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Tree Physiology 28, 785–795
© 2008 Heron Publishing—Victoria, Canada
Impact of an exceptionally hot dry summer on photosynthetic traits in
oak (Quercus pubescens) leaves
P. HALDIMANN,1–3 A. GALLÉ1 and U. FELLER1
1
Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland
2
Present address: Department of Plant Molecular Biology, Biophore – Biology building, University of Lausanne, CH-1015 Lausanne, Switzerland
3
Corresponding author ([email protected])
Received June 3, 2007; accepted August 16, 2007; published online March 3, 2008
Summary Climatic constraints on diurnal variations in
photosynthetic traits were investigated in oaks (Quercus pubescens Willd.) growing in the Swiss Alps. The measurement
period included the summer of 2003, when central Europe experienced a record-breaking heat wave. During the summer, a
combination of moderate heat and drought caused a reduction
in photosynthetic CO2 assimilation rate (Pn ) by mid-morning,
which increased by the afternoon. More extreme drought and
heat caused a sharp day-long reduction in Pn. These effects
were closely related to changes in stomatal conductance (gs ),
but low gs was unaccompanied by low intercellular CO2 concentrations (Ci ). Around midday, a combination of heat and
drought increased Ci, indicating metabolic limitation of photosynthesis. Chlorophyll a (Chl a) fluorescence measurements
revealed reversible down-regulation of photosystem (PS) II activity during the day, which was accentuated by heat and
drought and correlated with diurnal variation in zeaxanthin accumulation. A combination of heat and drought reduced leaf
Chl a + b concentrations and increased ratios of total carotenoids, xanthophyll-cycle carotenoids and lutein to Chl a + b.
The combination of summertime heat and drought altered the
77 K Chl fluorescence emission spectra of leaves, indicating
changes in the organization of thylakoid membranes, but it
had no effect on the amounts of the major light-harvesting
Chl-a/b-binding protein of PSII (LHCII), Rubisco, Rubisco
activase, Rubisco-binding protein (cpn-60), phosphoribulokinase and chloroplast ATP synthase. The results demonstrate
that Q. pubescens can maintain photosynthetic capacity under
adverse summer conditions.
Keywords: chlorophyll fluorescence, chloroplast ATP synthase, drought, heat, leaf pigments, photosynthesis, Rubisco,
xanthophyll cycle.
Introduction
Pubescent oak (Quercus pubescens Willd.) is a thermophilous
tree found throughout central Europe, most abundantly in areas with a hot dry Mediterranean summer. In Switzerland,
Q. pubescens is mainly found on warm and sunny south-facing
slopes of the Wallis, Jura and Tessin regions, typically on
rocky shallow soils with limited water-holding capacity, where
it plays an important role in protecting slopes from erosion and
scree-fall. Quercus pubescens is a winter-deciduous species,
and summer drought and elevated temperatures limit photosynthetic yield. Such limitation may be expected to intensify
in the event of continued climate warming (Luterbacher et al.
2004, Schär et al. 2004).
The progressive reduction in stomatal aperture in response
to water stress (i.e., negative tisssue water potential) reduces
water loss but limits the influx of CO2 and, hence, net photosynthetic rate (Pn ) (Epron and Dreyer 1993a, 1993b, Damesin
and Rambal 1995, Valentini et al. 1995). In addition, water
stress may reduce Pn by reducing diffusion of CO2 from leaf
intercellular spaces to the site of carboxlation within the
mesophyll (Epron and Dreyer 1993b, Epron et al. 1995, Flexas
et al. 2002). Whether water stress affects Pn primarily through
limitation on transcellular diffusion of CO2 or by impairment
of the carboxylation process remains subject to debate (Cornic
2000, Flexas and Medrano 2002, Lawlor 2002, Lawlor and
Cornic 2002, Medrano et al. 2002, Tang et al. 2002, Flexas et
al. 2004). A study with sunflower (Helianthus annuus L.) indicated that drought-dependent inhibition of Pn is largely due to
a reduction in in the concentration of ribulose-1,5-bisphosphate, which is, in turn, the result of reduced ATP synthesis
due to a decrease in chloroplast ATP synthase (Tezara et al.
1999). Our objective was to investigate the existence of a comparable mechanism of drought-inhibited photosynthesis in
Q. pubescens.
Photosynthesis is sensitive to inhibition by high temperature
(Berry and Björkman 1980, Salvucci and Crafts-Brandner
2004a, Sharkey 2005), and high solar irradiance can raise leaf
temperature above air temperature (Singsaas and Sharkey 1998,
Leakey et al. 2003), an effect enhanced by drought, which limits transpiration and evaporative cooling of foliage. Under natural conditions, climatic constraints act in concert, and the
combination of drought, elevated temperature and high solar
irradiance may limit photosynthesis (Ludlow 1987, Havaux
1992). An acute and long-lasting combination of high temperature, drought and high solar irradiance may also affect photo-
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synthesis by altering amounts of key photosynthetic proteins.
Such effects may be reversed only slowly, so that carbon assimilation may remain low for some time following the termination of extreme conditions.
Here we investigated effects of environmental stress during
the summer, including the summer of 2003, the warmest in Europe in the last 500 years (Luterbacher et al. 2004, Schär et al.
2004, Ciais et al. 2005), on various photosynthetic traits of
Q. pubescens in a warm dry region of the Swiss Alps.
Materials and methods
Study site and plant material
The study site is located near Salgesch in the Wallis valley in
the Swiss Alps (46°19′27″ Ν, 7°34′40″ Ε, 975 m a.s.l.) in one
of the warmest and driest regions of Switzerland. It is located
on a steep, south-facing oak– pine wooded slope with a shallow rocky soil with limited water-holding capacity, where
Quercus pubescens trees (less than 70 years old) develop as
shrubs, typically with two to four stems. Observations focused
on a single tree, about 3 m tall, with three trunks with basal diameters of about 0.10 m. The tree was located 10 m from a
meteorological station. Measurements were carried out on four
sunny days: August 15, 2002, June 24 and July 15, 2003 and
May 18, 2004.
Additional measurements on the primary sample tree in late
July and August 2003 were precluded by exceptionally hot
weather which caused leaf mortality by the second half of July.
Measurements on July 23, 2003 were therefore performed on
another tree, about 3 m tall and at a distance of about 50 m
from the weather station, which had leaves that remained
photosynthetically active in late July. By early August 2003,
none of the oaks in the stand had photosynthetically active foliage, with the exception of trees adjacent to a creek that
crosses the slope 250 m north of the meteorological station. It
was among these trees that measurements were made on August 6, 2003. Measurements carried out on several trees located around the meteorological station under non-stressful
conditions on August 15, 2002 and May 18, 2004 indicated
that Pn was similar in all trees, whether or not they were adjacent to the creek, which suggests uniformity of response to environmental constraints among trees at the site. All measurements were made on randomly selected, fully sun-exposed
leaves.
