Download How do all the parts of photosynthesis work together

How do all the parts of photosynthesis work together - control, limitation
and energy loss
Jeremy Harbinson,
University of Wageningen
Thursday, 2 April 2009
What does photosynthesis do?
CHLOROPLAST cyclic
Q
starch
+
b /f
6
PSII
NADPH
linear
Fd
H + NADP
PSI
ATP
H+
H + : ∆pH + ∆ψ = ∆µ H +
BC cycle
thylakoid
Pi
+ ADP
stroma
RuBP
CO2
MESOPHYLL CELL
water flows from
xylem into mesophyll
LEAF
water evaporates
from the cell-wall/
air interface
boundary layer
XYLEM
PHLOEM
Pi
Pi
sucrose
Pi
Cc
sucrose
synthesis
CO2 diffuses from the intercellular air space
to the site of carboxylation in the stroma
sub-stomatal cavity air
space: [CO2] = Ci
Ci
lower
epidermis
water vapour and CO2 exchange between the boundary layer and the intercellular air space via the stomata
water vapour and CO exchange between the boundary layer and the free air
2
Thursday, 2 April 2009
free air with turbulent
mixing: [CO2] = Ca
Basic steps:
1. absorption of light,
2. conversion of
absorbed energy into
chemical energy,
3. conversion of
chemical energy into a
useful form
4. fixation of CO₂
5. processing and
transport
6. replacement of fixed
CO₂
Why does photosynthesis need control?
1. Stress/damage avoidance - the oxygen model.
a. oxygenic photosynthesis is an aerobic process and ³O₂ can be
transformed into more reactive species (Reactive Oxygen Species - ROS)
by reduction, or conversion to ¹O₂.
b. the operation of photosynthesis generates ³chl, and reductants that can
reduce O₂.
c. even for ‘sun plants’ the capacity for photosynthesis is limiting
compared to the absorbed quantum flux: the absorbed energy cannot be
dissipated via metabolism.
d. the environment is dynamic and unpredictable
e. imbalances will result in the build-up of driving forces (³chl and
reductant) that will drive ROS formation.
Thursday, 2 April 2009
Why does photosynthesis need control?
2. Optimisation of the photosynthetic process
a. The photosynthetic electron/proton transport system generates
REDUCTANT and ATP - these are needed in differing amounts by
different processes.
b. Redox poising of the system
Thursday, 2 April 2009
ROS 1. Singlet O₂ (¹O₂) formation.
1. !chl* can convert to "chl:
a. backreactions in RC (reaction centre)
of both PSII and PSI
b. relaxation from !chl* in pigment bed
2. "chl can convert "O# to !O#, the la$er is much more
reactive than the former.
3. RC formation of !O# is associated with
photodamage to PSII
4. Increased lifetime of !chl* - as can occur in PSII as
the Qa pool becomes reduced - will increase the yield
of "chl and thus !O#.
k[ chl ] ! chl;
1
*
3
car + chl ! car + chl; carotenoid valve
3
3
1
O2 + chl ! O2 + chl; singlet oxygen formation
3
Thursday, 2 April 2009
3
1
1
fixation
ROS 2: superoxide formation:
O2 + e! = O2!
Em, 7 = !0.33V
pK a = 4.88
Thursday, 2 April 2009
ROS 2: superoxide formation - reduction can also occur in the stroma.
!
O₂/O₂⁻
modern value for
redox pot. of O₂⁻ : -160mV
Thursday, 2 April 2009
metabolic pathways that use
Fd- as a reductant
Photosystem free-energy in
II dimer
H+
flow of reducing
power: freeenergy out
thioredoxins
O$
malate
NADP reduction
linear e- transport
H+
etc
cytochrome
b6/f dimer
Q-cycle
Fd
free-energy in
cyclic
reduction of
oxaloacetate to malate
by NADP-MDH
ohmic H"
e#ux
oxaloacetate
plastocyanin
H+ flow
trans-thylakoid proton potential
(comprised of trans-membrane
!pH and !") drives ATP
synthesis
Lumen
Photosystem I
Cl- influx, Mg!"
