Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol15.1995:
Tree Physiology 15, 151--158
© 1995 Heron Publishing----Victoria, Canada
Recovery of photosynthesis in 1-year-old needles of unfertilized and
fertilized Norway spruce (Picea abies (L.) Karst.) during spring
M. STRAND1 and T. LUNDMARK2
1
Department of Forest Genetics and Plant Physiology, The Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden
2
Vindeln Experimental Forests, The Swedish University of Agricultural Sciences, S-922 91 Vindeln, Sweden
Received June 8, 1994
Summary Photosynthetic O2 evolution and chlorophyll a
fluorescence were measured in 1-year-old needles of unfertilized and fertilized trees of Norway spruce (Picea abies (L.)
Karst.) during recovery of photosynthesis from winter inhibition in northern Sweden. Measurements were made under
laboratory conditions at 20 °C. In general, the CO2-saturated
rate of O2 evolution was higher in needles of fertilized trees
than in needles of unfertilized trees over a wide range of
incident photon flux densities. Furthermore, the maximum
photochemical efficiency of photosystem (PS) II, as indicated
by the ratio of variable to maximum fluorescence (FV/FM) was
higher in needles of fertilized trees than in needles of unfertilized trees. The largest differences in FV/FM between the two
treatments occurred before the main recovery of photosynthesis from winter inhibition in late May. The rate of O2 evolution
was higher in needles of north-facing branches than in needles
of south-facing branches in the middle of May.
Simultaneous measurements of O2 exchange and chlorophyll fluorescence indicated that differences in the rate of O2
evolution between the two treatments were paralleled by differences in the rate of PS II electron transport determined by
chlorophyll fluorescence. We suggest that, during recovery of
photosynthesis from winter inhibition, the balance between
carbon assimilation and PS II electron transport was maintained largely by adjustments in the nonphotochemical dissipation of excitation energy within PS II.
Keywords: chlorophyll fluorescence, fertilization, fluorescence quenching, winter inhibition.
Introduction
The light-saturated rate of photosynthesis at both normal and
high intercellular concentrations of CO2 is often strongly related to leaf nitrogen content (Field and Mooney 1986, Seemann et al. 1987, Evans 1989). This relationship arises because
the majority of leaf nitrogen is associated with proteins involved in photosynthesis (Evans 1989, Evans and Seemann
1989). Analyses of gas exchange have indicated that the balance between the capacities of Rubisco and electron transport
is maintained in vivo in plants grown at different nitrogen
concentrations (e.g., Makino et al. 1992). Furthermore, simul-
taneous measurements of CO2 assimilation and chlorophyll a
fluorescence in leaves of maize (Zea mays L.) with different
nitrogen contents have shown that the lower rate of CO2 assimilation in nitrogen-deficient leaves at high irradiances was
accompanied by an increased nonphotochemical quenching
(qN) of chlorophyll fluorescence (Khamis et al. 1990). At high
irradiances, qN mainly reflects nonradiative dissipation of excitation energy in photosystem (PS) II (Krause and Weis 1991).
Therefore, an increased thermal dissipation of excitation energy in nitrogen-deficient leaves under saturating light conditions provides a mechanism for adjusting the rate of PS II
photochemistry to that of CO2 assimilation (Weis and Berry
1987, Genty et al. 1989, Foyer et al. 1990). Photoinhibitory
damage to PS II reaction centers can thereby be minimized
(Krause 1988, Baker 1991, Demmig-Adams and Adams
1992). Nevertheless, nitrogen stress has been found to increase
susceptibility to photoinhibition in shade-acclimated plants
(Ferrar and Osmond 1986, Seemann et al. 1987). An increased
sensitivity to excess light has been related to an increased
proportion of closed PS II reaction centers, i.e., decreased
photochemical quenching (qP) (see Krause and Weis 1991).
Because qP (at high irradiances) in nitrogen-deficient leaves of
maize was higher than in leaves well supplied with nitrogen, it
was suggested that nitrogen stress did not increase susceptibility to photoinhibition in this species (Khamis et al. 1990).
Photoinhibition of PS II is also believed to occur in evergreen conifers during winter (Öquist et al. 1987). It has been
suggested that the gradual inhibition of the potential for CO2
assimilation by low and subfreezing temperatures during
autumn and winter predisposes PS II to photoinhibition (Ottander and Öquist 1991). Recovery from winter inhibition of
photosynthesis takes place during spring and early summer
(Troeng and Linder 1982, Leverenz and Öquist 1987, Lundmark et al. 1988). In boreal forests, nitrogen is often regarded
as the mineral nutrient most limiting for growth. Fertilization
of conifers resulting in an increased foliar nitrogen content
often stimulates photosynthetic capacity (see Linder and Rook
1984). However, the influence of mineral nutrient stress on the
photosynthetic performance of conifers during winter and
spring does not seem to have been investigated previously. In
the present study, the photosynthetic properties of 1-year-old
needles of unfertilized and fertilized trees of Norway spruce
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STRAND AND LUNDMARK
(Picea abies (L.) Karst.) were studied during recovery from
winter inhibition. The relationship between PS II activity and
carbon assimilation was examined under nonphotorespiratory
conditions by simultaneously measuring chlorophyll a fluorescence and O2 exchange.
Materials and methods
Experimental site and plant material
This investigation took place at the Flakaliden research area
(N 64°07′, E 19°27′, altitude 310 m above sea level) between
April and August 1992. The area, which is located about 50 km
northwest of Umeå in northern Sweden, was planted with
Norway spruce seedlings of local provenance in 1963 after
clear-felling. The following treatments were applied to 50 m2
plots: control (C), irrigation (I), solid fertilization (F) and
irrigation plus liquid fertilization (IL). In the IL treatment, a
complete nutrient solution was injected into the irrigation
water during the growing season (see Linder and Flower-Ellis
1992). The treatments commenced in 1987 when the mean
height of the trees was 2.6 m. In the present study, only trees
from the C and IL treatments were investigated. Air temperature in the stand was measured at 1.7 m above ground in the
center of a 15-m diameter opening. The temperature probe was
a thermistor with a continuously ventilated radiation shield.
Global radiation above the canopy was measured with a solarimeter (CM-5, Kipp and Zonen, Delft, The Netherlands).
Sensors were read each minute, and averages were recorded
every 10 min by a datalogger (CR-10, Campbell Scientific
Inc., Logan, UT, USA).
Second-order shoots were collected from south-facing
branches of the seventh whorl down from the tops of five trees
in one of the C and one of the IL plots from April 6 to
August 11, 1992. In addition, shoots were collected from
north-facing branches of the seventh whorl on May 11 and
June 1. The same trees were sampled on all occasions. Samples
were stored in darkness at 0 °C until measurements were made.
These were completed within 2 to 3 days. No consistent
change in the measured parameters occurred during that time.
Measurements of oxygen exchange
Measurements of O2 exchange by needles at 20 °C were made
with a leaf-disc oxygen electrode (LD 2, Hansatech Ltd.,
King’s Lynn, U.K.) described by Delieu and Walker (1983).
