High Intensity White Light Research Articles

Mid-Infrared Spectral Photoresponse of Semi-Insulating GaAs

AbstractWe have developed a characterization technique using spectral photoconductivity that is capable of identifying deep energy levels in gallium arsenide. Our technique requires less sample processing than deep level transient spectroscopy (DLTS) and can be used on semi-insulating (si) gallium arsenide. The technique uses a mid-infrared fourier transform spectrometer for rapid photoresponse versus wavelength measurements. Using this technique we have observed evidence of several below mid-gap energy levels in undoped si gallium arsenide by the spectral PC technique. Spectral PC measurements were made on si GaAs before and after illuminating the cooled samples with high intensity white light at 8K. The PC spectrum typically showed a broad photoresponse that was attributed to multiple energy levels being present in the material. The observed energy levels were: 0.54 eV, 0.44 eV, 0.25 eV, 0.17 eV, 0.14 eV and ~0.10 eV. The shallow photoresponse from the carbon acceptor was not observed. The most striking spectral feature was a sharp ionization edge at about 0.44 eV in all of the samples studied. This energy level correlates to the 0.43 eV intrinsic defect level that has been observed by temperature dependent Hall effect measurements. The 0.44 eV level was also observed in our PC spectrum of a n-type Bridgman grown sample that had shown the 0.43 Hall effect level. The presence of additional deep levels in concentrations comparable to those of EL2 and carbon indicates that the simple model for compensation in si GaAs which invokes only EL2 and shallow impurities needs revision.

Changes in the organization of stroma membranes induced by in vivo state 1-state 2 transition

Stroma lamellae were isolated from mesophyll thylakoids of maize plants exposed to dark (state 1) and high-intensity white light (state 2). Freeze-fracture electron microscopy established that the preparations consisted of small, spherical, right-side-out vesicles, and that the extent of contamination by grana appreassed lamellae was less than 0.1%. The state 1–state 2 transition resulted in a decrease in the chlorophyll a/b ratio of the isolated stroma lamellae due to lateral migration of light-harvesting chlorophyll a/b protein of Photosystem II (LHC II) from appressed grana lamellae. Fluorescence spectroscopy showed that this ‘mobile LHC II’ became functionally attached to the Photosystem I reaction centre, with a resulting 15% increase in antenna size, as determined by measurement of the chlorophyll/P-700 ratio and kinetics of P-700 photo-oxidation under light-limiting conditions. The mobile LHC II could be resolved by gel electrophoresis into four polypeptides with an approximate 1:1:1:1 stoichiometry. This differed significantly from the polypeptide composition of grana LHC II, which had a different stoichiometry, and an additional polypeptide of lower apparent molecular mass which did not become phospharylated. The freeze-fracture particle density data showed that the phosphorylated LHC II was not visible as discrete small particles, as found in appressed grana membranes, and may have become incorporated into the large PF particles of the stroma lamellae, which contain the Photosystem I reaction centre.

Effects of Thiopentone Halothane-Nitrous Oxide Anaesthesia Compared to Ketamine-Xylazine Anaesthesia on the DC Recorded Dog Electroretinogram

Eleven ophthal-moscopically healthy dark adapted dogs were examined by DC ERG technique with single flash full field illumination starting with near b-wave threshold blue (tests 1-3) and white (tests 4-6) stimuli of different intensity and ending with 30 Hz photopic flicker smuli (test 7) after light adaptation. All animals were anaesthetized using 2 different anaesthetic methods: Anaesthesia I (A I): Induction with thiopentone sodium, continued with halothane and nitrous oxide in oxygen. Anaesthesia II (A II): Praemedication with xylazine hydrochloride followed by anaesthesia with ketamine hydrochloride. A minimum interval of 1 week was kept between all anaesthesias. The a- and b-wave amplitudes and latencies were determined. Statistical analysis of results indicated that the a- and b-waves were elicited by weaker intensities in A II. In Tests 3-6 the a-wave was highly significantly (P < 0.001), higher in amplitude in AII than in A I. Differencies in b-wave amplitudes were not statistically significant (except Test 1). The b-wave latencies were longer in AI in Test 2 (using low intensity blue light). The a-wave latencies were slightly shorter in AII in Test 6 (using high intensity white light). In additional experiments the selective action of the different agents (except N2O) used in AI and AII was studied. Thiopentone alone given to 3 dogs seemed to depress the a-wave selectively. Halothane given separately to 3 dogs lowered both the a- and b-wave amplitudes. Ketamine given with a neuromuscular blocking agent to three dogs resulted in responses almost identical to those in AII. Xylazine with vecuronium given to 4 dogs resulted in responses with slighly depressed a- and b-waves in comparison to ketamine with vecuronium. The results indicate that when developing an animal model for the electrophysiologic study of human retinal dystropies, the actions of different anaesthetics upon the ERG components are of great importante.