e#ux
ATPase
NADPH for pathways
other than the BC cycle
ion channels
NADPH for BC
cycle
ATP synthesis
Thylakoid
coupled to H"
e#ux
PGO fed into
photorespiratory
pathway
ribulose1,5-bisphosphate + CO$ ! 2 PGA
ribulose1,5-bisphosphate + O$ ! PGA + PGO
ATP synthesis:
free-energy out
ATP for
BC cycle
Fixation
ribulose1,5-bisphosphate + ADP
Regeneration
ribulose-5-phosphate + ATP
Stroma
PGA + ATP ! 1,3-bisphosphogycerate
Reduction
1,3-bisphosphogycerate + NADPH !
glyceraldehyde-3-phosphate + NADP
BC cycle
ATP for pathways other
than the BC cycle
Chloroplast
Pi
Cytosol
Thursday, 2 April 2009
triose
phosphate
l and the thylakoids, while the electron transport capacer unit of chlorophyll declines. If growth irradiance
ences the relationship between photosynthetic capacity
nitrogen content, predicting nitrogen distribution ben leaves in a canopy becomes more complicated. When
light-saturation
photosynthetic capacity and leaf nitrogen content are
essed 6
on the basis of leaf area, considerable variation
e photosynthetic capacity for a given leaf nitrogen conis found between species. low
The variation
differlight-usereflects
efficiency
strategies of nitrogen partitioning, the electron transcapacity per unit of chlorophyll and the specific activi4 carboxylase. Survival in certain environments
f RuBP
ly does not require maximising photosynthetic capacity
a given leaf nitrogen content. Species that flourish in
hade partition relatively more nitrogen into the thylakalthough this is associated with lower photosynthetic
2 unit of nitrogen.
city per
plant Summary.
have different
relationships.
Wheat,
rice species
and Panicum
and growth conditions. While this is true in a general
The PhOtosynthetic
capacity of leaves
is related
sense, the
it conceals
variation that may be of significance for
the nitrogenand
content
primarily1983,
becausenot
the proteins
of show
laxumto(Brown
Wilson
shown)
high-
CO2 fixation, µmol m-2 s-1
The global limitation of photosynthesis - evolution of plants in ecosystems.
light-limitation
words: RuBP carboxylase - Chlorophyll - Thylakoid
high
light-use efficiency
gen - Nitrogen
partitioning
JA270793 mnja27b 2008.data
0
plants that have specialised for particular environments.
the Calvin cycle and thylakoids represent the majority of
I wish to consider this variation by analysis of the partitionleaf nitrogen. To a first approximation, thylakoid nitrogen
ing of nitrogen between various major components in the
is proportional to the chlorophyll content (50 mol thylakleaf.
oid N m o l - ~ Chl). Within species there are strong linear
The variation becomes strikingly apparent when photorelationships
between
nitrogen
and both RuBP carboxylase
plants
with
shortsynthesis and nitrogen content are both expressed per unit
and chlorophyll. With increasing nitrogen per unit leaf area,
of leaf area (Fig. 1). Numerous other species could have
the proportion
of total leaf
nitrogen in the thylakoids relived leaves
(eg
been included, but the feature that should be noted is the
mains the same while the proportion in soluble protein in4O creases.annuals)
[] under lower irradiance
10-fold variation in the CO2 assimilation rate at, for examIn many species, growth
ple, 100 mmol N m 2. It is clear that different types of
greatly increases the partitioning of nitrogen into chloro[]
plant have different relationships. Wheat, rice and Panicum
phyll and the thylakoids, while the
[] electron transport capaclaxum (Brown and Wilson 1983, not shown) show the highity per unit of chlorophyll
irradiance
E
9
[]declines.
9 1 4 9If growth
A
influences the relationship between photosynthetic capacity
-6
A distribution
~6,
and nitrogen content, predicting nitrogen
beE
=L 30 tween leaves in a canopy becomes more complicated. When
both photosynthetic capacity and leaf nitrogen content are
expressed on the basis of leaf area, considerable variation
4O
[]
m
in the photosynthetic capacity for a given leaf nitrogen con[]
[]
tent is found between species. The variation reflects differE
9
[]
9149
A
ent strategies of nitrogen partitioning, the electron trans-6
A ~6,
E
port capacity per unit of chlorophyll and the specific activi=L 30
environments
2o ty of RuBP carboxylase. Survival in certain
plants with long~
A
9
clearly does not require maximising photosynthetic capacity
t.3
m
for a given leaf nitrogen content. Species lived
that flourish
in (eg
leaves
the shade partition relatively more nitrogen into the thylakIp
2o
oids, although this is associated with lowerevergreens)
photosynthetic
~
A
9
t.3
capacity per unit of nitrogen.