The cuvette was flushed with humidified 5% CO2 in air before
each measurement and whenever the concentration of CO2
decreased to about 3%. Illumination was provided from an
array of red light-emitting diodes (LH36U together with control box LS3, Hansatech Ltd., King’s Lynn, U.K.). Photon flux
density (PFD) was lowered stepwise to darkness after a pretreatment at 130--160 µmol m −2 s −1 for about 20 min. Subsequently, PFD was raised to near light saturation (780--840
µmol m −2 s −1) for 15--20 min to obtain a steady-state rate of
O2 evolution. Measurements of O2 exchange were also made
simultaneously with chlorophyll fluorescence (see below) using an adapted top window for the fiber-optic cable of the
fluorometer. In this case, white light was provided from a
Schott fiber illuminator (FL 101, H. Walz, Effeltrich, Germany). Each sample with a projected leaf area of about 2.7 cm2
was subjected to a range of PFDs in an ascending order. A
15-min exposure was necessary to obtain steady-state rates of
O2 evolution at the lowest PFD, whereas 10 min was sufficient
at the higher PFDs. Rate of photosynthesis was expressed as
gross rate of O2 evolution, i.e., the sum of O2 evolution in light
and O2 uptake in darkness. Dark respiration in light was estimated after 5--6 min in darkness. The apparent quantum yield
(øO2) during combined measurements of O2 exchange and
chlorophyll fluorescence was obtained by dividing the gross
rate of O2 evolution by the incident PFD. Maximum apparent
quantum yield (ømaxO2) in both red and white light was obtained from the initial slope of the relationship between gross
photosynthesis and incident PFD by linear regression. Four to
five PFD values below 130 µmol m −2 s −1 were used in red light,
whereas three PFD values below 190 µmol m −2 s −1 were used
in white light. Photon flux density over the range of 400 to
700 nm was measured with a quantum sensor (LI-190SB,
Li-Cor Inc., Lincoln, NE, USA). The projected area of needles
was determined with a leaf area meter (Delta-T Devices, Cambridge, U.K.), and needle dry weight was determined after
drying for 24 h at 80 °C.
Measurements of chlorophyll fluorescence
In vivo chlorophyll a fluorescence was measured simultaneously with O2 exchange at 20 °C and 5% CO2 with a modulation fluorometer (PAM 101 Chlorophyll Fluorometer with
accessory units PAM 102 and PAM 103, H. Walz, Effeltrich,
Germany) equipped with a polyfurcated fiber-optic system
(101 F, H. Walz, Effeltrich, Germany) as described by
Schreiber et al. (1986). The nomenclature of van Kooten and
Snel (1990) was adopted. Each sample consisted of needles
from shoots stored in darkness at 0 °C for at least 2 h. One or
two samples from each tree were assessed. Minimal (dark)
fluorescence yield (F0) was obtained on excitation with a weak
measuring beam from a pulsed light-emitting diode. Maximal
fluorescence yield (FM) was determined after exposure to a 1-s
saturating pulse of white light (about 8000 µmol m −2 s −1) from
a modified Schott fiber illuminator KL 1500 (FL 103, H. Walz,
Effeltrich, Germany) to close all reaction centers. Variable
fluorescence (FV) was calculated by subtracting F0 from FM.
White actinic light (FL 101, H. Walz, Effeltrich, Germany) was
then switched on, and 1-s near saturating pulses (about 8000
µmol m −2 s −1) were applied automatically at 100-s intervals for
periodic determination of FM′ during induction. Values of FM′
did not significantly increase when the extrapolation method
suggested by Markgraf and Berry (1990) was used. When
steady-state conditions were reached, actinic light was
switched off to enable determination of F0′. Fluorescence
induction kinetics were recorded on a chart recorder (Seconic
SS-250 F, Tokyo, Japan). Components of fluorescence quenching at steady state, photochemical quenching (qP) and nonphotochemical quenching (qN) were determined according to van
Kooten and Snel (1990). Quenching of F0 (q0) was calculated
as described by Bilger and Schreiber (1986). The quantum
PHOTOSYNTHESIS IN NORWAY SPRUCE NEEDLES
yield of PS II electron transport (øe) was determined according
to Genty et al. (1989).
Statistical analysis
Data were analyzed by analysis of variance (ANOVA) with
five trees from each treatment as replicates. Means from each
treatment were compared using Fisher’s protected LSD test.
Differences were considered significant at P ≤ 0.05. Paired
t-tests were used to test differences between north- and southfacing branches of the trees.
Results
The recovery of photosynthesis in 1-year-old needles of fertilized and unfertilized Norway spruce trees was followed from
April to August 1992. The main recovery from winter inhibition occurred in late May between Days 132 and 146. This
coincided with increasing daily maximum and minimum temperatures (Figure 1A). Values of FV/FM were significantly
higher in needles of fertilized (IL) trees than in needles of
control (C) trees at all sampling dates (Figure 1B), with the
largest differences between the two treatments occurring before the middle of May (Day 132). The highest FV/FM values
were reached in August. For samples collected on Days 220
and 224, FV/FM in IL trees was 0.845 (± 0.005). Differences in
153
FV/FM between IL trees and C trees were due to differences in
FV (data not shown). Minimal fluorescence yield (F0) was not
significantly different between the two treatments on any sampling occasion. Maximum apparent quantum yield of O2 evolution (ømaxO2) was generally higher in needles of IL trees than
in needles of C trees (Figure 1C). As expected (see e.g., Evans
1987), values of ømaxO2 measured in white light were lower
than those obtained in red light. Furthermore, differences in
ømaxO2 between IL trees and C trees generally agreed less well
with corresponding differences in FV/FM when white light
instead of red light was used. Data on FV/FM and ømaxO2
normalized to maximal values of these parameters for IL trees
obtained on Days 220 and 224 indicated that ømaxO2 was
inhibited more than FV/FM during spring and early summer
(Figure 1D). This was especially evident for measurements in
white light. On most occasions, the rate of O2 evolution at near
saturating irradiances was significantly higher in needles from
IL trees than in needles of C trees, irrespective of whether red
or white light was used (Figure 1E). From April to August, the
mean difference in photosynthetic capacity between the two
treatments was 3.3 (± 0.4) µmol m −2 s −1 on a projected needle
area basis and 9.3 (± 0.8) nmol g −1 s −1 on a needle dry weight
basis.
Simultaneous measurements of O2 evolution and chlorophyll fluorescence revealed that an increased potential for
Figure 1. Recovery from winter
inhibition in 1-year-old needles of
unfertilized (open symbols) and
fertilized (filled symbols) Picea
abies. (A) Daily solar radiation
and daily air maximum and minimum temperature at 1.7 m above
ground. Sampling dates are indicated. (B) Ratio of variable to
maximum fluorescence (FV/FM).
