Host factor requirements and some properties of ΦXtB

Summary Replication of XtB, a capsid mutant of bacteriophage X174, depends on the host functions directed by the E. coli genes dnaE, dnaF, dnaG, dnaZ, lig and rep. The cellular products of dnaA, dnaB, dnaC(D), dnaI, dnaP, polA, polB and xth genes are, however, dispensable for the viral growth. In these host factor requirements, XtB resembles phages K and St-1, rather than X174. Host ranges of XtB, St-1 and K overlap considerably, and growth temperature of the three phages is somewhat higher than that of X174. Furthermore, XtB is, like K, inactivated by antiserum against St-1. XtB may thus fill an evolutionary gap between the X174 group and the St-1 group.     

Mechanisms for controlling balance between light input and utilisation in the salt tolerant alga Dunaliella C9AA

The yield of photosynthetic O₂ evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters _{PS I} and _{PS II} . _{PS I} is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. _{PS II} is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O₂ evolution decreased more than predicted by either _{PS I} or _{PS II}. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.Abbreviations F₀ dark level of chlorophyll fluorescence yield (PS II centres open) - F_m maximum level of chlorophyll fluorescence yield (PS II centres closed) - F_v variable fluorescence (F_m-F₀) - PS I Photosystem I - PS II Photosystem II - P₇₀₀ reaction centre chlorophyll(s) of PS I - q_N coefficient of non-photochemical quenching of chlorophyll fluorescence - q_P coefficient of photochemical quenching of fluorescence yield - q_E high-energy-state quenching coefficient - _{PS I} yield of PS I - _{PS II} yield of PS II - _S yield of photosynthetic O₂ evolution - _P intrinsic yield of open PS II centres     

Physiological relationship between the temperate phages lambda and phi80

Summary This work deals with the ability of phage 80 to provide defective mutants of with their missing functions. Functions Involved in Recombination. As shown by others, the Int mechanism of 80 cannot excise prophage . However, 80 efficiently excises recombinants from tandem dilysogens, using its Ter mechanism. Likewise, the nonspecific mechanism Red is interchangeable between 80 and . Maturation of DNA by 80. The Ter recombinants excised by 80 from tandem dilysogens are packaged into a 80 protein coat. This contrasts with the fact, already mentionned by Dove, that 80 is extremely inefficient for packaging phage superinfecting a -lysogen. The latter result is also found when the helper phage is a hybrid with the left arm of (80hy4 or 80hy41 — see Fig. 1). However, the maturation of the superinfecting is much more efficient if the 80hy used as a helper has the att-N region of (like 80hy1). Conversely a with the att-N region of 80 (hy6 — see Fig. 1) is packaged more efficiently by 80 or 80hy4 than by 80hy1. It is suggested that the maturation of chromosome superinfecting an immune cell requires a recombination with the helper phage. Vegetative Functions. Among the replicative functoons O and P, the latter only can be supplied by 80. That N mutants are efficiently helped by 80 does not tell that 80 provides the defective with an active N product; the chromosomes are simply packaged into a 80 coat. This shows that 80 is unable to switch on the late genes of . That neither 80 nor any of the 80hy tested can provide an active N product is shown in a more direct way by their complete failure to help N ^-r₁₄; this phage carries a polar mutation which makes the expression of genes O and P entirely N-dependant. The maturation of a N ^- by 80 contrasts with the fact that mutants affected in late genes (A, F or H) are not efficiently helped by 80. This suggests that the products coded by these genes are not interchangeable between 80 and , and that packaging of DNA into 80 coats is possible but inhibited when late proteins are present in the cell. Activation of the Late Genes. Among the im ₈₀ h ⁺ hybrids tested, only 80hy41 is able to switch on the late genes of a N defective mutant. This hybrid differs from the other hybrids studied here, by the fact that it has the Q-S-R region of (see Fig. 1). The results are consistant with the view that the product of Q gene is sufficient for activating the late genes of a DNA. N would thus control the expression of late genes only indirectly by controlling the expression of gene Q (Couturier & Dambly have independantly reached the same conclusion, 1970). Furthermore the failure of 80 and of the 80hy1 and 80hy4 to activate the late genes of would imply that these phages are unable to provide an Q product active on the chromosome Reciprocally, switches on the late genes of prophage 80hy41, but not of prophages 80hy1 and 80hy4. This suggests that the initiation of late genes expression takes place at a main specific site located in the Q-S-R region of the chromosome. The expression of the late genes would thus be sequential, and proceed through the left arm only when steaky ends cohere. Similar conclusions were reached independantly by Toussaint (1969) and by Herskowitz and Signer (1970).