Evaluation of a technique for the measurement of chlorophyll fluorescence from leaves exposed to continuous white light

Abstract An instrument for the generation and measurement of modulated chlorophyll fluorescence signals from leaves exposed to continuous, highintensity white light is described. Modulated fluorescence is generated in the leaf by pulsed diodes emitting low‐intensity yellow radiation and is detected with a photodiode whose output is fed to an amplifier locked in to the frequency of the lightemitting diodes. Comparisons are made between the modulated fluorescence signals measured with this instrument and the continuous fluorescence signals emitted from dark‐adapted leaf tissue and isolated thylakoids when photosynthetic activity is induced by exposure to a range of intensities of continuous broad‐band, blue‐green light. The modulated fluorescence signals were similar to the continuous fluorescence signals, but they were not always identical. The small differences between the two signals are mainly attributable to differences in the populations of chloroplasts being monitored in the two measurements as a result of differential penetration of the modulated and actinic light sources into the sample.

Phytochrome control of RNA levels in developing pea and mung-bean leaves

We have examined phytochrome effects on the abundance of transcripts from several nuclear and chloroplast genes in buds of dark-grown pea seedlings and primary leaves of dark-grown mung-bean seedlings. Probes for nuclear-coded RNAs were selected from a library of cDNA clones and included those corresponding to the small subunit (SS) of ribulosebisphosphate carboxylase and a chlorophyll a/b binding protein (AB). Transcripts from chloroplast genes for RuBP carboxylase large subunit (LS) and a 32,000-dalton photosystem II polypeptide (PII) were assayed with cloned fragments of the chloroplast genome. In addition, we present data on transcripts from a number of other nuclear genes of unknown function, several of which change in abundance during light-induced development. Transcript levels were measured as a proportion of total RNA by a dot blot assay in which RNA from different tissues or stages is fixed to nitrocellulose and hybridized with (32)P-labeled probes prepared from cloned DNAs. Several patterns of induction can be seen. For example, although both SS and AB RNAs show positive, red/far-red reversible responses in both pea and mung bean, in pea buds the induction ratio for SS RNA is much higher than that for AB RNA, while just the reverse is true for mung-bean leaves. In addition, treatment with lowfluence red light produces full induction of the pea AB RNA, while SS RNA in the same tissue does not reach a maximum steady-state level until after about 24 h of supplementary high-intensity white light. In pea buds, chloroplast genes (LS, PII) also show clear responses to phytochrome, as measured by the steady-state levels of their RNA products. Chloroplast DNA levels (as a fraction of the total cellular DNA) show the same response pattern, which may indicate that in peas many of the light effects we see are related to a general stimulation of chloroplast development. In mung beans, the levels of plastid DNA and RNA are already quite high in the leaves of 7-d dark-grown seedlings, and light effects are much less pronounced. The results are consistent with the notion that chloroplast development is arrested at a later stage in dark-grown mung-bean leaves than in etiolated pea buds.