(C) Maximum apparent quantum
yield in red (circles) and white
(squares) light. (D) Maximum apparent quantum yield (symbols as
in C) and FV/FM (------ unfertilized, -------- fertilized) normalized
to values of fertilized trees obtained on Days 220 and 224. (E)
Gross rate of oxygen evolution at
near saturating red (circles) and
white (squares) light. The SE bars
are included when larger than the
symbols (n = 5). In C and E, asterisks represent significant differences between treatments at
P < 0.05 (*), P < 0.01 (**) and
P < 0.001 (***).
154
STRAND AND LUNDMARK
photosynthesis during recovery was accompanied by substantial changes in chlorophyll fluorescence parameters. Steadystate values of qP were lower and steady-state values of qN were
higher on May 11 than on June 1 and August 11, below an
incident PFD of approximately 500 µmol m −2 s −1 (Figures 2A
and 2B). A relatively high qN at low PFDs was also observed
for samples collected on June 26 (Figure 2H). Steady-state
values of q0 decreased substantially from May 11 to June 1 and
then remained relatively constant until August 11 (Figure 2C).
Furthermore, steady-state values of FV′/FM′ and øe (= qP ×
Figure 2. Gross rate of oxygen evolution and chlorophyll fluorescence parameters (photochemical quenching
(qP), nonphotochemical quenching (qN),
F0 quenching (q0), ratio of variable to
maximum fluorescence (FV′/FM′), and
quantum yield of PS II electron transport (øe)) in 1-year-old needles of unfertilized (open symbols) and fertilized
(filled symbols) Picea abies as a function of incident photon flux density.
Data for samples collected on May 11
(circles), June 1 (squares) and August
11 (triangles) are shown in A--E,
whereas data from June 26 are shown in
F--K. For clarity, symbols of fertilized
trees are offset by 30 µmol m −2 s −1 in
A--E. The SE bars are included when
larger than the symbols (n = 5). Asterisks represent significant differences between treatments at P < 0.05 (*), and
P < 0.01 (**).
PHOTOSYNTHESIS IN NORWAY SPRUCE NEEDLES
FV′/FM′) were much lower on May 11 than later during the
summer (Figures 2D and 2E).
Values of qP and qN were very similar in IL trees and C trees
over a wide range of PFDs (Figures 2A and 2B), except for
samples collected on June 26 (Figures 2G and 2H). In contrast,
q0 generally appeared to be lower in IL trees than in C trees
during recovery from winter inhibition (Figure 2C), whereas
FV′/FM′ and øe appeared to be higher in IL trees than in C trees
(Figures 2D and 2E). For samples collected on June 26, significant differences in rate of O2 evolution between the two treatments over the range of PFDs tested (Figure 2F) were
paralleled by significant differences in q0, FV′/FM′ and øe (Figures 2I--K).
The light regime to which the needles were exposed during
winter affected the rate of O2 evolution as well as the photochemical efficiency of PS II, at least before the middle of May.
For samples collected on May 11, needles from north-facing
branches of both treatments had higher rates of O2 evolution
than needles from south-facing branches (Figure 3). However,
significantly higher values of ømaxO2 were accompanied by
significantly higher ratios of FV/FM only in the north-facing
branches of IL trees (Table 1). Furthermore, steady-state values of FV′/FM′ were higher in north-facing branches than in
south-facing branches of IL trees (data not shown). For samples collected on June 1, the rate of O2 evolution at high PFDs
was higher in north-facing branches than in south-facing
branches of C trees, but not IL trees (Figure 3). Neither FV/FM
nor ømaxO2 were significantly affected by branch orientation on
June 1 (Table 1).
Mean values of øe had a significant linear correlation to
corresponding mean values of øO2 when needles of C and IL
trees collected between May and August were subjected to a
wide range of incident PFDs (Figure 4). The relationship
between øe and øO2 was not affected by differences in the light
Figure 3. Gross rate of oxygen evolution in 1-year-old needles of
unfertilized (open symbols) and fertilized (filled symbols) Picea abies
as a function of incident photon flux density. Samples were collected
on May 11 (circles) and June 1 (squares) from the south-facing (--------)
and north-facing (------) sides of the trees. The SE is indicated for
n = 4--5.
155
regime to which the needles were exposed in the field. These
results suggest that PS II activity was closely linked to the rate
of O2 evolution under nonphotorespiratory conditions. However, small deviations from the single curvilinear relationship
between øe and øO2 obtained by Seaton and Walker (1990) for
leaves from several species were observed (Figure 4). These
deviations were mainly caused by the tendency of the slope of
the relationship to decrease from June to August, especially in
needles of IL trees (data not shown).
Table 1. Maximum apparent quantum yield (ømaxO2) and the ratio of
variable to maximum fluorescence (FV/FM) in 1-year-old needles of
unfertilized (C) and fertilized (IL) Picea abies collected on May 11
and June 1. The SE is indicated for n = 4--5. Significant differences
between needles of branches oriented toward south and north are
indicated by different letters (paired t-test).
ParameterTreatmentBranch orientation
SouthNorth
May 11
ømaxO2C0.015 ± 0.002 a0.018 ± 0.002 b
IL0.022 ± 0.002 a0.029 ± 0.001 b
FV/FMC0.537 ± 0.014 a0.531 ± 0.016 a
IL0.615 ± 0.020 a0.659 ± 0.013 b
June 1
ømaxO2C0.046 ± 0.002 a0.048 ± 0.002 a
IL0.049 ± 0.002 a0.050 ± 0.001 a
Figure 4. The relationship between the quantum yield of PS II electron
transport (øe) and the apparent quantum yield of O2 evolution (øO2) in
1-year-old needles of unfertilized (s) and fertilized (d) Picea abies.
Measurements were made at incident photon flux densities ranging
from 130 to 980 µmol m −2 s −1. Samples were collected on May 11,
June 1, June 26 and August 11. Each point represents a mean steadystate value from 4--5 trees. The regression equation is y = 11.2x +
0.019 (r2 = 0.948, P = 0.0001). The broken line represents the relationship found in Figure 1B of Seaton and Walker (1990).
156
STRAND AND LUNDMARK
Discussion
Winter inhibition of photosynthesis is well known in conifers
(for a review, see Öquist and Martin 1986). In addition to low
and subfreezing temperatures causing depression of potential
photosynthetic activity during winter, the importance of excess
light in causing photoinhibition of photosynthesis has been
emphasized (Öquist et al. 1987). We observed that winter
inhibition was manifested as a depression of the maximal
photochemical efficiency of PS II (FV/FM) and ømaxO2 (Figures 1B and 1C) as well as of the rate of O2 evolution at high
PFDs (Figure 1E). The main recovery from winter inhibition
occurred in late May, although ømaxO2 tended to increase
throughout the study. Similarly, Kull and Koppel (1992) found
that the apparent quantum yield of CO2 uptake in Norway
spruce increased steadily from spring to autumn. Recovery
coincided with increasing maximum and minimum temperatures during spring (Figure 1A). A strong temperature dependence of recovery has been observed previously (Lundmark et
al. 1988).
The increase in FV/FM in needles from both C and IL trees
during recovery was accompanied by an increase in ømaxO2.
