Over-production of the D1:2 protein makes Synechococcus cells more tolerant to photoinhibition of photosystem II

Over-expression of the psbAIII gene encoding for the D1 protein (form II; D1:2) of the photosystem II reaction centre in the Synechococcus sp. PCC 7942 was studied using a tac promoter and the lacI ^Q system. Over-expression was induced with 40 g/ml IPTG in the growth medium for either 6 or 12 h at growth irradiance (50 mol photons m^-2 s^-1). This treatment doubled the amount of psbAII/III mRNA and the D1:2 protein in membranes but decreased the amount of psbAI messages and the D1:1 protein. The total amount of both heterodimeric reaction centre proteins, D1 and D2, remained constant under growth light conditions, indicating that the number of PSII centres in the membranes was not affected, only the form of the D1 protein was changed from D1:1 to D1:2 in most centres. When the cells were photoinhibited either at 500 or 1000 mol photons m^-2 s^-1, in the presence or absence of the protein synthesis inhibitor lincomycin, the D1:2 protein remained at a higher level in cells in which over-expression had been induced by IPTG. These cells were also less prone to photoinhibition of PSII. It is suggested that the tolerance of cells to photoinhibition increases when most PSII reaction centres contain the D1:2 protein at the beginning of high irradiance. This tolerance is further strengthened by maintaining psbAIII gene over-expression during the photoinhibitory treatment.     

Form II of D1 is important during transition from standard to high light intensity in Synechococcus sp. strain PCC 7942

In Synechococcus sp. strain PCC 7942 the D1 protein of Photosystem II is encoded by a multigene family; psbAI encodes Form I of D1 whereas both psbAII and psbAIII encode Form II. The psbA genes are differentially regulated in response to changes in light intensity, such that psbAI expression and Form I predominate at standard light intensity, whereas psbAII and psbAIII are induced at high light intensity, causing insertion of Form II into the thylakoids. The present study addressed whether high-light induced Form II is important for Synechococcus cells during adaptation to high light intensity. Wild-type Synechococcus, and mutants which produce only Form I (R2S2C3) or only Form II (R2K1), were co-cultured at standard light (130 E · m^–2 · s^–1) and then shifted to high light (750 E·m^–2·s^–1). Measurement of the proportion of each cell type at various time intervals revealed that the growth of R2S2C3, which has psbAII and psbAIII inactive, and thus lacks Form II, is transiently impaired upon shift to high light. Both mutants R2S2C3 and R2K1 maintained normal levels of psbA messages and D1 protein under standard and high light through an unknown mechanism that compensates for the inactive psbA genes. Thus, the impairment of R2S2C3 at high light is not due to a deficiency of D1 protein, but results from lack of Form II. We discounted the influence of possible secondary mutations by re-creating the psbA-inactivated mutants and testing the newly isolated strains. We conclude that Form II of D1 is intrinsically important for Synechococcus cells during a critical transition period after exposure to high light intensities.     

Photoinactivation of functional photosystem II and D1-protein synthesis in vivo are independent of the modulation of the photosynthetic apparatus by growth irradiance