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Ce travail a été réalisé dans le cadre du contrat d'association Euratom-U. L. B. 007-61-10 ABIB et avec l'aide du Fonds de la Recherche Fondamentale Collective.     

Virulence as a consequence of genome instability of a novel temperate bacteriophage, ΦRsG1, of Rhodobacter sphaeroides Y

A novel temperate bacteriophage, designated RsG1, was isolated from Rhodobacter sphaeroides Y (previously designated Rhodopseudomonas sphaeroides) following exposure to mitomycin C. The phage morphology, as revealed from electron microscopy, showed a hexagonal head (90 by 46.5 nm) connected with a tail (116 by 9.4 nm), to which a collar was proximally attached. A morphologically similar phage was also produced by spontaneous lysis of the cells. While RsG1 did not grow on any other bacterial strain tested, spontaneously produced phage particles propagated (and formed plaques) on R. sphaeroides Y still carrying RsG1 in the prophage state. The genome of RsG1 consisted of double stranded linear DNA with cohesive ends and a GC-content of 71.8 mol%. The DNA molecules formed circles in vitro with a mean contour length of 17.18±0.4 m, which corresponds to a size of 49 kbase pairs (kb). On the other hand, DNA extracted from the virulent phage particles was heterogeneous and consisted of two DNA species of different size, occurring in a ratio of about 1:1. These molecules also circularized having contour lengths of 17.18±0.4 m and 14.02±0.41 m corresponding to 49 and 40 kb, respectively. Restriction digest analysis of the two DNA species and DNA from RsG1 indicated that they are similar, and allowed the indentification of an 11.5 kb EcoRI fragment that carries the cohesive ends. Because DNA from RsG1 and the 49 kb DNA of the virulent phage particles were indistinguishable with the criteria applied, it is suggested that phage particles containing the 40 kb DNA represent the virulent type of phage, termed RsG1.1.     

The solution conformation of sialyl-α(2→6)-lactose studied by modern NMR techniques and Monte Carlo simulations

Summary We present a comprehensive strategy for detailed characterization of the solution conformations of oligosaccharides by NMR spectroscopy and force-field calculations. Our experimental strategy generates a number of interglycosidic spatial constraints that is sufficiently large to allow us to determine glycosidic linkage conformations with a precision heretofore unachievable. In addition to the commonly used {¹H,¹H} NOE contacts between aliphatic protons, our constraints are: (a) homonuclear NOEs of hydroxyl protons in H₂O to other protons in the oligosaccharide, (b) heteronuclear {¹H,¹³C} NOEs, (c) isotope effects of O¹H/O²H hydroxyl groups on¹³C chemical shifts, and (d) long-range heteronuclear scalar coupling across glycosidic bonds.We have used this approach to study the trisaccharide sialyl-(26)-lactose in aqueous solution. The experimentally determined geometrical constraints were compared to results obtained from force-field calculations based on Metropolis Monte Carlo simulations. The molecule was found to exist in 2 families of conformers. The preferred conformations of the (26)-linkage of the trisaccharide are best described by an equilibrium of 2 conformers with angles at –60° or 180° and of the 3 staggered rotamers of the angle with a predominantgt conformer. Three intramolecular hydrogen bonds, involving the hydroxyl protons on C8 and C7 of the sialic acid residue and on C3 of the reducing-end glucose residue, contribute significantly to the conformational stability of the trisaccharide in aqueous solution. Supplementary material available from the corresponding author: Table containing values for the dihedral angles , , , , and for bond angles , for the six lowest-energy conformations of sialyl-(26)-lactose (1 page).     