Lundmark et al. (1988) found that the apparent quantum yield
of CO2 assimilation was linearly related to FV/FM during recovery of lodgepole pine (Pinus contorta Dougl.) and Scots
pine (P. sylvestris L.) under laboratory conditions. Furthermore, seasonal changes in the quantum yield of CO2 uptake
and FV/FM were linearly related in Scots pine (Leverenz and
Öquist 1987). In contrast, Ottander and Öquist (1991) concluded that the recovery of FV/FM in Scots pine under both field
and laboratory conditions was much faster than that of ømaxO2.
When we normalized the FV/FM and ømaxO2 data to the maximal values of these parameters obtained in August, we found
that inhibition of ømaxO2, especially in white light, was more
severe than that of FV/FM during spring and early summer
(Figure 1D). This may indicate that ømaxO2 is influenced by
factors other than the primary photochemical reactions of PS
II (Adams et al. 1990).
Before the main recovery in late May, ømaxO2 was higher in
needles from north-facing branches of both C and IL trees than
in needles from the more exposed south-facing branches (Table 1). In addition, the FV/FM ratio was higher in needles from
north-facing branches than in needles from south-facing
branches of IL trees. These results are in general agreement
with the hypothesis that excessive light constitutes an important stress factor during winter (Öquist et al. 1987). However,
there was no difference in ømaxO2 between exposed and shaded
branches of Scots pine during recovery under field conditions
(Ottander and Öquist 1991). This was attributed to subfreezing
temperatures being the primary factor reducing ømaxO2. It was
previously found that freezing of frost-hardened Scots pine
seedlings in darkness at temperatures above those causing
permanent damage to the needles increased steady-state qN,
whereas steady-state qP was only slightly depressed (Strand
and Öquist 1988). We observed a similar response in Norway
spruce on May 11. At PFDs below approximately 500 µmol
m −2 s −1, the inhibition of O2 evolution was accompanied by a
decrease in qP and an increase in qN (Figures 2A and 2B).
In needle samples collected on June 26, qN was elevated at
low PFDs (Figure 2H); furthermore, qP was suppressed in
needles of C trees (Figure 2G), which could promote photoinhibition of PS II (cf. Foyer et al. 1990, Krause and Weis 1991,
Ögren 1991). Three major components of qN have been identified in leaves and isolated protoplasts according to their relaxation kinetics on darkening (see Krause and Weis 1991). The
fastest phase, referred to as energy-dependent quenching (qE),
is assumed to reflect nonradiative dissipation processes within
PS II (Krause et al. 1983). Increased qE quenching may serve
to minimize an accumulation of reduced primary acceptors
(QA) in PS II (Weis and Berry 1987). The slowly relaxing
components of qN are commonly attributed to an altered distribution of excitation energy between PS II and PS I (qT) and to
photoinhibition (qI) (see Krause and Weis 1991). Both the fast
and the slow components of qN can be associated with a
quenching of F0 (Demmig and Winter 1988). Elevated levels
of qN on May 11 and June 26 were associated with an increased
quenching coefficient for both a fast component (qE) and one
(or several) slow component(s), which did not relax in darkness within 5 min (data not shown). It should be noted that the
relaxation kinetics of these components were not resolved in
detail.
The lower rate of photosynthesis in needles of C trees than
in needles of IL trees at high PFDs (Figures 1E, 2F and 3) is
consistent with results obtained in a similar experiment with
Scots pine in central Sweden (Linder and Troeng 1980, Linder
and Rook 1984). Invariably, a lower photosynthetic capacity in
needles of C trees than in needles of IL trees was accompanied
by a lower FV/FM ratio, which seems to support the hypothesis
that nitrogen deficiency increases susceptibility to photoinhibition (Ferrar and Osmond 1986, Seemann et al. 1987).
However, a depression of FV/FM may also be viewed as a
protective mechanism of thermal energy dissipation (Demmig
and Björkman 1987, Krause 1988). The largest differences in
FV/FM between the two treatments were observed before the
middle of May (Figure 1B). Differences in ømaxO2 obtained in
red light were also most pronounced during that period (Figure 1C). The utilization of excitation energy in carbon metabolism was obviously much lower during winter and spring than
during summer, inter alia, as a result of low temperatures
(Figure 1A; cf. Troeng and Linder 1982). Thus, a protective
adjustment or down-regulation of PS II photochemistry may
be essential to reduce photoinhibitory damage to PS II (cf.
Ottander and Öquist 1991). The less pronounced increase in
thermal energy dissipation for IL trees indicates that other
mechanisms for photoprotection (see Demmig-Adams and
Adams 1992) may be involved during winter.
The marked increase in FV/FM during recovery was associated with large increases in FV′/FM′ and øe (Figures 2D and
2E), and a substantial decrease in q0 occurred concomitantly
(Figure 2C). In the model of Genty et al. (1989), FV′/FM′
represents the efficiency of excitation capture by open PS II
reaction centers, which is assumed to depend on nonradiative
processes in the antennae. In the present study, the increase in
FV′/FM′ and the decrease in q0 during recovery was related to
an increase in FV/FM rather than to a decrease in qN. However,
PHOTOSYNTHESIS IN NORWAY SPRUCE NEEDLES
the decreased nonphotochemical quenching associated with an
increase in FV/FM can be very similar to a decrease in qE (often
the major component of qN) in terms of energy dissipation
(Krause 1988, Krause and Weis 1991). Differences in thermal
energy dissipation associated with FV/FM may also largely
explain the differences in FV′/FM′ and q0 between the two
treatments (Figures 2C--D and 2I--J). These were most obvious
for samples collected on June 26. Despite the increased proportion of closed reaction centers observed at low to moderate
PFDs on May 11 (Figure 2A), the increase in øe during recovery (Figure 2E) was predominantly determined by an increase
in FV′/FM′. Differences in øe between the two treatments (Figures 2E and 2K) could also be largely attributed to differences
in FV′/FM′.
In Norway spruce needles collected between May and August, øe was linearly related to øO2 (Figure 4). The relationship
between øe and øO2 over a range in incident PFD is nearly
identical for a wide variety of species grown under favorable
conditions (Seaton and Walker 1990). However, the slope of
the relationship between øe and øO2, especially in IL trees,
tended to decrease from June to August (data not shown). This
may be explained by a decrease in whole-chain electron flow
to O2. In addition, several factors can modify the relationship
between øe and øO2 (see Genty et al. 1989, Harbinson et al.
1990, Öquist and Chow 1992). Nevertheless, it is suggested
that a proportionality between øe and øO2 during recovery was
largely achieved by adjustments in thermal energy dissipation,
as indicated by changes in FV′/FM′.
We conclude that the balance between CO2 assimilation and
PS II electron transport in unfertilized and fertilized trees of
Norway spruce was maintained during recovery from winter
inhibition largely by adjustments in the nonphotochemical
dissipation of excitation energy in PS II. However, measurements were made under nonphotorespiratory and otherwise
favorable conditions in the laboratory. Therefore, further studies on the relationship between CO2 assimilation and PS II
activity should be performed under conditions more similar to
those prevailing in the field.
Acknowledgments
This investigation was supported by the Swedish Council for Forestry
and Agricultural Research.