Microclimate, leaf temperature and leaf water potential
Microclimatic data were recorded with a locally installed meteorological station (see Zweifel et al. 2005). Global irradiance, air temperature (Ta ) and vapor pressure deficit (VPD)
were determined at 10-s intervals and averaged every 10 min.
Daily precipitation was estimated with a conventional rain
gauge. Photosynthetic photon flux (PPF) was measured with a
Li-Cor radiometer (LI-250, Li-Cor, Lincoln, NE). Leaf temperature (Tl ) was determined with an infrared thermometer
(Oakton TempTestr IR, Cole-Parmer International, Vernon
Hills, IL). On each date, predawn (Ψpd ) and midday (Ψm ) leaf
water potentials were measured on three leaves with a Scholander pressure chamber (SKPM, Skye Instruments, Powys,
U.K.).
Gas exchange and chlorophyll a fluorescence
Photosynthetic gas exchange measurements were made on attached leaves with an infrared gas analyzer (IRGA) (CIRAS1, PP-Systems, Hitchin, U.K.) operated in open mode. Temperature and relative humidity in the leaf chamber were close
to ambient values. Recorded data included Pn, stomatal conductance to water vapor (gs ), transpiration rate (E) and calculated intercellular CO2 concentration (Ci ). Measurements on
four to eight leaves were made throughout the day.
Chlorophyll (Chl) a fluorescence was measured with a pulse
amplitude modulated fluorometer (FMS-1, Hansatech, King’s
Lynn, Norfolk, U.K.). The fiber-optic was connected to a
leaf-clip that shielded the sample from ambient light. Records
were taken after 5 min of illumination with a PPF corresponding to that incident on the leaves, and after a subsequent 5–10
min exposure to a PPF of 500 µmol m – 2 s – 1. The following parameters were monitored: maximum quantum yield of photosystem (PS) II primary photochemistry in the dark- and lightadapted state (Fv /Fm and Fv′/Fm′, respectively); photochemical
quenching (qp ), which increases with the fraction of open PSII
reaction centers (RCs), being equal to it if no connectivity between PSII units is assumed; and quantum yield of PSII electron transport (φPSII ) (Maxwell and Johnson 2000). We measured Fv /Fm after 30 min of dark-adaptation. Measurements
were performed at different times of day, with three leaves
measured on each occasion. The quantum yield of PSII electron transport and Fv /Fm (n = 4–10) were measured on three
dates (August 15, 2002 and June 24 and July 15, 2003) with a
PAM-2000 fluorometer (Heinz Walz, Effeltrich, Germany),
and φPSII was directly determined under ambient light.
On completion of the physiological measurements, 1.4-cmdiameter leaf disks were excised, frozen in liquid nitrogen and
stored at –80 °C for later biochemical and biophysical analyses.
Low-temperature chlorophyll fluorescence emission spectra
Low temperature chlorophyll fluorescence was determined by
immersing a small piece of oak leaf tissue in liquid nitrogen
and mounting it in an LS50 luminescence spectrophotometer
(Perkin-Elmer) 1 cm above a reservoir of liquid nitrogen,
thereby maintaining the sample temperature between –190
and –180 °C (Sperling et al. 1998). The emission spectra were
recorded with an excitation wavelength of 440 nm. Measurements were performed on three to four leaves collected between 0530 and 0730 h local solar time (LST) and between
1330 and 1530 h.
Leaf pigment analysis
Leaf chlorophylls and carotenoids were extracted from the
frozen leaf disks with 80% ice-cold acetone containing sodium carbonate. The extract was centrifuged at 20,000 g for
2 min and the supernatant filtered through a 0.2-µm membrane
(Anatop 10, Merck, Darmstadt, Germany). Pigment separa-
TREE PHYSIOLOGY VOLUME 28, 2008
IMPACT OF CLIMATIC CONSTRAINTS ON PHOTOSYNTHETIC TRAITS IN OAK
tion and quantification were performed by high performance
liquid chromatography by the method of Gilmore and Yamamoto (1991) as modified by Färber et al. (1997). The de-epoxidation state of the pool of xanthophyll-cycle carotenoids (V +
A + Z) was calculated as the (Z + 0.5A)/(V + A + Z) molar ratio, where V, A and Z are violaxanthin, antheraxanthin and
zeaxanthin, respectively. Measurements were made on samples collected at different times of day, with three to five samples analyzed at each sampling time. If the concentration of a
pigment showed no diurnal variation, data were pooled (n = 21
to 33) and daily means calculated.
SDS-PAGE and Western-blot analysis
Proteins were extracted from the frozen leaf disks after grinding to a powder in liquid nitrogen with a Heidolph tissue homogenizer (Heidolph Instruments, Schwabach, Germany).
Proteins were extracted from the powder at 4 °C in a buffer
containing 20 mM sodium phosphate (pH 7.5), 1% (w/v)
polyvinylpolypyrrolidone and 0.1% (v/v) 2-mercaptoethanol.
Extracts were filtered through Miracloth (Calbiochem, Luzern,
Switzerland) and an aliquot used to determine total soluble
protein (TSP) concentration by the Bradford method (1976).
The remainder of each extract was stored at –20 °C, for later
protein analysis by gel electrophoresis. Measurements were
performed on three samples that had been collected between
0800 and 0900 h LST and between 1330 and 1530 h LST. Results presented are means for each date, as the concentration
did not change between morning and afternoon. Polypeptides
were analyzed in a single extract of three leaf disks (collected
between 1330 and 1530 h LST). Polypeptides were separated
by sodium dodecyl sulfate-polyacrylamide gel electrophoresis
(SDS-PAGE; 12%) according to Laemmli (1970) with a Mini
Protean II Dual slab Cell (Bio-Rad, Glattbrugg, Switzerland).
Immunoblot analysis of polypeptides was performed as described by Feller et al. (1998).
Soluble carbohydrates
Leaf water-soluble carbohydrate (WSC) concentration was
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determined as described by Stieger and Feller (1994), with
glucose as a standard. Measurements were performed on three
leaves collected in the morning between 0800 and 0900 h LST
and in the afternoon between 1330 and 1530 h LST, but we
present only the mean WSC concentration for each date
(n = 6), because concentrations did not change during the day.
Statistics
Although most measurements were made on leaves of the
same tree, data were considered to be independent, and we
performed variance analyses (ANOVAs) taking date or time
within date as factors for the different studied parameters. A
least-significant test (LSD; P < 0.05) was used to analyze the
differences between means.