10 Key words: RuBP carboxylase - Chlorophyll - Thylakoid
nitrogen - Nitrogen partitioning
Ip
10
0
100
200
300
400
500
Oecologia (1989) 78: 9 19
Plants evolve, and so does
photosynthesis. It is a
resource hungry process
using scarce mineral
nutrients. The maximum
rate of photosynthesis
found in leaves in
determined by
1.long term evolutionary
optimisation of economics
of photosynthesis and
2. the application of that
solution to local conditions
Oecologia
of nutrient supply etc.
9 Springer-Verlag1989
The strength of the relationship between the light-saturated
strength of the relationship between the light-saturated
1O0
200
300
400
photosynthetic rate in air and the nitrogen content of leaves
-2 -1
Total leaf nitrogen ( m m o l rn - 2 )
1O0 When both200
300
400
is widely recognised.
parameters are expressed
osynthetic rate in air and the nitrogen content of leaves
Fig. l. Rate of CO2 assimilation at high irradiance versus leaf nitroon the basis of leaf dry weight, data collected from a wide
Total
leaf
nitrogen
(
m
m
o
l
rn
2
)
gen content, both expressed per unit leaf area. 9 Triticum aestivum
dely recognised. When both parameters are expressed
range of C3 species cluster around a single straight line,
(Evans 1983, 1985) o Oryza (Cook and Evans 1983a, b) ~ Radeclining
toCO2
zero assimilation
at about 0.6 mmol
N g-1irradiance
(Field and versus
Fig.
l.
Rate
of
at
high
nitro-(K/ippers et al. 1988) zxDeath valley annuals
he basis of leaf dry weight, data collected from a wide
phanus leaf
raphanistrum
Photosynthesis
and
nitrogen
relationships
Mooney 1986). Field and Mooney have argued that this
(Mooney aestivum
et al. 1981) [] Illinois annuals (Mooney et al. 1981). 9
gen content,
expressed
per unit
area. of9 Triticum
e of C3 species cluster around a single straight line,
reflects
a both
fundamental
relationship
that isleaf
independent
in leaves
of Ca
plants
Alocasia macrorrhiza (Seemann et al. 1987) 9 Lepechinia calycina
(Evans
1983,
1985)
o
Oryza
(Cook
and
Evans
1983a,
b)Mooney
~ Ra(Field and
1983) ~, Californian evergreen trees and shrubs
J o h n R. E v a n s *
ning to zero at about 0.6 mmol N g-1 (Field and
address
and
address
forBoxoffprint
requests.
Plant
Environ(Field
et al.annuals
1983) and rainforest trees (Langenheim et al. 1984)
CSIRO, Division
of Plant
Industry,
G.P.O.
1600, Canberra,
A.C.T.
2601,
Australia
Oecologia (1989) 78:phanus
9 19 * Present
raphanistrum
(K/ippers
et
al.
1988)
zx
Death
valley
mental Biology Group, Research School of Biological Sciences,
v South African shrubs (Mooney et al. 1983)9 9 Springer-Verla
Prunus ilicifolia
ney 1986). Field and Mooney have argued that this
(Mooney
etP.O.
al. Box
1981)
[] Illinois
annuals
(Mooney (Field
etWhile
al.etthisal.1981).
9
A.N.U.,
475, Canberra
A.C.T. 2601,
Australia
1983)
species and growth conditions.
is true
in a general
Summary. The PhOtosynthetic capacity of leaves is related
irradiance, µmol m s
Oecologi
cts a fundamental relationship that is independent of
Thursday, 2 April 2009
it conceals variation that may be of significance for
the nitrogen content primarily because the proteins of
Alocasiato macrorrhiza
(Seemann et al. sense,
1987)
9 Lepechinia calycina
Electron transport capacity (Jmax) and carboxylation capacity (Vcmax) are o'en
Seasonal change in CO response of photosynthesis
correlated
160
2
0.6
Jmax (µmol m-2 s-1)
120
(a)
Cytchrome f / Rubisco (µmol g-1)
Vcmax (µmol m-2 s-1)
90
60
30
80
0
0
1
2
3
Rubisco (g m-2)
200
0
0
20
40
60
80
100
Vcmax (µmol m-2 s-1)
Fig. 5. The relationship between the in vivo maximum rate of electron
transport driving RuBP regeneration (Jmax) and the in vivo maximum rate
of RuBP carboxylation (Vcmax) in young-August (circles), young-October
(squares), and old-October (triangles) leaves of Polygonum cuspidatum
grown either at ambient CO2 (370 lmol mol!1; open symbols) or at
elevated CO2 (700 lmol mol!1; closed symbols). Regression lines:
y=1.10x+31.36 (R2=0.70) for August leaves; y=1.71x+16.59 (R2=0.95)
for October leaves.
