Partial decline of Arachis hypogaea L. photosynthesis triggered by drought stress

Photosynthetic capacity (PC) of three peanut cultivars (Arachis hypogaea L. cvs. 57-422, 73-30, and GC 8-35) decreased during drought stress (decline in relative water content from ca. 95 to 70 %) and recovered two days after rewatering. Mild water stress was not limiting for the total ribulose-1,5-bisphosphate carboxylase/oxygenase activity, since this enzyme activity increased under drought. Photosystem (PS) 2 and PS1 (the latter only in cv. GC 8-35) electron transport activities decreased under drought. The ratio of the variable to maximal chlorophyll fluorescence (Fv/Fm) decreased mainly in the cv. GC 8-35. All cultivars showed decreases in photochemical quenching (qP) and quantum yield of PS2 electron transport (Φe). Increase of basal fluorescence (F0) was observed in the cvs. 73-30 and GC 8-35, while the cv 57-422 showed a decrease. After rewatering a sharp increase was observed in the majority of the parameters. Thus under the present stress conditions, the cv GC 8-35 was the most affected for all the parameters under study. The cv. 57-422 showed a higher degree of tolerance being gradually affected in photosynthetic capacity (PC) in contrast to the two other cvs. which showed a sharp decrease in PC at the beginning of the drought cycle. This revised version was published online in September 2006 with corrections to the Cover Date.     

Drought Effects on Membrane Lipids and Photosynthetic Activity in Different Peanut Cultivars

The effects of drought on thylakoid acyl lipid composition, photosynthetic capacity (P _max), and electrolyte lekage were evaluated in two-months-old peanut cultivars (57-422, 73-30, GC 8-35) growing in a glasshouse. For lipid studies, plants were submitted to three treatments by withholding irrigation: control (C), mild water stress (S1), and severe water stress (S2). Concerning membrane and photosynthetic capacity stability, drought was imposed by polyethylene glycol (PEG 600). In the cv. 73-30 a sharp decrease in the content of thylakoid acyl lipids was observed, already under S1 conditions, whereas cv. 57-422 was strongly affected only under S2. Cv. GC 8-35 had the lowest content of acyl lipids under control conditions, a significant increase under S1 conditions, and only under S2 a decrease occurred. Thus concerning lipid stability, cv. 73-30 was the most sensitive. Among lipid classes, phospholipids and galactolipids were similarly affected, as was MGDG relatively to DGDG. Water deficit imposed by PEG induced a higher increase in electrolyte leakage in cv. 73-30 than in the other cvs. A positive relationship between acyl lipid concentration and membrane integrity was found in all studied cvs. A positive association between acyl lipid concentration, membrane integrity, and P _max was found in the cvs. 57-422 and 73-30.     

Peanut Photosynthesis Under Drought and Re-Watering

The photosynthetic response of three Arachis hypogaea L. cultivars (57-422, 73-30, and GC 8-35) grown for two months was measured under water available conditions, severe water stress, and 24, 72, and 93 h following re-watering. At the end of the drying cycle, all the cultivars reached dehydration, relative water content (RWC) ranging between 40 and 50 %. During dehydration, leaf stomatal conductance (g _s), transpiration rate (E), and net photosynthetic rate (P _N) decreased more in cvs. 57-422 and GC 8-35 than in 73-30. Instantaneous water use efficiency (WUE_i) and photosynthetic capacity (P _max) decreased mostly in cv. GC 8-35. Except in cv. GC 8-35, the activity of photosystem 1 (PS1) was only slightly affected. PS2 and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were the main targets of water stress. After re-watering, cvs. 73-30 and GC 8-35 rapidly regained g _s, E, and P _N activities. Twenty-four hours after re-watering, the electron transport rates and RuBPCO activity strongly increased. P _N and P _max fully recovered later. Considering the different photosynthetic responses of the studied genotype, a general characterisation of the interaction between water stress and this metabolism is presented.     