Results
Meteorological parameters and leaf temperature
Daily precipitation and maximum Ta measured at the study site
in Years 2002, 2003 and 2004 are shown in Figure 1. During
the heat wave of summer 2003, Ta exceeded 30 °C on most
days from the end of May to the end of August, whereas Ta values greater than 30 °C occurred infrequently in most other
years. Total May–August precipitation was 377 mm in 2002,
but only 201.1 and 213.7 mm in 2003 and 2004, respectively.
Annual precipitation was 690, 426.6 and 500.5 mm in 2002,
2003 and 2004, respectively (MeteoSwiss, Sion, 20 km WSW
of the study site). Mean annual precipitation between 1983 and
2002 was 623 mm.
Figure 2 shows the diurnal courses of global irradiance
(PPF), Tl, Ta and VPD on the dates of the physiological measurements. The hottest days were July 15 and August 6, 2003,
when Ta was close to or higher than 35 °C for much of the afternoon. In July and August 2003, Tl was several degrees
above Ta, and for several hours each day, was near or above
40 °C. In general, high Ta values correlated with high VPD
values.
Figure 1. Daily maximum air temperature
(Ta, 䊉, 䊊) and precipitation (bars) recorded in 2002, 2003 and 2004. Different
symbols were used to distinguish maximum Ta values higher (䊉) or lower (䊊)
than 30 °C. Arrows indicate the dates
when diurnal variations in leaf physiological traits were examined. DOY = day of
year.
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Figure 2. Diurnal variations in (A) global irradiance (䉱) and photosynthetic photon flux (PPF, 䉭); (B) air (Ta, 䊉) and leaf (Tl, 䊊) temperatures;
and (C) air vapor pressure deficit (VPD, 䉲) on each measurement date. The dotted line corresponds to 35 °C. Values of PPF and Tl are means ± SE
of 4–10 replicates.
Leaf water potential
In May 2004, Ψpd and Ψm were within the normal range
(Epron et al. 1992, Epron and Dreyer 1993a, Damesin and
Rambal 1995, Valentini et al. 1995), averaging –0.34 and
–1.85 MPa, respectively (Table 1). Similar values were measured on August 15, 2002 (Ψpd = –0.30 MPa; Ψm = –1.70
MPa). However, Ψpd as low as –2.50 MPa and Ψm as low as
–3.34 MPa were recorded in summer 2003, when significant
water deficit developed by June. Due to technical problems,
Ψpd and Ψm were not determined on July 15, 2003.
Leaf protein and water-soluble carbohydrate concentrations
and pigment composition
Total leaf soluble protein concentration did not vary greatly
among dates, whereas total WSC concentration was greater in
July 2003 than on the other dates (Table 1). Chlorophyll a + b
concentration, despite minor differences among dates, was
lower in June and July 2003 than in August 2003 and May
2004. In summer 2003, there was a large reduction in Chl concentration, and leaves acquired a yellowish appearance, unlike
leaves of the tree adjacent to the creek. Variations in Chl a/b
ratio were observed among dates, but there was no clear correlation between Chl a/b ratio and leaf water potential. In June
and July 2003, leaves had lower total carotenoid (X + C) concentrations than in the spring, but a higher (X + C)/Chl a + b
ratio (Table 1). Concentrations of lutein and xanthophyll-cycle carotenoids (V + A + Z) were greater in July 2003 than on
other dates. The concentrations of the other carotenoids and
Table 1. Mean ± SE predawn (Ψpd ) and midday (Ψm ) leaf water potentials (n = 3), concentrations of total leaf soluble protein (TSP; n = 6), total
leaf water-soluble carbohydrates (WSCs; n = 6) and chlorophyll (Chl a + b), Chl a/b ratio and concentration of total carotenoids (X + C; n = 21 to
33) in oak (Quercus pubescens) leaves on specific dates (month/day/year). Values a column followed by different letters differ significantly at P =
0.05. Leaf water potentials on 07/15/03 were not determined due to instrument failure.
Date
06/24/03
07/15/03
07/23/03
08/06/03
05/18/04
Ψpd
(MPa)
–2.05 ± 0.14 ab
–
–2.50 ± 0.24 a
–1.90 ± 0.11 b
–0.34 ± 0.02 c
Ψm
(MPa)
TSP
(g m –2 )
Total WSCs
(mol m –2 )
Chl a + b
(µmol m –2 )
Chl a/b
(mol mol –1 )
X+C
(µmol m –2 )
(X + C)/Chl a+ b
(mmol mol –1 )
–3.34 ± 0.05 a
–
–2.56 ± 0.10 b
–3.32 ± 0.18 a
–1.85 ± 0.09 c
1.26 ± 0.10 ab
1.41 ± 0.06 a
1.28 ± 0.15 ab
1.21 ± 0.13 b
1.34 ± 0.17 ab
503.6 ± 30.8 a
672.1 ± 43.3 ab
684.6 ± 38.4 b
566.3 ± 11.8 ab
500.6 ± 72.5 a
160.3 ± 4.6 a
135.3 ± 5.9 b
173.5 ± 9.2 a
258.5 ± 9.4 c
270.9 ± 7.5 c
4.15 ± 0.03 a
3.93 ± 0.04 b
3.55 ± 0.05 c
3.76 ± 0.05 d
4.27 ± 0.05 e
71.3 ± 2.2 a
67.3 ± 2.6 a
80.0 ± 3.8 b
104.0 ± 3.0 c
109.9 ± 2.7 c
445.9 ± 7.2 a
503.6 ± 8.8 b
465.5 ± 10.1 a
407.6 ± 7.5 c
407.3 ± 4.3 c
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Table 2. Carotenoid composition of oak (Quercus pubescens) leaves on five measurement dates. Means ± SE of concentrations of individual carotenoids are expressed in mmol (mol Chl a + b) –1 (n = 21–33). Abbreviations: V + A + Z denotes the pool of xanthophyll-cycle carotenoids, where
V, A and Z stand for violaxanthin, antheraxanthin and zeaxanthin, respectively; and X/C denotes the molar ratio of total xanthophylls to total carotenes. Values within a column followed by different letters differ significantly at P = 0.05.
