Photosynthetica - Latest articles
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Response of photosynthetic apparatus in Arabidopsis thaliana L. mutant deficient in phytochrome A and B to UV-BArticle
V. D. Kreslavski, A. N. Shmarev, V. Yu. Lyubimov, G. A. Semenova, S. K. Zharmukhamedov, G. N. Shirshikova, A. Yu. Khudyakova, S. I. Allakhverdiev
Photosynthetica 2018, 56(1):418-426 | DOI: 10.1007/s11099-017-0754-8
The effects of UV-B radiation (1 W m-2, 1 and 2 h) on PSII activity, chloroplast structure, and H2O2 contents in leaves of 26-d-old Arabidopsis thaliana phyA phyB double mutant (DMut) compared with the wild type (WT) were investigated. UV-B decreased PSII activity and affected the H2O2 content in WT and DMut plants grown under white light (WL). The chloroplast structure changes in DMut plants exposed to UV were more significant than that in WT. Reductions in maximal and real quantum photochemical yields and increase in the value of thermal dissipation of absorbed light energy per PSII RC...
Brandt iH026a plant growth regulatorArticle
A. M. Nonomura, A. Pedersen, D. P. Brummel, L. Loveless, A. Lauria, B. Haschemeyer, M. S. McBride
Photosynthetica 2018, 56(1):411-417 | DOI: 10.1007/s11099-017-0739-7
iH026a is a formulation containing a biochemical class of plant growth regulator that modulates glycoconjugation through the plant lectin cycle. While lectins are common to vascular plants, we observed, consistent with reversible binding of sugars from lectins, enhancements of quantities and qualities of various features, including significant enrichment of Brix soluble sugars compared to controls in cherry, grape, and melon in trials conducted in Arizona and California, USA.
Loss of photosynthesis signals a metabolic reprogramming to sustain sugar homeostasis during senescence of green leaves: Role of cell wall hydrolasesReview
B. Biswal, J. K. Pandey
Photosynthetica 2018, 56(1):404-410 | DOI: 10.1007/s11099-018-0784-x
Leaf senescence is always associated with decline in photosynthesis, consequently a loss of cellular sugar. On the other hand, execution of senescence program needs energy and leaves, therefore, tend to collect sugars from other sources to sustain energy homeostasis. This sugar reprogramming induced by loss of sugar involves operation of a complex catabolic network. The exact molecular mechanism of induction and regulation of the network, however, is not fully resolved but the current literature available suggests sugar starvation as a signal for induction of several senescence-associated genes including the genes coding for the enzymes for degradation...
Evaluating the link between photosynthetic capacity and leaf vascular organization with principal component analysisArticle
S. K. Polutchko, J. J. Stewart, B. Demmig-Adams, W. W. Adams
Photosynthetica 2018, 56(1):392-403 | DOI: 10.1007/s11099-017-0764-6
Significant linear relationships between photosynthetic capacity and principal components loaded by phloem cell numbers and tracheary elements per minor vein as well as the latter two normalized for vein density (proxy for apoplastic phloem loading capacity involving membrane transporters) were revealed for all apoplastic loaders (summer annuals and winter annual Arabidopsis thaliana). In addition, significant linear relationships between photosynthetic capacity and a principal component loaded by tracheary element cross-sectional areas and volumes per unit of leaf area (water flux capacity proxy) was present for symplastic and apoplastic loaders....
Tocopherols modulate leaf vein arrangement and composition without impacting photosynthesisArticle
J. J. Stewart, W. W. Adams, C. M. Cohu, B. Demmig-Adams
Photosynthetica 2018, 56(1):382-391 | DOI: 10.1007/s11099-017-0757-5
Growth of the tocopherol-deficient vte1 mutant and Col-0 wild type of Arabidopsis thaliana in a sunlit glasshouse revealed both similarities and differences between genotypes. Photosynthetic capacity and leaf mesophyll features did not differ between mutant and wild type. Likewise, the total volume of water conduits (tracheary elements, TEs), sugar conduits (sieve elements, SEs), and sugar-loading cells (companion and phloem parenchyma cells) on a leaf area basis were unaffected by tocopherol deficiency. However, tocopherol deficiency yielded smaller and more numerous minor veins with fewer phloem cells and smaller TEs, resulting in greater...
