Regulation of Photosystem Composition in Cyanobacterial Photosynthetic System: Evidence Indicating that Photosystem I Formation is Controlled in Response to the Electron Transport State

The quantitative composition of thylakoid components such asthe photosystem (PS) I/PS II ratio in the cyanobacterial photosyntheticsystem is regulated in response to the light regime. The regulationoccurs as changes in PS I content due to control of either PSI formation or decomposition. In order to determine which ofthese two is controlled in this regulation, experiments wereperformed to determine the light-induced PS I decrease in cellsof Synechocystis PCC 6714 under conditions where protein synthesiswas suppressed, i.e. the incubation without a nitrogen sourcefor cell growth or with chloramphenicol. The results revealedthat light-induced PS I decrease did not occur when synthesisof the thylakoid system was suppressed by incubation withouta nitrogen source or by the addition of chloramphenicol, indicatingthat (1) the thylakoid composition is regulted in the processof thylakoid formation and (2) the regulation is achieved bythe control of PS I formation. (Received November 6, 1987; Accepted March 2, 1988)     

Chromatic Regulation of Photosystem Composition in the Cyanobacterial Photosynthetic System: Kinetic Relationship between Change of Photosystem Composition and Cell Proliferation

Kinetics of the change of photosystem (PS) composition in cyanobacteriainduced by chromatic light were studied in relation to cellproliferation. The study was made for two unicellular strains,Synechococcus NIBB 1059 and Synechocystis (Aphanocapsa) PCC6714. We found that (1) the change to a higher or lower PS I/IIratio was due to acceleration or suppression of apparent PSI formation, and (2) it progressed on a similar time scale tothat of the cell proliferation. The apparent rate constant ofthe change in the PS I/II ratio was proportional to that ofcell proliferation, µ, when this was low, but at highvalues of µ the increase in the rate constant of the changein the PS I/II ratio became smaller, causing a deviation fromthe linear relationship. Results indicate that under autotrophicconditions, the photoregulated composition change occurs asa result of thylakoid development, which accompanies cell proliferation. (Received June 23, 1986; Accepted December 5, 1986)     

Regulation of Electron Transport Composition in Cyanobacterial Photosynthetic System: Stoichiometry among Photosystem I and II Complexes and Their Light-Harvesting Antennae and Cytochrome b6/fComplex

This study was done to confirm our previous observation withthe pattern of changes in electron transport composition inducedby an imbalance of the electron transport state. Contents ofphotosystem (PS) I and II complexes and their antennae and Cytb₆/f complex were determined for systems of cyanobacterium SynechocystisPCC 6714 of different PS I/PS II ratios. The results indicatedthat (1) the observed changes in the PS I/PS II ratio are not-dueto regulation of the activities of the respective PS's but tochanges in their contents, (2) the molar ratio between PS IIand Cyt b₆/f complexes was fairly constant when marked changesoccurred in the PS I content, and (3) the PS II and Cyt b₆/fcontents per cell remained fairly constant while the PS I contentchanged markedly. These findings agree with our previous observationwith autotrophic cells of Anacystis nidulans Tx 20 and supportour argument that in cyanobacterial and red algal electron transportsystems, the content of the terminalcomponent(s), such as PSI complex, is regulated in order to maintain a balance betweenthe electron influx by PS II action to the system and the effluxby PS I action from it. (Received June 3, 1987; Accepted September 20, 1987)     

Chromatic Regulation of Photosystem Composition in the Photosynthetic System of Red and Blue-Green Algae