resistance was high, Vcmax might have been overestimated.
As Jmax was less dependent on resistance, the Jmax:Vcmax
ratio would be increased by higher resistance without any
changes
in photosynthetic proteins (von Caemmerer, 2000).
Thursday,
2 April 2009
Jmax (µmol m-2 s-1)
40
(b)
100
50
0
P <0.001
P > 0.1
P > 0.1
0.4
0.3
0.2
0.1
0
YoungAugust
YoungOctober
OldOctober
Fig. 7. Ratio of cytochrome f to Rubisco in young-August,
October, and old-October leaves of Polygonum cuspidatum grow
at ambient CO2 (370 lmol mol!1; open columns) or at elevated C
lmol mol!1; closed columns). Bars represent 61 SE (n=3–
difference was tested with ANOVA.
150
0
0.5
Leafgroup (LG)
CO2
LG X CO2
0.2
0.4
0.6
Cytochrome f (µmol m-2)
0.8
Fig. 6. Relationship between the in vivo maximum rate of RuBP
carboxylation (Vcmax) and Rubisco content (a), and between the in vivo
maximum rate of electron transport driving RuBP regeneration (Jmax) and
cytochrome f content (b) in young-August (circles), young-October
(squares), and old-October (triangles) leaves of Polygonum cuspidatum
grown either at ambient CO2 (370 lmol mol!1; open symbols) or at
elevated CO2 (700 lmol mol!1; closed symbols). Regression lines: (a)
y=25.68x+20.20 (R2=0.83); (b) y=161.2x+44.8 (R2=0.77).
change in the Jmax:Vcmax ratio does not occur across growth
It has been believed that an increase in atmospheri
concentration would be of greater benefit to tropi
warm climate plants with respect to carbon gain
plants in a cool climate (Long, 1991; Sage et al.,
Kirschbaum, 1994, Hikosaka and Hirose, 1998).
prediction is based on the kinetics of Rubisco and o
solubility of CO2 and O2; suppression of photorespi
with elevated CO2 is stronger at higher temperatures (
1991). It was supported by short-term experiments (se
to minutes) (Sage et al., 1995; Bunce, 1998) as well
some long-term elevated CO2 studies (Lewis et al.,
Ziska and Bunce, 1997). However, a number of s
reported that temperature dependence of relative CO2
lation of Agrowth was much weaker than theoretical p
tions (Greer et al., 1995; Teskey, 1997; Wayne et al.,
Bunce, 1998; Lewis et al., 2001). This mig
partly explained by temperature acclimation. Most m
that predicted the effects of climate change on plants d
gross CO2 fixation, µmol m!" s!#
Some plants have evolved very high rates of CO₂
fixation, much higher than crop plants.
The example shown is from a desert annual and
has an estimated Pmax (in 21% O₂) of about 170
µmol m⁻² s⁻¹ - a temperate zone crop plant will
have a Pmax of 20 - 30 µmol m⁻² s⁻¹. We do not
know how these extremely high rates of CO₂ are
achieved, but they will probably depend not only
on the chloroplasts - the organisation of
chloroplasts in the leaf will be important.
60
40
20
0
0
500
1000
1500
2000
irradiance,µmol m!" s!#
fit to Camissonia using J2 PVV n2 with Keq function
keD/f: 2950 Keq: 35 C: 0.057 Est Pmax 168 µmol m!" s!#
Photosynthesis Research 54: 5–23, 1997.
c 1997 Kluwer Academic Publishers. Printed in the Netherlands.