Photosynthesis and salinity: are these mutually exclusive?Review
S. Wungrampha, R. Joshi, S. L. Singla-Pareek, A. Pareek
Photosynthetica 2018, 56(1):366-381 | DOI: 10.1007/s11099-017-0763-7
Photosynthesis has walked into the path of evolution for over millions of years. Organisms relying directly on photosynthesis, when subjected to adverse environments for a long duration, experience retardation in their growth and development. Salinity stress is perceived as one of the major threats to agriculture as it can cause an irreversible damage to the photosynthetic apparatus at any developmental stage of the plant. However, halophytes, a special category of plants, carry out all life processes, including photosynthesis, without showing any compromise even under high saline environments. The fascinating mechanism for Na+ exclusion...
Photosynthetic efficiency in sun and shade plantsReview
S. Mathur, L. Jain, A. Jajoo
Photosynthetica 2018, 56(1):354-365 | DOI: 10.1007/s11099-018-0767-y
Photosynthesis is amongst the plant cell functions that are highly sensitive to any type of changes. Sun and shade conditions are prevalent in fields as well as dense forests. Dense forests face extreme sun and shade conditions, and plants adapt themselves accordingly. Sun flecks cause changes in plant metabolic processes. In the field, plants have to face high light intensity and survive under such conditions. Sun and shade type of plants develops a respective type of chloroplasts which help plants to survive and perform photosynthesis under adverse conditions. PSII and Rubisco behave differently under different sun and shade conditions. In this review,...
Leptolyngbya CCM 4, a cyanobacterium with far-red photoacclimation from Cuatro Ciénegas Basin, MéxicoArticle
C. Gómez-Lojero, L. E. Leyva-Castillo, P. Herrera-Salgado, J. Barrera-Rojas, E. Ríos-Castro, E. B. Gutiérrez-Cirlos
Photosynthetica 2018, 56(1):342-353 | DOI: 10.1007/s11099-018-0774-z
A cyanobacterium containing phycobiliproteins with far-red acclimation was isolated from Pozas Rojas, Cuatro Ciénegas, México. It was named Leptolyngbya CCM 4 after phylogenetic analysis and a description of its morphological characteristics. Leptolyngbya was grown in far-red light. Sucrose-gradient analysis of the pigments revealed two different colored bands of phycobiliproteins. A band at 60% sucrose was a phycocyanin containing phycobilisome; at 35% sucrose, a new type of phycobiliprotein absorbed at 710 nm. SDS-PAGE revealed the presence of two types of core-membrane linkers. Analysis of the hydrophobic pigments extracted from the...
Factors affecting photobiological hydrogen production in five filamentous cyanobacteria from ThailandArticle
P. Yodsang, W. Raksajit, E-M. Aro, P. Mäenpää, A. Incharoensakdi
Photosynthetica 2018, 56(1):334-341 | DOI: 10.1007/s11099-018-0789-5
We report here the screening of sixteen cyanobacterial and three green algal strains from Thailand for their potential biohydrogen production. Five filamentous cyanobacterial species, namely Calothrix elenkinii, Fischerella muscicola, Nostoc calcicola, Scytonema bohneri, and Tolypothrix distorta, all possessing nitrogenase activity, showed potentially high biohydrogen production. These five strains showed higher hydrogen production in the absence than in the presence of nitrogen. In particular, F. muscicola had a 17-fold increased hydrogen production under combined nitrogen and sulfur deprived conditions. Among...
Function, regulation and distribution of IsiA, a membrane-bound chlorophyll a-antenna protein in cyanobacteriaReview
H. Y.S. Chen, A. Bandyopadhyay, H. B. Pakrasi
Photosynthetica 2018, 56(1):322-333 | DOI: 10.1007/s11099-018-0787-7
IsiA is a membrane-bound Chl a-antenna protein synthesized in cyanobacteria under iron deficiency. Since iron deficiency is a common nutrient stress in significant fractions of cyanobacterial habitats, IsiA is likely to be essential for some cyanobacteria. However, the role it plays in cyanobacteria is not fully understood. In this review paper, we summarize the research efforts directed towards characterizing IsiA over the past three decades and attempt to bring all the pieces of the puzzle together to get a more comprehensive understanding of the function of this protein. Moreover, we analyzed the genomes of over 390 cyanobacterial strains...
