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T V Marsho

Publications and source records attributed to T V Marsho.

15 recordsLinked to original sources
PubMed1985

The effect of low osmotic potential on nitrite reduction in intact spinach chloroplasts.

The effect of water stress (reduced osmotic potential) on photosynthetic nitrite reduction was investigated using intact, isolated spinach (Spinacia oleracea) chloroplasts. Nitrite-dependent O(2) evolution was inhibited 39% at -29.5 bars osmotic potential, relative to a control at -11 bars. In the presence of an uncoupler of photophosphorylation this inhibition was not seen. Reduced osmotic potential did not inhibit either methyl viologen reduction or photosynthetic O(2) reduction. These results indicate that an inhibition of electron transport to ferredoxin cannot account for the observed inhibition of nitrite-dependent O(2) evolution. In vitro assay of nitrite reductase activity showed that the interaction of the enzyme with nitrite was not affected by changes in the concentrations of ions or molecules that might be caused by water stress conditions. These results indicate that the most likely site for the effect of water stress on chloroplastic nitrite reduction is the interaction of ferredoxin with nitrite reductase.

Journal Article↗
PubMed1982

Photosynthetic o(2) exchange kinetics in isolated soybean cells.

Light-dependent O(2) exchange was measured in intact, isolated soybean (Glycine max. var. Williams) cells using isotopically labeled O(2) and a mass spectrometer. The dependence of O(2) exchange on O(2) and CO(2) was investigated at high light in coupled and uncoupled cells. With coupled cells at high O(2), O(2) evolution followed similar kinetics at high and low CO(2). Steady-state rates of O(2) uptake were insignificant at high CO(2), but progressively increased with decreasing CO(2). At low CO(2), steady-state rates of O(2) uptake were 50% to 70% of the maximum CO(2)-supported rates of O(2) evolution. These high rates of O(2) uptake exceeded the maximum rate of O(2) reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O(2)-uptake process (i.e. photorespiration).Rates of O(2) exchange in uncoupled cells were half-saturated at 7% to 8% O(2). Initial rates (during induction) of O(2) exchange in uninhibited cells were also half-saturated at 7% to 8% O(2). In contrast, steady-state rates of O(2) evolution and O(2) uptake (at low CO(2)) were half-saturated at 18% to 20% O(2). O(2) uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O(2) for photoreductant.These results suggest that two mechanisms (O(2) reduction and photorespiration) are responsible for the light-dependent O(2) uptake observed in uninhibited cells under CO(2)-limiting conditions. The relative contribution of each process to the rate of O(2) uptake appears to be dependent on the O(2) level. At high O(2) concentrations (>/=40%), photorespiration is the major O(2)-consuming process. At lower (ambient) O(2) concentrations (</=20%), O(2) reduction accounts for a significant portion of the total light-dependent O(2) uptake.

Journal Article↗
PubMed1980

Relationship of Ribulose-1,5-bisphosphate Carboxylase-Oxygenase Specific Activity to Subunit Composition.

Ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBPCase, EC 4.1.1.39) was isolated from Nicotiana sylvestris and from two cultivars and three nuclear substitution lines of Nicotiana tabacum. Isoelectric focusing patterns, supported by amino acid analyses and tryptic peptide mapping, were used to divide these enzymes into two categories: (a) RuBPCase with variable large subunits and identical small subunits; and (b) RuBPCase with identical large but different small subunits. Specific activities for both the carboxylation and oxygenation reactions were determined for all six RuBPCase enzymes under standard conditions of activation and assay. High, intermediate, and low levels of carboxylase (880, 530, and 340 nanomoles HCO(3) (-) per milligram per minute) and oxygenase (66, 45, and 35 nanomoles O(2) per milligram per minute) activity were noted. The carboxylase to oxygenase ratios ranged from 9 to 14.

Journal Article↗
PubMed1980

Regulation of Photosynthetic Electron Transport in Intact Spinach Chloroplasts: I. INFLUENCE OF EXOGENOUS SALTS ON OXALOACETATE REDUCTION.