Enhancement of susceptivity to photoinhibition and photooxidation in rice chlorophyll <Emphasis Type="Italic">b</Emphasis>-less mutants

Two rice chlorophyll (Chl) b-less mutants (VG28-1, VG30-5) and the respective wild type (WT) plant (cv. Zhonghua No. 11) were analyzed for the changes in Chl fluorescence parameters, xanthophyll cycle pool, and its de-epoxidation state under exposure to strong irradiance, SI (1 700 μmol m⁻² s⁻¹). We also examined alterations in the chloroplast ultrastructure of the mutants induced by methyl viologen (MV) photooxidation. During HI (0–3.5 h), the photoinactivation of photosystem 2 (PS2) appeared earlier and more severely in Chl b-less mutants than in the WT. The decreases in maximal photochemical efficiency of PS2 in the dark (F_v/F_m), quantum efficiency of PS2 electron transport (Φ_PS2), photochemical quenching (q_P), as well as rate of photochemistry (P_rate), and the increases in de-epoxidation state (DES) and rate of thermal dissipation of excitation energy (D_rate) were significantly greater in Chl b-mutants compared with the WT plant. A relatively larger xanthophyll pool and 78–83 % conversion of violaxanthin into antheraxanthin and zeaxanthin in the mutants after 3.5 h of HI was accompanied with a high ratio of inactive/total PS2 (0.55–0.73) and high 1–q_P (0.57–0.68) which showed that the activities of the xanthophyll cycle were probably insufficient to protect the photosynthetic apparatus against photoinhibition. No apparent difference of chloroplast ultrastructure was found between Chl b-less mutants and WT plants grown under low, LI (180 μmol m⁻² s⁻¹) and high, HI (700 μmol m⁻² s⁻¹) irradiance. However, swollen chloroplasts and slight dilation of thylakoids occurred in both mutants and the WT grown under LI followed by MV treatment. These typical symptoms of photooxidative damage were aggravated as plants were exposed to HI. Distorted and loose scattered thylakoids were observed in particular in the Chl b-less mutants. A greater extent of photoinhibition and photooxidation in these mutants indicated that the susceptibility to HI and oxidative stresses was enhanced in the photosynthetic apparatus without Chl b most likely as a consequence of a smaller antenna size.     

Cytochrome b6-f complex : The carburettor of exciton distribution in oxygenic photosynthesis

Efficient oxygenic photosynthesis not only requires synchronous turover and operation of photosystem I (PS I) and photosystem II (PS II) but also the preferential turnover of PS I for cyclic photophosphorylation to maintain required ATP and NADPH ratio during carbon dioxide reduction. Ohe initial higher rate of turnover of PS IIin viva is accounted by the fact that (i) PS I contains only about one-third of total chlorophylls, (ii) about 90% of light harvesting a/b protein (LAC) which accounts for about 50% of the total chlorophylls, remains associated with PS II as PS II-LHC II complexes (PS II_α and (iii) the ratio of PS II/PS I is always greater than unity, in the range of 1–2 : 1 under different environmental regimes. Ohe initial preferential feeding of PS II, due to its larger antenna, is bound to result in faster rate of turn over of PS II than PS I, leading to higher rate of reduction of an intersystem carrier than the rate of its oxidation by PS I. Ohe light dependent phosphorylation of a ‘mobile’ and small pool (−20%) of LHC II of PS II_α (possibly located at the edge of appressed regions of the membranes) increases the repulsive forces of LHC II resulting in its migration to non-appressed region associating itself with PS 1. Ohe phosphorylation itself is controlled by the redox state of an intermediate of electron transport. Several experimental approaches have provided evidence which suggest that (i) phosphorylation of LAC II involves interaction of cyt b5-f complex with LAC II kinase and the interaction of QA with cyt b₅-f complex and (ii) different kinases may be involved in phosphorylation of LHC IIversus PS II polypeptides. Ohe major purpose of light dependent LAC II phosphorylation and its consequent migration close to PS I appears to balance the rate of cyclicversus non-cyclic photophosphorylation. Ohe mechanism by which cyt b₅-f complex controls the activation of LAC II is not known. Ohe role of membrane bound ealmodulin, electron transfer through cyt b₆-f complex in activation of LAC II kinase should be explored.     