Date
Neoxanthin
Lutein
V+A+Z
α-carotene
β-carotene
X/C
06/24/03
07/15/03
07/23/03
08/06/03
05/18/04
34.5 ± 0.6 a
32.1 ± 1.2 b
35.8 ± 0.5 a
34.6 ± 0.5 a
32.9 ± 0.6 ab
121.4 ± 1.6 a
141.3 ± 2.8 b
135.0 ± 3.4 c
120.5 ± 1.3 a
120.1 ± 1.1 a
155.1 ± 5.2 a
196.5 ± 5.6 b
181.0 ± 7.4 b
132.7 ± 6.6 c
146.2 ± 3.2 ac
3.4 ± 0.2 a
2.3 ± 0.2 b
3.8 ± 0.2 a
6.7 ± 0.3 c
2.3 ± 0.1 b
131.5 ± 1.0 a
131.3 ± 2.1 a
109.8 ± 1.0 b
113.1 ± 1.2 b
105.8 ± 1.1 c
2.30 ± 0.04 a
2.78 ± 0.06 b
3.10 ± 0.10 c
2.41 ± 0.06 a
2.77 ± 0.04 b
the ratio of total xanthophylls to total carotenes (X/C) showed
only slight variation among dates (Table 2). None of these parameters showed diurnal variations.
Gas exchange parameters
Except when trees were subject to either drought or high temperature (August 2002 and May 2004), Pn increased early in
the day as PPF increased, reaching a maximum (Pn,max ) by
midmorning that was greater in August (15.4 µmol m – 2 s – 1 )
than in May (12.7 µmol m – 2 s –1 ), indicating that leaf aging had
not affected CO2 assimilation capacity. After midmorning, Pn
declined to a transitory plateau then declined further late in the
day as photosynthesis became light-limited (Figure 3). When
trees were subject to moderate drought (June and August
2003), midmorning Pn,max was reduced and fell sharply by
early afternoon, when high temperature and VPD exerted additional constraints. During the severe drought with high temperatures in July 2003, Pn was suppressed throughout the day.
Diurnal changes in Pn and gs correlated closely (Figure 3A).
As with Pn, maximal gs (gs,max ) was highest in August 2002 and
May 2004 and lowest in summer 2003. Compared with August
15, 2002, gs,max was 60% lower on August 6, 2003, whereas
Pn,max was only 30% lower, indicating that drought had increased water-use efficiency.
The diurnal course of E diverged from that of gs and Pn (Figures 3A and 3B), especially in the late morning and much of
the afternoon, when declines in gs were unaccompanied by reductions in VPD.
During periods of drought and high temperature, C i either
remained constant or increased, despite a reduction in gs (Figure 3C), indicating metabolic limitation of Pn . On July 1, 2003,
when the CO2 assimilation rate was negative, Ci remained high
throughout the day.
Chlorophyll a fluorescence parameters
The Fv /Fm ratio was always high at 0800 h (around 0.80) but
declined to around 0.2 by midday; the reduction being most
pronounced during hot dry weather (Figure 3D). The lowest
Fv /Fm ratio was recorded in the afternoon of July 23, 2003. On
each date, recovery in fluorescence ratio was nearly, if not
fully, complete by the end of the day, except on July 15, 2003,
when Fv /Fm was about 15% lower at 1830 than at 0600 h. The
daily time course of φPSII was closely related to incident PPF
(cf. Figures 2A and 3D).
Diurnal changes in φPSII (φPSII = (Fv′/Fm′)qP ) were associated
with changes in both Fv′/Fm′ and qP (Figure 4). However, at a
PPF of 500 µmol m – 2 s – 1, diurnal changes in qP, if any, did not
always correlate with those in φPSII. For instance, the reduction
in φPSII observed around midday was due mostly to a decrease
in Fv′/Fm′ that reflected enhanced thermal dissipation of absorbed excitation energy at the PSII level. At a PPF of
500 µmol m – 2 s – 1, φPSII was lower throughout hot days during
dry weather (summer 2003) than under non-stressful conditions (May 2004). Appreciable φPSII values were measured
even when Pn was completely suppressed.
Xanthophyll-cycle carotenoids
The leaf concentration of Z + A showed large diurnal changes
(Figure 5A). From a low value in the morning, Z + A concentration reached a peak around midday, which was higher during hot dry weather than during milder conditions. In July
2003, the de-epoxidation state (DEPS) of the V + A + Z pool
reached 98% around noon (Figure 5B). High DEPS values
were also observed in June (up to 83%) and August 2003 (up
to 93%), whereas in May 2004, DEPS remained below 63%.
In response to severe drought (July 2003), massive de-epoxidation occurred before 0900 h when solar irradiance was low
or moderate.
Low temperature fluorescence emission spectra
Figure 6 shows the ~77 K chlorophyll fluorescence emission
spectra of oak leaves in the morning (Figure 6A) and afternoon (Figure 6B). Emission spectra are plotted on an equal
area basis to facilitate comparison within and among dates. On
each date, spectra had three major peaks at 685 nm (F685 ),
695 nm (F695 ) and 735 nm (F735 ). In higher plants, the F685 and
F695 peaks are due to the CP-43 and CP-47 components of PSII
RCs, respectively, whereas F735 is due to PSI (Briantais et al.
1986, Dekker et al. 1995). When trees were subject to moderate drought (June 24 and August 6, 2003), low temperature
emission spectra were comparable to those observed during a
period of negligible plant water stress (May 18, 2004). In the
morning, but not the afternoon, leaves of trees subject to severe
drought (July 15 and 23, 2003) had a significantly lower
F735 /F685 ratio, a measure that characterizes the fluorescence
emission of PSI relative to that of PSII (e.g., Schrader et al.
2004), under severe drought (July 2003) (Figure 6C).
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HALDIMANN, GALLÉ AND FELLER
Figure 3. Diurnal variations in (A) net photosynthetic CO2 assimilation rate (Pn, 䊉) and stomatal conductance (gs, 䊊); (B) transpiration rate (E,
䊏); (C) intercellular CO2 concentration (Ci, 䊐); and (D) the maximum quantum yield of photosystem (PS) II primary photochemistry (Fv /Fm, 䉱)
and quantum yield of PSII electron transport (φPSII, 䉭) measured in oak (Quercus pubescens) leaves on measurement dates. Values are means ± SE
of 3–10 replicates.
Heat, drought and proteins of the photosynthetic apparatus
Western blot analyses indicated that the large (RLS) and small
(RSS) subunits of Rubisco, Rubisco activase (RA), phosphoribulokinase (PRK), the major light-harvesting Chl-a/b-binding protein of PSII (LHCII) and the γ-subunit of the chloroplast ATP-synthase (γ-CF1 ) differed little among dates,
whereas amounts of RLS, RSS, RA and PRK were slightly
lower in May 2004 than in other months (Figure 7). The
cpn-60 Rubisco-binding protein (RBP) was more abundant in
July 2003 than on other dates. The antibodies raised against
γ-CF1 cross-reacted with β-CF1 , indicating that the amount of
β-CF1 did not differ among dates (data not shown), thus providing evidence that drought and high temperature did not alter the amount of CF1.
