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T V Marsho

Publications and source records attributed to T V Marsho.

8 recordsLinked to original sources
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↗