacements were recorded every 5 min. Plant extension rate was
calculated from a running mean over 1 h (synchronized with
the transpiration measurements).
Auxanometers set up as described here actually measure a
combination of leaf and internodal extension (Gallagher,
1979). Hereafter this is termed plant extension rate (PER).
Approximately 80% of plant extension in M.×giganteus is
attributable to leaf development (Clifton-Brown and Jones,
1997).
Outputs from LVDTs, the psychrometer and thermocouples
were recorded every 5 min by a Campbell datalogger (Type
21X Campbell Scientific, Leicestershire, UK ).
Data handling and statistical analysis
The responses of leaf extension rate were plotted with Sigma
Plot (Jandel Corp., Erkrath, Germany) and peak area analysis
was performed within NIH Image V1.60 (National Institutes
of Health, USA). Data were analysed by analysis of variance
and the Bonferroni post-hoc test (Data Desk, 4.1, Ithaca).
Cabinet and auxanometer performance
The performance of the controlled environment system and
auxanometers were tested in the absence of plants at temperatures of 5 °C and 20 °C. An LVDT was tied to a 0.5 kg weight
resting on the base of the cabinet and the model plant with the
artificial freely evaporating leaf surfaces was placed on the
balance. The VPD of the cabinet air was then stepped between
the two extremes possible at these two temperatures.
Treatments
The plants, which were grown at 20 °C in hydroponics, were
transferred to the controlled environment cabinet for experimental treatment (Fig. 1) and exposed to constant radiation.
For each experimental run, five plants for leaf extension
measurements were placed in separate 0.5 l containers. A further
two plants for transpiration measurements were placed in a
similar container. The containers were filled with fresh aerated
hydroponics solution at 20 °C. The shoots of the plants were
held steady by the lid of the container. To prevent evaporation
from the containers they were enclosed within polythene bags.
After transfer of plants, air temperature in the cabinet was then
maintained for 4 h at 20 °C to allow plants to settle before the
1396 Clifton-Brown and Jones
cabinet was adjusted to the lower treatment temperatures of 6,
10, and 15 °C. With the exception of the 20 °C treatment,
temperatures were reduced at a rate of 3 °C h−1 to the treatment
temperatures. 12 h after the treatment temperature was reached
VPD was stepped to a higher level for 8 h and then back to the
original level.
In an additional treatment, to investigate the effect of VPD
steps on PER after increased exposure time to low temperature,
plants were exposed to 6 °C for 38 h. During this time VPD
was stepped twice between 0.3 and 0.49 kPa for 2 h periods
after 13 and 23 h exposure to 6 °C had begun. Here, in contrast
to other treatments, PER and VPD were calculated from a
moving point average over a period of 20 min, because VPD
steps were only for 2 h periods. This period was insufficient
for stable measurements of transpiration rate at this low
temperature.
Results
Cabinet and auxanometer performance
The time-course of the effects of VPD steps on an
immobilized transducer and the boundary layer conductance at 20 °C and 5 °C indicated that accumulative electrical and mechanical noise in the auxanometer system
amounted to apparent extension rates below 0.02 mm h−1
irrespective of treatment (data not shown). Evaporation
(E ) from the ‘leaf ’ model was used to calculate boundary
layer conductance ( g ) under the full range of temperature
a
and VPD conditions using equation 3. g averaged
a
549±1.6 and 539±2.8 mmol m−2 s−1 throughout the
VPD steps at 20 and 5 °C, respectively.
Plant extension responses
Figures 2 to 5 show the time-courses of PER, transpiration, leaf temperature, and leaf conductance during a
25 h period of continuous light when air temperature was
maintained close to 6, 10, 15, and 20 °C, respectively, as
VPD was stepped from a lower to a higher level (for 8 h)
and back to the lower level. When air temperatures were
maintained at 20 °C, the 0.3 kPa VPD change (from 0.82
to 1.12 kPa) led to the expected increase in transpiration
rate, but there was no effect on leaf conductance to water
vapour and PER was only transiently affected (Fig. 2).
When air temperatures were maintained close to 15 °C a
0.3 kPa VPD shift (between 0.57 and 0.87 kPa) caused a
change in the transpiration rate between about 9 and
12 mg m−2 s−1 and a transient change in the PER (Fig. 3).
