PubMed · 5598606
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E Arondel. [Carbon dioxide welding. Initial observations].. https://pubmed.ncbi.nlm.nih.gov/5598606/
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Diffusion of CO2 inside leaves is generally regarded to be from the substomatal cavities to the assimilating tissues, i.e. in the vertical direction of the leaf blades. However, lateral gas diffusion within intercellular air spaces may be much more effective than hitherto considered. In a previous work it was demonstrated that, when 'clamp-on' leaf chambers are used, leaf internal 'CO2 leakage' beyond the leaf chamber gaskets may seriously affect gas exchange measurement. This effect has been used in the present paper to quantify gas conductance (g(leaf,l), mmol m(-2) s(-1)) in the lateral directions within leaves and significant differences between homo- and heterobaric leaves were observed. For the homobaric leaves, lateral gas conductance measured over a distance of 6 or 8 mm (the widths of the chamber gaskets) was 2-20% of vertical conductance taken from published data measured over much smaller distances of 108-280 microm (the thickness of the leaves). The specific internal gas diffusion properties of the leaves have been characterized by gas conductivities (g*(leaf), micromol m(-1) s(-1)). Gas conductivities in the lateral directions of heterobaric leaves were found to be small but not zero. In homobaric leaves, they were between 67 and 209 micromol m(-1) s(-1) and thus even larger than those in the vertical direction of the leaf blades (between 15 and 78 micromol m(-1) s(-1)). The potential implications for experimentalists performing gas exchange measurements are discussed.
To function, the catalytic sites of Rubisco (EC 4.1.1.39) need to be activated by the reversible carbamylation of a lysine residue within the sites followed by rapid binding of magnesium. The activation of Rubisco in vivo requires the presence of the regulatory protein Rubisco activase. This enzyme is thought to aid the release of sugar phosphate inhibitors from Rubisco's catalytic sites, thereby influencing carbamylation. In C3 species, Rubisco operates in a low CO2 environment, which is suboptimal for both catalysis and carbamylation. In C4 plants, Rubisco is located in the bundle sheath cells and operates in a high CO2 atmosphere close to saturation. To explore the role of Rubisco activase in C4 photosynthesis, activase levels were reduced in Flaveria bidentis, a C4 dicot, by transformation with an antisense gene directed against the mRNA for Rubisco activase. Four primary transformants with very low activase levels were recovered. These plants and several of their segregating T1 progeny required high CO2 (>1 kPa) for growth. They had very low CO2 assimilation rates at high light and ambient CO2, and only 10% to 15% of Rubisco sites were carbamylated at both ambient and very high CO2. The amount of Rubisco was similar to that of wild-type plants. Experiments with the T1 progeny of these four primary transformants showed that CO2 assimilation rate and Rubisco carbamylation were severely reduced in plants with less than 30% of wild-type levels of activase. We conclude that activase activity is essential for the operation of the C4 photosynthetic pathway.
Preservation of pulpal health is the primary prerequisite for successful application of laser systems in the hard tissue management of vital teeth. The purpose of this study was to investigate the short and long-term pulpal effects to cavity preparations in healthy human teeth using carbon dioxide (CO2) laser. A total of seven, healthy, third molars that were scheduled to be removed due to space problems were used. After the laser drilling, the occlusal cavities were closed temporarily, and the teeth were extracted 7 days (n=5) and 3 months (n=2) after the operation. The specimens were fixed, decalcified, subdivided and processed for light and transmission electron microscopy. Seven days postoperatively all the five teeth that had been irradiated with the CO2 laser did not reveal any pathological changes in the pulpo-dentine complex. Three months postoperatively the two teeth that were prepared with the laser showed subtle but distinct apposition of tertiary dentine that was lined with intact odontoblasts. One of the specimens at 3 months revealed the presence of a mild, but very circumscribed, pulpal infiltration of chronic inflammatory cells subjacent to the cavity preparation. The latter is unlikely to be due to a direct effect of the laser irradiation but a possible consequence of microleakage of oral antigens and/or other tissue-irritating molecules through the temporary restoration and the remaining dentine thickness (RDT). Although these preliminary histological results suggest that the CO2 laser under investigation induced only minimal response of the dentine-pulp complex when used as a hard-tissue drilling tool, with specific energy settings, pulse duration within thermal relaxation time and emitting radiations at 9.6 microm of wavelength, larger clinical trials involving various types of teeth are necessary to reach definite conclusions for large-scale clinical application of the laser device.