Relatively high concentrations of monovalent salts (150 millimolar) stimulated light-saturated uncoupled rates of O(2) evolution linked to oxaloacetic acid (OAA) reduction by intact chloroplasts 2-to 3-fold. In contrast, monovalent salts partially inhibited light-saturated rates of O(2) evolution coupled to CO(2) fixation and uncoupled rates of nitrite reduction. In the presence of high salt concentration, light-saturated rates of electron transport were about equivalent for all three terminal electron acceptors. It is inferred that exogenous monovalent salts have at least two effects on photosynthetic electron transport, independent of photophosphorylation and CO(2) metabolism: a partial inhibitory effect common to OAA, NO(2) (-) and CO(2) reduction and a marked stimulatory effect unique to the photoreduction of OAA.The stimulation of electron transport to OAA was effected by certain exogenous monovalent salts (KCl or NaCl, but not LiCl). Divalent salts (MgCl(2) or CaCl(2)) and high osmotic strength were ineffective. The salt-induced stimulation was eliminated by low concentrations of phosphate or sulfate (>/= millimolar) and by higher concentrations of magnesium (>/=30 millimolar). These results suggest that the ion content of the medium (or cytosol) is potentially important in modulating photosynthetic electron transport events in intact chloroplasts.

Journal Article↗
PubMed1980

Regulation of Photosynthetic Electron Transport in Intact Spinach Chloroplasts: II. MECHANISM OF SALT-INDUCED INCREASE IN OXALOACETATE PHOTOREDUCTION.

The main focus of this study was to determine the mechanism by which certain exogenous monovalent salts stimulate rates of net O(2) evolution linked to oxaloacetate reduction in intact spinach chloroplasts. The influence of salts on the dicarboxylate translocator involved in the transport of oxaloacetate and on the activity and activation of the chloroplast enzyme NADP-malate dehydrogenase, which mediates electron transport to oxaloacetate, was examined. High concentrations of KCl (155 millimolar) increased the apparent K(m) for oxaloacetate but did not significantly alter the maximal velocity of uptake. Likewise, external salts (KCl, MgCl(2), or KH(2)PO(4)) had minimal effects on the magnitude of light activation of NADP-malate dehydrogenase. In contrast, measurements of chloroplast NADP-malate dehydrogenase activity (after release by osmotic shock) showed a marked dependence on salt concentration. Rates were stimulated approximately 2-fold by both monovalent (optimally 75 millimolar) and divalent (optimally 20 millimolar) salts. It was inferred that the salt-induced increase in net rates of O(2) evolution linked to oxaloacetate reduction is due, at least in part, to stimulation of NADP-malate dehydrogenase caused by monovalent cation permeability of the chloroplast inner envelope membrane.

Journal Article↗
PubMed1979

Photosynthetic oxygen reduction in isolated intact chloroplasts and cells in spinach.

The time course of light-induced O(2) exchange by isolated intact chloroplasts and cells from spinach was determined under various conditions using isotopically labeled O(2) and a mass spectrometer. In dark-adapted chloroplasts and cells supplemented with saturating amounts of bicarbonate, O(2) evolution began immediately upon illumination. However, this initial rate of O(2) evolution was counterbalanced by a simultaneous increase in the rate of O(2) uptake, so that little net O(2) was evolved or consumed during the first approximately 1 minute of illumination. After this induction (lag) phase, the rate of O(2) evolution increased 3- to 4-fold while the rate of O(2) uptake diminished to a very low level. Inhibition of the Calvin cycle, e.g. with dl-glyceraldehyde or iodoacetamide, had negligible effects on the initial rate of O(2) evolution or O(2) uptake; both rates were sutained for several minutes, and about balanced so that no net O(2) was produced. Uncouplers had an effect similar to that observed with Calvin cycle inhibitors, except that rates of O(2) evolution and photoreduction were stimulated 40 to 50%.These results suggest that higher plant phostosynthetic preparations which retain the ability to reduce CO(2) also have a significant capacity to photoreduce O(2). With near-saturating light and sufficient CO(2), O(2) reduction appears to take place primarily via a direct interaction between O(2) and reduced electron transport carriers, and occurs principally when CO(2)-fixation reactions are suboptimal, e.g. during induction or in the presence of Calvin cycle inhibitors. The inherent maximum endogenous rate of O(2) reduction is approximately 25 to 50% of the maximum rate of noncyclic electron transport coupled to CO(2) fixation. Although the photoreduction of O(2) is coupled to ion transport and/or phosphorylation, this process does not appear to supply significant amounts of ATP directly during steady-state CO(2) fixation in strong light.

Journal Article↗
PubMed1977

Slow fluorescence quenching of type A chloroplasts. Resolution into two components.