When air temperatures were maintained close to 10 °C
(Fig. 4) a VPD shift of 0.3 kPa had the effect of increasing
transpiration rate from about 3.3 to 7.3 mg m−2 s−1 at
VPDs of 0.38 and 0.68 kPa, respectively. These changes
in transpiration rate altered PER transiently, but the
duration of the transience was longer than that observed
at the higher temperatures as shown in Figs 2 and 3.
Contrary to the marked effects on PER, changes in leaf
conductance were not detected.
Fig. 2. Time-course of plant extension rate (PER) (D); leaf (,), air
(%) and shoot ($) temperature (A); transpiration (+, E ) and
conductance (#, g ) (C ); of M.×giganteus when air temperature was
l
maintained close to 20 °C (A) as VPD was stepped between 0.82 and
1.12 kPa (B). Error bars in (D)=±1 sem, n=5.
With an air temperature of 6 °C (Fig. 5) PER was
barely detectable for the first 8 h after the plant was
brought down to this temperature. However, gradually,
after about 12 h (6 h as plotted in Fig. 5 since the first
6 h of the time-course was omitted) at the new temperature of 6 °C, the PER was detectable (though very low
at less than 0.1 mm h−1). When VPD was increased from
0.25 to 0.45 kPa this increased the evaporation rate from
3 to 6 mg m−2 s−1. Simultaneous to the rise in VPD the
PER halted for several hours before gradually recovering
(rates 0.05 to 0.1 mm h−1). On return to a lower VPD of
0.25 kPa there was a 3-fold increase in PER as transpiration rates dropped to about half that at 0.45 kPa.
Although the transient response of PER to the change in
VPD was consistent with that at higher temperatures the
duration of the effect of the VPD step was longer in this
low temperature treatment than at higher temperatures
as shown in Figs 2, 3 and 4.
A summary of the relationships between PER and g
l
VPD effects on leaf extension 1397
Fig. 3. Time-course of plant extension rate (PER) (D); leaf (,), air
(%) and shoot ($) temperature (A); transpiration (+, E) and leaf
conductance (#, g ) (C ); of M.×giganteus when air temperature was
l
maintained close to 15 °C (A) as VPD was stepped between 0.57 and
0.87 kPa (B). All values of conductance ( g) have been offset by
−10 mmol m−2 s−1 to avoid overlap with transpiration. Error bars in
(D)=±1 sem, n=5.
and temperature, taken from the different time-courses in
Figs 2 and 5 are shown in Fig. 6A, B. The values plotted
in Fig. 6 were obtained from averaging periods of 2 h
immediately before increases or decreases in VPD. The
thermal response of PER shows the characteristic exponential shape reported earlier (Clifton-Brown and Jones,
1997) ( Fig. 6A). Leaf conductance ( g ) appears to be
l
unaffected by temperature ( Fig. 6B). Leaf conductance
was also unaffected by the air VPD ( Fig. 6C ) with the
result that the transpiration rate is almost 151 related to
the VPD (Fig. 6D).
The relative effects on PER of the stepped decrease in
VPD over the range of temperatures used is shown in
Fig. 7. The area under the PER time-courses, above
steady-state rates of extension, shown in Fig. 7A integrates the relative additional amount of leaf extension at
each of the shoot temperatures when VPD was decreased
Fig. 4. Time-course of plant extension rate (PER) (D); leaf (,), air
(%) and shoot ($) temperature (A); transpiration (+, E ) and leaf
conductance (#, g ) (C ); of M.×giganteus when air temperature was
l
maintained close to 10 °C (A) as VPD was stepped between 0.38 and
0.68 kPa (B). Error bars in (D)=±1 sem, n=5.
( Figs 2–5). When the relative areas under the PER timecourses were calculated it was found that the area
decreased with increasing temperatures (P>0.001,
Fig. 7A). The relative peak height (Fig. 7B) also
decreased with increasing temperatures and at each temperature the transient peaks were significantly different
from each other (P>0.006).
The results presented in Fig. 7 show that the transient
effects on PER brought about by a decrease in VPD is
greatest at low temperatures. Figure 8 shows the results
of a treatment which tests the effect of increasing time of
exposure to low temperature on the response to decrease
in VPD. The first reduction in VPD, after 13 h at 6 °C
(4 h as plotted in Fig. 8), increased PER from zero to a
peak of 0.2±0.03 mm h−1 before VPD levels were raised
again and PER was reduced once more almost to zero.
