Kinetics Of Light-induced Changes Research Articles

Acclimation of shade-tolerant and light-resistant Tradescantia species to growth light: chlorophyll a fluorescence, electron transport, and xanthophyll content.

In this study, we have compared the photosynthetic characteristics of two contrasting species of Tradescantia plants, T. fluminensis (shade-tolerant species), and T. sillamontana (light-resistant species), grown under the low light (LL, 50-125 µmol photons m-2s-1) or high light (HL, 875-1000µmol photons m-2s-1) conditions during their entire growth period. For monitoring the functional state of photosynthetic apparatus (PSA), we measured chlorophyll (Chl) a emission fluorescence spectra and kinetics of light-induced changes in the heights of fluorescence peaks at 685 and 740nm (F 685 and F 740). We also compared the light-induced oxidation of P700 and assayed the composition of carotenoids in Tradescantia leaves grown under the LL and HL conditions. The analyses of slow induction of Chl a fluorescence (SIF) uncovered different traits in the LL- and HL-grown plants of ecologically contrasting Tradescantia species, which may have potential ecophysiological significance with respect to their tolerance to HL stress. The fluorometry and EPR studies of induction events in chloroplasts in situ demonstrated that acclimation of both Tradescantia species to HL conditions promoted faster responses of their PSA as compared to LL-grown plants. Acclimation of both species to HL also caused marked changes in the leaf anatomy and carotenoid composition (an increase in Violaxanthin + Antheraxantin + Zeaxanthin and Lutein pools), suggesting enhanced photoprotective capacity of the carotenoids in the plants grown in nature under high irradiance. Collectively, the results of the present work suggest that the mechanisms of long-term PSA photoprotection in Tradescantia are based predominantly on the light-induced remodeling of pigment-protein complexes in chloroplasts.

Theoretical analysis of inductive effects in higher plant photosynthesis

The proposed mathematical model of the light stages of photosynthesis in higher plants takes into account the regulatory process of excitation energy redistribution between photosystems depending on the redox state of electron carriers. Nonmonotonic changes in the intensity of pigment fluorescence upon switching on the excitation light (delayed fluorescence induction) are described. The time required for fluorescence to reach its peak intensity and the degree of fluorescence quenching depending on light intensity are analyzed. Th kinetics of light-induced changes in EPR I signal upon switching from the far-red to white light is simulated.

Evolution of Charged Gap Statesin a- Si:H Under Light Exposure

AbstractThe charge deep-level transient spectroscopy (Q-DLTS) experiments on undoped hydrogenated amorphous silicon (a-Si:H) demonstrate that during light soaking the states in the upper part of the gap disappear, while additional states around and below midgap are created. Since no direct correlation is observed in light-induced changes of the three groups of states that we identify from the Q-DLTS signal, we believe that we deal with three different types of defects. Positively charged states above midgap are related to a complex formed by a hydrogen molecule and a dangling bond. Negatively charged states below midgap are attributed to floating bonds. Various trends in the evolution of dark conductivity due to light soaking indicate that the kinetics of light-induced changes of the three gap-state components depend on their initial energy distributions and on the spectrum and intensity of light during exposure.

Kinetics of light-induced degradation in a-Si:H films investigated by computer simulation

In this work we investigated the stability of a-Si:H films under illumination and following recovery in darkness at different temperatures. The a-Si:H films were fabricated with 55 kHz PECVD and with conventional rf 13.56 MHz PECVD. We measured the steady-state photocurrent and the dark-current after switching off the light source as a function of time. We observed photocurrent degradation and the following recovery of the dark current. The kinetics of the photocurrent degradation as well as the dark-current recovery demonstrated stretched-exponential behavior. The results of these straightforward measurements in combination with computer simulation were used to determine the effect of light-induced degradation and thermal recovery on the density of states distribution in the band gap of a-Si:H. We have found that the photocurrent degradation and the corresponding increase in the total defect concentration have different kinetics. The different kinetics were also determined for the dark-current recovery and the corresponding decrease in the total defect concentration. The results point out that slow and fast types of defects in a-Si:H films control the kinetics of light-induced changes of the defect distribution in the band gap. A model is proposed that relates the origin of the fast and slow metastable defects with the distribution of Si-Si bond lengths.