To investigate whether the in-vivo photoinhibition of photosystem II (PSII) function by excess light is an intrinsic property of PSII, the maximal photochemical efficiency of PSII (Fv/Fm) and the content of functional PSII (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum sativum L.), grown in 50 (low light), 250 (medium light), and 650 (high light) mol photons·m^–2·s^–1. The modulation of PSII functionality in vivo was induced in 1.1% CO₂ by varying either (i) the duration (0–2 h) of light treatment (fixed at 1800 mol photons· m^–2·s^–1) or (ii) irradiance (0–3200 mol photons·m^–2·s^–1) at a fixed duration (1 h), after infiltration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), or a combination of lincomycin with nigericin (an uncoupler), through the cut petioles of leaves of 22-to 24-d-old plants. The reciprocity law of irradiance and duration of illumination for PSII function in vivo (Park et al. 1995, Planta 196: 401–411) holds in all differently light-grown peas, demonstrating that inactivation of functional PSII depends on photon exposure (mol photons·m^–2), not on the rate of photon absorption. In vivo, PSII acts as an intrinsic photon counter and at higher photon exposures is inactivated following absorption of about 3 × 10⁷ photons. There is a functional heterogeneity of PSII in vivo with 25% less-stable PSIIs that are inactivated at low photon exposure, compared to 75% more-stable PSIIs regardless of modulation of the photosynthetic apparatus. We suggest that the less-stable PSIIs represent monomers located in the nonappressed granal margins, while the more-stable PSIIs are dimers located in the appressed grana membrane cores. The capacity for D1-protein synthesis was the same in all the light-acclimated peas and saturated at low light, indicating that D1-protein repair is also an intrinsic property of PSII. This accounts for the low intensity required for recovery of photoinhibition in sun and shade plants which is independent of light-harvesting antennae size or PSII/PSI stoichiometries.Abbreviations D1-protein psbA gene product - D2 protein psbD gene product - Fo chlorophyll fluorescence corresponding to open PSII reaction centres - Fv, Fm variable and maximum fluorescence after dark incubation, respectively - PS photosystem - Q_B secondary quinone electron acceptor Financial support for this research by the Department of Employment, Education and Training/Australian Research Council International Research Fellowships Program (Korea) is gratefully acknowledged.     

Photoinhibition and repair in Dunaliella salina acclimated to different growth irradiances

The light-dependent rate of photosystem-II (PSII) damage and repair was measured in photoautotrophic cultures of Dunaliella salina Teod. grown at different irradiances in the range 50–3000 mol photons · m^–2· s^–1. Rates of cell growth increased in the range of 50–800 mol photons·m^–2·s^–1, remained constant at a maximum in the range of 800–1,500 mol photons·m^–2 ·s^–1, and declined due to photoinhibition in the range of 1500–3000 mol photons·m^–2·s^–1. Western blot analyses, upon addition of lincomycin to the cultures, revealed first-order kinetics for the loss of the PSII reaction-center protein (D1) from the 32-kDa position, occurring as a result of photodamage. The rate constant of this 32-kDa protein loss was a linear function of cell growth irradiance. In the presence of lincomycin, loss of the other PSII reaction-center protein (D2) from the 34-kDa position was also observed, occurring with kinetics similar to those of the 32-kDa form of D1. Increasing rates of photodamage as a function of irradiance were accompanied by an increase in the steady-state level of a higher-molecular-weight protein complex ( 160-kDa) that cross-reacted with D1 antibodies. The steady-state level of the 160-kDa complex in thylakoids was also a linear function of cell growth irradiance. These observations suggest that photodamage to D1 converts stoichiometric amounts of D1 and D2 (i.e., the D1/D2 heterodimer) into a 160-kDa complex. This complex may help to stabilize the reaction-center proteins until degradation and replacement of D1 can occur. The results indicated an intrinsic half-time of about 60 min for the repair of individual PSII units, supporting the idea that degradation of D1 after photodamage is the rate-limiting step in the PSII repair process.Abbreviations Chl chlorophyll - PSI photosystem I - PSII photosystem II - D1 the 32-kDa reaction-center protein of PSII, encoded by the chloroplast psbA gene - D2 the 34-kDa reactioncenter protein of PSII, encoded by the chloroplast psbD gene - Q_A primary electron-accepting plastoquinone of PSII The work was supported by grant 94-37100-7529 from the US Department of Agriculture, National Research Initiative Competitive Grants Program.     