During the next 4 h, although the VPD continued to rise
slowly, PER became detectable at about 0.04 mm h−1.
The second step down in VPD caused a larger peak in
1398 Clifton-Brown and Jones
Fig. 6. Mean thermal response of PER (A) and leaf conductance (B)
of M.×giganteus at temperatures between 6 °C and 20 °C. The influence
of air VPD on leaf conductance (g ) and transpiration rate (E ) are
l
shown in (C ) and (D), respectively. Error bars=±1 sem, n=5.
Fig. 5. Time-course of plant extension rate (PER) (D); leaf (,), air
(%) and shoot ($) temperature (A); transpiration (+, E) and leaf
conductance (#, g ) (C ); of M.×giganteus when air temperature was
l
maintained close to 6 °C (A) as VPD was stepped between 0.25 and
0.45 kPa (B). Error bars in (D)=±1 sem, n=5.
PER (peak=0.35±0.07 mm h−1) than the previous one.
It is evident that PER becomes more sensitive to shifts in
VPD after longer exposure to a temperature of 6 °C. Leaf
conductance ( g ) and transpiration (E ) during this treatl
ment were less stable than those shown in Figs 2–5 and,
consequently, conclusions cannot be drawn about the
response of g to changing VPD.
l
Discussion
The results presented here show that changes in transpiration rate, caused by an alteration of VPD without change
in temperature, effected PER, but these effects were small
at temperatures of 15 °C and above and were most marked
at low temperatures. If the concept of the soil–plant–air
continuum holds (Richter, 1997) then an increase in
transpiration rate (water flux) should cause a decrease
in leaf water potential ( Y ) if the hydraulic conductance
l
between the hydroponic solution and the stomata is
Fig. 7. Relative areas under the PER time-course (A) and relative peak
heights (B) over the range of mean shoot temperatures within 5 cm of
the growing point when VPD was stepped down to promote a decrease
in transpiration rate. Different letters immediately above the bars
indicate statistically significant differences between means at P<0.006,
n=5.
assumed constant and the solute concentration does not
change. This concept can be expressed in the form of an
equation (Slatyer, 1967):
Y =Y −flux(r )
l
s
s,l
(4)
VPD effects on leaf extension 1399
Fig. 8. Time-course of plant extension rate (PER) of M.×giganteus
when air temperature was maintained close to 6 °C (B) as VPD (A)
was stepped between 0.49 and 0.29 kPa. Error bars in (B)=±1 sem,
n=3.
where Y is the soil water potential and r is the resistance
s
s,l
to flow between the soil and leaf. Stomatal resistance (the
reciprocal of conductance) is a key factor in regulating
the transpiration flux in plants (Jones, 1992). It was
found that throughout the treatments illustrated in Figs
2–5 leaf conductance remained virtually constant and
therefore the stomata exerted no variable influence on
transpiration rates. Leaf conductances were at the lower
end of the range reported for cultivated C grasses (Jones,
4
1992). Although it has been demonstrated that stomata
can respond to changes in humidity (Lösch and
Tenhunen, 1981; Meinzer et al., 1997; Yong et al., 1997)
the data presented here show a complete lack of stomatal
response to humidity. This is consistent with observations
of some other C plants such as Dactyloctenium aegyptium
4
and Eragrotis tremula made in leaf chambers (Maroco et
al., 1997).
Current models of the water relations of growing cells
are based on the Lockhart equation (Lockhart, 1965). A
simple formulation of this equation assumes that
hydraulic conductivity into the cells does not limit growth,
supported by experimental evidence (Cosgrove and
Steudle, 1981; Malone and Tomas, 1992), is as follows:
G=m(P−Y )
(5)
where G is the growth rate (here measured as PER with
an LVDT ), P is the turgor pressure of the cell, Y is the
yield threshold pressure for growth, and m is cell extensibility. The water potential of the cell is equal to the turgor
pressure minus the osmotic potential (Salisbury and Ross,
1992). Though strictly formulated for single cells, equation 5 can be used to model growth of multicellular
tissues such as leaves (Passioura and Fry, 1992). This
model predicts that during steady leaf growth, the rate of
water influx and rate of cell wall yielding to the turgor
pressure generated must be equal in the expanding cells.