5
Minireview
The ‘high’ concentrations of enzymes within the chloroplast
Gary C. Harris & Martina Königer
Department of Biological Sciences, Wellesley College, Wellesley, MA 02181, USA
Received 4 February 1997; accepted in revised form 13 June 1997
Key words:
Thursday,
2 Aprilchanneling,
2009 enzyme kinetics, ‘high’ concentrations of enzymes, metabolite binding, photosynthetic
What are the limits for Pmax at the chloroplast
level? Not known, but some limits are obvious: the
active site concentration in a Rubisco crystal is 10.4
or 8.7 mM (spinach and tobacco), while the
concentration in spinach chloroplasts is 4mM.
Regulation.
free-energy in
Photosystem
II dimer
Lumen
H"
linear e- transport
H"
cytochrome
b6/f dimer
flow of
reducing
power: freeenergy out
Fd
Fd
free-energy in
ohmic H"
e#ux
cyclic
Q-cycle
plastocyanin
H+ flow
trans-thylakoid proton potential
(comprised of trans-membrane $pH
Photosystem I
Cl- influx, Mg!"
and $%) drives ATP synthesis
e#ux
ATPase
Thylakoid
ion channels
ADP + Pi ! ATP
ATP synthesis:
free-energy out
Thursday, 2 April 2009
Stroma
ATP synthesis
coupled to H"
e#ux
Electron and photosynthesis are tightly coupled: ‘photosynthesis’ in this context
The
value
of
chlorophyll
fluorescence
1:
measurement
of
means metabolism, and for a leaf 90% of that is processing the products of the
efficiency of light driven electron transport (by PSII ΦP S II )
carboxylation and oxygenation of RuBP - algae are more complex.
light saturation
light limitation
electron transport through PSII
Thursday, 2 April 2009
Intrathylakoid pH as a regulatory factor:
free-energy in
Photosystem
II dimer
Lumen
H"
flow of
reducing
power: freeenergy out
linear e- transport
H"
cytochrome
b6/f dimer
Fd
Fd
free-energy in
ohmic H"
e#ux
cyclic
Q-cycle
plastocyanin
H+ flow
trans-thylakoid proton potential
(comprised of trans-membrane $pH
Photosystem I
Cl- influx, Mg!"
and $%) drives ATP synthesis
e#ux
ATPase
Thylakoid
ion channels
ADP + Pi ! ATP
ATP synthesis:
free-energy out
ATP synthesis
coupled to H"
e#ux
Stroma
the limiting reaction of linear electron transport, deprotonation of the bound PQH₂ pH sensitive and NPQ (PSII) is pH sensitive
Thursday, 2 April 2009
The effect of lowered intrathylakoid pH
Opinion
TRENDS in Plant Science
Vol.8 No.1 January 2003
Balancing the central roles of the
thylakoid proton gradient
David M. Kramer, Jeffrey A. Cruz and Atsuko Kanazawa
Institute of Biological Chemistry, Washington State University, Pullman, WA 99163, USA
Thursday, 2 April 2009
The photosynthetic electron transfer chain generates
Low pH is damaging:
pKa O₂⁻ is 4.9
OEC damaged < pH 6
Pc Cu loss < pH 5.5
BUT
e⁻ transport restricts < pH 6.5
violaxanthin de-epoxidase (NPQ)
active below pH 6.5 - max at pH 5.8
pH for ATP synthesis (50% of pmf as
Δψ) < 6.5 - 7.0
if ke does not change with increasing
irradiance, pH does not fall much
below 6.5 - no evidence for control of
electron transport with increasing
irradiance - limitation shared!
27
gradient, or proton motive force ( pmf ), which serves
0.8
Fv’/Fm’
0.6
qP
0.4
ΦPSI
ΦPSII
0.2
0.0
0
100
200
300
400
500
irradiance, µmol m s
-2
-1
40.0
6
4
20.0
2
CO2 fixation rate
rate constant for P700+ reduction
via the cytochrome b6/f complex
0
0
100
200
300
400
0.0
500
rate constant for P700+ reduction
1.0
gross CO2 fixation, µmol m!" s!#
8
from the PQH2 pool
!PSI, !PSII, qP and Fv'/Fm'
What happens to light harvesting and electron transport with increasing irradiance ?
irradiance,µmol m!" s!#
Increasing irradiance: Fv’/Fm’ decreases due to increasing NPQ, qP decreases due to Qa- reduction, ΦPSI decreases
due to P-700 oxidation. The limiting rate constant for electron transport does not change - stromal redox state is
relatively constant and NADPH/NADP is low. Fv’/Fm’ - a measure of the efficiency of charge separation by open
PSII RC. qP - a measure of the probability that a ¹chl* will find an open RC. qP x Fv’/Fm’ = ΦPSII
Thursday, 2 April 2009
more light,
more
light,
fixation
more reduction
more oxidation
Redox state:
more light more NPQ
- together with more Qa
reduction (less qP) associated with lowered ΦPSII
more P-700+,
less ΦPSI
!