Proline Rich Polypeptide (PRP-1) Increases the Superoxide-Producing and Ferrihemoglobin Reducing Activities of Cytochrome B<Subscript>558</Subscript> Isoforms from Human Lymphosarcoma Tissue Cells

The two cytochromes (cyt) b₅₅₈ of acidic nature, one—95–100 kDa and another one, 60–70 kDa were isolated for the first time from the human’s lymphosarcoma tissue cells using gel filtration and ion exchange chromatography. These hemoproteins possess NADPH dependent O₂ ⁻-producing and ferrihemoglobin-reducing activities. The incubation of neuropeptide PRP-1 (5 μg) with cytochrome b₅₅₈, caused elevation of these activities. The gel filtration results indicated possible binding of PRP-1 to these cytochromes b₅₅₈. PRP-1 activated both NADPH dependent O₂ ⁻-producing and ferriHb-reducing activities of the cyt b¹ ₅₅₈ and cyt b² ₅₅₈, obtained from human lymphosarcoma tissue cells. One can assume that PRP-1 associated with cyt b₅₅₈ on the surface of the tumor cells by increasing both NADPH dependent O₂ ⁻-producing and ferriHb-reducing activities of cyt b₅₅₈, increases the oxidation- reduction status. Changing the oxidation–reduction status and oxygen homeostasis of the tumor cells by PRP-1 can serve as one of the possible explanation of antitumorigenic effect of this cytokine.     

Chilling stress and chilling tolerance of sweet potato as sensed by chlorophyll fluorescence

We studied changes in the chlorophyll (Chl) fluorescence components in chilling-stressed sweet potato (Ipomoea batatas L. Lam) cv. Tainung 57 (TN57, chilling-tolerant) and cv. Tainung 66 (TN66, chilling-susceptible). Plants under 12-h photoperiod and 400 μmol m⁻² s⁻¹ irradiance at 24/20 °C (day/night) were treated by a 5-d chilling period at 7/7 °C. Compared to TN66, TN57 exhibited a significantly greater basic Chl fluorescence (F₀), maximum fluorescence (F_m), maximum fluorescence yield during actinic irradiation (F_m′ ), and the quantum efficiency of electron transport through photosystem 2, PS2 (Φ_PS2). Chilling stress resulted in decrease in the potential efficiency of PS2 (F_v/F_m), Φ_PS2, non-photochemical fluorescence quenching (NPQ), non-photochemical quenching (q_N), and the occurrence of chilling injury in TN66. Chilling increased the likelihood of photoinhibition, characterized by a decline in the Chl fluorescence of both cultivars, and photoinhibition during low temperature stress generally occurred more rapidly in TN66.     

Competition between the photosynthetic and the (chloro)respiratory electron transport chains in cyanobacteria, green algae and higher plants. Effect of heat stress

By measuring the effect of cyanide on the flash-induced redox reactions of the cytochrome (cyt) b ₆/f complex we carried out a comparative study in order to characterize the interaction between the photosynthetic and the respiratory electron transport systems in cyanobacterial (Synechococcus sp. PCC 6301) and green algal (Chlamydomonas reinhardtii) cells, and in tobacco (Nicotiana tabacum L. cv. Petit Havana SR1) protoplasts. We found that the addition of 1 mM KCN resulted in a significant acceleration of the rereduction-rate of cyt f ⁺. This enhancement of the activity of the cyt b ₆/f complex apparently occurred with the same mechanism in prokaryotes and eukaryotes, and its dependence on the concentration of KCN in eukaryotes ruled out an origin in mitorespiration, superoxide dismutase and plastocyanin, strongly suggesting that a cyanide-sensitive terminal oxidase, a putative component of chlororespiration, competes with photosystem 1 (PS1) for electrons from the plastoquionone (PQ) pool. Concerning the physiological role of the competition between the (chloro)respiratory and the photosynthetic electron transport systems, our data obtained with cyanobacterial and algal cells incubated at elevated temperatures (30–50 °C) showed that the respiratory control over photosynthesis became significant in cells exposed to heat-stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic studies of electron transfer in the cyt b 5 : metHb system