When transpiration rate is altered the change in Y is
l
accompanied by a change in P. Turgor pressure changes
in response to alteration in transpiration rate were demonstrated in expanding vine leaves (Shackel et al., 1987)
and in Tradescantia virginiana L. (Frensch and Schulze,
1988) with a pressure probe. Here, as transpiration rate
was reduced by decreasing VPD, turgor pressures of the
expanding cells were shown to increase. This increase in
P caused the expected increase in leaf expansion rate
(consistent with equation 5). Hypothetical turgor changes
in the growing cells adequately explain the initial increase
in PER in our data resulting from decreased transpiration
( Figs 2–5) and also the subsequent decrease when transpiration rate was increased. Interestingly, in vine leaves,
although the turgor was more or less constant at a
particular transpiration rate the leaf expansion rate was
only transiently increased ( lasting 20 min) following a
reduction in transpiration rate (Shackel et al., 1987). In
the measurements reported here the changes in PER
following alteration in transpiration rate were also
transient (Figs 2–5).
The transience in leaf extension rates could occur as a
result of osmotic adjustment in the cells. However, it has
been shown that the osmotic potential in the vine leaves
did not adjust during the transient alteration of PER by
transpiration rate (Shackel et al., 1987) and by deduction
from equation 5, it was suggested that cell wall rheology
(wall yielding) was regulating cell growth rate rather than
water influx rate. Similarly, in these experiments, if it is
assumed that P remains stable at a constant evaporation
rate then the transience observed is probably due to
elastic effects or a biochemical alteration in wall rheology.
At temperatures higher than 10 °C the transients are
most likely to be entirely due to elastic effects (Malone,
personal communication). However, elastic effects are
unlikely to explain the large transients observed at temperatures as low as 6 °C. An explanation of the biophysical
mechanism by which rheology could alter following
changes in turgor pressure has been proposed (Passioura
and Fry, 1992; Passioura, 1994). This draws largely on
current views of the mechanism of cell wall extension
proposed by Cosgrove (Cosgrove, 1993). Here it is
suggested that cell walls contain load-bearing (taut)
and non-load-bearing (slack) molecules interconnecting
the cellulose microfibrils. When turgor (P) is increased,
growth rate increases because the wall stretches, which
1400 Clifton-Brown and Jones
recruits more slack molecules into the load-bearing contingent and thereby stiffens the wall. If enzyme action
releasing the connections of the load-bearing molecules
does not change, then, after a period, the rate should
become similar to that before the change in P. At this
stage after the shift in P, growth (G) is governed by the
rate of enzyme cleavage of these molecular linkages in
the cell wall.
PER of plants exposed to the low temperature of 6 °C
were found to become more sensitive to changes in VPD
after longer periods (Fig. 8). This increased sensitivity
with time-exposure at low temperatures (between 5 °C
and 6 °C ) to sudden shifts in environmental conditions
has also been observed when switching between day and
night and also when the plants are re-warmed. Saturation
of this low temperature acclimation effect was shown to
be between 18 h and 24 h after the plant was brought to
temperatures between 5 °C and 6 °C in a rewarming test
(JC Clifton-Brown, unpublished results). Further study
of these acclimation effects could reveal more of the leaf
expansion mechanism.
In conclusion, the effects of VPD on leaf extension
observed in these experiments, which were carried out at
temperatures close to the threshold for growth, can be
largely attributed to changes in transpiration rate and
hence leaf water status. However, the transient changes
in PER following the step changes in VPD suggest that
there are also changes in cell extensibilty. The reason why
these are proportionately greater at lower temperatures
is not clear and requires further investigation. Finally,
since VPD effects were found to be transient, it is anticipated that models relating leaf growth with temperature
only (Clifton-Brown and Jones, 1997), are adequate
for the prediction of leaf area development of
Miscanthus×giganteus in the range of temperature
conditions between 6 °C and 20 °C.
Acknowledgements
We thank Brendan Kennelly for assistance with development
of the VPD control system, the staff at Trinity College Botanic
Gardens for the maintenance of the plant material and Dr Iris
Lewandowski for facilitating the completion of this paper. This
work was partially funded by the EU under contract number
AIR-CT92–0294.
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