Annals of Botany 97: 831–835, 2006
doi:10.1093/aob/mcl031, available online at www.aob.oxfordjournals.org
SHORT COMMUNICATION
Stomatal Oscillations in Orange Trees under Natural Climatic Conditions
K A T H Y S T E P P E 1,*, S E B I N A S I D Z I K I T I 2, R A O U L L E M E U R 1 and J A M E S R . M I L F O R D 2
1
Faculty of Bioscience Engineering, Laboratory of Plant Ecology, Ghent University, Coupure links 653,
B-9000 Ghent, Belgium and 2Agricultural Meteorology Group, Physics Department, University of Zimbabwe,
PO Box MP167, Mt Pleasant, Harare, Zimbabwe
Received: 6 October 2005 Returned for revision: 2 November 2005 Accepted: 5 January 2006 Published electronically: 14 February 2006
Background and Aims Stomatal oscillations have been reported in many plant species, but they are usually induced
by sudden step changes in the environment when plants are grown under constant conditions. This study shows that
in navel orange trees (Citrus sinensis) pronounced stomatal oscillations occur and persist under natural climatic
conditions.
Methods Oscillations in stomatal conductance were measured, and related to simultaneous measurements of leaf
water potential, and flow rate of sap in the stems of young, potted plants. Cycling was also observed in soil-grown,
mature orchard trees, as indicated by sap flow in stem and branches.
Key Results Oscillations in stomatal conductance were caused by the rapid propagation and synchronization of
changes in xylem water potential throughout the tree, without rapid changes in atmospheric conditions.
Conclusions The results show marked stomatal oscillations persisting under natural climatic conditions and
underscore the need to discover why this phenomenon is so pronounced in orange trees.
Key words: Citrus sinensis, orange tree, sap flow, transpiration, leaf water potential, stomatal conductance, stomatal
oscillation, natural climatic conditions.
INTROD UCTION
Stomatal oscillations are defined as the phenomenon of
cyclic opening and closing movements of stomates (Barrs,
1971). They are believed to result from instability in the
negative feedback loops that control stomatal aperture via
leaf water status (Barrs, 1971; Herppich and von Willert,
1995). Such instability can be caused by a lag and/or an
overshoot in response to the feedback signal (Kaiser and
Kappen, 2001).
Based on the work of Apel (1967), Barrs (1971) proposed
a tentative grouping of oscillations based on water or
CO2. Short-period stomatal oscillations (<10 min) with
small amplitudes are associated with control of CO2
status and appear to depend on the external CO2 concentration. Slower oscillations (30–50 min) with larger amplitudes are associated with control of the plant–water
status. However, the general validity of this grouping is
uncertain, because short-period oscillations related to
CO2 content have been observed only in a small number
of plant species.
Stomatal oscillations, described as water-based by Barrs
(1971), have been reported in at least 30 plant species.
However, most of these oscillations occurred when plants
were grown in a constant environment that was suddenly
changed (Barrs, 1971; Cowan, 1972; Farquhar and Cowan,
1974; Herppich and von Willert, 1995; Kaiser and Kappen,
2001; Prytz et al., 2003; West et al., 2005). Although there is
no a priori reason why stomatal oscillations may not occur
under field conditions, there is little evidence for their
occurrence in plants grown under field conditions (Levy
and Kaufmann, 1976; Hirose et al., 1994). If oscillations
in stomatal aperture and conductance are a result of
* For correspondence. E-mail [email protected]
unbalanced water supply and demand, caused by sudden
large increase in water loss from the plant, then such conditions might arise under natural conditions and result in
oscillations.
This short communication shows that in navel orange
trees pronounced stomatal oscillations occur and persist
under naturally occurring atmospheric conditions unaffected by any specific environmental disturbance.
MATERIALS AND METHODS
Experiments were done in the open-air laboratory on the
flat roof of the Physics Department of the University of
Zimbabwe (1779 S, 3104 W, elevation 1450 m a.s.l.)
using six actively growing 2-year-old navel orange trees
[Citrus sinensis (L.) Osbeck] of the baianinha selection
grafted on a troyer citrange rootstock (Citrus sinensis ·
Poncirus trifoliate). The trees were approx. 10 m high
and their stem diameter at soil surface ranged from 10
to 12 mm. The trees were planted in asbestos pots 07 m
diameter and 05 m high filled with dark-red clayey loam
soil (Hussein, 1982). The soil water content was maintained
close to field capacity (45 % v/v) by watering twice a week
in the evening. The trees were grown under a transparent
plastic shelter to exclude rain, but with all sides of the
shelter open to allow free circulation of air.