Light inactivation of functional photosystem II in leaves of peas grown in moderate light depends on photon exposure

To determine the dependence of in vivo photosystem (PS) II function on photon exposure and to assign the relative importance of some photoprotective strategies of PSII against excess light, the maximal photochemical efficiency of PSII (Fv/Fm) and the content of functional PSII complexes (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum satlvum L.) grown in moderate light. The modulation of PSII functionality in vivo was induced by varying either the duration (from 0 to 3 h) of light treatment (fixed at 1200 or 1800 mol photons · m^-2 · s^-1) or irradiance (from 0 to 3000 mol photons · m^-2 · s^-1) at a fixed duration (1 h) after infiltration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), nigericin (an uncoupler), or dithiothreitol (an inhibitor of the xanthophyll cycle) through the cut petioles of leaves of 22 to 24-day-old plants. We observed a reciprocity of irradiance and duration of illumination for PSII function, demonstrating that inactivation of functional PSII depends on the total number of photons absorbed, not on the rate of photon absorption. The Fv/Fm ratios from photoinhibitory light-treated leaves, with or without inhibitors, declined pseudo-linearly with photon exposure. The number of functional PSII complexes declined multiphasically with increasing photon exposure, in the following decreasing order of inhibitor effect: lincomycin > nigericin > DTT, indicating the central role of D1 protein turnover. While functional PSII and Fv/Fm ratio showed a linear relationship under high photon exposure conditions, in inhibitor-treated leaves the Fv/Fm ratio failed to reveal the loss of up to 25% of the total functional PSII under low photon exposure. The loss of this 25% of less-stable functional PSII was accompanied by a decrease of excitation-energy trapping capacity at the reaction centre of PSII (revealed by the fluorescence parameter, 1/Fo-1/Fm, where Fo and Fm stand for chlorophyll fluorescence when PSII reaction centres are open and closed, respectively), but not by a loss of excitation energy at the antenna (revealed by the fluorescence parameter, 1/Fm). We conclude that (i) PSII is an intrinsic photon counter under photoinhibitory conditions, (ii) PSII functionality is mainly regulated by D1 protein turnover, and to a lesser extent, by events mediated via the transthylakoid pH gradient, and (iii) peas exhibit PSII heterogeneity in terms of functional stability during photon exposure.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - Fo chlorophyll fluorescence corresponding to open PSII reaction centres - Fv, Fm variable and maximum fluorescence after dark incubation, respectively - Fs, Fm steady-state and maximum fluorescence during illumination, respectively - P680 reactioncentre chlorophyll and primary electron donor of PSII - PS photosystem Financial support of this work by Department of Employment, Education and Training/Australian Research Council International Research Fellowships Program (Korea) is gratefully acknowledged.     

Functional analysis of the two homologous psbA gene copies in Synechocystis PCC 6714 and PCC 6803

The cyanobacteria Synechocystis 6803 and 6714 contain three genes (psbA) coding for the D₁ protein. This protein is an essential subunit of photosystem II (PSII) and is the target for herbicides. We have used herbicide-resistant mutants to study the role of the two homologous copies of the psbA genes in both strains (the third copy is not expressed). Several herbicide resistance mutations map within the psbAI gene in Synechocystis 6714 (G. Ajlani et al.), Plant Mol. Biol. 13 (1989): (469–479). We have looked for mutations in copy II. Results show that in Synechocystis 6714, only psbAI contains herbicide resistance mutations. Relative expression of psbAI and psbAII has been measured by analysing the proportions of resistant and sensitive D₁ in the thylakoid membranes of the mutants. In normal growth conditions, 95% resistant D₁ and 5% sensitive D₁ were found. In high light conditions, expression of psbAII was enhanced, producing 15% sensitive D₁. This enhancement is specifically due to high light and not to the decrease of D₁ concentration caused by photoinhibition. Copy I of Synechocystis 6714 corresponds to copy 2 of Synechocystis 6803 since it was always psbA2 which was recombined in Synechocystis 6803 transformants. PSII of the transformant strains was found to be 95% resistant to herbicides as in resistant mutants of Synechocystis 6714.     

