David T. Canvin

CARBOXYSOME CONTENT OF SYNECHOCOCCUS LEOPOLIENSIS (CYANOPHYTA) IN RESPONSE TO INORGANIC CARBON1

The carboxysome content of chemostat grown Synechococcus leopoliensis (Racib.) Komarek increases under inorganic carbon limitation. At growth rates of ca. 85%μmax the carboxysome content (±SE) was 0.57 ± 0.09 carboxysomes·cell section−1. Under severe carbon limitation (ca. 13%μmax) this increased to 3.4 · 0.3 carboxysomes·cell section−1. Corresponding to this change is a three order of magnitude decrease in the half‐saturation constant of photosynthesis for dissolved inorganic carbon. Nitrogen and phosphorus limitation had no effect on carboxysome content or the kinetics of photosynthesis with respect to inorganic carbon. These results are discussed in light of the apparent lack of photorespiration in these organisms.

Pyruvate dehydrogenase complex activity in proplastids and mitochondria of developing castor bean endosperm

Summary Proplastids and mitochondria from developing castor bean endosperm have been separated using both prolonged centrifugation in continuous gradients and brief centrifugation in discontinuous sucrose density gradients. Continuous gradients showed some contamination of proplastids by mitochondria but no contamination was found in the discontinuous gradients. Catalase, a microsomal marker, was found between the mitochondria and proplastids in the discontinuous gradients. The pyruvate dehydrogenase complex activity was found coincident with both the mitochondria and proplastid peaks in both gradients.

Changes in specific radioactivities of sunflower leaf metabolites during photosynthesis in 14CO2 and 12CO2 at normal and low oxygen

SummaryThe 12CO2- and 14CO2-exchange of illuminated corn leaf discs were measured at normal (21%) and low (1%) oxygen. After periods of exposure to 14CO2 or to 14CO2 followed by 12CO2, the discs were killed and the specific activities of some metabolites were determined. At both O2 concentrations the specific activity of 3-PGA increased and decreased rapidly during the first 5 min of 14CO2-feeding or 12CO2-flushing but did not equilibrate with that of the CO2 in the assimilation chamber even after 15 min. The specific activity of aspartic acid also showed bimodal kinetics during both feeding and flushing. The specific activities of 3-phosphoglyceric acid (3-PGA), aspartic acid and alanine were higher at 1% O2 than at 21% O2, but glycine and serine were lower in specific activity at 1% O2. The results are in agreement with the proposed initial fixation of CO2 into C4-dicarboxylic acids and subsequent transfer of this carbon to 3-PGA. Indirect evidence supports the idea that at 21% O2, CO2 was produced by the corn leaf discs in the light and was refixed into C4-dicarboxylic acids. At 1% O2, the photorespiratory process could also have been active although the flux of carbon through the glycolate pathway was probably smaller than at 21% O2.

Oleic acid synthesis by a particulate preparation from developing castor oil seeds

1. 1. Oleic acid was the major fatty acid formed from either acetyl-CoA or malonyl-CoA by a particulate subcellular fraction from developing castor oil seeds. 2. 2. Required cof actors, with optimum concentrations, for acetyl-CoA as substrate were: ATP, 1 mM; Mn2+, 0.5 mM; HCO3−, > 20 mM; NADPH, < 0.1 mM and NADH, 0.5 mM. With malonyl-CoA as substrate NADPH, 0.1 mM, NADH, 0.3 mM and acetyl-CoA, < 9 mM were required. 3. 3. While oleic acid was the major product formed from either acetyl-CoA or malonyl-CoA when both NADH and NADPH were included in the incubation, stearic acid was the chief product in the presence of NADH alone. 4. 4. O2 was required for synthesis of oleic acid. In its absence, stearic acid was formed.

Changes in specific radioactivity of sunflower leaf metabolites during photosynthesis in 14CO2 and 12CO2 at three concentrations of CO2

SummarySunflower (Helianthus annuus L.) leaf discs were exposed to 14CO2 or 14CO2 followed by 12CO2 at 21% O2 and three different CO2 concentrations. After intervals of up to 15 min, the specific activity of some photosynthetic intermediates was determined. At all CO2 concentrations, the specific activity of 3-phosphoglyceric acid (3-PGA) increased most rapidly and after 15 min of 14CO2 feeding was 92% (967 ppm CO2), 87% (400 ppm CO2) and 53% (115 ppm CO2) of CO2 supplied to the assimilation chamber. The specific activity of glycine, serine and the photorespiratory CO2 was similar at all CO2 concentrations, in aggreement with their proposed close metabolic relationship in the glycolate pathway. However, the kinetics of serine and glycine labelling suggested that serine was not totally derived from glycine. Because the specific activity of these glycolate-pathway intermediates was very differnet from that of 3-PGA at all CO2 concentrations, not all of the carbon traversing this pathway came directly from the Calvin cycle. The non-equilibration of the 3-PGA with the feeding gas reflects the recycling of C from the glycolate pathway into the photosynthetic reduction cycle. Measurements of the rates of CO2 evolution in the light and estimates of the C flux through the glycolate pathway suggest that the photorespiratory activity was high and similar at 115 ppm CO2 and 400 ppm CO2 but inhibited at 967 ppm CO2.