Although the results for one tree are presented in this
short communication, observations in the other five trees
were similar on other days. All the trees could not be
measured at the same time due to limited equipment. Observations were made on mature, healthy and fully expanded leaves with equal exposure to solar radiation. The
oscillations in stomatal conductance were related to
changes in leaf water potential, and sap flow rate through
The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
For Permissions, please email: [email protected]
Steppe et al. — Stomatal Oscillations in Orange Trees
832
A
Solar radiation
VPD
600
1·5
400
1·0
300
200
VPD (kPa)
Solar radiation (W m–2)
500
0·5
100
0·0
0
150
Stem sap flow (g h–1)
120
B
0·15
0·12
90
0·09
60
0·06
0·03
30
Stomatal conductance (mol m–2 s–1)
Stem sap flow
Stomatal conductance
0·00
0
6
8
10
12
Time (h)
14
16
18
F I G . 1. (A) Natural variation in the microclimate of a young orange tree on a winter day in Zimbabwe (roof experiment in Zimbabwe, 6 Jul. 2005). Incoming
solar radiation and vapour pressure deficit (VPD) of the air are depicted, indicating clear sky conditions. (B) Oscillations in stem sap flow and stomatal
conductance of the orange tree related to the conditions shown in (A).
the stem. Leaf water potential was measured using thermocouple psychrometers: four standard C-52 sample chambers (Wescor Inc., Logan, UT, USA), connected to a
microvoltmeter (HR-33T; Wescor Inc.), kept at constant
temperature in the laboratory. Measurements were made
in the psychrometric (wet bulb depression) mode. The
psychrometers were calibrated with NaCl solutions over
the range 0 to 455 MPa. Samples were taken from leaves
adjacent to those selected for porometer measurements
(see below). In order to determine the changes in leaf
water potential, measurements were taken at 10-min intervals. With four C-52 sample chambers, equilibration intervals were limited to approx. 40 min. Trials showed that
equilibration was reached within this time interval. As a
consequence, water potential measurement could not be
replicated. However, the changes in water potential were
much larger than the errors. Stomatal conductance was
measured with a dynamic diffusion porometer (AP4;
Delta-T devices, Cambridge, UK) on four selected leaves.
Readings were taken in continuous cycles with each
leaf measured at least once every 5 min. Sap flow rate in
the stems was continuously measured with a Dynagage
heat balance sap flow sensor (SGA9-WS, Dynamax Inc.,
Houston, TX, USA). Signals from the sap flow sensor, and
from instruments for measuring air temperature, relative
humidity and incoming short-wave radiation, were recorded
automatically at 5-s intervals and 5-min averages were
stored on a data logger (CR23X; Campbell Scientific
Ltd, Shepshed, UK).
Oscillations in sap flow have also been measured in
mature, 45-year-old trees in an orchard at the Mazowe
Citrus Estate (Zimbabwe). Sap flow rates in stem and
branches of selected, soil-grown orchard trees were continuously measured to study the effects of different irrigation strategies. Cycling occurred in the orchard trees, as
indicated by the daily records of sap flow in stem and
branches. A Delta-T weather station was used for recording
microclimatic data.
RESULTS AND DISCUSSION
Examples of measurements of stomatal conductance and
stem sap flow in a young orange tree during a winter
day (6 Jul. 2005) in Zimbabwe are given in Fig. 1B. The
833
Steppe et al. — Stomatal Oscillations in Orange Trees
1200
A
Solar radiation
VPD
1000
800
2
600
400
VPD (kPa)
Solar radiation (Wm–2)
3
1
200
0
0
B
120
90
400
60
200
30
Branch sap flow
Stem sap flow
0
6
8
10
Stem sap flow (g h–1)
Branch sap flow (g h–1)
600
0
12
14
16
18
Time (h)
F I G . 2. (A) Natural variation in the microclimate of a mature, 45-year-old navel orange tree in the commercial orchard at Mazowe Citrus Estate (Zimbabwe).
Incoming solar radiation and vapour pressure deficit (VPD) of the air are displayed. (B) Typical pattern of cyclic changes in branch and stem sap flow of the
soil-grown orchard tree under the conditions shown in (A).
sky was largely clear with occasional patchy clouds as
shown by the small fluctuations in conditions (Fig. 1A).
The stomatal conductance (Fig. 1B) ranged from 0007 to
0145 mol m–2 s–1. Levy and Kaufmann (1976) reported
oscillations in stomatal conductance varying from 0004
to 0042 mol m–2 s–1 for 4-year-old citrus trees under
controlled greenhouse conditions with root systems at
different temperatures. Seven clear oscillations were
observed in a time interval of approx. 8 h giving a mean
period of about 70 min per oscillation (Fig. 1B). Oscillations
generally have a period of up to 50 min (see Barrs, 1971).
Also stem sap flow oscillated similarly with an identical
cycle of about 70 min. However, the oscillations in stem
sap flow lagged behind those in stomatal conductance
by about 30 min (i.e. water uptake lagged behind water
loss).