 The kinetics of methemoglobin reduction by cytochrome b ₅ has been studied by stopped-flow and saturation transfer NMR. A forward rate constant k _f = 2.44×10⁴ M^–1 s^–1 and a reverse rate constant k _b = 540 M^–1s^–1 have been observed at 10 mm, pH 6.20, 25  °C. The ratio k _f/k _b = k _eq = 43.6 is in good agreement with the equilibrium constant calculated from the electrochemical potential between cyt b ₅ and methemoglobin. A bimolecular collisional mechanism is proposed for the electron transfer from cyt b ₅ to methemoglobin based on the kinetic data analysis. The dependence of the rate constants on ionic strengths supports such collisional mechanism. It is also found that the reaction rate strongly depends on the conformations of methemoglobin. Received: 20 February 1996 / Accepted: 4 June 1996     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Characterization of acclimation of Hordeum vulgare to high irradiation based on different responses of photosynthetic activity and pigment composition

The ability of spring barley (Hordeum vulgare cv. Akcent) to adjust the composition and function of the photosynthetic apparatus to growth irradiances of 25–1200 μmol m⁻² s⁻¹ was studied by gas exchange and chlorophyll a fluorescence measurements and high-performance liquid chromatography. The increased growth irradiance stimulated light- and CO₂-saturated rates of CO₂ assimilation expressed on a leaf area basis up to 730 μmol m⁻² s⁻¹ (HL730), whereas at an irradiance of 1200 μmol m⁻² s⁻¹ (EHL1200) both rates decreased significantly. Further, the acclimation to EHL1200 was associated with an extremely high chlorophyll a/b ratio (3.97), a more than doubled xanthophyll cycle pool (VAZ) and a six-fold higher de-epoxidation state of the xanthophyll cycle pigments as compared to barley grown under 25 μmol m⁻² s⁻¹ (LL25). EHL1200 plants also exhibited a long-term inhibition of Photosystem II (PS II) photochemical efficiency (F _v/F _m). Photosynthetic capacity, chlorophyll a/b and VAZ revealed a linear trend of dependence on PS II excitation pressure in a certain range of growth irradiances (100–730 μmol m⁻² s⁻¹). The deviation from linearity of these relationships for EHL1200 barley is discussed. In addition, the role of increased VAZ and/or accumulation of zeaxanthin and antheraxanthin in acclimation of barley to high irradiance is studied with respect to regulation of non-radiative dissipation and/or photochemical efficiency within PS II. This revised version was published online in June 2006 with corrections to the Cover Date.     

Zinc sulphate improved microspore embryogenesis in barley

The effect of ZnSO₄ concentration on barley (Hordeum vulgare L.) microspore embryogenesis was investigated using cultivars of different androgenetic response. Concentrations from 0 (control) to 600 μM in the stress pre-treatment medium alone or in combination with 30 (control) to 600 μM in the embryo induction medium were assayed in anther culture. Incorporation of Zn²⁺ in the pre-treatment medium itself did not affect microspore embryogenesis. The optimum concentration in the stress pre-treatment and induction media was 180 μM for cultivars (cvs.) Igri and Reinette, and 90 μM for cv. Hop. A significant increase of 30 and 300% in cv. Igri and Reinette, respectively, were produced with 180 μM ZnSO₄ in both the number of embryos and green plants. In order to confirm the effect of Zn²⁺ on microspore embryogenesis this micronutrient was incorporated in the induction medium of isolated microspore cultures of cv. Igri. Concentrations of 90–300 μM ZnSO₄ resulted in an increase of 40–53% in the number of embryos and green plants. All these results indicate that the beneficial effect of Zn²⁺ is exerted mainly during the culture phase, increasing the number of embryos, leading to an increased number of green plants, but it had no effect on percentage of regeneration or green plants.     