Discussion
Under favorable conditions of temperature and water supply,
Pn of oak leaves varied diurnally (Figure 3A), as observed in
many other species (Epron et al. 1992, Damesin and Rambal
1995, Valentini et al. 1995). The higher value of Pn,max in Au-
gust 2002 than in May 2004 probably reflects a difference in
leaf maturity, as indicated by the increase in Rubisco, PRK and
RA that occurred after May (Figure 7). Moderate heat and
drought (June and August 2003) not only reduced Pn,max at
midmorning, but reduced Pn (Figure 3A) from before noon,
when Tl and VPD were high (Figures 2b and 2C), until sunset.
These results and the inhibition of Pn during the period of severe drought and high temperature in July 2003 agree with earlier findings (Damesin and Rambal 1995).
The changes in Pn within and among dates correlated with
changes in gs (Figure 3A). If drought-dependent inhibition of
Pn resulted primarily from a reduction in gs, reduced Ci values
in stressed leaves would have been expected, but were not observed (Figure 3C). Intercellular CO2 concentration in
stressed leaves is difficult to determine when Pn and gs are low
(Cornic 2000, Lawlor 2002, Lawlor and Cornic 2002) and
may have been overestimated. Alternatively, Ci may have remained high because of drought-induced metabolic limitations (Lawlor 2002, Cornic and Lawlor 2002, Tang et al. 2002)
or a drought-dependent increase in mesophyll resistance to the
diffusion of CO2 (Epron and Dreyer 1993b, Flexas et al.
2004).
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791
Figure 4. Diurnal variations in (A) quantum yield of photosystem (PS) II electron
transport (φPSII), (B) maximum quantum
yield of the PSII primary photochemistry
in the light-adapted state (Fv′/Fm′) and (C)
photochemical quenching (qP) determined
in oak (Quercus pubescens) leaves on
measurement dates. Measurements were
performed in a photosynthetic photon flux
(PPF) corresponding to that naturally incident on the leaves (closed symbols) or in a
fixed PPF of 500 µmol m – 2 s – 1 (open
symbols). Values are means ± SE of three
replicates.
Drought-induced inhibition of Pn is associated with reduced
ATP synthesis (Lawlor 2002, Lawlor and Cornic 2002, Tang et
al. 2002), which may be associated with low chloroplast ATP
synthase activity (Tezara et al. 1999). Not all studies, however,
support this interpretation (Wise et al. 1990, Ortiz-Lopez et al.
1991). Moreover, we observed no change in ATPase content in
Q. pubescens during inhibition of Pn by drought (Figure 7).
Reduced E (Figure 3B), due to a reduced gs , raised Tl several
degrees above Ta (Figure 2B), thus intensifying temperaturedependent suppression of Pn. High-temperature inhibition of
Pn in Q. pubescens is associated with the reversible inactivation of Rubisco (Haldimann and Feller 2004), which was
likely the reason for the increase in Ci on hot days during dry
weather when gs was low (Figures 3A and 3C), although increased mesophyll resistance may also have been a factor
(Bernacchi et al. 2002).
That φPSII remained high (Figures 3D and 4A) when Pn was
inhibited indicates that electrons were flowing to alternative
sinks, among which photorespiration was probably the most
important (Valentini et al. 1995, Ort and Baker 2002). However, even when Pn was zero, carboxylation likely served as a
sink for photosynthetic electron flow in the fixation of CO2
produced by mitochondrial respiration and photorespiration.
The variability in Fv′/Fm′ within and among dates (Figure 4B)
Figure 5. Diurnal variations in (A)
total concentration of zeaxanthin
and antheraxanthin (Z + A) and in
(B) the de-epoxidation state
(DEPS) of the pool of xanthophyll
cycle carotenoids determined in
oak (Quercus pubescens) leaves
between June 2003 and May
2004. Values are means ± SE of
3–5 replicates.
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HALDIMANN, GALLÉ AND FELLER
Figure 7. Western blot analysis showing cpn-60 Rubisco-binding protein (RBP), the large (RLS) and small (RSS) subunits of Rubisco,
Rubisco activase (RA), phosphoribulokinase (PRK), the major lightharvesting chlorophyll a/b binding protein of photosystem II (LHCII)
and the γ-subunit of the chloroplast ATP-synthase (γ-CF1 ) determined
in extracts of oak (Quercus pubescens) leaves. Lanes within the same
blot were loaded with extracts from equal leaf areas.
Figure 6. Low temperature (77 K) chlorophyll fluorescence emission
spectra of oak (Quercus pubescens) leaves collected in the (A) morning between 0600 and 0800 h local solar time (LST) or in the (B) afternoon between 1300 and 1500 h LST. Samples were collected: June
24, 2003 (dashes); July 15, 2003 (dash, dot, dot); July 23, 2003 (dots);
August 6, 2003 (dash dot); and May 18, 2004 (solid). Each curve is
the mean spectrum of three to four leaves. (C) The fluorescence emission of photosystem (PS) I relative to that of PSII (F735 /F685 ratio)
was determined for samples collected in the morning (open bars) and
in the afternoon (filled bars). Values are means + SE of 3–4 replicates.
Values followed by different letters differ significantly at P = 0.05.
reflects changes in the thermal dissipation of excitation energy
at the PSII level. It follows that carboxylation activity, electron
transfer to alternative sinks and thermal energy dissipation allowed leaves to keep a large fraction of their PSII RCs in an
open configuration during periods of drought and high temperature (Figure 4). Thermal energy dissipation leading to
down-regulation of PSII activity was most pronounced during
periods of drought and high temperature and was associated
with diurnal variations in the accumulation of zeaxanthin
(Figure 5), which plays a central role in thermal energy dissi-
pation and in protecting thylakoid membranes against photooxidative damage (Demmig-Adams and Adams 1996, Havaux
and Niyogi 1999, Niyogi 1999, Demmig-Adams and Adams
2000). Zeaxanthin may also have improved the thermal stability of the thylakoid membranes (Havaux 1998). Unlike other
species (Barker et al. 2002), Q. pubescens leaves subject to
high temperature and drought stress do not retain large quantities of zeaxanthin overnight (Figure 5A).
Leaves subject to drought and high temperature had reduced
Chl a + b concentrations (Table 1), an effect unaccompanied
by a reduction in LHCII (Figure 7). Stressed leaves displayed
an increased (X + C)/Chl a + b ratio (Table 1), which correlated with both a higher (V + A + Z)/Chl a + b ratio and a
higher lutein/Chl a + b ratio (Table 2). This reflects enhanced
photoprotection, because both the V + A + Z pool and lutein
(Niyogi 1999, Baroli et al. 2004) protect the photosynthetic
apparatus against photo-oxidative damage.