A chromatic adaptation in the photosynthetic quantum yield forthe light mainly absorbed by chlorophyll a (Chl a light) firstfound by Yocum (1951) was studied with one red and three blue-greenalgal strains. When the cells were grown under a weak Chl alight, the quantum yield in all the strains increased. Comparisonof photosystem (PS) compositions, including phycobilin (PBP)and Chl a antennae, reaction centers I and II, in the cellsgrown under the light mainly absorbed by PBP and Chl a revealedthat changes in quantum yield could be attributed to changesin the ratio of PS I/II; PS I/II becomes larger than 1 underPBP light but decreases to 1 in most cases under Chl a light.The change in the PS I/II ratio is due solely to the changesin the PS I population in the cell; PS II remains constant.These results are similar to the intensity-dependent responsein PS composition. A common hypothesis for both the chromatic and intensity-inducedregulation of PS composition was proposed based on the ideaof balance between the electron flow from H₂O to NADP drivenby PS I and II and the cyclic one driven by PS I. (Received May 16, 1985; Accepted September 4, 1985)     

Steady State of Photosynthesis in Cyanobacterial Photosynthetic Systems Before and After Regulation of Electron Transport Composition: Overall Rate of Photosynthesis and PSI/PS II Composition

The effect of regulation of photosystem (PS) composition onthe photosynthetic steady state was examined using the cyanobacteriumSynechocystis PCC 6714. Photosynthetic rates under orange lightabsorbed by phycobiliprotein (PBP) (PBP light) and under redlight absorbed mainly by chlorophyll a (Chi a light) were comparedfor the cells before and after adaptation to the respectivelight regimes. Results were as follows: (1) Photosynthetic ratesper absorbed light quantum became higher after adaptation thanthose before adaptation. (2) Under Chi a light, the low turnoverrate of PS I before adaptation was markedly enhanced after adaptation(decrease in PS I content), but in the case of adaptation toPBP light (increase in PS I content), a marked enhancement ofPS II turnover occurred after adaptation. (3) In the formercase, a low turnover rate of PS I before adaptation was dueto the occurrence of a large number of closed PS I complexes,but in the latter, limited excitation of PS I caused a largenumber of closed PS II complexes before adaptation. Resultsfor the latter case indicate that the energy transfer from phycobilisome(PBS) to one PS I complex is far smaller than that from PBSto one PS II complex, and that the imbalance of energy distributionfrom PBS to the two photosystems is compensated for by the increasein the number of PS I complexes. (Received September 10, 1987; Accepted December 9, 1987)     

Controlling Electron Transfer between the Two Cofactor Chains of Photosystem I by the Redox State of One of Their Components

Two functional electron transfer (ET) chains, related by a pseudo-C₂ symmetry, are present in the reaction center of photosystem I (PSI). Due to slight differences in the environment around the cofactors of the two branches, there are differences in both the kinetics of ET and the proportion of ET that occurs on the two branches. The strongest evidence that this is indeed the case relied on the observation that the oxidation rates of the reduced phylloquinone (PhQ) cofactor differ by an order of magnitude. Site-directed mutagenesis of residues involved in the respective PhQ-binding sites resulted in a specific alteration of the rates of semiquinone oxidation. Here, we show that the PsaA-F689N mutation results in an ∼100-fold decrease in the observed rate of PhQA− oxidation. This is the largest change of PhQA− oxidation kinetics observed so far for a single-point mutation, resulting in a lifetime that exceeds that of the terminal electron donor, P700+. This situation allows a second photochemical charge separation event to be initiated before PhQA− has decayed, thereby mimicking in PSI a situation that occurs in type II reaction centers. The results indicate that the presence of PhQA− does not impact the overall quantum yield and leads to an almost complete redistribution of the fractional utilization of the two functional ET chains, in favor of the one that does not bear the charged species. The evolutionary implications of these results are also briefly discussed.     