Effect of green light on the amount and activity of NDH-1-PSI supercomplex in Synechocystis sp. strain PCC 6803Article
F. Gao, T. Ogawa, W. Ma
Photosynthetica 2018, 56(1):316-321 | DOI: 10.1007/s11099-018-0790-z
Cyanobacterial NDH-1 interacts with PSI to form NDH-1-PSI supercomplex. CpcG2, a linker protein for the PSI-specific peripheral antenna CpcG2-phycobilisome, is essential for stabilization of the supercomplex. Green light (GL) increased the expression of CpcG2 but had little effect, if any, on the expression of NDH-1 and PSI, when compared to the abundance of these components under red light (RL). The increased expression of CpcG2 intensified the band of NDH-1-PSI supercomplex after blue-native gel electrophoresis of the thylakoid membrane, possibly by stabilizing the supercomplex. The activity of NDH-1-dependent cyclic electron transport around PSI...
Characterization of isolated photosystem I from Halomicronema hongdechloris, a chlorophyll f-producing cyanobacteriumArticle
Y. Li, N. Vella, M. Chen
Photosynthetica 2018, 56(1):306-315 | DOI: 10.1007/s11099-018-0776-x
Halomicronema hongdechloris is a chlorophyll (Chl) f-producing cyanobacterium. Chl f biosynthesis is induced under far-red light, extending its photosynthetically active radiation range to 760 nm. In this study, PSI complexes were isolated and purified from H. hongdechloris, grown under white light (WL) and far-red light (FR), by a combination of density gradient ultracentrifugation and chromatographic separation. WL-PSI showed similar pigment composition as that of Synechocystis 6803, using Chl a in the reaction center. Both Chl a and f were detected in the FR-PSI, although Chl f was a...
Isolation of the cyanobacterial YFP-tagged photosystem I using GFP-Trap®Brief Communications
A. Strašková, J. Knoppová, J. Komenda
Photosynthetica 2018, 56(1):300-305 | DOI: 10.1007/s11099-018-0771-2
A strain of Synechocystis sp. PCC 6803 expressing the yellow fluorescent protein (YFP) fused to the C-terminus of the PsaF subunit of PSI has been constructed and used to isolate native PSI complexes employing the GFP-Trap®, an efficient immunoprecipitation system which recognizes the green fluorescent protein (GFP) and its variants. The protein analysis and spectroscopic characterization of the preparation revealed an isolate of trimeric and monomeric PSI complexes, which showed minimal unspecific contamination as demonstrated by comparison with the wild type control. Interestingly, we detected CP43 subunits of PSII and small amounts...
Determination of PS I oligomerisation in various cyanobacterial strains and mutants by non-invasive methodsArticle
T. Zakar, L. Kovacs, S. Vajravel, E. Herman, M. Kis, H. Laczko-Dobos, Z. Gombos
Photosynthetica 2018, 56(1):294-299 | DOI: 10.1007/s11099-018-0795-7
PSI trimer to monomer ratio in intact cyanobacterial cells and isolated thylakoids was analysed by two noninvasive, in vivo methods; low-temperature fluorescence emission and circular dichroism spectroscopy. We measured fluorescence emission spectra of cells upon chlorophyll (Chl, 436 nm) excitation. All three species - Synechocystis sp. PCC 6803, Anabaena sp. PCC 7120, and Spirulina platensis - showed shifted Chl peak, indicating they have different spectral properties. CD spectroscopy revealed the highest intensity at 515 nm (PSI peak) in Spirulina platensis cells, which may originate from PSI multi-oligomerisation....
Ferredoxin: the central hub connecting photosystem I to cellular metabolismReview
J. Mondal, B. D. Bruce
Photosynthetica 2018, 56(1):279-293 | DOI: 10.1007/s11099-018-0793-9
Ferredoxin (Fd) is a small soluble iron-sulfur protein essential in almost all oxygenic photosynthetic organisms. It contains a single [2Fe-2S] cluster coordinated by four cysteine ligands. It accepts electrons from the stromal surface of PSI and facilitates transfer to a myriad of acceptors involved in diverse metabolic processes, including generation of NADPH via Fd-NADP-reductase, cyclic electron transport for ATP synthesis, nitrate reduction, nitrite reductase, sulfite reduction, hydrogenase and other reductive reactions. Fd serves as the central hub for these diverse cellular reactions and is integral to complex cellular metabolic networks....