Acclimation of Arabidopsis thaliana to the light environment: changes in photosynthetic function

Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta was grown under light regimes of differing spectral qualities, which results in differences in the stoichiometries of the two photosynthetic reaction centres. The acclimative value of these changes was investigated by assessing photosynthetic function in these plants when exposed to two spectrally distinct actinic lights. Plants grown in an environment enriched in far-red light were better able to make efficient use of non-saturating levels of actinic light enriched in long-wavelength red light. Simultaneous measurements of chlorophyll fluorescence and absorption changes at 820 nm indicated that differences between plants grown under alternative light regimes can be ascribed to imbalances in excitation of photosystems I and II (PSI, PSII). Measurements of chlorophyll fluorescence emission and excitation spectra at 77 K provided strong evidence that there was little or no difference in the composition or function of PSI or PSII between the two sets of plants, implying that changes in photosynthetic stoichiometry are primarily responsible for the observed differences in photosynthetic function.Abbreviations Chl chlorophyll - FR far-red light - HF highirradiance FR-enriched light (400 mol·m^–2·s^–1, RFR = 0.72) - HW high-irradiance white light (400 mol·m^–2 1·1 s^–1RFR = 1.40) - LHCI, LHCII light-harvesting complex of PSI, PSII - qO quenching of dark-level chlorophyll fluorescence - qN non-photochemical quenching of variable chlorophyll fluorescence - qP photochemical quenching of variable chlorophyll fluorescence - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase We thank Dr. Sasha Ruban for assistance with the 77 K fluorescence measurements and for helpful discussions. This work was supported by Natural Environment Research Council Grant GR3/7571A.     

Control of formation of active soluble or inactive insoluble baker''s yeast α-glucosidase PI in Escherichia coli by induction and growth conditions

Summary Using standard growth conditions (LB medium, 37°C, induction with 5 mM IPTG) yeast -glucosidase PI expressed under the control of the regulated tac-hybrid promoter results in the synthesis of insoluble aggregated -glucosidase granules in Escherichia coli. Under these conditions active soluble -glucosidase amounts to less than 1% of the heterologously produced protein. However, the amount of soluble active -glucosidase was dramatically increased when the strong tac-hybrid promoter was to a limited extent induced. This was achieved at concentrations of 0.01 mM IPTG or of 1% lactose or lower in a lactosepermease deficient host strain containing the lacI ^qrepressor gene on an R-plasmid. The formation of active soluble -glucosidase was almost 100% when E. coli cells induced in this manner were cultivated under conditions that reduced growth rate, i.e. at decreased temperature, extreme pH values or in minimal and complete media supplemented with different carbon sources.Abbreviations IPTG isopropyl--D-thiogalactopyranoside - RB refractile body - t-PA tissue type plasminogen activator - IFN interferon - X--glucoside 5-bromo-4-chloro-indolyl--D-glucopyranoside     

Light and the maintenance of photosynthetic competence in leaves of Populus balsamifera L. during short-term exposures to high concentrations of sulfur dioxide

Leaves of Populus balsamifera grown under full natural sunlight were treated with 0, 1, or 2 l SO₂·1^-1 air under one of four different photon flux densities (PFD). When the SO₂ exposures took place in darkness or at 300 mol photons·m^-2·s^-1, sulfate accumulated to the levels predicted by measurements of stomatal conductance during SO₂ exposure. Under conditions of higher PFD (750 and 1550 mol·m^-2·s^-1), however, the predicted levels of accumulated sulfate were substantially higher than those obtained from anion chromatography of the leaf extracts. Light-and CO₂-saturated capacity as well as the photon yield of photosynthetic O₂ evolution were reduced with increasing concentration of SO₂. At 2 l SO₂·1^-1 air, the greatest reductions in both photosynthetic, capacity and photon yield occurred when the leaves were exposed to SO₂ in the dark, and increasingly smaller reductions in each occurred with increasing PFD during SO₂ exposure. This indicates that the inhibition of photosynthesis resulting from SO₂ exposure was reduced when the exposure occurred under conditions of higher light. The ratio F _v/F _M (variable/maximum fluorescence emission) for photosyntem II (PSII), a measure of the photochemical efficiency of PSII, remained unaffected by exposure of leaves to SO₂ in the dark and exhibited only moderate reductions with increasing PFD during the exposure, indicating that PSII was not a primary site of damage by SO₂. Pretreatment of leaves with SO₂ in the dark, however, increased the susceptibility of PSII to photoinhibition, as such pretreated leaves exhibited much greater reductions inF _V/F _M when transferred to moderate or high light in air than comparable control leaves.Abbreviations and symbols A₁₂₀₀ photosynthetic capacity (CO₂-saturated rate of O₂ evolution at 1200 mol photons·m^-2·s^-1) - F_o instantaneous fluorescence emission - F_M maximum fluorescence emission - F_V variable fluorescence emission - PFD photon flux density (400–700 nm) - PSII photosystem II     