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© 1995 Heron Publishing----Victoria, Canada
Recovery of photosynthesis in 1-year-old needles of unfertilized and
fertilized Norway spruce (Picea abies (L.) Karst.) during spring
M. STRAND1 and T. LUNDMARK2
1
Department of Forest Genetics and Plant Physiology, The Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden
2
Vindeln Experimental Forests, The Swedish University of Agricultural Sciences, S-922 91 Vindeln, Sweden
Received June 8, 1994
Summary Photosynthetic O2 evolution and chlorophyll a
fluorescence were measured in 1-year-old needles of unfertilized and fertilized trees of Norway spruce (Picea abies (L.)
Karst.) during recovery of photosynthesis from winter inhibition in northern Sweden. Measurements were made under
laboratory conditions at 20 °C. In general, the CO2-saturated
rate of O2 evolution was higher in needles of fertilized trees
than in needles of unfertilized trees over a wide range of
incident photon flux densities. Furthermore, the maximum
photochemical efficiency of photosystem (PS) II, as indicated
by the ratio of variable to maximum fluorescence (FV/FM) was
higher in needles of fertilized trees than in needles of unfertilized trees. The largest differences in FV/FM between the two
treatments occurred before the main recovery of photosynthesis from winter inhibition in late May. The rate of O2 evolution
was higher in needles of north-facing branches than in needles
of south-facing branches in the middle of May.
Simultaneous measurements of O2 exchange and chlorophyll fluorescence indicated that differences in the rate of O2
evolution between the two treatments were paralleled by differences in the rate of PS II electron transport determined by
chlorophyll fluorescence. We suggest that, during recovery of
photosynthesis from winter inhibition, the balance between
carbon assimilation and PS II electron transport was maintained largely by adjustments in the nonphotochemical dissipation of excitation energy within PS II.
Keywords: chlorophyll fluorescence, fertilization, fluorescence quenching, winter inhibition.
Introduction
The light-saturated rate of photosynthesis at both normal and
high intercellular concentrations of CO2 is often strongly related to leaf nitrogen content (Field and Mooney 1986, Seemann et al. 1987, Evans 1989). This relationship arises because
the majority of leaf nitrogen is associated with proteins involved in photosynthesis (Evans 1989, Evans and Seemann
1989). Analyses of gas exchange have indicated that the balance between the capacities of Rubisco and electron transport
is maintained in vivo in plants grown at different nitrogen
concentrations (e.g., Makino et al. 1992). Furthermore, simul-
taneous measurements of CO2 assimilation and chlorophyll a
fluorescence in leaves of maize (Zea mays L.) with different
nitrogen contents have shown that the lower rate of CO2 assimilation in nitrogen-deficient leaves at high irradiances was
accompanied by an increased nonphotochemical quenching
(qN) of chlorophyll fluorescence (Khamis et al. 1990). At high
irradiances, qN mainly reflects nonradiative dissipation of excitation energy in photosystem (PS) II (Krause and Weis 1991).
Therefore, an increased thermal dissipation of excitation energy in nitrogen-deficient leaves under saturating light conditions provides a mechanism for adjusting the rate of PS II
photochemistry to that of CO2 assimilation (Weis and Berry
1987, Genty et al. 1989, Foyer et al. 1990). Photoinhibitory
damage to PS II reaction centers can thereby be minimized
(Krause 1988, Baker 1991, Demmig-Adams and Adams
1992). Nevertheless, nitrogen stress has been found to increase
susceptibility to photoinhibition in shade-acclimated plants
(Ferrar and Osmond 1986, Seemann et al. 1987). An increased
sensitivity to excess light has been related to an increased
proportion of closed PS II reaction centers, i.e., decreased
photochemical quenching (qP) (see Krause and Weis 1991).
Because qP (at high irradiances) in nitrogen-deficient leaves of
maize was higher than in leaves well supplied with nitrogen, it
was suggested that nitrogen stress did not increase susceptibility to photoinhibition in this species (Khamis et al. 1990).
Photoinhibition of PS II is also believed to occur in evergreen conifers during winter (Öquist et al. 1987). It has been
suggested that the gradual inhibition of the potential for CO2
assimilation by low and subfreezing temperatures during
autumn and winter predisposes PS II to photoinhibition (Ottander and Öquist 1991). Recovery from winter inhibition of
photosynthesis takes place during spring and early summer
(Troeng and Linder 1982, Leverenz and Öquist 1987, Lundmark et al. 1988). In boreal forests, nitrogen is often regarded
as the mineral nutrient most limiting for growth. Fertilization
of conifers resulting in an increased foliar nitrogen content
often stimulates photosynthetic capacity (see Linder and Rook
1984). However, the influence of mineral nutrient stress on the
photosynthetic performance of conifers during winter and
spring does not seem to have been investigated previously. In
the present study, the photosynthetic properties of 1-year-old
needles of unfertilized and fertilized trees of Norway spruce
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STRAND AND LUNDMARK
(Picea abies (L.) Karst.) were studied during recovery from
winter inhibition. The relationship between PS II activity and
carbon assimilation was examined under nonphotorespiratory
conditions by simultaneously measuring chlorophyll a fluorescence and O2 exchange.
Materials and methods
Experimental site and plant material
This investigation took place at the Flakaliden research area
(N 64°07′, E 19°27′, altitude 310 m above sea level) between
April and August 1992. The area, which is located about 50 km
northwest of Umeå in northern Sweden, was planted with
Norway spruce seedlings of local provenance in 1963 after
clear-felling. The following treatments were applied to 50 m2
plots: control (C), irrigation (I), solid fertilization (F) and
irrigation plus liquid fertilization (IL). In the IL treatment, a
complete nutrient solution was injected into the irrigation
water during the growing season (see Linder and Flower-Ellis
1992). The treatments commenced in 1987 when the mean
height of the trees was 2.6 m. In the present study, only trees
from the C and IL treatments were investigated. Air temperature in the stand was measured at 1.7 m above ground in the
center of a 15-m diameter opening. The temperature probe was
a thermistor with a continuously ventilated radiation shield.
Global radiation above the canopy was measured with a solarimeter (CM-5, Kipp and Zonen, Delft, The Netherlands).
Sensors were read each minute, and averages were recorded
every 10 min by a datalogger (CR-10, Campbell Scientific
Inc., Logan, UT, USA).
Second-order shoots were collected from south-facing
branches of the seventh whorl down from the tops of five trees
in one of the C and one of the IL plots from April 6 to
August 11, 1992. In addition, shoots were collected from
north-facing branches of the seventh whorl on May 11 and
June 1. The same trees were sampled on all occasions. Samples
were stored in darkness at 0 °C until measurements were made.
These were completed within 2 to 3 days. No consistent
change in the measured parameters occurred during that time.
Measurements of oxygen exchange
Measurements of O2 exchange by needles at 20 °C were made
with a leaf-disc oxygen electrode (LD 2, Hansatech Ltd.,
King’s Lynn, U.K.) described by Delieu and Walker (1983).