The Effects of Red and White Light During the Prebreeder and Breeder Periods on Egg Production and Feed Consumption in Large White Turkeys

Large White turkey hens were conditioned with 5 lx of white or red incandescent light from 20 to 32 weeks of age. Hens from both of these groups were given white or red incandescent light at two intensities (approximately 85 and 160 lx) during a 20-week breeder period.Feed consumption was greatest for hens on white light during the prebreeder period (P≤.01), but hens on red light consumed the greatest amount of feed during the breeder period.The only consistent finding concerning egg production was that hens exposed to high intensity (160 lx) white light, regardless of whether the prebreeder light was white or red, had the poorest egg production (P≤.01). It is concluded that red light during the prebreeder or breeder periods is not beneficial for egg production over that achieved with low intensity white incandescent light, and that bright white incandescent light is deleterious to egg production

THE EFFECT OF CYTOKININS ON GROWTH and PIGMENT ACCUMULATION OF RADISH SEEDLINGS (RAPHANUS SATIVUS L.) GROWN IN THE DARK and AT DIFFERENT LIGHT QUANTA FLUENCE RATES*

Abstract— The dependency of cytokinin effects upon irradiance was studied with radish seedlings (Raphanus sativus L. cv. Saxa Treib). Kinetin (6‐furfurylamino‐purine) or BAP (6‐benzylamino‐purine) were applied via the roots of plants growing either in continuous darkness or under high (90 Wm‐2) or low intensity white light (10Wm‐2). Apart from the different development of plants at low and high fluence rates, the following cytokinin effects were found:(1) Both cytokinins acted in a similar manner on growth characteristics and pigment accumulation at high and low light conditions, BAP being in many cases more effective than kinetin.(2) When compared with the control, the cytokinins suppressed hypocotyl and root lengthening in the dark and light‐grown plants. In darkness they led to increased cotyledon areas, whereas in the light the leaf expansion was suppressed.(3) In the etiolated and low light grown plants, the anthocyanin content of the hypocotyls was enhanced due to the action of cytokinins, whereas under high light the anthocyanin accumulation was decreased.(4) In the cotyledons of etiolated plants, more phototransformable protochlorophyll(ide) and more carotenoids were formed when cytokinins were present. In green leaves the carotenoid content was diminished due to the action of cytokinins, particularly in plants grown in strong light. The chlorophyll a/b ratio was increased in the cytokinin‐treated plants in most cases.The results suggest a light dependency of the cytokinin effects. It is believed that the response of a plant towards exogenously applied cytokinins is similar to that with high intensity light.

Mechanische und energetische Grundlagen der lichtäbhingigen Gestältanderung des Mougeotia-Chloroplasten

Summary The chloroplast of the green alga Mougeotia changes its shape and size under the influence of light. In high-intensity white or blue light, the chloroplast volume decreases, which can be recognized by its shortening and which is called «contraction». In subsequent low-intensity white light, the chloroplast regains its original size, called «dilatation». This investigation deals with the mechanics and energetics of contraction and dilatation. (1) Inhibitors of actomyosin action do not decrease but rather increase contraction. Thus, microfilaments cannot be the mechanical cause of contraction; rather they are assumed to counteract it. Microtubules are not involved in contraction. (2) Strong light must act continuously for contraction to proceed. Moreover, inhibitors of photosynthesis inhibit contraction. Thus, photosynthesis is necessary for the response. It is proposed that photosynthetic energy acts via transmembranic ion pumps, which finally results in osmotic water efflux from the chloroplast. (3) Since the light effect is restricted to the blue region in causing contraction, photosynthetic energy conversion cannot be the only factor involved. Therefore, a second light effect is assumed to be necessary for contraction, acting via a blue-light absorbing cytoplasmic pigment. As a result of light absorption, anchoring of the chloroplast by actin microfilaments is loosened, making contraction possible. (4) According to this hypothesis, contraction requires a dual action of light, viz. energy conversion by chlorophyll and signal transduction by a cytoplasmic photoreceptor, which probably is a flavin. (5) In contrast to contraction, which is completed in one hour, dilatation requires at least 24 hours for completion. It is complicated by cell growth, which cannot be neglected during such a long time, and which has to be compensated for by the dilatating chloroplast in addition. (6) During a first phase of about one hour, dilatation is not influenced by inhibitors neither of actomyosin activity nor of photosynthesis. Thus, neither mechanical forces nor metabolic energy are necessary for the initial phase of dilatation; it is assumed to be a passive process initiated by the dissipation of the proton gradient and its consequences. In this phase about 50 percent of the contraction are reversed. (7) After one hour, dilatation becomes increasingly sensitive to all inhibitors tested, without any specificity, and is completely blocked after 3 to 5 hours. In addition, all these substances also inhibit cell growth to nearly zero. Thus these effects appear to be very unsepcific, and nothing can be concluded yet about the later phase of dilatation.