Daphnia galeata life history response to heterogeneous food conditions and dissolved chemicals in the Rímov Reservoir

Laboratory cultivation experiments were carried out in order to determine the response of D. galeata to food conditions and dissolved chemicals in the water collected in three contrasting sites of the Rímov Reservoir. There were two types of cultivation media: (1) lake water with natural seston, filtered through a 40 m mesh size sieve, (2) lake water filtered through a membrane filter 0.45 m (or 0.22 m), with Scenedesmus subspicatus culture added to a concentration of 1.0 mg POC l^–1. In the treatment with natural seston, the parameters investigated (body length, filtering setae length, clutch size, egg volume and postembryonic development time) were determined mainly by the food level, which was higher in the upper part of the reservoir than near the dam. In one series, significantly larger clutches and smaller eggs together with longer postembryonic development (PED) were detected in daphnids cultivated in hypolimnetic water, compared to those in epilimnetic water. As particulate carbon concentrations were similar, this indicates that in that case food quality played a role shaping life histories in Daphnia. In some of the experiments with the filtered water, there was a detectable effect of dissolved chemical(s), which induced shorter PED and smaller primiparae in the animals cultivated in water from the upper part of the reservoir.     

The involvement of the photoinhibition of photosystem II and impaired membrane energization in the reduced quantum yield of carbon assimilation in chilled maize

In this study we investigated the basis for the reduction in the quantum yield of carbon assimilation in maize (Zea mays L. cv. LG11) caused by chilling in high light. After chilling attached maize leaves at 5° C for 6 h at high irradiance (1000 mol photons·m^–2·s^–1) chlorophyll fluorescence measurements indicated a serious effect on the efficiency of photochemical conversion by photosystem II (PSII) and measurements of [¹⁴C]atrazine binding showed that the plastoquinone binding site was altered in more than half of the PSII reaction centres. Although there were no direct effects of the chilling treatment on coupling-factor activity, ATP-formation capacity was affected because the photoinhibition of PSII led to a reduced capacity to energize the thylakoid membranes. In contrast to chilling at high irradiance, no photoinhibition of PSII accompanied the 20% decrease in the quantum yield of carbon assimilation when attached maize leaves were chilled in low light (50 mol photons·m^–2·s^–1). Thus it is clear that photoinhibition of PSII is not the sole cause of the light-dependent, chillinduced decrease in the quantum yield of carbon assimilation. During the recovery of photosynthesis from the chilling treatment it was observed that full [¹⁴C]atrazinebinding capacity and membrane-energization capacity recovered significantly more slowly than the quantum yield of carbon assimilation. Thus, not only is photoinhibition of PSII not the sole cause for the decreased quantum yield of carbon assimilation, apparently an appreciable population of photoinhibited PSII centres can be tolerated without any reduction in the quantum yield of carbon assimilation.Abbreviations and Symbols PPFD photosynthetically active photon flux density - PSII photosystem II - F_v/F_m ratio of variable to maximal fluorescence - quantum yield of carbon assimilation This work was supported in part by grants from the UK Agricultural and Food Research Council (AG 84/5) to N.R.B. and from the U.S. Department of Agriculture (Competitive Research Grant 87-CRCR-1-2381) to D.R.O. G.Y.N. was the recipient of a British Council scholarship and N.R.B. received a fellowship from the Organization for Economic Co-operation and Development (Project on Food Production and Preservation).     