Molecular modeling of cytochrome b₅ with a single cytochrome c-like thioether linkage

Bovine liver cytochrome b ₅ (cyt b ₅), with heme bound noncovalently, has been converted into a cyt c-like protein (cyt b ₅ N57C) by constructing a thioether linkage between the heme and the engineered cysteine residue. With no X-ray or NMR structure available, we herein performed a molecular modeling study of cyt b ₅ N57C. On the other hand, using amino acid sequence information for a newly discovered member of the cyt b ₅ family, domestic silkworm cyt b ₅ (DS cyt b ₅), we predicted the protein structure by homology modeling in combination with MD simulation. The modeling structure shows that both Cys57 in cyt b ₅ N57C, and Cys56, a naturally occurring cysteine in DS cyt b ₅, have suitable orientations to form a thioether bond with the heme 4-vinyl group, as the heme is in orientation A. In addition to providing structural information that was not previously obtained experimentally, these modeling studies provide insight into the formation of cyt c-like thioether linkages in cytochromes, and suggest that c-type cyt b ₅ maturation involves a b-type intermediate.     

Interactions of uranyl ion with cytochrome <Emphasis Type="Italic">b</Emphasis><Subscript>5</Subscript> and its His39Ser variant as revealed by molecular simulation in combination with experimental methods

The biological toxicity of uranyl ion (UO₂²⁺) lies in interacting with proteins and disrupting their native functions. The structural and functional consequences of UO₂²⁺ interacting with cytochrome b ₅ (cyt b ₅), a small membrane heme protein, and its heme axial ligand His39Ser variant, cyt b ₅ H39S, were investigated both experimentally and theoretically. In experiments, although cyt b ₅ was only slightly affected, UO₂²⁺ binding to cyt b ₅ H39S with a K _D of 2.5 μM resulted in obvious alteration of the heme active site, and led to a decrease in peroxidase activity. Theoretically, molecular simulation proposed a uranyl ion binding site for cyt b ₅ at surface residues of Glu37 and Glu43, revealing both coordination and hydrogen bonding interactions. The information gained in this study provides insights into the mechanism of uranyl toxicity toward membrane protein at an atomic level.     

Photosynthetic response of different pea cultivars to low and high temperature treatments

The thermo-sensitivity of two new pea (Pisum sativum L.) cultivars—Afila (mutant in the gene transforming leaves into mustaches) and Ranen (mutant for early ripening)—as compared to the control cultivar Pleven-4 to either low (4 °C, T₄) or high temperature (38 °C, T₃₈) was investigated by means of chlorophyll (Chl) fluorescence kinetics. The low temperature treatment decreased the photosynthetic activity, measured via a decline of the Chl fluorescence decrease ratios R_Fd690 and R_Fd735, and this was mainly due to a decline of the Chl fluorescence decrease parameter F_d and maximum Chl fluorescence F_m. In the new cv. Ranen the R_Fd ratios at first decreased and increased again after 24-h exposure to 4 °C, indicating its good acclimation ability to low temperature. The cold-induced changes in the photosynthetic performance of all cultivars were reversed after transferring plants back to 23 °C for 48 h. In the Chl and carotenoid (Car) contents no or little changes occurred during the T₄ treatment, except for a slight but clear increase of the ratio Chl a/b and a decrease in the ratio Chl/Car. In contrast to this, the T₃₈ treatment for 72 h decreased the R_Fd ratios more strongly than the T₄ exposure did. In fact, an irreversible injury of the photosynthetic apparatus was caused in the control pea cv. Pleven-4 by a 48-h T₃₈ exposure and for the new cv. Afila after a 72-h T₃₈ exposure. In contrast, the cv. Ranen was less and little sensitive to the T₃₈ exposure. In the heat-sensitive cvs. Pleven-4 and Afila, the decrease in R_Fd values at T₃₈ was associated with a strong decline of the Chl a+b and total Car contents. The Chl a+b decline could also be followed via an increase of the Chl fluorescence ratio F₆₉₀/F₇₃₅. Parallel to this, a strong decline of Chl a/b from ca. 3.0 (range 2.85–3.15) to ca. 1.9 (range 1.85–1.95) occurred indicating a preferential decline of the Chl a-pigment proteins but not of the Chl a/b-pigment protein LHC2. In the relatively heat-tolerant cv. Ranen, however, the ratio Chl a/b declined only partially. After the T₄ treatment the stress adaptation index Ap was higher in cv. Ranen than in controls and reached in heat-treated Ranen plants almost the starting value indicating a cold and heat stress hardening of the treated plants. The Chl fluorescence parameters and pigment contents were influenced by T₃₈ and T₄ treatments in various ways indicating that the mechanisms of low and high temperature injury of the photosynthetic apparatus are different. The new cv. Ranen exhibited a cross tolerance showing a fairly good acclimation ability to both T₄ and T₃₈, hence it is a very suitable plant for outdoor growth and for clarification of the acclimation mechanisms to unfavourable temperatures.     