The Instron technique as a measure of immediate-past wall extensibility

The relationship between the plastic-extensibility values (PEx) obtained in the Instron technique and the growth parameter, wall extensibility () has been evaluated for Avena sativa L. coleoptile cell walls. The possibility that PEx is proportional to the growth rate rather than to has been eliminated by showing that turgor-driven changes in the growth rate do not cause comparable changes in PEx. For Avena coleoptiles, PEx appears to be a measure of the average over the previous 60–90 min rather than a measure of the instantaneous of the growth equation. This is indicated by the fact that while PEx and the growth rate start to change simultaneously after addition of indole-3-acetic acid or KCN, the growth rate reaches a new, constant value 60–90 min before a new plateau value of PEx is obtained. Similar results are obrained with soybean (Glycine max L.) hypocotyl walls, indicating that the relationship between PEx and the parameter is a general one, although the period over which is averaged differs from tissue to tissue. In addition, it is shown that PEx can be measured more than once on the same section; a new potential for plastic extension is regenerated whenever the force vectors are changed even slightly. It is concluded that PEx is a measure of those domains in the wall where a wall-loosening event has occurred which has not been eliminated by further wall synthesis or other biochemical events.Abbreviations and symbols DP Instron plastic compliance - IAA indole-3-acetic acid - PEx Instron plastic extensibility - instantaneous wall extensibility     

Isolation of a halobacterial phage with a fully cytosine-methylated genome

Summary A new bacteriophage from Halobacterium halobium has been isolated and partially characterized. It is not homologous to the phage H (Schnabel, et al. 1982) which infects the same bacterium, though it appeared spontaneously in a culture of H adapted to H. halobium NRL/JW. The size and morphology of N are comparable to that of other known halophages. The genome of N consists of linear double-stranded DNA, 56 kb in size, whose dCMP is totally replaced by 5-methyl-dCMP. This is the second case of a fully cytosine-methylated genome, the bacteriophage XP12 from Xanthomonas oryzae, being so far the only one reported. Like H, the N, genome seems to have terminal redundancy and circular permutation. N is the first halobacterial phage which survives prolonged exposure to low ionic strength environments. After 48 h incubation in distilled water a loss in infectivity of less than 50% is observed.     

Quantifying Protein Folding Transition States with ΦT

A number of reaction coordinates have been proposed for reduced-dimensionalityrepresentations of a protein's folding free energy surface. We discuss in detail the entropic reaction coordinate _T = SS, recently introduced to quantify the conservation of mutations and the location of the folding transition state based on experimental temperature-tuning data. Numerical simulations illustrate the advantages as well as the limitations of _T. _T can be determined from experiment,computation, and analytical theory; _T can also be used to investigate structurally localized perturbations of the free energy surface. However, _T is only a relative reaction cordinate; furthermore, proteins undergo cold denaturation at sufficiently low temperatures, and care must be taken ininterpreting _T near the region where G/T = 0, particularly if the heat capacity change upon folding is small.     

An improved method,using saturating light pulses,for the determination of photosystem I quantum yield via P700+-absorbance changes at 830 nm

An improved method is introduced for the determination of the quantum yield of photosystem I. The new method employs saturating light pulses with steep rise characteristics to distinguish, in a given physiological state, centers with an open acceptor side from centers with a reduced acceptor side. The latter do not contribute to PSI quantum yield (_I). Oxidation of P700 is measured by a rapid modulation technique using the absorbance change around 830 nm. The quantum yield _I is calculated from the amplitude of the rapid phase of absorbance change (A; 830 nm) upon application of a saturation pulse in a given state, divided by the maximal A (830 nm) which is induced by a saturation pulse with far-red background illumination. Using this technique, _I can be determined even under conditions of acceptor-side limitation, as for example in the course of a dark-light induction period or after elimination of CO₂ from the gas stream. Thus determined _I values display a close-to-linear relationship with those for the quantum yield of PSII (_II) calculated from chlorophyll fluorescence parameters. It is concluded that the proposed method may provide new information on the activity of the PSI acceptor side and thus help to separate the effects of acceptorside limitation from those of cyclic PSI, whenever a non-linear relationship between _II and the P700-reduction level is observed.Abbreviations and Symbols A absorbance change - _I quantum yield of photosystem I - _II quantum yield of photosystem II - PAR photosynthetically active radiation This work was supported by the Deutsche Forschungsgemeinschaft (SFB 176 Molekulare Grundlagen der Signalübertragung und des Stofftransportes in Membranen and SFB 251 Ökologie, Physiologie und Biochemie pflanzlicher Leistung unter Streß).     