That Fv /Fm reached values around 0.80 in the morning on all
dates (Figure 3D) shows that PSII was well protected against
photoinactivation or thermal damage, or both, in agreement
with earlier findings (Epron et al. 1992, Damesin and Rambal
1995). The diurnal depression in Fv /Fm probably reflects a
slow relaxation kinetic of the down-regulation of PSII activity.
The finding that PSII was resistant to inactivation by heat accords with previous studies showing that both water stress
(Havaux 1992, Epron 1997, Ladjal et al. 2000) and pre-accli-
TREE PHYSIOLOGY VOLUME 28, 2008
IMPACT OF CLIMATIC CONSTRAINTS ON PHOTOSYNTHETIC TRAITS IN OAK
mation to high temperature (Süss and Yordanov 1986, Haldimann and Feller 2005) occur under natural conditions, significantly improving the thermotolerance of PSII and increasing
the thermal stability of the thylakoid membranes (Ghouil et al.
2003, Haldimann and Feller 2005).
In July 2003, leaves displayed 77 K Chl fluorescence emission spectra in the morning that differed from those measured
on other dates (Figure 6A), indicating that severe drought
changed the organization of the thylakoid membranes, thus altering fluorescence emission of PSI relative to that of PSII
(Figure 6C).
Emission spectra are influenced by fluorescence reabsorption, which is most pronounced at short wavelengths, the extent of reabsorption is dependent, in part, on Chl concentration
(Krause and Weis 1984, Weis 1985). Differences in thylakoid
membrane organization rather than Chl concentration appear
to have been responsible for the changes in the emission spectra observed in July 2003, as spectra of leaves with a reduced
Chl concentration in June 2003 were similar to those of leaves
with a high Chl concentration in August 2003 and May 2004.
Diurnal variations in emission spectra in July 2003 (Figure 6)
may be due to high temperature-induced State-1 to State-2
transitions (Mohanty et al. 2002, Schrader et al. 2004) or to
other high temperature-induced changes that affected energy
distribution between PSI and PSII. In oak, however, these postulated temperature-dependent changes occurred only in severely water-stressed leaves, as they were not seen on the hot
day of August 6, 2003.
That extreme heat and drought did not affect amounts of
PRK and Rubisco (Figure 7) contrasts with studies of other
species where drought reduced Rubisco concentration (Parry
et al. 2002, Tezara et al. 2002). The increased accumulation of
cnp-60 RBP observed in severely stressed leaves (Figure 7)
may have prevented stress-dependent inhibition of Rubisco
accumulation, because cpn-60 RBP is known to be involved in
Rubisco assembly (Spreitzer 1999, Roy and Andrews 2000).
However, drought may have reduced Rubisco activity, which
could in turn limit Pn (Parry et al. 2002, Tezara et al. 2002).
Our finding that drought and heat did not alter the amount of
RA (Figure 7), a protein that plays an essential role in Rubisco
activation (Portis 2003, Salvucci and Crafts-Brandner 2004a,
2004b) and is sensitive to thermal denaturation (Feller et al.
1998, Salvucci et al. 2001), shows that RA of oak leaves is resistant to, or protected against, heat-dependent denaturation.
The finding that the TSP concentration was similar on all dates
(Table 1) indicates that drought and high temperature did not
inhibit protein synthesis, in contrast with their effects in sunflower (Tezara et al. 2002).
In conclusion, we found that drought and high temperatures
strongly suppressed Pn in oak leaves, without reducing leaf
content of key photosynthetic proteins. The inhibition of Pn by
drought was unaccompanied by a reduction in chloroplast
ATP synthase. We found that severe drought changed the organization of thylakoid membranes and that leaves were protected against photo-oxidative damage by the accumulation of
protective carotenoids, a reduction in chlorophyll concentration, down-regulation of PSII and the dissipation of excess ex-
793
citation energy through alternative pathways. Quercus pubescens is able to preserve the functional capacity of the photosynthetic apparatus during periods of drought and high temperature and solar irradiance, thus, ensuring rapid recovery of
Pn upon alleviation of stress. Nevertheless, the exceptionally
hot dry summer of 2003 resulted in premature leaf mortality in
July, the first time summer leaf fall has occurred in the Swiss
Alps in 70 years.
Acknowledgments
We thank Dr. S.J. Crafts-Brandner and Dr. M.E. Salvucci, WCRL,
USDA/ARS, Phoenix, AZ (PRK and RA), Prof. K. DemirevskaKepova, Bulgarian Academy of Sciences, Sofia, Bulgaria (RBP), PD
Dr. G. Groth, Heinrich-Heine-University, Düsseldorf, Germany
(γ-CF1 ) and Prof. S. Gepstein, Technion Israel Institute of Technology, Haifa, Israel (RLS, RSS and LHCII) for the primary antibodies
used in this study. We thank Prof. P. Jahns (Heinrich-Heine-University, Düsseldorf, Germany) and Prof. K. Apel (ETH-Zürich, Switzerland) for providing facilities to perform the pigment analyses and the
77 K fluorescence measurements, respectively. We thank Dr. R. Zweifel for providing local climatic data, Dr. M. Tsimilli-Michael for helpful discussions and L. Zimmermann and S. Bangerter for their assistance. This work was part of the project “THERMOAK” within the
“NCCR Climate,” a cooperative research program supported by the
Swiss National Science Foundation. PH had a Swiss Federal Research Fellowship.
References
Barker, D.H., W.W. Adams, III, B. Demmig-Adams, B.A. Logan,
A.S. Verhoeven and S.D. Smith. 2002. Nocturnally retained zeaxanthin does not remain engaged in a state primed for energy dissipation during the summer in two Yucca species growing in the
Mojave Desert. Plant Cell Environ. 25:95–103.
Baroli, I., B.L. Gutman, H.K. Ledford, J.W. Shin, B.L. Chin, M. Havaux and K.K. Niyogi. 2004. Photo-oxidative stress in a xanthophyll-deficient mutant of Chlamydomonas. J. Biol. Chem. 279:
6337–6344.
Bernacchi, C.J., A.R. Portis, H. Nakano, S. von Caemmerer and
S.P. Long. 2002. Temperature response of mesophyll conductance.
Implications for the determination of Rubisco enzyme kinetics and
for limitations to photosynthesis in vivo. Plant Physiol. 130:
1992–1998.
Berry, J.A. and O. Björkman. 1980. Photosynthetic response and adaptation to temperature in higher plants. Annu. Rev. Plant Physiol.
31:491–543.
Bradford, M. 1976. A rapid and sensitive method for the quantitation
of microgram quantities of protein utilizing the principle of protein-dye binding. Ann. Biochem. 72:248–254.