Regulation of Photosystem Stoichiometry in the Photosynthetic System of the Cyanophyte Synechocystis PCC 6714 in Response to Light-Intensity

Light-induced changes in stoichiometry among three thylakoidcomponents, PS I, PS II and Cyt b₆-f complexes, were studiedwith the cyanophyte Synechocystis PCC 6714. Special attentionwas paid to two aspects of the stoichiometric change; first,a comparison of the patterns of regulation in response to differencesin light-intensity with those induced by differences in light-quality,and second, the relationship between regulation of the stoichiometryand the steady state of the electron transport system. Resultsfor the former indicated that (1) the abundance of PS I on aper cell basis was reduced under white light at the intensityas high as that for light-saturation of photosynthesis, butPS I per cell was increased under low light-intensity, (2) PSII and Cyt b₆-f complexes remained fairly constant, and (3)changes in the abundance of PS I depended strictly on proteinsynthesis. The pattern was identical with that of chromaticregulation. For the second problem, the redox steady-statesof Cyt f and P700 under white light of various intensities weredetermined by flash-spectroscopy. Results indicated that (1)Cyt f and P700 in cells grown under low light-intensity [highratio of PS I to PS II (PS I/PS II)] were markedly oxidizedwhen the cells were exposed to high light-intensity, while theyremained in the reduced state under low light-intensity. (2)After a decrease in the abundance of PS I, most of P700 remainedin the reduced state even under high light-intensity, whilethe level of reduced Cyt f remained low. (3) Both Cyt f andP700 in cells of low PS I/PS II were fully reduced under lowlight-intensity, and Cyt f reduction following the flash wasrapid, which indicates that the turnover of PS I limits theoverall rate of electron flow. After an increase in the abundanceof PS I, the electron transport recovered from the biased state.(4) The redox steady-state of the Cyt b₆-f complex correlatedwell with the regulation of PS I/PS II while the state of thePQ pool did not. Based on these results, a working model ofthe regulation of assembly of the PS I complex, in which theredox steady-state of the Cyt b₆-f complex is closely relatedto the primary signal, is proposed. (Received August 2, 1990; Accepted December 10, 1990)     

Relationships between the Efficiencies of Photosystems I and II and Stromal Redox State in CO(2)-Free Air : Evidence for Cyclic Electron Flow in Vivo

The responses of the efficiencies of photosystems I and II, stromal redox state (as indicated by NADP-malate dehydrogenase activation state), and activation of the Benson-Calvin cycle enzymes ribulose 1,5-bisphosphate carboxylase and fructose 1,6-bisphosphatase to varying irradiance were measured in pea (Pisum sativum L.) leaves operating close to the CO₂ compensation point. A comparison of the relationships among these parameters obtained from leaves in air was made with those obtained when the leaves were maintained in air from which the CO₂ had been removed. P700 was more oxidized at any measured irradiance in CO₂-free air than in air. The relationship between the quantum efficiencies of the photosystems in CO₂-free air was distinctly curvilinear in contrast to the predominantly linear relationship obtained with leaves in air. This nonlinearity may be consistent with the operation of cyclic electron flow around photosystem I because the quantum efficiency of photosystem II was much more restricted than the quantum efficiency of photosystem I. In CO₂-free air, measured NADP-malate dehydrogenase activities varied considerably at low irradiances. However, at high irradiance the activity of the enzyme was low, implying that the stroma was oxidized. In contrast, fructose-1,6-bisphosphatase activities tended to increase with increasing electron flux through the photosystems. Ribulose-1,5-bisphosphate carboxylase activity remained relatively constant with respect to irradiance in CO₂-free air, with an activation state 50% of maximum. We conclude that, at the CO₂ compensation point and high irradiance, low redox states are favored and that cyclic electron flow may be substantial. These two features may be the requirements necessary to trigger and maintain the dissipative processes in the thylakoid membrane.     