!
2
O2 + e = O
Em, 7 = !0.33V
pK a = 4.88
Thursday, 2 April 2009
fluorescence signal, relative units
The consequences of NPQ for ¹chl* lifetime
Fs or Fss
Fmax
150
Fs in absence of NPQ
100
50
0
0
100
200
300
400
-2
500
-1
irradiance, µmol m s
Thursday, 2 April 2009
Fs (Fss) - the steady state
fluorescence yield - remains
relatively constant with increasing
irradiance despite the large
changes in ΦPSII; it may increase or
decrease slightly with irradiance.
A consequence of this is the
decrease in ³chl formation in PSII
and less photodamage to the PSII
RC relative to the non-NPQ state.
1.0
0.8
0.6
Fv’/Fm’
qP
0.4
ΦPSI
ΦPSII
0.2
0.0
0
100
200
300
400
CO2 concentration, µmol/mol
model for control - the pH sensitivity of
the bc1 class of proteins, of which
cytochrome b6/f is a member
Thursday, 2 April 2009
index of PSI electron transport (!PSI x irradiance)
!PSI, !PSII, qP and Fv'/Fm'
What about down-regulation; eg metabolism slows down?
120
80
40
0
JA280793 mainja28 2008
0
10
20
30
40
50
ke: rate constant for electron transport
from the PQH! pool
t½ = 15ms - ke = 46s⁻¹
t½ = 50ms - ke = 13.8s⁻¹
Thursday, 2 April 2009
index of PSI electron transport (!PSI x irradiance)
BUT are the pH changes too large to be safe:
120
hi CO₂
80
lo CO₂
40
ke varies by a factor of 4
0
JA280793 mainja28 2008
0
10
20
30
40
50
ke: rate constant for electron transport
from the PQH! pool
Is there a problem understanding the regulation of NPQ?
Fv'/Fm' - a measure of NPQ
0.8
21% O!, CO! fixed, var irr
Begonia hi O!
0.7
high CO₂
0.6
4
2
120
20.0
CO2 fixation rate
rate constant for P700+ reduction
via the cytochrome b6/f complex
0
0
100
200
80
300
0.0
500
400
irradiance,µmol m!" s!#
0.5
high light, low CO₂
0.4
0.6
0.8
qP (660)
Thursday, 2 April 2009
6
index of PSI electron transport (!PSI x irradiance)
lo O!, var CO!
lo O!, CO! fixed var irr
40.0
1.0
40
0
rate constant for P700+ reduction
from the PQH2 pool
low light
gross CO2 fixation, µmol m!" s!#
8
hi CO₂
lo CO₂
JA280793 mainja28 2008
0
10
20
30
40
50
ke: rate constant for electron transport
from the PQH! pool
la-
J,
ess
fa
la-
ug-
SI
Other samples indicate an apparent increase in 4l with
9)
A
comparison
of
(Fig.
CO2
concentration.
.reasing
01,
Electron
transport
changes
pH
and
redox
states
th ol suggests that for most samples the relationship between
efficiencies ofboth photosystems is linear. The exceptional
contribute
to
control?
the
line
of
are
three
that
do
not
lie
on
the
principal
nples
rrelation between X, and CO2 partial pressure (Fig. 8). The
m.
se
nd
se
of
ity
not
0
0
0
0
0
0
0
J1,
his
2.
gas
of
es-
at
(1R
Irradiance)
Figure 4. Relationship between J, (the product of X, and irradiance)
and the activity of NADP-MDH extracted from pea leaves subjected
to a range of irradiances in air. The square of the correlation coefficient
between J, and NADP-MDH activity is 0.82.
J
..
- do redox state changes
Several enzymes of the stroma (and
the ATPase) are reductively activated
in an incremental fashion (not the
ATPase!)
max NADP-MDH (NADP linked
malate dehydrogenase) activity in
these leaves: 123 µmol h⁻¹ mg⁻¹ chl).