Briantais, J.-M., C. Vernotte, G.H. Krause and A. Weis. 1986. Chlorophyll a fluorescence of higher plants: chloroplasts and leaves. In
Light Emission by Plants and Bacteria. Eds. Govindjee, J. Amesz
and D.C. Fork. Academic Press, Orlando, San Diego, pp 539–583.
Ciais, P., M. Reichstein, N. Viovy et al. 2005. Europe-wide reduction
in primary productivity caused by the heat and drought in 2003.
Nature 437:529–533.
Cornic, G. 2000. Drought stress inhibits photosynthesis by decreasing
stomatal aperture—not by affecting ATP synthesis. Trends Plant
Sci. 5:187–188.
Damesin, C. and S. Rambal. 1995. Field study of leaf photosynthetic
performance by a Mediterranean deciduous oak tree (Quercus
pubescens) during a severe summer drought. New Phytol. 131:
159–167.
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com
794
HALDIMANN, GALLÉ AND FELLER
Dekker, J.P., A. Hassolt, A. Petterson, H. van Roon, M.-L. Groot and
R. van Grondelle. 1995. On the nature of the F695 and F685 emission
of photosytem II. In Photosynthesis: From Light to Biosphere,
Vol. I. Ed. P. Mathis. Kluwer Academic Publishers, Dordrecht,
pp 63–56.
Demmig-Adams, B. and W.W. Adams, III. 1996. The role of xanthophyll cycle carotenoids in the protection of photosynthesis. Trends
Plant Sci. 1:21–26.
Demmig-Adams, B. and W.W. Adams, III. 2000. Harvesting sunlight
safely. Nature 403:371–374.
Epron, D. 1997. Effects of drought on photosynthesis and on the
thermotolerance of photosystem II in seedlings of cedar (Cedrus
atlantica and C. libani). J. Exp. Bot. 48:1835–1841.
Epron, D. and E. Dreyer. 1993a. Long-term effects of drought on photosynthesis of adult oak trees (Quercus petraea (Matt.) Liebl. and
Quercus robur L.) in a natural stand. New Phytol. 125:381–389.
Epron, D. and E. Dreyer. 1993b. Photosynthesis of oak leaves under
water stress: maintenance of high photochemical efficiency of
photosystem II and occurrence of non-uniform CO2 assimilation.
Tree Physiol. 13:107–117.
Epron, D., E. Dreyer and N. Bréda. 1992. Photosynthesis of oak trees
(Quercus petraea (Matt.) Liebl.) during drought under field conditions: diurnal course of net CO2 assimilation and photochemical efficiency of photosystem II. Plant Cell Environ. 15:809–820.
Epron, D., D. Godard, G. Cornic and B. Genty. 1995. Limitation of
net CO2 assimilation rate by internal resistances to CO2 transfer in
the leaves of two tree species (Fagus sylvatica L. and Castanea
sativa Mill.). Plant Cell Environ. 18:43–51.
Färber, A., A.J. Young, A. Ruban, P. Horton and P. Jahns. 1997. Dynamics of xanthophyll-cycle activity in different antenna subcomplexes in the photosynthetic membranes of higher plants. Plant
Physiol. 115:1609–1618.
Feller, U., S.J. Crafts-Brandner and M.E. Salvucci. 1998. Moderately
high temperatures inhibit ribulose-1,5-bisphosphate carboxylase/
oxygenase (Rubisco) activase-mediated activation of Rubisco.
Plant Physiol. 116:539–546.
Flexas, J. and H. Medrano. 2002. Drought-inhibition of photosynthesis in C3 plants: stomatal and non-stomatal limitations revisited.
Ann. Bot. 89:183–189.
Flexas, J., J. Bota, J.M. Escalona, B. Sampol and H. Medrano. 2002.
Effects of drought on photosynthesis in grapevines under field conditions: an evaluation of stomatal and mesophyll limitations. Funct.
Plant Biol. 29:461–471.
Flexas, J., J. Bota, F. Loroteo, G. Cornic and T.D. Sharkey. 2004. Diffusive and metabolic limitations to photosynthesis under drought
and salinity in C3 plants. Plant Biol. 6:269–279.
Ghouil, H., P. Montpied, D. Epron, M. Ksontini, B. Hanchi and
E. Dreyer. 2003. Thermal optima of photosynthetic functions and
thermostability of photochemistry in cork oak seedlings. Tree
Physiol. 23:1031–1039.
Gilmore, A.M. and H.Y. Yamamoto. 1991. Resolution of lutein and
zeaxanthin using a non-endcapped, lightly carbon-loaded C18 high
performance liquid chromatographic column. J. Chromatogr. 543:
137–145.
Haldimann, P. and U. Feller. 2004. Inhibition of photosynthesis by
high temperature in oak (Quercus pubescens L.) leaves grown under natural conditions closely correlates with a reversible heat-dependent reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase. Plant Cell Environ. 27:1169–1183.
Haldimann, P. and U. Feller. 2005. Growth at moderately elevated
temperature alters the physiological response of the photosynthetic
apparatus to heat stress in pea (Pisum sativum) leaves. Plant Cell
Environ. 28:302–317.
Havaux, M. 1992. Stress tolerance of photosystem II in vivo. Antagonistic effects of water, heat and photoinhibition stresses. Plant
Physiol. 100:424–432.
Havaux, M. 1998. Carotenoids as membrane stabilizers in chloroplasts. Trends Plant Sci. 3:147–151.
Havaux, M. and K.K. Niyogi. 1999. The violaxanthin cycle protects
plants from photooxidative damage by more than one mechanism.
Proc. Nat. Acad. Sci. USA 96:8762–8767.
Krause, G.H. and E. Weis. 1984. Chlorophyll fluorescence as a tool in
plant physiology. II. Interpretations of fluorescence signals.
Photosynth. Res. 5:139–157.
Ladjal, M., D. Epron and M. Ducrey. 2000. Effects of drought preconditioning on thermotolerance of photosystem II and susceptibility
of photosynthesis to heat stress in cedar seedlings. Tree Physiol.
20:1235–1241.
Laemmli, U.K. 1970. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227:680–685.
Lawlor, D.W. 2002. Limitations to photosynthesis in water-stressed
leaves: stomata vs. metabolism and the role of ATP. Ann. Bot.
89:871–885.
Lawlor, D.W. and G. Cornic. 2002. Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in
higher plants. Plant Cell Environ. 25:275–294.
Leakey, A.D.B., M.C. Press and J.D. Scholes. 2003. High-temperature inhibition of photosynthesis is greater under sunflecks than
uniform irradiance in a tropical rain forest tree seedling. Plant Cell
Environ. 26:1681–1690.