Is There a Field-Theoretic Explanation For Precursor Biopolymers?

A Hu-Barkana-Gruzinov cold dark matter scalarfield may enter a weak isospin invariant derivative interactionthat causes the flow of right-handed electrons to align parallelto (). Hence, in the outer regions of galaxies where () islarge, as in galactic halos, the derivative interaction mayinduce a chirality-imbued quantum chemistry. Such a chirality-imbued chemistry would in turn be conducive to the formation ofabundant precursor biopolymers on interstellar dust grains,comets and meteors in galactic halo regions, with subsequentdelivery to planets in the inner galactic regions where and() are concomitantly near zero and left-right symmetricterrestrial quantum chemistry prevails.     

Light dependence of quantum yields of Photosystem II and CO2 fixation in C3 and C4 plants

The light dependence of quantum yields of Photosystem II (_II) and of CO₂ fixation were determined in C₃ and C₄ plants under atmospheric conditions where photorespiration was minimal. Calculations were made of the apparent quantum yield for CO₂ fixation by dividing the measured rate of photosynthesis by the absorbed light [A/I=_CO2 and of the true quantum yield by dividing the estimated true rate of photosynthesis by absorbed light [(A+R_l)/I_a=_CO2·], where R_L is the rate of respiration in the light. The dependence of the _II/_CO2 and _II/_CO2 ^* ratios on light intensity was then evaluated. In both C₃ and C₄ plants there was little change in the ratio of _II/_CO2 at light intensities equivalent to 10–100% of full sunlight, whereas there was a dramatic increase in the ratio at lower light intensities. Changes in the ratio of _II/_CO2 can occur because respiratory losses are not accounted for, due to changes in the partitioning of energy between photosystems or changes in the relationship between PS II activity and CO₂ fixation. The apparent decrease in efficiency of utilization of energy derived from PS II for CO₂ fixation under low light intensity may be due to respiratory loss of CO₂. Using dark respiration as an estimate of R_L, the calculated _II/_CO2 ^* ratio was nearly constant from full sunlight down to approx 5% of full sunlight, which suggests a strong linkage between the true rate of CO₂ fixation and PS II activity under varying light intensity. Measurements of photosynthesis rates and _II were made by illuminating upper versus lower leaf surfaces of representative C₃ and C₄ monocots and dicots. With the monocots, the rate of photosynthesis and the ratio of _II/_CO2 exhibited a very similar patterns with leaves illuminated from the adaxial versus the abaxial surface, which may be due to uniformity in anatomy and lack of differences in light acclimation between the two surfaces. With dicots, the abaxial surface had both lower rates of photosynthesis and lower _II values than the adaxial surface which may be due to differences in anatomy (spongy versus palisade mesophyll cells) and/or light acclimation between the two surfaces. However, in each species the response of _II/_CO2 to varying light intensity was similar between the two surfaces, indicating a comparable linkage between PS II activity and CO₂ fixation.Abbreviations A measured rate of CO₂ assimilation - A+R_L true rate of CO₂ assimilation; e - CO₂ estimate of electrons transported through PSII per CO₂ fixed by RuBP carboxylase - f fraction of light absorbed by Photosystem II - F'_m yield of PSII chlorophyll fluorescence due to a saturating flash of white light under steady-state photosynthesis - F_s variable yield of fluorescence under steady-state photosynthesis; PPFD-photosynthetic photon flux density - I_a absorbed PPFD - PS II Photosystem II - R_d rate of respiration in the dark - R_I rate of respiration in the light estimated from measurement of R_d or from analysis of quantum yields - apparent quantum yield of CO₂ assimilation under a given condition (A/absorbed PPFD) - true quantum yield of CO₂ assimilation under a given condition [(A+R_L)/(absorbed PPFD)] - quantum yield for photosynthetic O₂ evolution - electrons transported via PS II per quantum absorbed by PS II Supported by USDA Competitive Grant 90-37280-5706.     