The deep red state of photosystem II in Cyanidioschyzon merolaeBrief Communications
J. Langley, J. Morton, R. Purchase, L. Tian, L. Shen, G. Han, J. R. Shen, E. Krausz
Photosynthetica 2018, 56(1):275-278 | DOI: 10.1007/s11099-017-0760-x
We identified and characterised the deep red state (DRS), an optically-absorbing charge transfer state of PSII, which lies at lower energy than P680, in the red algae Cyanidioschyzon merolae by means of low temperature absorption and magnetic circular dichroism spectroscopies. The photoactive DRS has been previously studied in PSII of the higher plant Spinacia oleracea, and in the cyanobacterium Thermosynechococcus vulcanus. We found the DRS in PSII of C. merolae has similar spectral properties. Treatment of PSII with dithionite leads to reduction of cytochrome (cyt) b559 and the PsbV-based cyt c550...
The phycobilisome terminal emitter transfers its energy with a rate of (20 ps)-1 to photosystem IIArticle
A. M. Acuña, P. Van Alphen, R. Van Grondelle, I. H. M. Van Stokkum
Photosynthetica 2018, 56(1):265-274 | DOI: 10.1007/s11099-018-0779-7
Ultrafast time resolved emission spectra were measured in whole cells of a PSI-deficient mutant of Synechocystis sp. PCC 6803 at room temperature and at 77K to study excitation energy transfer and trapping. By means of a target analysis it was estimated that the terminal emitter of the phycobilisome, termed allophycocyanin 680, transfers its energy with a rate of (20 ps)-1 to PSII. This is faster than the intraphycobilisome energy transfer rates between a rod and a core cylinder, or between the core cylinders.
Increased thermal stability of photosystem II and the macro-organization of thylakoid membranes, induced by co-solutes, associated with changes in the lipid-phase behaviour of thylakoid membranesArticle
C. Kotakis, P. Akhtar, O. Zsiros, G. Garab, P. H. Lambrev
Photosynthetica 2018, 56(1):254-264 | DOI: 10.1007/s11099-018-0782-z
The principal function of the thylakoid membrane depends on the integrity of the lipid bilayer, yet almost half of the thylakoid lipids are of non-bilayer-forming type, whose exact functions are not fully understood. Non-bilayer lipids can be extruded from the membrane in the presence of high concentrations of co-solutes. We applied 2 M sucrose to induce lipid phase separation in isolated thylakoid membranes, following consequent structural and physiological effects. Circular dichroism spectroscopy indicated significant changes in the chiral macro-arrangement of the pigment-protein complexes, which were reversed after washing out the co-solute. Similarly,...
Photooxidation and photoreduction of exogenous cytochrome c by photosystem II preparations after various modifications of the water-oxidizing complexArticle
A. A. Khorobrykh, D. V. Yanykin, V. V. Klimov
Photosynthetica 2018, 56(1):244-253 | DOI: 10.1007/s11099-017-0762-8
The redox interaction of exogenous cytochrome c550 (Cyt) with PSII isolated from spinach was studied. Illumination of PSII particles in the presence of Cyt led to: (1) Cyt photooxidation by PSII reaction center (demonstrated at the first time), (2) Cyt photoreduction via O2-* photoproduced on the acceptor side of PSII, and (3) Cyt photoreduction by reduced electron carriers of PSII. A step-by-step removal of components of water-oxidizing complex was accompanied by the appearance of Cyt photooxidation, an increase in the superoxide dismutase (SOD)-dependent Cyt photoreduction (related...
Influence of the disaccharide trehalose on the oxidizing side of photosystem IIArticle
M. D. Mamedov, E. S. Nosikova, L. A. Vitukhnovskaya, A. A. Zaspa, A. Yu. Semenov
Photosynthetica 2018, 56(1):236-243 | DOI: 10.1007/s11099-017-0750-z
The steady-state oxygen evolution rate was previously shown to be stimulated by the disaccharide trehalose in PSII suspension. Here we showed a similar increase in the rate of oxygen evolution in PSII core complexes from spinach in solution and in proteoliposomes in the presence of trehalose. Using direct electrometrical technique, we also revealed that trehalose had no effect on the kinetics of electron transfer from Mn to redox-active-tyrosyl radical, YZ* (S1 → S2 transition), while it accelerated the kinetics of electrogenic proton transport during S2 → S3 and S4...