Stress-induced chlorosis and increase in D1-protein turnover precede photoinhibition in spinach suffering under magnesium/sulphur deficiency

To test wether chlorosis is induced by photoinhibitory damage to photosystem II (PSII), onset of chlorosis and loss of PSII function were compared in young spinach (Spinaciae oleracea L.) plants suffering under a combined magnesium and sulphur deficiency. Loss of chlorophyll already occurred after the first week of deficiency and preceded any permanent functional inhibition of the photosynthetic apparatus. Permanent disturbancies of photosynthetic electron transport measured in isolated thylakoids and of PSII function, determined via the ratio of variable fluorescence to maximal fluorescence, F_v/F_m, could be detected only after the second week of deficiency. After the third week, the plants had lost about 60% of their chlorophyll; even so, fluorescence data indicated that 85% of the existing PSII was still capable of initiating photosynthetic electron transport. However, quenching analysis of steady-state fluorescence showed an early increase in non-photochemical quenching and in down-regulated PSII centres with low steady-state quantum efficiency. Together with the down-regulation of PSII centres, a 1.4-fold increase in D1-protein synthesis, measured as incorporation of [¹⁴C]leucine, could be observed at the end of the first week before any loss of D1 protein, chlorophyll or photosynthetic activity could be detected. Immunological determiation by Western-blotting did not show a change in D1-protein content; thus, at this time, D1 protein was not only faster synthesised but was also faster degraded than before the imposition of mineral deficiency. The increased turnover was high enough to prevent any loss or functional inhibition of PSII. After 3 weeks, D1-protein synthesis on a chlorophyll basis was further stimulated by a factor of 2. However, this was not enough to prevent a net loss of D1 protein of about 70%, showing that the D1-protein was now degraded faster than it was synthesised. Immunological determination and electron-transport measurements showed that together with the loss of D1 protein the other polypetides of PSII were also degraded, resulting in a specific loss of PSII centres. The degradation of PSII centres prevented a large accumulation of damaged PSII centres. We assume that the decrease in PSII centres initiates the breakdown of the other thylakoid proteins.Abbreviations Fo yield of intrinsic fluorescence when all PSII centres are open in the dark - F_m yield of maximal fluorescence when all reaction centres are closed - F_m fluorescence yield when all reaction centres are closed under steady-state conditions - F_v yield of variable fluorescence, (difference between F_o and F_m) - F yield of variable fluorescence under steady-state conditions, difference between F_m and F_t, the fluorescence yield under steady-state conditions - PFD photon flux density - Q_A primary quinone acceptor of PSII - Q_B secondary quinone acceptor of PSII - q_p photochemical quenching - q_n non-photochemical quenching This work was supported by grants from the Bundesminister für Forschung und Technologie and the German Israeli Foundation. The authors thank Prof. I. Ohad (Department of Biological Chemistry, Hebrew University, Jerusalem, Israel) for fruitful discussions.     

Increased taxane accumulation in callus cultures of Taxus cuspidata and Taxus × media by some elicitors and precursors

In Taxus cuspidata callus, vanadyl sulfate (10 mg l^–1) induced a high (146 g g^–1 dry wt) production of 10-deacetylbaccatin III in comparison to 7 g g^–1 dry wt of the control. The content of paclitaxel in this species increased from 16 g g^–1 to 74 g g^–1 dry wt when 20 mg phenylalanine l^–1 was used. In T. media, p-aminobenzoic acid induced the highest content of 10-deacetylbaccatin III (481 g g^–1 dry wt) versus 181 g g^–1 in the control. Paclitaxel increased from 89 to 139 g g^–1 dry wt after adding chitosan (20 mg l^–1) to the cultures.     