Ludlow, M.M. 1987. Light stress at high temperature. In Photoinhibition. Eds. D.J. Kyle, C.B. Osmond and C.J. Arntzen.
Elsevier, Amsterdam, pp 89–109.
Luterbacher, J., D. Dietrich, E. Xoplaki, M. Grosjean and H. Wanner.
2004. Europrean seasonal and annual temperature variability,
trends and extremes since 1500. Science 303:1499–1503.
Maxwell, K. and G.N. Johnson. 2000. Chlorophyll fluorescence—a
practical guide. J. Exp. Bot. 51:659–668.
Medrano, H., J.M. Escalona, J. Botza, J. Gulías and J. Flexas. 2002.
Regulation of photosynthesis of C3 plants in response to progressive drought: stomatal conductance as a reference parameter. Ann.
Bot. 89:895–905.
Mohanty, P., B. Vani and J.S.S. Prakash. 2002. Elevated temperature
treatment induced alteration in the thylakoid membrane organization and energy distribution between the two photosystems in
Pisum sativum. Z. Naturforsch. 57c:836–842.
Niyogi, K.K. 1999. Photoprotection revisited: genetic and molecular
approach. Annu. Rev. Plant Physiol. Plant Mol. Biol. 50:333–359.
Ort, D.R. and N.R. Baker. 2002. A photoprotective role for O2 as an
alternative sink in photosynthesis. Curr. Opin. Plant Biol. 5:
193–198.
Ortiz-Lopez, A., D.R. Ort and J.S. Boyer. 1991. Photophosphorylation in attached leaves of Helianthus annuus at low water potentials. Plant Physiol. 96:1018–1025.
Parry, A.J., P.J. Andralojc, S. Khan, P.J. Leaand and A.J. Keys. 2002.
Rubisco activity: effects of drought stress. Ann. Bot. 89:833–839.
Portis, A.R., Jr. 2003. Rubisco activase—Rubisco’s catalytic chaperone. Photosynth. Res. 75:11–27.
Roy, H. and T.J. Andrews. 2000. Rubisco: assembly and mechanism.
In Photosynthesis: Physiology and Metabolism. Eds. R.C. Leegood, T.D. Sharkey and S. von Caemmerer. Kluwer, Dordrecht,
pp 53–83.
Salvucci, M.E. and S.J. Crafts-Brandner. 2004a. Inhibition of photosynthesis by heat stress: the activation state of Rubisco as a limiting
factor in photosynthesis. Physiol. Plant. 120:66–71.
TREE PHYSIOLOGY VOLUME 28, 2008
IMPACT OF CLIMATIC CONSTRAINTS ON PHOTOSYNTHETIC TRAITS IN OAK
Salvucci, M.E. and S.J. Crafts-Brandner. 2004b. Relationship between the heat tolerance of photosynthesis and the thermal stability
of Rubisco activase in plants from contrasting thermal environments. Plant Physiol. 134:1460–1470.
Salvucci, M.E., K.W. Osteryoung, S.J. Crafts-Brandner and E. Vierling. 2001. Exceptional sensitivity of Rubisco activase to thermal
denaturation in vitro and in vivo. Plant Physiol. 127:1053–1064.
Schär, C., P.L. Vidale, D. Luthi, C. Frei, C. Haberli, M.A. Liniger and
C. Appenzeller. 2004. The role of increasing temperature variability in European summer heat waves. Nature 427:332–336.
Schrader, S.M., R.R. Wise, W.F. Wacholtz, D.R. Ort and T.D. Sharkey. 2004. Thylakoid membrane responses to moderately high leaf
temperature in Pima cotton. Plant Cell Environ. 27:725–735.
Sharkey, T.D. 2005. Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species and thermotolerance provided by isoprene.
Plant Cell Environ. 28:269–277.
Singsaas, E.L. and T.D. Sharkey. 1998. The regulation of isoprene
emission responses to rapid leaf temperature fluctuations. Plant
Cell Environ. 21:1181–1188.
Sperling, U., F. Franck, B. Van Cleve, K. Apel and G.A. Armstrong.
1998. Etioplast differentiation in Arabidopsis: both PORA and
PORB restore the prolamellar body and photoactive protochlorophyllide-F655 to the cop1 photomorphic mutant. Plant Cell 10:
283–296.
Spreitzer, R.J. 1999. Questions about the complexity of chloroplast
ribulose-1,5-bisphosphate carboxylase/oxygenase. Photosynth. Res.
60:29–42.
Stieger, P.A. and U. Feller. 1994. Senescence and protein remobilisation in leaves of maturing wheat plants grown on waterlogged soil.
Plant Soil 166:173–179.
795
Süss, K.-H. and I.T. Yordanov. 1986. Biosynthetic cause of in vivo acquired thermotolerance of photosynthetic light reactions and metabolic responses of chloroplasts to heat stress. Plant Physiol. 81:
192–199.
Tang, A.C., Y. Kawamitsa, M. Kanechi and J.S. Boyer. 2002. Photosynthesis at low water potential in leaf discs lacking epidermis.
Ann. Bot. 89:788–791.
Tezara, W., V.J. Mitchell, S.P. Driscoll and D.W. Lawlor. 1999. Water
stress inhibits plant photosynthesis by decreasing coupling factor
and ATP. Nature 401:914–917.
Tezara, W., V. Mitchell, S.P. Driscoll and D.W. Lawlor. 2002. Effects
of water deficit and its interaction with CO2 supply on the biochemistry and physiology of photosynthesis in sunflower. J. Exp. Bot.
53:1781–1791.
Valentini, R., D. Epron, P. De Angelis, G. Matteucci and E. Dreyer.
1995. In situ estimation of net CO2 assimilation, photosynthetic
electron flow and photorespiration in turkey oak (Q. cerris L.)
leaves: diurnal cycles under different levels of water supply. Plant
Cell Environ. 18:631–640.
Weis, E. 1985. Chlorophyll fluorescence at 77 K in intact leaves: characterization of a technique to eliminate artifacts related to self-absorption. Photosynth. Res. 6:73–86.
Wise, R.R., J.R. Frederick, D.M. Alm, D.M. Kramer, J.D. Hesketh,
A.R. Crofts and D.R. Ort. 1990. Investigation on the limitations to
photosynthesis induced by leaf water deficit in field-grown sunflower (Helianthus annuus L.). Plant Cell Environ. 13:923–931.
Zweifel, R., L. Zimmermann and D.M. Newbery. 2005. Tree water
deficit as a measure for drought stress and its modeling with microclimate. Tree Physiol. 25:147–156.
TREE PHYSIOLOGY ONLINE at http://heronpublishing.com