Chloroplast division in spinach leaves examined by scanning electron microscopy and freeze-etching.

Spinach leaf disks were cultured for 5 days in low-intensity green light and then were transferred to high-intensity white light. Harvests over the next 16 h established that cell area increased by about 80% and chloroplast number per cell increased by about 65%, while the percentage of dumbbell-shaped chloroplasts per cell decreased by 65%. Freeze-etch replicas of fixed and unfixed leaf disks, as well as scanning electron-microscope preparations of fixed material, contained dumbbell-shaped chloroplasts constricted to various degrees. Freeze-etch replicas of unfixed cells from young leaf bases, in which the number of chloroplasts per cell is known to be rapidly increasing, also contained many constricted chloroplasts. It is concluded that dumbbell-shaped chloroplasts occur in vivo and represent a stage in the division of chloroplasts.

Photosensitivity of Puccinia graminis f.sp. tritici urediniospores in vitro and on the leaf surface

The effect of light on germination of Puccinia graminis f.sp. tritici Eriks. & Henn. urediniospores was investigated in vitro on agar blocks and in vivo on the surface of wheat leaves. Under high intensity white light, in excess of 10 000 lux, spores were completely inhibited, although after 24 h continuous irradiation partial recovery was observed. The percentage spore germination on leaves was always less than on agar and the threshold light intensities required for complete photo-inhibition were 10 000 lux and 4500 lux for spores sown on agar and leaves respectively. Fully dehydrated spores failed to germinate and there was a positive correlation between hydration time and percentage germination of dark incubated spores. Light affects germination at a point prior to emergence of germ tube papillae and possible links between photo-inhibition and endogenous autoinhibitors are discussed.

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to -196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.Light quality also exerted a strong influence on the structural and functional organization of the photochemical apparatus. Data on the relative optical cross-sections of PSI and PSII as a function of excitation wavelength indicate that cells grown in light absorbed primarily by the phycobilisomes package a large fraction of their Chl into PSI (PSI Chl/PSII Chl approximately 20), whereas cells grown in light absorbed by Chl distribute their Chl much more equitably (PSI Chl/PSII Chl approximately 1.5). In both types of cells the phycobilisomes transfer their excitation energy predominantly to PSII Chl with little or no direct energy transfer to PSI, but the yield of energy transfer from PSII to PSI is approximately twice as large for cells grown in the phycobilin wavelengths of light. These differences in functional organization and energy distribution account for the physiological expressions of chromatic adaptation. The effects of chromatic adaptation on O(2) evolution can be predicted from our calculations of energy distribution between PSI and PSII for cells grown in the different colors of light.

Effects of the Herbicide San 9789 on Photomorphogenic Responses

The herbicide, 4-chloro-5-(methylamino)-2-(alpha,alpha,alpha-trifluoro-m-tolyl)- 3(2H)-pyridazinone (San 9789), an inhibitor that prevents both carotenoid and chlorophyll accumulation and normal chloroplast development in white light, does not affect the physiological effectiveness of phytochrome in dark-and light-grown plants. Red/far red reversibility of growth inhibition, stimulation of anthocyanin synthesis, and stimulation of phenylalanine ammonia-lyase synthesis are not significantly different in plants grown with and without San 9789. Despite the complete absence of photosynthesis, flowering could be induced in the long day plant Hordeum vulgare L. when sucrose was provided to the leaves. Since the nonphotochemical reactions of phytochrome also are not affected by the herbicide, San 9789 may be used as a tool to study the phytochrome system spectrophotometrically in plants grown for relatively long periods under high intensity white light.

