Intense Background Illumination Research Articles

A System for Registering Shadow Images of Rapid Processes in the UV Spectral Range under the Conditions of Intense Background Illumination

A system is presented for registering shadow images obtained from dense pulsed plasma with a high background illumination level, wherein an LGI-21 molecular nitrogen laser is used as an illuminator (laser pulse power 12 µW, duration 20 ns), and a modified Canon EOS 1000D camera is used as a registering unit. A micropinch discharge such as a high-current low-inductance vacuum spark (HCLIVS) has been used as the plasma object under investigation.

Light reduces the excitation efficiency in the nss mutant of the sheep blowfly Lucilia.

The nss (no steady state) phototransduction mutant of the sheep blowfly Lucilia was studied electrophysiologically using intracellular recordings. The effects of the nss mutation on the receptor potential are manifested in the following features of the light response. (a) The responses to a flash or to dim lights are close to normal, but the receptor potential decays close to the baseline level during prolonged illumination after a critical level of light intensity is reached. (b) The decline of the response is accompanied by a large reduction in responsiveness to light that recovers within 20 s in the dark. (c) The full reduction in responsiveness to light is reached when approximately 13% of the photopigment molecules are converted from rhodopsin (R) to metarhodopsin (M). (d) A maximal net pigment conversion from R to M by blue light induces persistent inactivation in the dark, without an apparent voltage response. This inactivation could be abolished at any time by M-to-R conversion with orange light. The above features of the mutant indicate that the effect of the nss mutation on the light response of Lucilia is very similar to the effects of the transient receptor potential (trp) mutation on the photoreceptor potential of Drosophila. Noise analysis and voltage measurements indicate that the decay of the receptor potential is due to a severe reduction in the rate of occurrence of the elementary voltage responses (bumps). The bumps are only slightly modified in shape and amplitude during the decline of the response to light of medium intensity. There is also a large increase in response latency during intense background illumination. These results are consistent with the hypothesis that separate, independent mechanisms determine bump triggering and bump shape and amplitude. The nss mutation affects the triggering mechanism of the bump.

Computer-linked automated method for measurement of the reversal frequency in phototaxis of Halobacterium halobium.

A computer-linked automated system was developed for counting the number of moving microorganisms under a microscope, that discriminates whether they showed reversals or smooth swimming. The features of the system are 1) commercial I/O devices, and a microcomputer can be used to process the moving pictures, 2) fast processing is accomplished by bit opera-tions for computing the correlation matrix, and pseudo moving objects for processing on the later frames, and 3) a number of cells on the frames are processed in a short time, occluded individuals on a frame are neglected.The system was used to study the phototactic behavior of Halobacterium halobium. High intensity background illumination caused a decrease in the sensitivity of the behavioral response of the cell to the attractant light.

Sub-microsecond chlorophyll A delayed fluorescence from Photosystem I Magnetic field-induced increase of the emission yield

(1) In photosystem I (PS I) particles in the presence of dithionite and intense background illumination at 290 K, an external magnetic field (0–0.22 T) induced an increase, ΔF, of the low chlorophyll a emission yield, F ( ΔF F ⋍ 1–1.5% ). Half the effect was obtained at about 35–60 mT and saturation occurred for magnetic fields higher than about 0.15 T. In the absence of dithionite, no field-induced increase was observed. Cooling to 77 K decreased ΔF at 685 nm, but not at 735 nm, to zero. Measuring the emission spectra of F and ΔF, using continuous excitation light, at 82, 167 and 278 K indicated that the spectra of F and ΔF have about the same maximum at about 730, 725 and 700 nm, respectively. However, the spectra of ΔF show more long-wavelength emission than the corresponding spectra of F. (2) Only in the presence of dithionite and with (or after) background illumination, was a luminescence (delayed fluorescence) component observed at 735 nm, after a 15 ns laser flash (530 nm), that decayed in about 0.1 μs at room temperature and in approx. 0.2 μs at 77 K. A magnetic field of 0.22 T caused an appreciable increase in luminescence intensity after 250 ns, probably mainly caused by an increase in decay time. The emission spectra of the magnetic field-induced increase of luminescence, ΔL, at 82, 167 and 278 K coincided within experimental error with those of ΔF mentioned above. The temperature dependence of ΔF and ΔL was found to be nearly the same, both at 685 and at 735 nm. (3) Analogously to the proposal concerning the 0.15 μs luminescence in photosystem II (Sonneveld, A., Duysens, L.N.M. and Moerdijk, A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 5889–5893), we propose that recombination of the oxidized primary donor P-700 + and the reduced acceptor A −, probably A − 1, of PS I causes the observed fast luminescence. The effect of an external magnetic field on this emission may be explained by the radical pair mechanism. The field-induced increase of the 0.1–0.2 μs luminescence seems to be at least in large part responsible for the observed increase of the total (prompt + delayed) emission measured during continuous illumination in the presence of a magnetic field.

Evidence for two photoreceptors controlling growth in de-etiolated seedlings

The effect of blue light on hypocotyl extension in de-etiolated seedlings of lettuce, cucumber and tomato was investigated under conditions which precluded the involvement of phytochrome. Small but highly inhibitory amounts of blue light were added to a high intensity background illumination from low pressure sodium lamps. A log-linear response for inhibition of hypocotyl extension against the blue light fluence rate was obtained for lettuce and cucumber, and inhibition in tomato was also related to the blue light fluence rate. The added blue light did not alter phytochrome photostationary state and its effect was independent of the total fluence rate. Growth inhibition by Pfr could be demonstrated in tomato and cucumber but not in lettuce. The results indicate that two photoreceptors may normally be involved in the control of seedling growth but their relative importance varies greatly between species.