The cuvette was flushed with humidified 5% CO2 in air before
each measurement and whenever the concentration of CO2
decreased to about 3%. Illumination was provided from an
array of red light-emitting diodes (LH36U together with control box LS3, Hansatech Ltd., King’s Lynn, U.K.). Photon flux
density (PFD) was lowered stepwise to darkness after a pretreatment at 130--160 µmol m −2 s −1 for about 20 min. Subsequently, PFD was raised to near light saturation (780--840
µmol m −2 s −1) for 15--20 min to obtain a steady-state rate of
O2 evolution. Measurements of O2 exchange were also made
simultaneously with chlorophyll fluorescence (see below) using an adapted top window for the fiber-optic cable of the
fluorometer. In this case, white light was provided from a
Schott fiber illuminator (FL 101, H. Walz, Effeltrich, Germany). Each sample with a projected leaf area of about 2.7 cm2
was subjected to a range of PFDs in an ascending order. A
15-min exposure was necessary to obtain steady-state rates of
O2 evolution at the lowest PFD, whereas 10 min was sufficient
at the higher PFDs. Rate of photosynthesis was expressed as
gross rate of O2 evolution, i.e., the sum of O2 evolution in light
and O2 uptake in darkness. Dark respiration in light was estimated after 5--6 min in darkness. The apparent quantum yield
(øO2) during combined measurements of O2 exchange and
chlorophyll fluorescence was obtained by dividing the gross
rate of O2 evolution by the incident PFD. Maximum apparent
quantum yield (ømaxO2) in both red and white light was obtained from the initial slope of the relationship between gross
photosynthesis and incident PFD by linear regression. Four to
five PFD values below 130 µmol m −2 s −1 were used in red light,
whereas three PFD values below 190 µmol m −2 s −1 were used
in white light. Photon flux density over the range of 400 to
700 nm was measured with a quantum sensor (LI-190SB,
Li-Cor Inc., Lincoln, NE, USA). The projected area of needles
was determined with a leaf area meter (Delta-T Devices, Cambridge, U.K.), and needle dry weight was determined after
drying for 24 h at 80 °C.
Measurements of chlorophyll fluorescence
In vivo chlorophyll a fluorescence was measured simultaneously with O2 exchange at 20 °C and 5% CO2 with a modulation fluorometer (PAM 101 Chlorophyll Fluorometer with
accessory units PAM 102 and PAM 103, H. Walz, Effeltrich,
Germany) equipped with a polyfurcated fiber-optic system
(101 F, H. Walz, Effeltrich, Germany) as described by
Schreiber et al. (1986). The nomenclature of van Kooten and
Snel (1990) was adopted. Each sample consisted of needles
from shoots stored in darkness at 0 °C for at least 2 h. One or
two samples from each tree were assessed. Minimal (dark)
fluorescence yield (F0) was obtained on excitation with a weak
measuring beam from a pulsed light-emitting diode. Maximal
fluorescence yield (FM) was determined after exposure to a 1-s
saturating pulse of white light (about 8000 µmol m −2 s −1) from
a modified Schott fiber illuminator KL 1500 (FL 103, H. Walz,
Effeltrich, Germany) to close all reaction centers. Variable
fluorescence (FV) was calculated by subtracting F0 from FM.
White actinic light (FL 101, H. Walz, Effeltrich, Germany) was
then switched on, and 1-s near saturating pulses (about 8000
µmol m −2 s −1) were applied automatically at 100-s intervals for
periodic determination of FM′ during induction. Values of FM′
did not significantly increase when the extrapolation method
suggested by Markgraf and Berry (1990) was used. When
steady-state conditions were reached, actinic light was
switched off to enable determination of F0′. Fluorescence
induction kinetics were recorded on a chart recorder (Seconic
SS-250 F, Tokyo, Japan). Components of fluorescence quenching at steady state, photochemical quenching (qP) and nonphotochemical quenching (qN) were determined according to van
Kooten and Snel (1990). Quenching of F0 (q0) was calculated
as described by Bilger and Schreiber (1986). The quantum
PHOTOSYNTHESIS IN NORWAY SPRUCE NEEDLES
yield of PS II electron transport (øe) was determined according
to Genty et al. (1989).
Statistical analysis
Data were analyzed by analysis of variance (ANOVA) with
five trees from each treatment as replicates. Means from each
treatment were compared using Fisher’s protected LSD test.
Differences were considered significant at P ≤ 0.05. Paired
t-tests were used to test differences between north- and southfacing branches of the trees.
Results
The recovery of photosynthesis in 1-year-old needles of fertilized and unfertilized Norway spruce trees was followed from
April to August 1992. The main recovery from winter inhibition occurred in late May between Days 132 and 146. This
coincided with increasing daily maximum and minimum temperatures (Figure 1A). Values of FV/FM were significantly
higher in needles of fertilized (IL) trees than in needles of
control (C) trees at all sampling dates (Figure 1B), with the
largest differences between the two treatments occurring before the middle of May (Day 132). The highest FV/FM values
were reached in August. For samples collected on Days 220
and 224, FV/FM in IL trees was 0.845 (± 0.005). Differences in
153
FV/FM between IL trees and C trees were due to differences in
FV (data not shown). Minimal fluorescence yield (F0) was not
significantly different between the two treatments on any sampling occasion. Maximum apparent quantum yield of O2 evolution (ømaxO2) was generally higher in needles of IL trees than
in needles of C trees (Figure 1C). As expected (see e.g., Evans
1987), values of ømaxO2 measured in white light were lower
than those obtained in red light. Furthermore, differences in
ømaxO2 between IL trees and C trees generally agreed less well
with corresponding differences in FV/FM when white light
instead of red light was used. Data on FV/FM and ømaxO2
normalized to maximal values of these parameters for IL trees
obtained on Days 220 and 224 indicated that ømaxO2 was
inhibited more than FV/FM during spring and early summer
(Figure 1D). This was especially evident for measurements in
white light. On most occasions, the rate of O2 evolution at near
saturating irradiances was significantly higher in needles from
IL trees than in needles of C trees, irrespective of whether red
or white light was used (Figure 1E). From April to August, the
mean difference in photosynthetic capacity between the two
treatments was 3.3 (± 0.4) µmol m −2 s −1 on a projected needle
area basis and 9.3 (± 0.8) nmol g −1 s −1 on a needle dry weight
basis.
Simultaneous measurements of O2 evolution and chlorophyll fluorescence revealed that an increased potential for
Figure 1. Recovery from winter
inhibition in 1-year-old needles of
unfertilized (open symbols) and
fertilized (filled symbols) Picea
abies. (A) Daily solar radiation
and daily air maximum and minimum temperature at 1.7 m above
ground. Sampling dates are indicated. (B) Ratio of variable to
maximum fluorescence (FV/FM).
(C) Maximum apparent quantum
yield in red (circles) and white
(squares) light. (D) Maximum apparent quantum yield (symbols as
in C) and FV/FM (------ unfertilized, -------- fertilized) normalized
to values of fertilized trees obtained on Days 220 and 224. (E)
Gross rate of oxygen evolution at
near saturating red (circles) and
white (squares) light. The SE bars
are included when larger than the
symbols (n = 5). In C and E, asterisks represent significant differences between treatments at
P < 0.05 (*), P < 0.01 (**) and
P < 0.001 (***).