Effects of Protein Synthesis Inhibitors on ent-Kaurene Biosynthesis during Photomorphogenesis of Etiolated Pea Seedlings

Excised shoot tips from 10-day-old etiolated pea (Pisum sativum L. cv. Alaska) seedlings were incubated in solutions of chloramphenicol, cycloheximide, and lincomycin at different concentrations during periods of 0, 4, 8, and 12 hours of irradiation with high intensity white light. Enzyme extracts were prepared from the whole shoot tips and compared with extracts from nontreated shoot tips for their capacity to synthesize ent-kaurene from mevalonate. In control samples, kaurene synthesis increased during the first 8 hours of irradiation and decreased after 12 hours. Chlorophyll content increased steadily up to 12 hours of irradiation. Chloramphenicol and cycloheximide reduced both kaurene synthesis and chlorophyll formation to a similar extent during all periods of irradiation, the reduction being greatest after 8 hours of irradiation. Lincomycin, a specific inhibitor of the formation of chloroplast ribosomes in detached pea shoot tips, did not significantly affect kaurene synthesis activity but strongly inhibited chlorophyll formation. It is tentatively concluded that the increase in kaurene synthesis activity during normal photomorphogenesis in pea seedlings is due to photoinduction of de novo synthesis of one or more proteins involved in the biosynthetic pathway from mevalonate to kaurene.

Photokilling of Micrococcus roseus.

Abstract. Previous work showed that the bacterium Micrococcus roseus is killed by photodynamic action when an exogenous photosensitizer is present, but when a photosensitizer is not added the organism survives long exposure to high intensity (22,000ft‐c, 348 J/s/m2) white light. Experiments designed to demonstrate the presence of a mechanism to repair damage caused by photodynamic action failed to reveal such a mechanism. However, the organism was killed by light of a very high intensity (32,000ft‐c, 506 J/s/m2) in the absence of added photosensitizer, indicating that cells have an effective endogenous photosensitizer(s). Two carotenoid‐deficient mutants were killed via photodynamic action more rapidly than the fully pigmented wild‐type in the presence or absence of an exogenous photosensitizer. Thus, resistance of M. roseus to photodynamic action is not due to a repair mechanism, nor to lack of an effective endogenous photosensitizer, but to the protective action of carotenoid pigments.

Effects of Quality, Intensity, and Duration of Light Breaks during a Long Night on Dormancy in Blue Spruce (Picea pungens Engelm.) Seedlings

Blue spruce (Picea pungens Engelm.) seedlings grow continuously when exposed to photoperiods exceeding 16 hours and enter dormancy within 4 weeks under photoperiods of 12 hours or less. Dormancy was prevented under 12-hour photoperiods by 2-hour light breaks of red light (1.70 muw/cm(2) at 650 nm) or high intensity white light (2,164.29 muw/cm(2) at 400 to 800 nm) given in the middle of the 12-hour night, and by continuous low intensity white light (204.76 muw/cm(2) at 400 to 800 nm). Two-hour light breaks of far red light (1.80 muw/cm(2) at 730 nm), red light followed by far red light, or low intensity white light were not effective in delaying dormancy. The results imply that the phytochrome system mediates the photoperiodic control of dormancy in blue spruce seedlings. The similarity of results obtained using the low intensity, long duration as against the high intensity, short duration light treatments suggests that the law of reciprocity applies in this response.

Nature of light requirement for the flowering of Chenopodium rubrum L. (Ecotype 60� 47? N)

Seedlings of C. rubrum were irradiated with different light qualities and intensities following a single inductive dark period. Our results show that relatively low intensity white light (35–100 ft. c.) does not support flower development while high intensity white light (650–800 ft. c.) permits 100% flowering. We have shown that the low intensity light inhibiton of flower development is not due to suboptimal photosynthesis. Relatively low intensities of light rich in far-red or blue wavebands sustains optimum flower development, whereas red light is totally ineffective in this respect. Considering that the intensity dependent High Energy Reaction (HER) has its action maxima in the blue and far-red we propose that HER may be positively involved in the flower development of C. rubrum. Our study further suggests that there may be some flower inhibitory component at play in relatively low intensity white light conditions and HER may be required to counteract this flower inhibitory effect.