Chromosomal location of genes for resistance to powdery mildew in common wheat (Triticum aestivum L. em Thell.) 6. Alleles at the Pm5 locus

Genetic characterization of powdery mildew resistance genes were conducted in common wheat cultivars Hope and Selpek possessing resistance gene Pm5, cvs. Ibis and Kormoran expressing resistance gene Mli, a backcross-derived line IGV 1–455 and a Triticum sphaerococcum var. rotundatum Perc. line Kolandi. Monosomic analyses revealed that one major recessive gene is located on chromosome 7B in the lines IGV 1–455 and Kolandi. Allelism tests of the F₂ and F₃ populations involving the tested resistant lines crossed with either cv. Hope or Selpek indicated that their resistance genes are alleles at the Pm5 locus. The alleles are now designated Pm5a in Hope and Selpek, Pm5b in Ibis and Kormoran, Pm5c in T. sphaerococcum var. rotundatum line Kolandi, and Pm5d in backcross-derived line IGV 1–455, respectively. Received: 5 November 1999 / Accepted: 14 April 2000     

Effects of charged peptides on electron transfer from [Fe(CN)6]4– to cytochrome c or plastocyanin

 Interactions of charged peptides, such as aspartic acid peptides (Aspptds) and lysine peptides (Lysptds), with cytochrome c (cyt c) or plastocyanin (PC) have been studied by measuring electron transfer between [Fe(CN)₆]^4– and cyt c or PC in the presence of these peptides. Aspptds, up to penta-aspartic acid, served as competitive inhibitors of electron transfer from [Fe(CN)₆]^4– to oxidized cyt c, while Lysptds, up to penta-lysine, promoted electron transfer from [Fe(CN)₆]^4– to oxidized PC. The electron transfer inhibitory effects of Aspptds are explained as competitive inhibition due to neutralization of the positively charged amino acid residues at the surface of cyt c by electrostatic interactions, whereas the electron transfer promoting effects of Lysptds may be due to formation of PC·Lysptd or Lysptd·[Fe(CN)₆]^4– complexes subsequently forming an electron transferring complex, PC·Lysptd·[Fe(CN)₆]^4–, without repulsion of the negative charges. The inhibitory effect of Aspptds and promotional effect of Lysptds became significant as the net charge or concentration of the peptides increased. The promotional effects of Lysptds decreased as the net charge of the PC negative patch was decreased by mutagenesis. Thus, charged peptides may serve as a probe for investigation of the molecular recognition character of proteins. Received: 19 May 1998 / Accepted: 27 July 1998     