The quenching of chlorophyll a fluorescence as a consequence of the transport of inorganic carbon by the cyanobacterium Synechococcus UTEX 625

Abstract The chlorophyll a fluorescence yield of the cyanobacterium Synechococcus UTEX 625 decreased upon the initiation of inorganic carbon transport. The fluorescence yield recovered upon the depletion of inorganic carbon from the medium or upon the addition of DCMU. The inhibition of photosynthetic CO 2 fixation by iodoacetamide did not prevent this reduction of fluorescence yield. Similar results were obtained for both Na + -stimulated HCO 3 − transport and for the transport (presumably of CO 2 ) that is stimulated by carbonic anhydrase. A transient lowering of the fluorescence yield was also observed when cell suspensions were pulsed with CO 2 . In cells not inhibited with iodoacetamide, a very close quantitative relationship existed between the net rate of O 2 evolution and the maximum extent of fluorescence quenching seen as a function of the inorganic carbon concentration. The fluorescence quenching, however, was not due to CO 2 fixation but rather to the transport of inorganic carbon or the accumulation of the internal pool of inorganic carbon. If quenching is due to the latter it is not surprising that the extent of quenching corresponds to the maximum rate of photosynthesis as the rate of photosynthesis also depends on the size of the internal pool. The results with DCMU suggest that the quenching is Q quenching and transport must provide a mechanism for the oxidation of Q other than CO 2 fixation.

Subcellular localization of oleic acid biosynthesis enzymes in the developing castor bean endosperm

Abstract The complete enzyme complement for oleic acid synthesis from acetyl-CoA or malonyl-CoA was associated with subcellular particles (density 1.21g/cc) isolated from developing castor seed endosperm. These particles were either the proplastids or a new unique particle.

Effect of carbon dioxide and temperature on photosynthetic CO2 uptake and photorespiratory CO2 evolution in sunflower leaves

Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 μl/l external CO2. The ratio of PR/TPS and the solubility ratio of O2/CO2 in the intercellular spaces both decreased with increasing CO2. The rate of PR was not affected by the CO2 concentration in the leaves and was independent of the solubility ratio of oxygen and CO2 in the leaf cell. At photosynthesis-limiting concentrations of CO2, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO2 dm-2 h-1 at 18°C to 8.6 mg CO2 dm-2 h-1 at 30°C. The specific activity of photorespired CO2 was CO2-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O2 and CO2 around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.

Hybrid grass-clump dwarfness in wheat: Physiology and genetics

SummaryThe growth of all grass-clump dwarfs is sensitive to temperature with low temperature giving rise to the grass-clump phenotype and high temperature producing normal phenotype. A continuous temperature of 26°C is required for normal growth of Type 1 dwarfs, a continuous temperature of 21°C is required for normal growth of Ty[e 2 dwarfs and a continuous temperature of 16°C is required for normal growth of Type 3 dwarfs.Genetic studies show that the inheritance of the grass-clump characteristic is due to three complementary dominant genes.The grass-clump growth habit is produced as a result of the temperature sensitivity of the apical meristem. In grass-clump plants low temperature treatment results in the cessation of cell division, DNA synthesis and phospholipid synthesis in the apical meristem. The primary temperature lesion has not been identified. Prolonged low temperature treatment of grass-clump plants results in extensive cell necrosis in a region just below the apical meristem; this cell death results in the permanent inactivation of the apical meristem.

Inorganic Carbon Accumulation Stimulates Linear Electron Flow to Artificial Electron Acceptors of Photosystem I in Air-Grown Cells of the Cyanobacterium Synechococcus UTEX 625.

The effect of inorganic carbon (Ci) transport and accumulation on photosynthetic electron transport was studied in air-grown cells of the cyanobacterium Synechococcus UTEX 625. When the cells were depleted of Ci, linear photosynthetic electron flow was almost completely inhibited in the presence of the photosystem I (PSI) acceptor N,N-dimethyl-p-nitrosoaniline (PNDA). The addition of Ci to these cells, in which CO2 fixation was inhibited with glycolaldehyde, greatly stimulated linear electron flow and resulted in increased levels of photochemical quenching and O2 evolution. In aerobic conditions substantial quenching resulted from methyl viologen (MV) addition and further quenching was not observed upon the addition of Ci. In anaerobic conditions MV addition did not result in quenching until Ci was added. Intracellular Ci pools were formed when MV was present in aerobic or anaerobic conditions or PNDA was present in aerobic conditions. There was no inhibitory effect of Ci depletion on electron flow to 2,6-dimethylbenzoquinone and oxidized diaminodurene, which accept electrons from photosystem II. The degree of stimulation of PNDA-dependent O2 evolution varied with the Ci concentration. The extracellular Ci, concentration required for a half-maximum rate (K1/2) was 3.8 [mu]M and the intracellular K1/2 was 1.4 mM for the stimulation of PNDA reduction. These values agreed closely with the K1/2 values of extracellular and intracellular Ci for O2 photoreduction. Linear electron flow to artificial electron acceptors of PSI was enhanced by intracellular Ci, which appeared to exert an effect on PSI or on the intersystem electron transport chain.