Regulation of the Stoichiometry of Thylakoid Components in the Photosynthetic System of Cyanophytes: Model Experiments Showing that Control of the Synthesis or Supply of Chl a can Change the Stoichiometric Relationship between the Two Photosystems

Regulation of the assembly of the photosystem I (PS I) complexin response to the light regime in the photosynthetic systemof cyanophytes was studied in Synechocystis PCC 6714. The relationshipbetween the assembly of the PS I complex and synthesis of Chla was examined by model experiments in which synthesis of Chla was controlled by two inhibitors, gabaculine (GAB) and 2,2'-dipyridyl(DP). Both inhibitors caused a change to a lower ratio of PSI to PS II even under light that normally induces a high ratioof PS I to PS II. The change in stoichiometry induced by theseinhibitors was suppressed when protein synthesis was inhibitedby chloram-phenicol, similarly to the change in the stoichiometryinduced by light that excites mainly PS I (PS I light). Comparisonof the levels of PS I, PS II and Cyt b₆-f complexes per cellindicated that a selective suppression of the assembly of thePS I complex was induced by the inhibitors: the stoichiometricrelationship among PS I, PS II and Cyt b₆-f complexes was identicalto that induced by PS I light or white light of high intensity.GAB induced a decrease in size of the phycobilisome also, whileDP did not, similarly to PS I light. The results indicate thatthe ratio of PS I to PS II can be changed by the control ofsynthesis of Chl a. They also suggest that control of the synthesisor supply of Chl a probably exerted at site(s) in or after theprocess of the Mg-protoporphyrin branch, is involved in themechanism of regulation of the assembly of the PS I complexin cyanophytes. (Received September 7, 1989; Accepted November 20, 1989)     

Regulation of the Photosynthesis Rhythm in Euglena gracilis: II. Involvement of Electron Flow through Both Photosystems

Rhythmic changes in the light reactions of Euglena gracilis have been found which help to explain the basic reactions effected in the circadian rhythm of O₂ evolution. Diurnal changes in the slope of light intensity plots indicated that the maximal rate of photosynthesis changed throughout the circadian cycle. No evidence was obtained consistent with the premise that changes in chlorophyll content, as measured by total chlorophyll or chlorophyll a/b ratio, or photosynthetic unit size are responsible for this rhythim.     

Regulation of Stoichiometry between PSI and PSII in Response to Light Regime for Photosynthesis Observed with Synechocystis PCC 6714: Relationship between Redox State of Cyt b6-f Complex and Regulation of PSI Formation

The effect of the Cyt b₆-f redox state on the PSI formationwas examined with the cyanophyte Synechocystis PCC 6714 by usinga Q-cycle inhibitor, HQNO (2-n-heptyl-4-hydroxyquinoline N-oxide).HQNO inhibited the rapid reduction of flash-oxidized Cyt f,the reaction correlating with the stimulation of PSI formation,on one hand, and accumulated reduced Cyt b₆, on the other, indicatingthat the electron flow in the Q-cycle correlates with regulationof PSI synthesis. HQNO also inhibited the stimulation of PSIformation under PSII light, resulting in a low PSI/PSII ratioeven under PSII light, while the PSI formation under PSI lightwas not suppressed by HQNO. Simultaneous inhibition of Cyt b₆oxidation through the Q-cycle and the stimulated PSI formationby HQNO suggests that an HQNO-sensitive Cyt b₆ oxidation isinvolved in the mechanism of monitoring the state of electrontransport system for regulation of PSI formation. (Received March 3, 1993; Accepted August 9, 1993)     

Interaction of Anions and Cations in Regulating Energy Distribution between the Two Photosystems

Cations such as Mg²⁺ regulate spillover of absorbed excitation energy mainly in favour of photosystem (PS) 2. Effect of low concentration (<10 mM) of the monovalent cation Na⁺ on chlorophyll (Chl) a fluorescence was completely overridden by divalent cation Mg²⁺ (5 mM). Based on Chl a fluorescence yield and 77 K emission measurements, we revealed the role and effectiveness of anions (Cl^-, SO₄ ^2-, PO₄ ^3-) in lowering the Mg²⁺-induced PS2 fluorescence. The higher the valency of the anion, the lesser was the expression of Mg²⁺ effect. Anions may thus overcome Mg²⁺ effects up to certain extent in a valency dependent manner, thereby diverting more energy to PS1 even in the presence of MgCl₂. They may do so by reversing Mg²⁺-induced changes.     