Volume changes and electrostriction in the primary photoreactions of various photosynthetic systems: estimation of dielectric coefficient in bacterial reaction centers and of the observed volume changes with the Drude–Nernst equation

Photoacoustics (PA) allows the determination of enthalpy and volume changes of photoreactions in photosynthetic reaction centers on the 0.1–10 μs time scale. These include the bacterial centers from Rb. sphaeroides, PS I and PS II centers from Synechocystis and in whole cells. In vitro and in vivo PA data on PS I and PS II revealed that both the volume change (–26 A³) and reaction enthalpy (–0.4 eV) in PS I are the same as those in the bacterial centers. However the volume change in PS II is small and the enthalpy far larger, –1 eV. Assigning the volume changes to electrostriction allows a coherent explanation of these observations. One can explain the large volume decrease in the bacterial centers with an effective dielectric coefficient of ∼4. This is a unique approach to this parameter so important in estimation of protein energetics. The value of the volume contraction for PS I can only be explained if the acceptor is the super- cluster (Fe₄S₄)(Cys₄) with charge change from –1 to –2. The small volume change in PS II is explained by sub-μs electron transfer from Y_Z anion to P₆₈₀ cation, in which charge is only moved from the Y_Z anion to the Q_A with no charge separation or with rapid proton transfer from oxidized Y_Z to a polar region and thus very little change in electrostriction. At more acid pH equally rapid proton transfer from a neighboring histidine to a polar region may be caused by the electric field of the P₆₈₀ cation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparison of resistance to drought of three bean cultivars

The aim of the present work was to evaluate oxidative stress and plant antioxidant system of three contrasting bean (Phaseolus vulgaris L.) genotypes in the response to drought. Drought was imposed 14 d after emergence, by withholding water, until leaf relative water content reached 65 %. Water stress increased lipid peroxidation (LPO), membrane injury index, H₂O₂ and OH^⋅ production in leaves of stressed plants. Activities of the antioxidative enzymes superoxide dismutase (SOD) and ascorbate peroxidase (APOX) increased significantly under water stress in all the studied cultivars, while catalase (CAT) increased in cvs. Plovdiv 10 and Prelom, but decreased in cv. Dobrudjanski ran. Furthermore cv. Plovdiv 10 which had the highest APOX and CAT activities also showed the lowest increase in H₂O₂ and OH^⋅ production and LPO while cv. Dobrudjanski ran showed the lowest increases (and often the lowest values) in the antioxidant enzyme activities and the highest increases of H₂O₂ and OH^⋅ production, and LPO. On the basis of the data obtained we could specify cv. Plovdiv 10 and cv. Prelom as drought tolerant and cv. Dobrudjanski ran as a drought sensitive.     

Efficient production of androgenic doubled-haploid mutants in barley by the application of sodium azide to anther and microspore cultures

 The aim of this study was to establish a protocol for an efficient production of agronomical and/or physiological mutants from model (cvs. Igri and Cobra) and low-androgenic-responding (cv. Volga) cultivars of barley through the application of a mutagenic agent, sodium azide, to anthers and isolated microspores cultured in vitro. This technology offers the possibilities of screening for recessive mutants in the first generation, selecting for novel genotypes from very large haploid populations, avoiding chimerism and rapidly fixing selected genotypes as fertile true breeding lines. The mutagenic treatment, 10^–3–10^–5 M sodium azide, was applied during the anther induction pre-treatment or immediately after the microspore isolation procedure. Out of 616 M₂ doubled-haploid lines characterised under field conditions, a total of 63 morphological and developmental independent mutant lines were identified. The percentage of M₂ doubled-haploid lines carrying mutations per line analysed was 3.8% when 10^–4 M sodium azide was applied to anthers from the low-responding cv. Volga; this increased to 8.6% and 15.6% when 10^–5 and 10^–4 M sodium azide were applied to freshly isolated microspores from model cultivars. Received: 18 April 2000 / Revision received: 28 September 2000 / Accepted: 28 September 2000