Chloroplast replication and chloroplast DNA synthesis in spinach leaves

Cultured leaf disks from young spinach leaves have been used to study chloroplast DNA synthesis in relation to chloroplast growth and replication. Autoradiographs prepared from thin sections show that labelling throughout disks is uniform. We consider that cultured leaf disks serve as a model system for studies of cell expansion and chloroplast replication during the development of spinach leaf cells. Leaf disks from young spinach leaves were incubated continuously for 5 days in a medium containing [ 3 H]thymidine. They were harvested daily and light microscope autoradiographs of separated cells prepared. In the expanding cells of disks all chloroplasts divide and the synthesis of chloroplast DNA as measured from grain counts was correlated with the rate of chloroplast formation. A doubling of chloroplast numbers was associated with a doubling of chloroplast silver grains, which is consistent with a duplication of chloroplast DNA during the chloroplast division cycle. When disks were grown in green light, chloroplast replication but not chloroplast growth ceased in 3–4 days. The large chloroplasts of disks grown in green light appeared locked in a dumb-bell shaped configuration. Light microscope autoradiographs have established that these chloroplasts can synthesize DNA. These chloroplasts divided when the disks were exposed to high intensity white light and again it appears that all chloroplasts in the one cell divide. The chloroplasts of disks grown in green light for periods up to 9 days became uneven in size although all were dumb-bell shaped. We suggest that the size differences represent ploidy classes. Chloroplast replication in dicotyledons has been discussed in relation to the division and growth of leaf cells.

Effects of Green Light on the Chloroplasts of Spinach Leaf Discs

The development and photosynthetic capacity of chloroplasts formed in green light in cultured spinach leaf discs has been studied. At intensities of 4 to 6 mW cm-2 green light stimulates chloroplast replication to about the same extent as white, blue, and red light. However, practically no chloroplast replication occurs in discs grown in low intensity green or white light but considerable chloroplast growth takes place. Ultrastructural studies have shown that these chloroplasts, which can be two to five times the area of control plastids (high intensity white light), have an essentially normal thylakoid system. Double isotope labelling experi ments have established that the synthesis of chloroplast ribosomal-RNA is similar in controls and in discs grown in low intensity green or white light. On a per unit chlorophyll basis the CO2 fixation rate of spinach discs grown in low intensity green (or white) light saturates with increasing light intensity or increasing CO2 concentration at values well below control discs. In this respect their photosynthetic characteristics bear a similarity to those of shade plants.

The effect of cell age on chloroplast structure and chlorophyll in cultured spinach leaf discs.

Discs from young spinach leaves show a 50-fold increase in fresh weight and a 10-fold increase in chlorophyll over 7 days when cultured in high intensity white light (6.5 mWcm−2). In darkness the increase in fresh weight is 10-fold while chlorophyll decreases. Discs from mature spinach leaves show only a 2-fold increase in fresh weight and a marked decrease in chlorophyll content when cultured for 7 days either in light or darkness. Loss of chlorophyll in cultured discs is closely correlated with leaf and cell age and is affected positively by temperature.

Correlations of Growth Rate and De-etiolation with Rate of Ent-Kaurene Biosynthesis in Pea (Pisum sativum L.)

Biosynthesis of the gibberellin precursor ent-kaurene-(14)C from mevalonic acid-2-(14)C was assayed in cell-free extracts of shoot tips of etiolated and light-grown Alaska (normal) and Progress No. 9 (dwarf) peas (Pisum sativum L.). During ontogeny of light-grown Alaska peas, kaurene-synthesizing activity increased from an undectectable level in 3-day-old epicotyls to a maximum in shoot tips of 9-day-old plants and remained relatively constant thereafter until postanthesis. The capacity for kaurene synthesis in extracts from shoot tips of 10-day-old etiolated Alaska seedlings increased approximately exponentially during the first 12 hr of de-etiolation in continuous high intensity white light and remained relatively constant during the succeeding 24 hr of irradiation. Extracts from light-grown Alaska (normal) shoot tips possessed greater capacity for kaurene synthesis than did extracts from light-grown Progress No. 9 (dwarf) shoot tips. Extracts from shoot tips of either light-grown cultivar displayed greater kaurene-synthesizing capacity than was observed in extracts from their dark-grown counterparts. It is concluded that gibberellin biosynthesis in pea shoot tips is subject to partial regulation by factors controlling the rate of biosynthesis of kaurene.