Oxygen Requirement and Inhibition of C4 Photosynthesis . An Analysis of C4 Plants Deficient in the C3 and C4 Cycles

The basis for O2 sensitivity of C4 photosynthesis was evaluated using a C4-cycle-limited mutant of Amaranthus edulis (a phosphoenolpyruvate carboxylase-deficient mutant), and a C3-cycle-limited transformant of Flaveria bidentis (an antisense ribulose-1,5-bisphosphate carboxylase/oxygenase [Rubisco] small subunit transformant). Data obtained with the C4-cycle-limited mutant showed that atmospheric levels of O2 (20 kPa) caused increased inhibition of photosynthesis as a result of higher levels of photorespiration. The optimal O2 partial pressure for photosynthesis was reduced from approximately 5 kPa O2 to 1 to 2 kPa O2, becoming similar to that of C3 plants. Therefore, the higher O2 requirement for optimal C4 photosynthesis is specifically associated with the C4 function. With the Rubisco-limited F. bidentis, there was less inhibition of photosynthesis by supraoptimal levels of O2 than in the wild type. When CO2 fixation by Rubisco is limited, an increase in the CO2 concentration in bundle-sheath cells via the C4 cycle may further reduce the oxygenase activity of Rubisco and decrease the inhibition of photosynthesis by high partial pressures of O2 while increasing CO2 leakage and overcycling of the C4 pathway. These results indicate that in C4 plants the investment in the C3 and C4 cycles must be balanced for maximum efficiency

Effect of Semicore Orbitals on the Electronic Band Gaps of Si, Ge, and GaAs within the GW Approximation

We study the effect of semicore states on the self-energy corrections and electronic energy gaps of silicon, germanium and GaAs. Self-energy effects are computed within the GW approach, and electronic states are expanded in a plane-wave basis. For these materials, we generate {\it ab initio} pseudopotentials treating as valence states the outermost two shells of atomic orbitals, rather than only the outermost valence shell as in traditional pseudopotential calculations. The resulting direct and indirect energy gaps are compared with experimental measurements and with previous calculations based on pseudopotential and ``all-electron'' approaches. Our results show that, contrary to recent claims, self-energy effects due to semicore states on the band gaps can be well accounted for in the standard valence-only pseudopotential formalism.Comment: 6 pages, 3 figures, submitted to Phys. Rev.

Electronic Structure of Calcium Hexaboride within the Weighted Density Approximation

We report calculations of the electronic structure of CaB$_6$ using the weighted density approximation (WDA) to density functional theory. We find a semiconducting band structure with a sizable gap, in contrast to local density approximation (LDA) results, but in accord with recent experimental data. In particular, we find an $X$-point band gap of 0.8 eV. The WDA correction of the LDA error in describing the electronic structure of CaB$_6$ is discussed in terms of the orbital character of the bands and the better cancelation of self-interactions within the WDA.Comment: 1 figur

Plasmonic excitations in noble metals: The case of Ag

The delicate interplay between plasmonic excitations and interband transitions in noble metals is described by means of {\it ab initio} calculations and a simple model in which the conduction electron plasmon is coupled to the continuum of electron-hole pairs. Band structure effects, specially the energy at which the excitation of the $d$-like bands takes place, determine the existence of a subthreshold plasmonic mode, which manifests itself in Ag as a sharp resonance at 3.8 eV. However, such a resonance is not observed in the other noble metals. Here, this different behavior is also analyzed and an explanation is provided.Comment: 9 pages, 8 figure

The polaron-like nature of an electron coupled to phonons

When an electron interacts with phonons, the electron can exhibit either free electron-like or polaron-like properties. The latter tends to occur for very strong coupling, and results in a phonon cloud accompanying the electron as it moves, thus raising its mass considerably. We summarize this behaviour for the Holstein model in one, two and three dimensions, and note that the crossover occurs for fairly low coupling strengths compared to those attributed to real materials exhibiting conventional superconductivity.Comment: 5 pages; contains a summary of single particle results for the Holstein mode

BAs and boride III-V alloys

Boron arsenide, the typically-ignored member of the III-V arsenide series BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an X_1c-like indirect band gap, and its bond charge is distributed almost equally on the two atoms in the unit cell, exhibiting nearly perfect covalency. The reasons for these are tracked down to the anomalously low atomic p orbital energy in the boron and to the unusually strong s-s repulsion in BAs relative to most other III-V compounds. We find unexpected valence band offsets of BAs with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is significantly higher than that of AlAs, despite the much smaller bond length of BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects result from the unusually strong mixing of the cation and anion states at the VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and composition-independent band gap bowing. This means that while addition of small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing enthalpies which are smaller than in GaN-GaAs alloys. The unique features of boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for publication in Phys. Rev. B. Scheduled to appear Oct. 15 200

