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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