Sinking properties of some phytoplankton shapes and the relation of form resistance to morphological diversity of plankton – an experimental study

Form resistance () is a dimensionless number expressing how much slower or faster a particle of any form sinks in a fluid medium than the sphere of equivalent volume. Form resistance factors of PVC models of phytoplankton sinking in glycerin were measured in a large aquarium (0.6 × 0.6 × 0.95 m). For cylindrical forms, a positive relationship was found between and length/width ratio. Coiling decreased in filamentous forms. Form resistance of Asterionella colonies increased from single cells up to 6-celled colonies than remained nearly constant. For Fragilaria crotonensis chains, no such upper limit to was observed in chains of up to 20 cells (longer ones were not measured). The effect of symmetry on was tested in 1–6-celled Asterionella colonies, having variable angles between the cells, and in Tetrastrum staurogeniaeforme coenobia, having different spine arrangements. In all cases, symmetric forms had considerably higher form resistance than asymmetric ones. However, for Pediastrum coenobia with symmetric/asymmetric fenestration, no difference was observed with respect to symmetry. Increasing number and length of spines on Tetrastrum coenobia substantially increased . For a series of Staurastrum forms, a significant positive correlation was found between arm-length/cell-width ratio and : protuberances increased form resistance. Flagellates (Rhodomonas, Gymnodinium) had a < 1: they sank faster than the spheres of equivalent volume. Ceratium ( = 1.61) proved an exception among flagellates: in most forms tested in this study (ellipsoid flagellates, Staurastrum forms with no or very short protuberances, and Cosmarium forms), > 1. The highest value ( = 8.1) was established for a 20-celled Fragilaria crotonensis chain. Possible origin of the so-called `vital component' (a factor that shows how much slower viable populations sink than morphologically similar senescent or dead ones) is discussed, as is the role of form resistance in evolution of high diversity of plankton morphologies.An erratum to this article can be found at     

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (_p), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (q_I).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (_exe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q _N and the efficiency of excitation capture (_exe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.Abbreviations F₀ basic fluorescence - F₀ basic fluorescence, thylakoid in energized state - F_m maximal fluorescence - F_m maximum fluorescence in energized state - F_s steady state fluorescence - F_v maximal variable fluorescence - PFD photon flux density - PS II_rc Photosystem II reaction center - q_F0 quenching of basic fluorescence - q_E energy related quenching - q_N non-photochemical quenching:-q_f-total quenching - q_I inhibition-related quenching - q_p photochemical quenching - q_r quenching due to state transition - R_d dark respiration - _p PS II efficiency of excitation capture of open PS II_rc - _pe extrapolated minimal value of _p - _p0 extrapolated maximal value of _p - _si quantum efficiency of linear electron transport, calculated from gas exchange measurements based on incident light - _sf quantum efficiency of linear electron transport, calculated from fluorescence measurements, based on incident measuring light     

Conformational energy of the 5-membered ring. Implication of geometric and energetic properties in the conformational characteristics

In a previous article (8) a geometrical study of the five-membered ring showed that: a) for the case of the 20 symmetrical C₂ and C_s conformations, the pseudorotation formulae for the torsion angles are a geometrical property of the ring; b) geometrical considerations alone are unable to define the puckering amplitude, the bond angle values, and the pathway between two symmetrical conformations. Here we examine how the energy equations enable us to define the deformation amplitude _m, establish the bond angles expressions and check the energy invariability along the pseudorotation circuit. The problem is next developed fully in the case where the bond and torsional energy only are considered: the literal expression¹ of _m is then given as a function of the bond angle which cancels out the bond angle energy. A numerical application is carried out on cyclopentane and the values of the parameters K^t, K¹ and used in the Conformational energy calculations are considered.Notations used 1 _i bond lengths 1 in the case of the regular ring - _i torsional angles - _i bond angles - 3/5 = 108 - 4/5 = 144 - , _{i i} – = complement to the 108 bond angle _i - _T - E Conformational energy of the 5-membered ring - E Conformational energy difference between planar and deformed ring - A _n Coefficients of the energy development in terms of - E _i ^l Bond energy relative to atom i (associated with angle _i) - K _i ^l Bond constant relative to atom i (associated with angle _i) - E _i ^l Torsional energy relative to the i ^th bond (associated with angle _i) - k _i ^l Torsional constant relative to the i ^th bond (associated with angle _i) - _i Angle _i value corresponding to zero bond energy E _i ^l (when the 5 atoms of the ring are identical, _i ) - r _ij Distance between atoms i and j - q _i Charge carried by atom i - e Constant of proportionality including the effective dielectric constant - A _ij, B_ij, d_ij Coefficients dependent on the nature of the atoms i and j and accounted for in the Van der Waals energy and hydrogen bond expressions - S (r _ij) Electrostatic contribution to the hydrogen bond energy - P Pseudorotation phase angle - _m Maximum torsional angle value characterising the deformation amplitudeM     