Analysis of photosystem II electron transfer with natural PsbA-variants by redox polymer/protein biophotoelectrochemistryArticle
V. Hartmann, A. Ruff, W. Schuhmann, M. Rögner, M. M. Nowaczyk
Photosynthetica 2018, 56(1):229-235 | DOI: 10.1007/s11099-018-0775-y
Redox polymer/protein biophotoelectrochemistry was used to analyse forward electron transfer of isolated PSII complexes with natural PsbA-variants. PsbA1- or PsbA3-PSII was embedded in a redox hydrogel that allows diffusion-free electron transfer to the electrode surface and thus measurement of an immediate photocurrent response. The initial photocurrent density of the electrode is up to ~2-fold higher with PsbA1-PSII under all tested light conditions, the most prominent under high-light [2,300 μmol(photon) m-2 s-1] illumination with 5 μA cm-2 for PsbA3-PSII and 9.5 μA cm-2 for PsbA1-PSII. This indicates...
The multiplicity of roles for (bi)carbonate in photosystem II operation in the hypercarbonate-requiring cyanobacterium Arthrospira maximaArticle
G. Ananyev, C. Gates, G. C. Dismukes
Photosynthetica 2018, 56(1):217-228 | DOI: 10.1007/s11099-018-0781-0
Arthrospira maxima is unique among cyanobacteria, growing at alkaline pH (<11) in concentrated (bi)carbonate (1.2 M saturated) and lacking carbonic anhydrases. We investigated dissolved inorganic carbon (DIC) roles within PSII of A. maxima cells oximetrically and fluorometrically, monitoring the light reactions on the donor and acceptor sides of PSII. We developed new methods for removing DIC based on a (bi)carbonate chelator and magnesium for (bi)carbonate ionpairing. We established relative affinities of three sites: the water-oxidizing complex (WOC), non-heme iron/QA-, and solvent-accessible arginines throughout...
Quantification of bound bicarbonate in photosystem IIArticle
K. Tikhonov, D. Shevela, V. V. Klimov, J. Messinger
Photosynthetica 2018, 56(1):210-216 | DOI: 10.1007/s11099-017-0758-4
In this study, we presented a new approach for quantification of bicarbonate (HCO3-) molecules bound to PSII. Our method, which is based on a combination of membrane-inlet mass spectrometry (MIMS) and 18O-labelling, excludes the possibility of "non-accounted" HCO3- by avoiding (1) the employment of formate for removal of HCO3- from PSII, and (2) the extremely low concentrations of HCO3-/CO2 during online MIMS measurements. By equilibration of PSII sample to ambient CO2 concentration of dissolved CO2/HCO3-,...
PsbY is required for prevention of photodamage to photosystem II in a PsbM-lacking mutant of Synechocystis sp. PCC 6803Article
S. Biswas, J. J. Eaton-Rye
Photosynthetica 2018, 56(1):200-209 | DOI: 10.1007/s11099-018-0788-6
The PsbM (3.9 kDa) and PsbY (4.2 kDa) proteins are membrane-spanning, single-helix, subunits associated with the chlorophyll-binding CP47 pre-complex of photosystem II (PSII). Removal of PsbM resulted in accumulation of PSII pre-assembly complexes and impaired electron transfer between the primary (QA) and secondary (QB) plastoquinone electron acceptors of PSII indicating that the QB-binding site and bicarbonate binding to the non-heme iron were altered in this strain. Removal of PsbY alone had only a minor impact on PSII activity but deleting PsbY in the ΔPsbM background led to additional modification of the...
Regulating photoprotection improves photosynthetic growth and biomass production in QC-site mutant cells of the cyanobacterium Synechocystis sp. PCC 6803Article
J. Y. Huang, N. T. Hung, K. M. Lin, Y. F. Chiu, H. A. Chu
Photosynthetica 2018, 56(1):192-199 | DOI: 10.1007/s11099-018-0765-0
We characterized the photosynthetic growth of wild-type (WT) and QC-site mutant cells of the cyanobacterium Synechocystis sp. PCC 6803 grown in a photobioreactor under medium-intensity [~70 μmol(photon) m-2 s-1] and high-intensity [~200 μmol(photon) m-2 s-1] light conditions. Photosynthetic growth rate (the exponential phase) increased about 1.1-1.2 fold for the A16FJ, S28Aβ, and V32Fβ mutant compared with WT cells under medium-intensity light and about 1.2-1.3 fold under high-intensity light. Biomass production increased about 17-20% for A16FJ and S28Aβ mutant cells as compared...