Increased activity of this enzyme
implies increased reduction of
ferredoxin and increased export of
reductant from the chloroplast.
Plant Physiol. (1990) 94, 545-553
0032-0889/90/94/0545/09/$01 .00/0
Received for publication February 9, 1990
Accepted May 31, 1990
Relationship between Photosynthetic Electron Transport
and Stromal Enzyme Activity in Pea Leaves1
Toward an Understanding of the Nature of Photosynthetic Control
Thursday, 2 April 2009
Jeremy Harbinson*2, Bernard Genty, and Christine H. Foyer
Department of Applied Genetics, Johns Innes Institute, Colney Lane, Norwich, NR4 7UH, United Kingdom (J.H.);
Department of Biology, University of Essex, Colchester, C04 3SQ, United Kingdom (B.G.); and Laboratoire du
M6tabolisme, INRA, Route de St-Cyr, 78026 Versailles, France (C.H.F).
The distribution of reductant in the stroma
!
O₂/O₂⁻
modern value for
redox pot. of O₂⁻ : -160mV
Thursday, 2 April 2009
Balancing the formation of ATP and reductant.
export to cytosol,
mitochondria
Stoichiometries:
linear electron transport H + /e- : 3 if Q-cycle is engaged
O2 reduction
cyclic electron transport H + /e- : 2 if Q-cycle is engaged
ATP synthesis H + / ATP: 14/3 or 4?
cyclic path
PSII
cyt. b!/f
CO 2 fixation: 2NADPH (4e- ), 3ATP.
PSI
Fd"
linear electron transport in support of metabolism
coupled to oxygenation/ carboxylation of ribulose-1,5bisphosphate: both ATP and NADPH required.
linear electron transport coupled to purely
reductive processes: adds to !µH+ to but no
ATP consumed
NO2- etc
reduction
NADPH
if H + /e- is 3 and H + / ATP is 4 ATP/NADPH is 3/2 - OK
if H + /e- is 3 and H + / ATP is 4.67 ATP/NADPH is 2.7/2 - unbalanced
carboxylation,
oxygenation of
RuBP;
Agross=Vc-0.5Vo
VJ = (4 + 4! )VC
=
(4
+
4
)V
J
C
VV
=
(2
+
2
!
)V
NADPH
C
VV
!(2
)VC + 2 )V
ATP = (3 + 3.5
=
NADPH
C
VO
where ! = V
V = (3C+ 3.5 )V
!
cyclic electron flux: adds to !µH+
!
!
ATP
where ! =
Thursday, 2 April 2009
VO
VC
C
AND
there is always Vo (φ at 25°C is about
0.35, and when stomata are closed it is
about 2) which means the ATP/NADP
must be higher than 3/2!
HOWEVER there is always some O₂
reduction and there is always some
export to the cytosol, and there is nearly
always some NO₂⁻ reduction, and
cyclic....?
Cyclic and state transitions
relative quantum yield for PSII (!PSII)
1.0
JA2707
No evidence for
adjustable cyclic or
state transitions in the
responses shown here
(Juanulloa mexicana) other plants could be
different (eg pea)
JA2807
0.8
JA2607
0.6
0.4
0.2
0.0
0.0
0.4
0.8
1.2
relative quantum yield for PSI (!PSI)
Thursday, 2 April 2009
absolute quantum yield for CO! fixation
How about yield?
absorbed light
0.08
0.04
0.00
400
500
600
700
wavelength
data from Cucumber (by Sander Hogewoning, a hard-working PhD student....)
Thursday, 2 April 2009
The magic number for the quantum yield for CO₂ fixation is 0.125 - not
achievable (or maybe not? John Raven’s diatoms). The highest yield
measured by us is 0.095 in the red region of the spectrum (absorbed
light basis) in 2% O₂ (ie Vo is zero). How good is this?
losses due to photochemical limits (0.9?): 0.125 x 0.9 = 0.1125
loss of CO₂ yield due to nitrite reduction etc (10%?): 0.1125 x 0.9 = 0.10125
loss due to excess PSII (ΦPSII falls in low light due to Qa reduction) (5%?):
0.10125 x 0.95 = 0.0962
treat these values as an example!
Thursday, 2 April 2009