A Salt-Tolerant Cultivar of Wheat Maintains Photosynthetic Activity by Suppressing Sodium Uptake

The effects of NaCl treatment on the photosynthetic machinery in wheat (Triticum aestivum L.) cultivars differing in salt tolerance were investigated by comparison with iso-osmotic PEG treatment. Both cultivars similarly reduced the photosystem 2 (PS2) energy conversion efficiency (_PS2) rapidly when plants were exposed to a 100 mM NaCl solution, though no decline was detected under the iso-osmotic PEG treatment. There was no correlation between the reduction of the leaf relative water content (RWC) and the _PS2 in the two iso-osmotic stress treatments. In contrast, a decline of _PS2 along with the increase of the leaf sodium content above 4 % dry matter was detected under the NaCl treatment, while no such correlation was detected with other cations. The recovery of _PS2 after photoinhibitory irradiation was repressed by the NaCl treatment as the increase of the duration of the treatment. Norin 61 subjected to the 100 mM NaCl treatment for 10 d showed a decline of the _PS2 after 1 h moderate irradiation of 400 mol m^–2 s^–1 PPFD. Thus the concentrated Na⁺ within a leaf under salinity treatments may decrease the stability of PS2 functions and lead to photochemical inactivation.     

Leaf anthocyanin content changes in Zea mays L. grown at low temperature: Significance for the relationship between the quantum yield of PS II and the apparent quantum yield of CO2 assimilation

The quantum yield of non-cyclic electron transport from PS II (PS II) and the apparent quantum yield of CO2 fixation (CO2) were measured in the maize genotype, R-CH HOPI, which shows a high leaf anthocyanin content when grown at a temperature slightly below 20 °C. Thus, the leaf anthocyanin content was thirty-five times higher in plants grown at 18 °C when compared to plants grown at 23 °C. The relationship between PS II and CO2 obtained at different CO2 partial pressure was linear for plants with both high and low leaf anthocyanin content. The PS II/CO2 ratio was about 16 in plants with high leaf anthocyanin content and about 10 in plants with low leaf anthocyanin content. The leaf light absorptance in the 400–700 nm region was higher in plants with higher leaf anthocyanin content. Since leaf absorptance between 400 and 600 nm and leaf anthocyanin content also resulted in a strict linear relationship, an indirect estimation of the absorbed light by leaf anthocyanins and thus at chloroplasts was derived. Using the correct estimation of the absorbed light at chloroplasts, to obtain CO2, differences in PS II/CO2 ratios between plants with different leaf anthocyanin content were eliminated. The modulation of leaf anthocyanin content by growth temperature is regarded as an effective strategy to modulate the light available at the chloroplasts.     

The relationship between Photosystem II intrinsic quantum yield and millisecond luminescence in thylakoids

The relationship between charge recombination at Photosystem II (PS II), as indicated by millisecond luminescence, and PS II quantum yield was studied in spinach thylakoids during electron flow to methylviologen. Under the low magnesium conditions used, a decrease in quantum yield was observed in the absence of non-photochemical excitation quenching, and therefore cannot be due to a restriction in excitation delivery to the reaction centre. It was found that the decrease of the parameter _p, which is a measure of the intrinsic quantum yield of open PS II centers, correlates with an increase in luminescence per open center. The relationship between these two parameters was the same whether _p was manipulated by dissipation of the transthylakoid pH gradient or of the electrical potential. This indicates that the mechanism by which _p decreases depends in the same way on the two components of the protonmotive force as does the charge recombination at PS II. Calculation of the yield of luminescence with respect to the back reaction will be necessary to determine whether the charge recombination occurs at a sufficiently high rate to be directly responsible for the _p decrease.