Specificity of recA441-mediated (tif-1) mutational events.

Summary To investigate the impact of SOS induction on the distribution of spontaneous mutation, 111 recA441-mediated mutations were characterized at the DNA sequence level in the lacI gene of Escherichia coli. A 2.6-fold enhancement in lacI ^– mutation frequency was observed after induction of the SOS system in the absence of mutagenic treatment, and specific classes of mutational events were induced. G : C C : G, G : C T : A and A : T T : A transversion events were specifically enhanced after SOS induction. A preferential 5-Y-Purine-3 neighbouring base specificity for these transversion events is reported here (normalised for mutation of the purine residue). In addition, a preference for transversion events at 5-C/GTGG-3 sequences is also observed. Fifty events were recovered at the lacI frameshift hotspot site and were equally represented by 4 bp addition and deletion events. This 1:1 ratio deviates significantly from the 4:1 distribution characteristic of spontaneous frameshift mutation in the RecA⁺ background and is a consequence of the fourfold induction of the (–)4 event. This abberrant distribution was confirmed by oligomeric probing of 474 independent recA441-mediated spontaneous lacI ^– mutations.     

Arsenic concentrations in water and fish from Chautauqua Lake,New York

Synopsis Arsenic persists in Chautauqua Lake, New York waters 13 years after cessation of herbicide (sodium arsenite) application and continues to cycle within the lake. Arsenic concentrations in lake water ranged from 22.4–114.81 g l^–1, = 49.0 ag l^–1. Well water samples generally contained less than 10 g l^–1 arsenic. Arsenic concentrations in lake water exceeded U.S. Public Health Service recommended maximum concentrations (10 g l^–1) and many samples exceeded the maximum permissible limit (50 g l^–1). Fish accumulated arsenic from water but did not magnify it. Fish to water arsenic ratios ranged from 0.4–41.6. Black crappie (Pomoxis nigromaculatus) contained the highest arsenic concentrations (0.14–2.04 g g^–1 ), X = 0.7 g g^–1) while perch (Perca flavescens), muskellunge (Esox masquinongy) and largemouth bass (Micropterus salmoides) contained the lowest concentrations (0.02–0.13 g g^–1). Arsenic concentrations in fish do not appear to pose a health hazard for human consumers.     

Changes of amino acid sequence in PEST-like area and QEEET motif affect degradation rate of D1 polypeptide in photosystem II

The degradation rate of the D1 polypeptide was measured in threeSynechocystis PCC 6803 mutantsin vivo. Mutations were introduced into a putative cleavage area of the D1 polypeptide (QEEET motif) and into the PEST-like area. PEST sequences are often found in proteins with a high turnover rate. The QEEET-motif mutants are CA1 [(E242-E244);Q241H] and E243K, and the third mutation, E229D, was directed to the PEST-like area. During high-light illumination (1500 mol photons m^-2s^-1) that induced photoinhibition of photosystem II (PSII), the half-life time of the D1 polypeptide in mutant E229D (t _1/2=35 min) was about twice as long as in AR (control strain) cells (t _1/2=19 min). In growth light (40 mol photons m^-2s^-1), the degradation rate of the D1 polypeptide in E229D and AR strains was the same (t _1/25 h). In growth light the D1 polypeptide was degraded faster in both QEEET-motif mutants than in the AR strain, but in photoinhibitory light the degradation rates were similar. According to these results, the highly conservative QEEET motif as such is not required for the proteolytic cut of the D1 polypeptide, but it does affect the rate of degradation. No simple correlation existed between the degradation rate of the D1 polypeptide and the susceptibility of PSII to photoinhibition in mutant and AR cells under our experimental conditions.     