154
STRAND AND LUNDMARK
photosynthesis during recovery was accompanied by substantial changes in chlorophyll fluorescence parameters. Steadystate values of qP were lower and steady-state values of qN were
higher on May 11 than on June 1 and August 11, below an
incident PFD of approximately 500 µmol m −2 s −1 (Figures 2A
and 2B). A relatively high qN at low PFDs was also observed
for samples collected on June 26 (Figure 2H). Steady-state
values of q0 decreased substantially from May 11 to June 1 and
then remained relatively constant until August 11 (Figure 2C).
Furthermore, steady-state values of FV′/FM′ and øe (= qP ×
Figure 2. Gross rate of oxygen evolution and chlorophyll fluorescence parameters (photochemical quenching
(qP), nonphotochemical quenching (qN),
F0 quenching (q0), ratio of variable to
maximum fluorescence (FV′/FM′), and
quantum yield of PS II electron transport (øe)) in 1-year-old needles of unfertilized (open symbols) and fertilized
(filled symbols) Picea abies as a function of incident photon flux density.
Data for samples collected on May 11
(circles), June 1 (squares) and August
11 (triangles) are shown in A--E,
whereas data from June 26 are shown in
F--K. For clarity, symbols of fertilized
trees are offset by 30 µmol m −2 s −1 in
A--E. The SE bars are included when
larger than the symbols (n = 5). Asterisks represent significant differences between treatments at P < 0.05 (*), and
P < 0.01 (**).
PHOTOSYNTHESIS IN NORWAY SPRUCE NEEDLES
FV′/FM′) were much lower on May 11 than later during the
summer (Figures 2D and 2E).
Values of qP and qN were very similar in IL trees and C trees
over a wide range of PFDs (Figures 2A and 2B), except for
samples collected on June 26 (Figures 2G and 2H). In contrast,
q0 generally appeared to be lower in IL trees than in C trees
during recovery from winter inhibition (Figure 2C), whereas
FV′/FM′ and øe appeared to be higher in IL trees than in C trees
(Figures 2D and 2E). For samples collected on June 26, significant differences in rate of O2 evolution between the two treatments over the range of PFDs tested (Figure 2F) were
paralleled by significant differences in q0, FV′/FM′ and øe (Figures 2I--K).
The light regime to which the needles were exposed during
winter affected the rate of O2 evolution as well as the photochemical efficiency of PS II, at least before the middle of May.
For samples collected on May 11, needles from north-facing
branches of both treatments had higher rates of O2 evolution
than needles from south-facing branches (Figure 3). However,
significantly higher values of ømaxO2 were accompanied by
significantly higher ratios of FV/FM only in the north-facing
branches of IL trees (Table 1). Furthermore, steady-state values of FV′/FM′ were higher in north-facing branches than in
south-facing branches of IL trees (data not shown). For samples collected on June 1, the rate of O2 evolution at high PFDs
was higher in north-facing branches than in south-facing
branches of C trees, but not IL trees (Figure 3). Neither FV/FM
nor ømaxO2 were significantly affected by branch orientation on
June 1 (Table 1).
Mean values of øe had a significant linear correlation to
corresponding mean values of øO2 when needles of C and IL
trees collected between May and August were subjected to a
wide range of incident PFDs (Figure 4). The relationship
between øe and øO2 was not affected by differences in the light
Figure 3. Gross rate of oxygen evolution in 1-year-old needles of
unfertilized (open symbols) and fertilized (filled symbols) Picea abies
as a function of incident photon flux density. Samples were collected
on May 11 (circles) and June 1 (squares) from the south-facing (--------)
and north-facing (------) sides of the trees. The SE is indicated for
n = 4--5.
155
regime to which the needles were exposed in the field. These
results suggest that PS II activity was closely linked to the rate
of O2 evolution under nonphotorespiratory conditions. However, small deviations from the single curvilinear relationship
between øe and øO2 obtained by Seaton and Walker (1990) for
leaves from several species were observed (Figure 4). These
deviations were mainly caused by the tendency of the slope of
the relationship to decrease from June to August, especially in
needles of IL trees (data not shown).
Table 1. Maximum apparent quantum yield (ømaxO2) and the ratio of
variable to maximum fluorescence (FV/FM) in 1-year-old needles of
unfertilized (C) and fertilized (IL) Picea abies collected on May 11
and June 1. The SE is indicated for n = 4--5. Significant differences
between needles of branches oriented toward south and north are
indicated by different letters (paired t-test).
ParameterTreatmentBranch orientation
SouthNorth
May 11
ømaxO2C0.015 ± 0.002 a0.018 ± 0.002 b
IL0.022 ± 0.002 a0.029 ± 0.001 b
FV/FMC0.537 ± 0.014 a0.531 ± 0.016 a
IL0.615 ± 0.020 a0.659 ± 0.013 b
June 1
ømaxO2C0.046 ± 0.002 a0.048 ± 0.002 a
IL0.049 ± 0.002 a0.050 ± 0.001 a
Figure 4. The relationship between the quantum yield of PS II electron
transport (øe) and the apparent quantum yield of O2 evolution (øO2) in
1-year-old needles of unfertilized (s) and fertilized (d) Picea abies.
Measurements were made at incident photon flux densities ranging
from 130 to 980 µmol m −2 s −1. Samples were collected on May 11,
June 1, June 26 and August 11. Each point represents a mean steadystate value from 4--5 trees. The regression equation is y = 11.2x +
0.019 (r2 = 0.948, P = 0.0001). The broken line represents the relationship found in Figure 1B of Seaton and Walker (1990).
156
STRAND AND LUNDMARK
Discussion
Winter inhibition of photosynthesis is well known in conifers
(for a review, see Öquist and Martin 1986). In addition to low
and subfreezing temperatures causing depression of potential
photosynthetic activity during winter, the importance of excess
light in causing photoinhibition of photosynthesis has been
emphasized (Öquist et al. 1987). We observed that winter
inhibition was manifested as a depression of the maximal
photochemical efficiency of PS II (FV/FM) and ømaxO2 (Figures 1B and 1C) as well as of the rate of O2 evolution at high
PFDs (Figure 1E). The main recovery from winter inhibition
occurred in late May, although ømaxO2 tended to increase
throughout the study. Similarly, Kull and Koppel (1992) found
that the apparent quantum yield of CO2 uptake in Norway
spruce increased steadily from spring to autumn. Recovery
coincided with increasing maximum and minimum temperatures during spring (Figure 1A). A strong temperature dependence of recovery has been observed previously (Lundmark et
al. 1988).
The increase in FV/FM in needles from both C and IL trees
during recovery was accompanied by an increase in ømaxO2.