Responses of Photosynthetic Electron Transport to Drought and Re-watering in Two Maize Genotypes

Russian Journal of Plant Physiology - Effects of drought stress on photosynthesis have been well-documented. However, photosynthetic electron transport process in response to combined drought...     

Relationship between Chloroplast Membrane Fatty Acid Composition and Photosynthetic Response to a Chilling Temperature in Four Alfalfa Cultivars

The photosynthetic responses of four alfalfa (Medicago sativa L.) cultivars to 10 and 22 C air temperatures were examined and the relationship between the photosynthetic response at 10 C and the fatty acid composition of the chloroplast membranes was determined. Chilling-resistant cultivars exhibited moderate reductions in photosynthesis at 10 C, compared to 22 C, and contained a significantly greater percentage of polyunsaturated fatty acids in the chloroplast membrane and a greater double bond index than the chilling-sensitive cultivars. The chilling-sensitive cultivars exhibited severe reductions in photosynthesis at 10 C, compared to 22 C. The reduction in photosynthesis at 10 C is shown to be negatively correlated (r = −0.94) with the double bond index of the chloroplast membranes of the cultivars observed.

The results support the hypothesis that reduced photosynthesis due to chilling temperatures is influenced by the unsaturated fatty acid composition of the chloroplast membrane which affect temperature-induced phase changes in chloroplast membrane lipids.

     

Chromatic Regulation of Cytochrome Composition and Electron Transfer from Cytochrome b6-f Complex to Photosystem I in Anacystis nidulans (Tx 20)

Cytochrome composition of the cyanobacterial photosyntheticsystem was studied with Anacystis nidulans (Tx 20) in relationto the chromatic regulation of photosystem composition. Comparisonof cytochrome compositions in cells with a high PS I/II ratio(3.0, grown under weak orange light) and with a low ratio (1.6,grown under weak red light) indicated that cytochrome compositionwas also changed in the chromatic regulation of photosystemcomposition. Two types of cytochrome change were observed: 1)contents of cytochromes C₅₅₃ and c₅₄₈ were changed in parallelwith the changes in PS I content, and 2) cytochrome b₅₅₃ andcytochrome b₆-f complex were held at a constant molar ratioto PS II. The molar ratio, PS II : cytochrome b₅₅₉ : cytochromeb₆-f complex : cytochrome c₅₅₃ : PS I : cytochrome C₅₄₈, inthe red-grown cells was 1 : 2.5 : 1.3 : 0.17 : 1.6 : 0.67, andthe ratio in the orange-grown cells, 1:2.4:0.9:0.32:3.0:1.2.In both types of cells, almost all cytochrome f in the cytochromeb₆-f complex was rapidly oxidized after multiple flash activation,indicating that all cytochrome b₆-f complexes in cells of bothtypes are functionally connected to PS I, even when the molarratio to PS I is largely changed. The content of cytochromeC₅₅₃ was at most 0.14 of PS I, suggesting that the cytochrometurns over several times per one turnover of PS I. ¹Present address: Department of Biology, Faculty of Science,Tokyo Metropolitan University, Fukazawa 2-1-1, Setagaya, Tokyo158, Japan. (Received January 20, 1986; Accepted March 17, 1986)     

Size of the Plastoquinone Pool Functioning in Photosynthetic and Respiratory Electron Transport of Synechococcus sp.