These results show that, under atmospheric conditions
without specific or abrupt disturbance, oscillations in stomatal conductance persist in young orange trees. Further
sap flow measurements were made under field conditions
on mature navel orange trees grown in soil in a commercial
orchard at Mazowe Citrus Estate (Zimbabwe). A typical
pattern of cyclic variations in branch and stem sap flow
rate of a 45-year-old orange tree in the orchard is given
in Fig. 2. Sap flow in the stem lagged behind branch sap
flow by about 20 min. Despite the smooth increase in
vapour pressure deficit of the air from 0 to 34 kPa, again
pronounced cycling occurred.
Oscillations in leaf conductance for the potted trees
were accompanied by oscillations in leaf water potential
(Fig. 3): maximum conductance and minimum water potential were out of phase by 10–25 min. Stomates started
opening when leaf water potential was low (more negative)
and started closing when leaf water potential was high (less
negative) again. With roots of potted citrus trees at 5 C,
Levy and Kaufmann (1976) observed that maximum conductance preceded minimum xylem potential by 30 min,
while little time lag occurred with roots at 25 C. In the
present study the mean temperature of the soil was 16 C.
Because oscillations occurred in the whole tree, it is
Steppe et al. — Stomatal Oscillations in Orange Trees
834
0
Leaf water potential (MPa)
–1
0·15
0·12
–2
0·09
–3
0·06
–4
0·03
–5
Stomatal conductance (mol m–2 s–1)
Leaf water potential
Stomatal conductance
0·00
–6
9
10
11
12
13
Time (h)
14
15
16
17
18
F I G . 3. Phase-relationship between fluctuations in leaf water potential and stomatal conductance for the potted orange tree (roof experiment in Zimbabwe,
6 Jul. 2005).
probable that xylem water potential is the signal transmitting agent (Cowan, 1972). The synchronization of stomatal
movements by xylem water potential may occur by two
mechanisms. The first involves a hydropassive, positive
feedback response, which is purely hydraulic and causes
stomata to open when increased transpiration and/or
increased xylem tension reduces the turgor in guard and
epidermal cells. The increased stomatal aperture is positively related to increased turgor pressure of the guard cells
that form the pore, but negatively related to the pressure
of adjacent subsidiary or epidermal cells that surround
the guard cells. Guard and epidermal turgor is thereby
decoupled by the mechanical advantage of the epidermis
(Buckley, 2005). In other words, any increase of transpiration causes turgor to decline in both guard and epidermal cells and ensures an increase in stomatal aperture
due to the mechanical advantage of the epidermis. This
hydraulic effect increases the tendency of stomata to
oscillate (Kaiser and Kappen, 2001). It may also synchronize the stomatal movements in different parts of the
same leaf.
The second mechanism is the hydro-active, negative
feedback response where an increase in transpiration
will act as the closing stimulus. It involves an active (i.e.
biochemically mediated) physiological response of guard
cells to perturbations of the leaf water potential. With
increased transpiration, active regulation of guard cell
osmotic pressure will reduce guard cell turgor and cause
stomatal closure. This physiological response of guard cells
lags behind the stimulus (probably leaf water potential)
(Fig. 3) and is responsible for the relatively slow turgor
mechanism (Kaiser and Kappen, 2001). The hydraulic
effect acts rapidly and precedes the slower, physiological
feedback response which acts in the opposite direction
(Kaiser and Kappen, 2001; Buckley, 2005).
The juxtaposition of the positive and the negative feedback loops explains the stomatal oscillations because both
are involved in the stomatal responses to disturbances of the
water balance and cause the two-phase response of stomates
(Cowan, 1972; Farquhar and Cowan, 1974; Buckley, 2005).
The present results demonstrate that water uptake lags
behind water loss by approx. 30 min (Fig. 1B). Hence,
this temporal non-synchronization between water uptake
and loss leads to disturbances of the water balance which
seems to be an essential feature for the occurrence of oscillation as hypothesized by Lang et al. (1969) amongst others.
Cavitation in xylem conductive elements might play a role
in oscillations, but is a matter of speculation (Buckley,
2005). It is not possible to explain why such pronounced
oscillations in stomatal conductance occur in orange trees
under field conditions.
CONCLUSIONS
In orange trees stomatal oscillations occur and persist for
many hours under atmospheric conditions without sudden
changes, as shown by measurements on young potted
trees and verified under orchard conditions. Simultaneous
measurements of stomatal conductance and sap flow show
that xylem water potential is probably the signal transmitting agent, which is propagated throughout the tree. The
synchronization of stomatal movements by xylem water
potential may occur by hydropassive positive feedback
and by physiological negative feedback of stomates to
the leaf water potential. However, the detailed mechanism
of such pronounced oscillations in orange trees grown
under natural field conditions without specific and abrupt
disturbances in the atmospheric conditions remain to be
described.
ACKNOWLEDGEMENTS
Many thanks to the Mazowe Citrus Estate of Zimbabwe for
providing the plant material and management skills for the
trees and to the Belgian Inter-University Council (VLIR
Institutional Cooperation with the University of Zimbabwe)
for providing financial support and a travel grant for the
first author.
Steppe et al. — Stomatal Oscillations in Orange Trees
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