Effect of aluminium on yield and plant chemical concentrations of some temperate legumes

The aluminium (Al) tolerance of 34 temperate legume species (143 genotypes, including 57 from Trifolium repens) was determined in 60 experiments over a 3 year period in a low ionic strength (2.7 × 10^-3 M) solution culture. For each genotype, the relationship between solution Al³⁺ activity (M) and relative yield was determined and the Al³⁺ activity associated with a 50% reduction in yield (Al_RY50) calculated. In addition, plant chemical concentrations were determined in at least one genotype from most species. For white clover, Al_RY50 over all genotypes had an approximately normal distribution with mean of 1.31 M for the tops and 1.51 M for the roots, and a standard deviation of about 0.4. This suggested that Al tolerance had a polygenic inheritance. For the other species tested, Al_RY50 ranged from 0.15 to 4.53 M in the tops and from 0.21 to 4.89 M in the roots. In the tops and roots, 37% and 26% respectively of the genotypes had an Al_RY50 less than 1 M, including all species tested in the genera Melilotus and Medicago. Only 8% or 23% of the genotypes, based on the tops and roots respectively, had an Al_RY50 greater than 2, including all genotypes in the species Lotus pedunculatus. Except for Lotus, there were no consistent differences between genera in plant chemical concentrations. In Lotus, concentrations of Ca, Zn, Mn and Cu in the tops and of all elements except B in the roots were lower than that of the other species. The Al_RY50 of the species was not related to plant chemical concentrations in the absence of Al. Depending on the plant element, increasing solution Al concentrations had no significant effect on plant chemical concentrations for 56–94% of the species. When a significant effect did occur, increasing Al in solution generally decreased S and K concentrations and increased Mn, Zn, Cu Fe, B and Al concentrations in the tops and roots and decreased Ca concentrations in the tops. Plant P concentrations decreased in the tops but increased in the roots. Increasing Al in solution increase plant Al at the average rate of 44 g g^-1 M ^-1 (range 20–87) in the tops and 333 g M ^-1 (range 162–616) in the roots.     

In vitro plant regeneration via somatic embryogenesis from root culture of some rhizomatous irises

A method for plant regeneration of Iris via somatic embryogenesis is described. Root and leaf pieces from in vitro-grown plants of several genotypes of rhizomatous Iris sp. were cultured in vitro. Callus induction occurred only on root cultures incubated under low light intensity (35 mol m^-2 s^-1) on two induction media containing 2,4-D (4.5 or 22.5 M), NAA (5.4 M) and kinetin (0.5 M). Somatic embryos developed after transfer of callus onto four regeneration media containing 9 or 22 M BA, or 5 M kinetin and 2 M TIBA or 9 M BA and 4 M TIBA. Plantlets could be obtained from these somatic embryos. Genotypic differences were found both in callus induction and somatic embryo formation, with I. pseudacorus responding better than I. versicolor or I. setosa. Cytological analysis performed on root tips of 80 regenerated plants revealed that two of the I. pseudacorus regenerants were tetraploid.Abbreviations 2,4-D dichlorophenoxy acetic acid - NAA naphthaleneacetic acid - BA 6-benzyladenine - TIBA 2,3,5-triiodobenzoic acid - IBA indolebutyric acid     

The effect of triacontanol on micropropagation of Capsicum frutescens and Decalepis hamiltonii W & A

We studied the effect of triacontanol (TRIA) on shoot multiplication and rooting of in vitro derived shoot tips of Capsicum frutescens and Decalepis hamiltonii W & A. In both shoot multiplication and rooting phases, TRIA was administered at 2-20 g l^–1. TRIA resulted in highest promotion of axillary shoot proliferation at 2 g l^–1 in Capsicum frutescens and 20 g l^–1 in Decalepis hamiltonii while rooting was maximum at 5 and 10 g l^–1 for Capsicum frutescens and Decalepis hamiltonii respectively. TRIA enhanced shoot growth and chlorophyll content of leaves and also influenced root induction and supported growth of the roots. This work reveals that TRIA can be used as an effective growth regulator in the micropropagation of Capsicum frutescens and Decalepis hamiltonii, an endangered shrub of Deccan peninsular India.