The size of the plastoquinone pool on the reducing side of photosystem2 in the cyanobacterium Synechococcus sp. was estimated by measuringthe area over the fluorescence induction curve in the presenceof 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone relativeto the area in the presence of 3-(3,4-dichlorophenyl)-l,l-dimethylurea.Plastoquinone was found mostly in the reduced state in freshlyharvested cells but was oxidized by aeration of the cells inthe dark. The pool in the starved cells usually consisted offive or six plastoquinone molecules with a maximum of eightper photosystem 2 reaction center. Addition of glucose or fructoseto the starved cells completely reduced the plastoquinone poolunder anaerobic conditions or in the presence of KCN. The quinonereduced by brief illumination was rapidly and completely oxidizedin the dark. The dark oxidation proceeded at a rate comparableto that of respiratory O₂ uptake in the cyanobacterium and wasstrongly inhibited by KCN. It is concluded that a major populationof the plastoquinone molecules present in the cells functionsas the acceptor pool of photosystem 2 and that the pool is entirelyshared by respiratory electron transport in the cyanobacterium. (Received June 22, 1983; Accepted August 20, 1983)     

The Response of Cyclic Electron Flow around Photosystem I to Changes in Photorespiration and Nitrate Assimilation

Photosynthesis captures light energy to produce ATP and NADPH. These molecules are consumed in the conversion of CO₂ to sugar, photorespiration, and NO₃⁻ assimilation. The production and consumption of ATP and NADPH must be balanced to prevent photoinhibition or photodamage. This balancing may occur via cyclic electron flow around photosystem I (CEF), which increases ATP/NADPH production during photosynthetic electron transport; however, it is not clear under what conditions CEF changes with ATP/NADPH demand. Measurements of chlorophyll fluorescence and dark interval relaxation kinetics were used to determine the contribution of CEF in balancing ATP/NADPH in hydroponically grown Arabidopsis (Arabidopsis thaliana) supplied different forms of nitrogen (nitrate versus ammonium) under changes in atmospheric CO₂ and oxygen. Measurements of CEF were made under low and high light and compared with ATP/NADPH demand estimated from CO₂ gas exchange. Under low light, contributions of CEF did not shift despite an up to 17% change in modeled ATP/NADPH demand. Under high light, CEF increased under photorespiratory conditions (high oxygen and low CO₂), consistent with a primary role in energy balancing. However, nitrogen form had little impact on rates of CEF under high or low light. We conclude that, according to modeled ATP/NADPH demand, CEF responded to energy demand under high light but not low light. These findings suggest that other mechanisms, such as the malate valve and the Mehler reaction, were able to maintain energy balance when electron flow was low but that CEF was required under higher flow.Photosynthesis must balance both the amount of energy harvested by the light reactions and how it is stored to match metabolic demands. Light energy is harvested by the photosynthetic antenna complexes and stored by the electron and proton transfer complexes as ATP and NADPH. It is used primarily to meet the energy demands for assimilating carbon (from CO₂) and nitrogen (from NO₃⁻ and NH₄⁺; Keeling et al., 1976; Edwards and Walker, 1983; Miller et al., 2007). These processes require different ratios of ATP and NADPH, requiring a finely balanced output of energy in these forms. For example, if ATP were to be consumed at a greater rate than NADPH, electron transport would rapidly become limiting by the lack of NADP⁺, decreasing rates of proton translocation and ATP regeneration. Alternatively, if NADPH were consumed faster than ATP, proton translocation through ATP synthase would be reduced due to limiting ADP and the difference in pH between lumen and stroma would increase, restricting plastoquinol oxidation at the cytochrome b₆f complex and initiating nonphotochemical quenching (Kanazawa and Kramer, 2002). The stoichiometric balancing of ATP and NADPH must occur rapidly, because pool sizes of ATP and NADPH are relatively small and fluxes through primary metabolism are large (Noctor and Foyer, 2000; Avenson et al., 2005; Cruz et al., 2005; Amthor, 2010).The balancing of ATP and NADPH supply is further complicated by the rigid nature of linear electron flow (LEF). In LEF, electrons are transferred from water to NADP⁺, oxidizing water to oxygen and reducing NADP⁺ to NADPH. This electron transfer is coupled to proton translocation and generates a proton motive force, which powers the regeneration of ATP. The stoichiometry of ATP/NADPH produced by these reactions is thought to be 1.29 based on the ratio of proton pumping and the requirement for ATP synthase in the thylakoid (Sacksteder et al., 2000; Seelert et al., 2000). However, under ambient CO₂, oxygen, and temperature, the ATP/NADPH required by CO₂ fixation, photorespiration, and NO₃⁻ assimilation is approximately 1.6 (Edwards and Walker, 1983). The ATP/NADPH demand from central metabolism changes significantly from 1.6 if the ratio of CO₂ or oxygen changes, driving different rates of photosynthesis and photorespiration (see “Theory”). Such changes in energy demand require a flexible mechanism to balance ATP/NADPH that responds to environmental conditions.The difference between ATP/NADPH supply from LEF and demand from primary metabolism could be balanced via cyclic electron flow around PSI (CEF; Avenson et al., 2005; Shikanai, 2007; Joliot and Johnson, 2011; Kramer and Evans, 2011). During CEF, electrons from either NADPH or ferredoxin are cycled around PSI into the plastoquinone pool and regenerate ATP without reducing NADP⁺ (Golbeck et al., 2006). Therefore, CEF has been suggested to be important for optimal photosynthesis and plant growth, but its physiological role in energy balancing is not clear (Munekage et al., 2002, 2004; Livingston et al., 2010). For example, there was no shift in CEF in Arabidopsis (Arabidopsis thaliana) measured under low light (less than 300 μmol m⁻² s⁻¹) and different oxygen partial pressures, which would significantly change the ATP/NADPH demand of primary metabolism (Avenson et al., 2005). Similar results were seen under low light in leaves of barley (Hordeum vulgare) and Hedera helix (Genty et al., 1990). While CEF did not shift with energy demand in steady-state photosynthesis under low light, it did increase with photorespiration as expected at high light (Miyake et al., 2004, 2005). These observations could be explained if CEF becomes more important for energy balancing under high irradiances when other mechanisms become saturated.To determine under which conditions CEF responded to ATP/NADPH demand, we used biochemical models of leaf CO₂ fixation to model ATP and NADPH demand under a variety of conditions (see “Theory”). We then used in vivo spectroscopy to measure the relative response of CEF to modeled ATP/NADPH demand from CO₂ fixation and NO₃⁻ assimilation in hydroponically grown Arabidopsis. Our findings indicate that CEF responded to modeled ATP/NADPH demand under high light but not under low light or nitrate availability.     