Density functional theories and self-energy approaches

A purpose-designed microarray platform (Stressgenes, Phase 1) was utilised to investigate the changes in gene expression within the liver of rainbow trout during exposure to a prolonged period of confinement. Tissue and blood samples were collected from trout at intervals up to 648 h after transfer to a standardised confinement stressor, together with matched samples from undisturbed control fish. Plasma ACTH, cortisol, glucose and lactate were analysed to confirm that the neuroendocrine response to confinement was consistent with previous findings and to provide a phenotypic context to assist interpretation of gene expression data. Liver samples for suppression subtractive hybridisation (SSH) library construction were selected from within the experimental groups comprising “early” stress (2–48 h) and “late” stress (96–504 h). In order to reduce redundancy within the four SSH libraries and yield a higher number of unique clones an additional subtraction was carried out. After printing of the arrays a series of 55 hybridisations were executed to cover 6 time points. At 2 h, 6 h, 24 h, 168 h and 504 h 5 individual confined fish and 5 individual control fish were used with control fish only at 0 h. A preliminary list of 314 clones considered differentially regulated over the complete time course was generated by a combination of data analysis approaches and the most significant gene expression changes were found to occur during the 24 h to 168 h time period with a general approach to control levels by 504 h. Few changes in expression were apparent over the first 6 h. The list of genes whose expression was significantly altered comprised predominantly genes belonging to the biological process category (response to stimulus) and one cellular component category (extracellular region) and were dominated by so-called acute phase proteins. Analysis of the gene expression profile in liver tissue during confinement revealed a number of significant clusters. The major patterns comprised genes that were up-regulated at 24 h and beyond, the primary examples being haptoglobin, β-fibrinogen and EST10729. Two representative genes from each of the six k-means clusters were validated by qPCR. Correlations between microarray and qPCR expression patterns were significant for most of the genes tested. qPCR analysis revealed that haptoglobin expression was up-regulated approximately 8-fold at 24 h and over 13-fold by 168 h.This project was part funded by the European Commission (Q5RS-2001-02211), Enterprise Ireland and the Natural Environment Research Council of the United Kingdom

Phenotypic, cytogenetic and spike fertility characterization of a population of male-sterile triticale

Triticale (X Triticosecale Wittmack) is a good cereal for production of flour and feed. A segregating population of triticale was developed from a male-sterile (MS) plant. To determine whether this new source of male sterility in triticale is appropriate for use in breeding programs the expression of the male sterility phenotype was characterized through spike fertility, meiotic behavior, and pollen. Controlled crosses between male-sterile plants and control varieties male-fertile (MF) of triticale were also conducted, and cytological analyses were performed in the F2 and backcross plants. Plants with male-sterile phenotypes displayed reduced spike fertility when compared to plants with male-fertile phenotypes. Compared to male-fertile plants, male-sterile plants exhibited a lower percentage of normal meiotic cells, a reduced meiotic index and reduced pollen viability. The F2 plants had improved pollen fertility when compared to the male-sterile population; however there were no corresponding improvements in the percentage of normal meiotic cells or in the meiotic index. A single generation of backcrosses resulted in an improved meiotic index and increased pollen viability. However, no changes in the percentage of normal meiotic cells were observed. Meiotic instability, which was shown to be inheritable, was the likely cause of male sterility. Therefore, the use of this population in triticale breeding was considered to be inappropriate because it could promote or contribute to the maintenance of meiotic instability, which is commonly observed in this species

Biodistribution and Pharmacokinetics of O-Palmitoyl Tilisolol, a Lipophilic Prodrug of Tilisolol, after Intravenous Administration in Rats

The purpose of this study was to modify the biodistribution and pharmacokinetics of tilisolol, a β-blocker, using the palmitoyl prodrug approach. After intravenous administration of tilisolol and O-palmitoyl tilisolol in rats, drug concentrations were determined in blood, bile, urine, and several tissues. The concentration-time profiles of tilisolol and O-palmitoyl tilisolol were analyzed pharmacokinetically. The blood concentrations of O-palmitoyl tilisolol after intravenous administration of O-palmitoyl tilisolol were about 10-fold higher than those of tilisolol after intravenous administration of tilisolol. The biliary excretion rates of O-palmitoyl tilisolol and tilisolol after intravenous administration of O-palmitoyl tilisolol were about 10- to 100-fold larger than those of tilisolol after intravenous administration of tilisolol. In addition, the hepatic uptake clearance of O-palmitoyl tilisolol after intravenous administration of O-palmitoyl tilisolol was 3.6-fold higher than that of tilisolol after the intravenous administration of tilisolol. In the in vitro experiments, it was demonstrated that the distribution ratios between blood cells and plasma (blood/plasma) of O-palmitoyl tilisolol and tilisolol was 95.7 and 55.5%, respectively. These findings suggest that O-palmitoyl tilisolol exists as a binding form with biological components, especially blood cells, in systemic circulation. In conclusion, the palmitoyl prodrug approach is useful as a drug delivery system to deliver the parent drug to the liver