The divalent-cation-specific ionophore A23187 is used to define two components of the slow fluorescence quenching of type a spinach chloroplasts: ionophore-reversible and ionophore-resistant quenching. Ionophore-reversible quenching predominates at relatively low light intensities and approaches saturation as light levels are increased. It is sensitive to uncouplers and to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and is dark reversible. At high light intensities the bulk (greater than 80%) of slow fluorescence quenching is ionophore-resistant. Ionophore-resistant quenching is stimulated by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) at pH 7.6 and by both CCCP and methylamine at pH 9.0. It is insensitive to DCMU and is not reversed in subsequent darkness. Taken together, the two components account for all quenching observed in Type A chloroplasts. Ionophore-reversible quenching is identified with the Mg2+-mediated fluorescence quenching described by Krause (Biochim. Biophys. Acta (1974) 333, 301-313) and by Barber and Telfer (in Membrane Transport in Plants (Dainty, J., AND Zimmermann, U., eds.), pp. 281-288, Springer-Verlag, Berlin, 1974). Ionophore-resistant quenching, a first-order process requiring high light, resembles the quenching reported by Jennings et al. (Biochim. Biophys. Acta (1976) 423, 264-274). The resolution of the fluorescence quenching phenomenon into two distinct components reconciles the apparently contradictory observations of these earlier investigations.

Calcimycin↗
PubMed1976

Ascorbate-independent carotenoid de-epoxidation in intact spinach chloroplasts.

Slow (greater 1 s) light-induced absorbance changes in the 475-5300 nm spectral region were examined in Type A chloroplasts from spinach. The most prominent absorption change occurred at 505 nm. The difference spectrum for this light-induced increase, its absence in osmotically shocked chloroplasts and restoration by ascorbate, and its sensitivity to dithiothreitol indicate that the absorption change is due to carotenoid de-epoxidatiion. The reaction in intact chloroplasts is characterized by its independence of exogenous ascorbate and a rate constant 3- to 8-fold higher than that reported previously for chloroplasts supplemented with ascorbate. The relevance of carotenoid de-epoxidation to other photosynthetic processes was examined by comparing their sensitivities to dithiothreitol. Levels of dithiothreitol that eliminate the 505 nm shift are without effect on saturated rates of CO2 fixation and do not appreciably inhibit fluorescence quenching. We conclude that carotenoid de-epoxidation is not directly involved in the reactions of photosynthesis or in the regulation of excitation allocation between the photosystems.

Ascorbic Acid↗
PubMed1975

Slow 514 nm absorption phases and oxygen exchange transients in Ulva.

1. The slow 514-nm spectral changes in Ulva were studied using bright continuous 650-nm light. Transient and steady-state absorption changes were compared with changes in net rate of O2 exchange in a system designed to measure both parameters simultaneously. 2. Time courses of the 514-nm absorption change show three phases following the onset of light: one rapid increase and two slower (larger than or equal to 1 s) transient increases. Upon cessation of the light three transient absorption phases also follow: a rapid decrease and two slower (greater than 1 s) transient increases. Parallel transient phases (but opposite in sign) were found at 480 nm. 3. The kinetics of the slow 514-nm absorption transients correlate with the characteristic induction transients in net O2 exchange. 4. Similar difference spectra and the restoration kinetics of the light-on and light -off transient phases indicate that the slow 514-nm absorption changes reflect the same component(s) and process(es). 5. The experimental results are discussed in terms of the electrochromic hypothesis for the 515-nm absorption shift. We interpret the slow 514-nm absorption changes in Ulva as a reflection of relatively slow ionic readjustments across the photosynthetic membranes.

Chlorophyta↗
PubMed1975

Nitrogen fixation in the Rhode River estuary of Chesapeake Bay.

The distribution, seasonal variation, origin, and significance of biological nitrogen fixation has been determined for a Chesapeake Bay estuary using the acetylene reduction technique. Studies over a 15-month period have shown that nitrogen fixation occurs predominantly in the surface intertidal (marsh) and subtidal sediments. Negligible activity was found in surface waters. A marked seasonal variation in nitrogen fixation was observed for intertidal sediments incubated at a standard 20 degrees C. Average rates of about 37 and 12 ng N/g dry sediment per hour were observed in the late fall months of 1972 and 1973, respectively, and less than or equal to 5 ng N/g dry sediment per hour during other seaons. Peaks of activity were lowered considerably when samples were incubated at ambient temperatures (in situ). Activity in the subtidal sediments was low (less than or equal to 6.8 ng N/g dry sediment per hour but showed a similar (approximately twofold) seasonal variation in nitrogen fixation potential. Light-dark and substrate addition experiments suggest that heterotrophic bacteria are the principal agents for nitrogen fixation in sediments. Integrated estimates of nitrogen fixation in the estuary indicate that biological fixation probably accounts for less than 5% of the total influx of nitrogen into the system. Rates of activity in the intertidal sediments are insufficient to account for the high productivity of marine angiosperms found in the marsh.

Acetylene↗