Lundmark et al. (1988) found that the apparent quantum yield
of CO2 assimilation was linearly related to FV/FM during recovery of lodgepole pine (Pinus contorta Dougl.) and Scots
pine (P. sylvestris L.) under laboratory conditions. Furthermore, seasonal changes in the quantum yield of CO2 uptake
and FV/FM were linearly related in Scots pine (Leverenz and
Öquist 1987). In contrast, Ottander and Öquist (1991) concluded that the recovery of FV/FM in Scots pine under both field
and laboratory conditions was much faster than that of ømaxO2.
When we normalized the FV/FM and ømaxO2 data to the maximal values of these parameters obtained in August, we found
that inhibition of ømaxO2, especially in white light, was more
severe than that of FV/FM during spring and early summer
(Figure 1D). This may indicate that ømaxO2 is influenced by
factors other than the primary photochemical reactions of PS
II (Adams et al. 1990).
Before the main recovery in late May, ømaxO2 was higher in
needles from north-facing branches of both C and IL trees than
in needles from the more exposed south-facing branches (Table 1). In addition, the FV/FM ratio was higher in needles from
north-facing branches than in needles from south-facing
branches of IL trees. These results are in general agreement
with the hypothesis that excessive light constitutes an important stress factor during winter (Öquist et al. 1987). However,
there was no difference in ømaxO2 between exposed and shaded
branches of Scots pine during recovery under field conditions
(Ottander and Öquist 1991). This was attributed to subfreezing
temperatures being the primary factor reducing ømaxO2. It was
previously found that freezing of frost-hardened Scots pine
seedlings in darkness at temperatures above those causing
permanent damage to the needles increased steady-state qN,
whereas steady-state qP was only slightly depressed (Strand
and Öquist 1988). We observed a similar response in Norway
spruce on May 11. At PFDs below approximately 500 µmol
m −2 s −1, the inhibition of O2 evolution was accompanied by a
decrease in qP and an increase in qN (Figures 2A and 2B).
In needle samples collected on June 26, qN was elevated at
low PFDs (Figure 2H); furthermore, qP was suppressed in
needles of C trees (Figure 2G), which could promote photoinhibition of PS II (cf. Foyer et al. 1990, Krause and Weis 1991,
Ögren 1991). Three major components of qN have been identified in leaves and isolated protoplasts according to their relaxation kinetics on darkening (see Krause and Weis 1991). The
fastest phase, referred to as energy-dependent quenching (qE),
is assumed to reflect nonradiative dissipation processes within
PS II (Krause et al. 1983). Increased qE quenching may serve
to minimize an accumulation of reduced primary acceptors
(QA) in PS II (Weis and Berry 1987). The slowly relaxing
components of qN are commonly attributed to an altered distribution of excitation energy between PS II and PS I (qT) and to
photoinhibition (qI) (see Krause and Weis 1991). Both the fast
and the slow components of qN can be associated with a
quenching of F0 (Demmig and Winter 1988). Elevated levels
of qN on May 11 and June 26 were associated with an increased
quenching coefficient for both a fast component (qE) and one
(or several) slow component(s), which did not relax in darkness within 5 min (data not shown). It should be noted that the
relaxation kinetics of these components were not resolved in
detail.
The lower rate of photosynthesis in needles of C trees than
in needles of IL trees at high PFDs (Figures 1E, 2F and 3) is
consistent with results obtained in a similar experiment with
Scots pine in central Sweden (Linder and Troeng 1980, Linder
and Rook 1984). Invariably, a lower photosynthetic capacity in
needles of C trees than in needles of IL trees was accompanied
by a lower FV/FM ratio, which seems to support the hypothesis
that nitrogen deficiency increases susceptibility to photoinhibition (Ferrar and Osmond 1986, Seemann et al. 1987).
However, a depression of FV/FM may also be viewed as a
protective mechanism of thermal energy dissipation (Demmig
and Björkman 1987, Krause 1988). The largest differences in
FV/FM between the two treatments were observed before the
middle of May (Figure 1B). Differences in ømaxO2 obtained in
red light were also most pronounced during that period (Figure 1C). The utilization of excitation energy in carbon metabolism was obviously much lower during winter and spring than
during summer, inter alia, as a result of low temperatures
(Figure 1A; cf. Troeng and Linder 1982). Thus, a protective
adjustment or down-regulation of PS II photochemistry may
be essential to reduce photoinhibitory damage to PS II (cf.
Ottander and Öquist 1991). The less pronounced increase in
thermal energy dissipation for IL trees indicates that other
mechanisms for photoprotection (see Demmig-Adams and
Adams 1992) may be involved during winter.
The marked increase in FV/FM during recovery was associated with large increases in FV′/FM′ and øe (Figures 2D and
2E), and a substantial decrease in q0 occurred concomitantly
(Figure 2C). In the model of Genty et al. (1989), FV′/FM′
represents the efficiency of excitation capture by open PS II
reaction centers, which is assumed to depend on nonradiative
processes in the antennae. In the present study, the increase in
FV′/FM′ and the decrease in q0 during recovery was related to
an increase in FV/FM rather than to a decrease in qN. However,
PHOTOSYNTHESIS IN NORWAY SPRUCE NEEDLES
the decreased nonphotochemical quenching associated with an
increase in FV/FM can be very similar to a decrease in qE (often
the major component of qN) in terms of energy dissipation
(Krause 1988, Krause and Weis 1991). Differences in thermal
energy dissipation associated with FV/FM may also largely
explain the differences in FV′/FM′ and q0 between the two
treatments (Figures 2C--D and 2I--J). These were most obvious
for samples collected on June 26. Despite the increased proportion of closed reaction centers observed at low to moderate
PFDs on May 11 (Figure 2A), the increase in øe during recovery (Figure 2E) was predominantly determined by an increase
in FV′/FM′. Differences in øe between the two treatments (Figures 2E and 2K) could also be largely attributed to differences
in FV′/FM′.
In Norway spruce needles collected between May and August, øe was linearly related to øO2 (Figure 4). The relationship
between øe and øO2 over a range in incident PFD is nearly
identical for a wide variety of species grown under favorable
conditions (Seaton and Walker 1990). However, the slope of
the relationship between øe and øO2, especially in IL trees,
tended to decrease from June to August (data not shown). This
may be explained by a decrease in whole-chain electron flow
to O2. In addition, several factors can modify the relationship
between øe and øO2 (see Genty et al. 1989, Harbinson et al.
1990, Öquist and Chow 1992). Nevertheless, it is suggested
that a proportionality between øe and øO2 during recovery was
largely achieved by adjustments in thermal energy dissipation,
as indicated by changes in FV′/FM′.
We conclude that the balance between CO2 assimilation and
PS II electron transport in unfertilized and fertilized trees of
Norway spruce was maintained during recovery from winter
inhibition largely by adjustments in the nonphotochemical
dissipation of excitation energy in PS II. However, measurements were made under nonphotorespiratory and otherwise
favorable conditions in the laboratory. Therefore, further studies on the relationship between CO2 assimilation and PS II
activity should be performed under conditions more similar to
those prevailing in the field.
Acknowledgments
This investigation was supported by the Swedish Council for Forestry
and Agricultural Research.
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