Effect of the Pool Size of Stromal Reductants on the Alternative Pathway of Electron Transfer to Photosystem I in Chloroplasts of Intact Leaves

The effect of elevated temperature on electron flow to plastoquinone pool and to PSI from sources alternative to PSII was studied in barley (Hordeum vulgare L.) and maize (Zea mays L.) leaves. Alternative electron flow was characterized by measuring variable fluorescence of chlorophyll and absorption changes at 830 nm that reflect redox changes of P700, the primary electron donor of PSI. The treatment of leaves with elevated temperature resulted in a transient increase in variable fluorescence after cessation of actinic light. This increase was absent in leaves treated with methyl viologen (MV). The kinetics of P700⁺ reduction in barley and maize leaves treated with DCMU and MV exhibited two exponential components. The rate of both components markedly increased with temperature of the heat pretreatment of leaves when the reduction of P700⁺ was measured after short (1 s) illumination of leaves. The acceleration of both kinetic components of P700⁺ reduction by high-temperature treatment was much less pronounced when P700⁺ reduction rate was measured after illumination of leaves for 1 min. Since the treatment of leaves with DCMU and MV inhibited both the electron flow to PSI from PSII and ferredoxin-dependent cycling of electrons around PSI, the accelerated reduction of P700⁺ indicated that high temperature treatment activated electron flow to PSII from reductants localized in the chloroplast stroma. We conclude that the lesser extent of activation of this process by elevated temperature after prolonged illumination of heat-inhibited leaves is caused by depletion of the pool stromal reductants in light due to photoinduced electron transfer from these reductants to oxygen.