The PsbQ' protein affects the redox potential of the QA in photosystem IIArticle
M. Yamada, R. Nagao, M. Iwai, Y. Arai, A. Makita, H. Ohta, T. Tomo
Photosynthetica 2018, 56(1):185-191 | DOI: 10.1007/s11099-018-0778-8
Red alga contains four extrinsic proteins in photosystem II (PSII), which are PsbO, PsbV, PsbU, and PsbQ'. Except for the PsbQ', the composition is the same in cyanobacterial PSII. Reconstitution analysis of cyanobacterial PSII has shown that oxygen-evolving activity does not depend on the presence of PsbQ'. Recently, the structure of red algal PSII was elucidated. However, the role of PsbQ' remains unknown. In this study, the function of the acceptor side of PSII was analyzed in PsbQ'-reconstituted PSII by redox titration of QA and thermoluminescence. The redox potential of QA was positively shifted when PsbQ' was attached to...
PsbP-induced protein conformational changes around Cl- ions in the water oxidizing center of photosystem IIArticle
J. Kondo, T. Noguchi
Photosynthetica 2018, 56(1):178-184 | DOI: 10.1007/s11099-017-0749-5
PsbP is an extrinsic protein of PSII having a function of Ca2+ and Cl- retention in the water-oxidizing center (WOC). In order to understand the mechanism how PsbP regulates the Cl- binding in WOC, we examined the effect of PsbP depletion on the protein structures around the Cl- sites using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S1→S2 transition were obtained using Cl--bound and NO3--substituted PSII membranes in the presence and absence of PsbP. A clear difference in the amide I band changes...
Early emergence of the FtsH proteases involved in photosystem II repairArticle
S. Shao, T. Cardona, P. J. Nixon
Photosynthetica 2018, 56(1):163-177 | DOI: 10.1007/s11099-018-0769-9
Efficient degradation of damaged D1 during the repair of PSII is carried out by a set of dedicated FtsH proteases in the thylakoid membrane. Here we investigated whether the evolution of FtsH could hold clues to the origin of oxygenic photosynthesis. A phylogenetic analysis of over 6000 FtsH protease sequences revealed that there are three major groups of FtsH proteases originating from gene duplication events in the last common ancestor of bacteria, and that the FtsH proteases involved in PSII repair form a distinct clade branching out before the divergence of FtsH proteases found in all groups of anoxygenic phototrophic bacteria. Furthermore, we...
Control of the maximal chlorophyll fluorescence yield by the QB binding siteArticle
O. Prášil, Z. S. Kolber, P. G. Falkowski
Photosynthetica 2018, 56(1):150-162 | DOI: 10.1007/s11099-018-0768-x
Differences in maximal yields of chlorophyll variable fluorescence (Fm) induced by single turnover (ST) and multiple turnover (MT) excitation are as great as 40%. Using mutants of Chlamydomonas reinhardtii we investigated potential mechanisms controlling Fm above and beyond the QA redox level. Fm was low when the QB binding site was occupied by PQ and high when the QB binding site was empty or occupied by a PSII herbicide. Furthermore, in mutants with impaired rates of plastoquinol reoxidation, Fm was reached rapidly during MT excitation. In PSII particles with no mobile...
On the quantitative relation between dark kinetics of NPQ-induced changes in variable fluorescence and the activation state of the CF0.CF1.ATPase in leavesArticle
W.J. Vredenberg
Photosynthetica 2018, 56(1):139-149 | DOI: 10.1007/s11099-018-0772-1
The variable fluorescence at the maximum Fm of the fluorescence induction (Kautsky) curve is known to be substantially suppressed shortly after light adaption due to nonphotochemical qE quenching. The kinetic pattern of the dark decay at Fm consists of three components with rates ~20, ~1, and ~0.1 s-1, respectively. Light adaptation has no or little effect on these rate constants. It causes a decrease in the ratio between the amplitudes of the slow and fast one with negligible change in the small amplitude of the ultra-slow component. Results add to evidence for the hypothesis that the dark-reversible decrease...