Kinetics of Light-Induced Changes in P-I-N Cells with Protocrystalline Si:H

AbstractStudies have been carried out on the thickness dependent transition between the amorphous and microcrystalline phases in intrinsic Si:H materials (i-layers) and its effect on p-i-n solar cell performance [1]. P(a-SiC:H)-i(a-Si:H)-n(μcSi:H) cell structures were deposited with the intrinsic Si:H layer thickness and the flow ratio of hydrogen to silane, R=[H2]/[SiH4], guided by an evolutionary phase diagram obtained from real-time spectroscopic ellipsometry. The thickness range over which the fill factors are controlled by the bulk was established and their characteristics investigated with different protocrystalline i-layer materials (i.e., materials prepared near the amorphous to microcrystalline boundary but on the amorphous side). Insights into the properties of these materials and the effects of the transition to the microcrystalline phase were obtained from the systematic changes in the initial fill factors, their light-induced changes, and their degraded steady states for cells with i-layers of different thickness and H2 dilution.

Light-induced changes in hydrogen-diluted a-Si : H materials and solar cells: A new perspective on self-consistent analysis

We report a study on a-Si: H materials and p(a-SiC : H)/i(a-Si : H)/n(μc-Si) solar cells prepared without and with hydrogen dilution (10 : 1) at substrate temperatures between 240°C and 130°C. In contrast to previously reported studies, the cell characteristics in the annealed state of these ∼4000 Å thick cells could be directly correlated with the properties of their corresponding i-layer materials. Also, despite the importance of the p/i interface regions, very similar kinetics of light-induced changes are observed in the cells and the corresponding films. In particular, both cells and films fabricated with hydrogen dilution reach a degraded steady state in less than 100 h of AM 1 illumination, which offers a well-defined “marker” for the direct correlation of their respective light-induced changes. Advantage is also taken of the differences in degradation kinetics between diluted and undiluted materials in fabricating custom-designed cells in which these well-characterized intrinsic materials are incorporated into either the bulk or the p/i interface regions.

Effect of light-induced changes in thylakoid voltage on chlorophyll fluorescence of Aegopodium podagraria leaves

The millisecond kinetics of light-induced changes in thylakoid voltage as detected by absorbance changes at 520 nm were compared with the kinetics of the fluorescence yield on Aegopodium podagraria leaves. Frequency response measurements with red actinic light and the analysis of the response to far-red light flashes reveal that the light induced increase in thylakoid voltage is coupled to an increase in fluorescence yield. An increase in fluorescence yield of 5.4% per 10 mV increase in thylakoid voltage is found after calibration of the 520 nm absorbance changes by single-turnover flashes which are assumed to induce an increase in thylakoid voltage by 25 mV. The measured voltage sensitivity of the fluorescence yield compares well to calculations based on the assumption that the rate constant of PS II charge separation is sensitive to electric fields as proposed by Schatz et al. (Biophys. J. 54 (1988) 379–405). Based on the results of the frequency response measurements, it is estimated that about 7% of the O-I phase of the fluorescence kinetics and 10% of the fluorescence response to single-turnover flashes are due to the voltage effect. It is discussed whether transthylakoid voltage might be of relevance for the control of PS II electron flux.

Detection of rapid induction kinetics with a new type of high-frequency modulated chlorophyll fluorometer

A newly developed modulation fluorometer is described which operates with 1 μsec light pulses from a light-emitting diode (LED) at 100 KHz. Special amplification circuits assure a highly selective recording of pulse fluorescence signals against a vast background of non-modulated light. The system tolerates ratios of up to 1:10(7) between measuring light and actinic light. Thus it is possible to measure the "dark fluorescence yield" and record the kinetics of light-induced changes. A high time resolution allows the recording of the rapid relaxation kinetic following a saturating single turnover flash. Examples of system performance are given. It is shown that following a flash the reoxidation kinetics of photosystem II acceptors are slowed down not only by the inhibitor DCMU, but by a number of other treatments as well. From a light intensity dependency of the induction kinetics the existence of two saturated intermediate levels (I1 and I2) is apparent, which indicates the removal of three distinct types of fluorescence quenching in the overall fluorescence rise from F0 to Fmax.

The kinetics of light-induced changes of C-550, cytochrome b559 and fluorescence yield in chloroplasts at low temperature

The kinetics of the photoreduction of C-550, the photooxidation of cytochrome b 559 and the fluorescence yield changes during irradiation of chloroplasts at −196 °C were measured and compared. The photoreduction of C-550 proceeded more rapidly than the photooxidation of cytochrome b 559 and the fluorescence yield increase followed the cytochrome b 559 oxidation. These results suggest that fluorescence yield under these conditions indicates the dark reduction of the primary electron donor to Photosystem II, P680 +, by cytochrome b 559 rather than the photoreduction of the primary electron acceptor. The photoreduction of C-550 showed little if any temperature dependence over the range of −196 to −100 °C. The amount of cytochrome b 559 photooxidized was sensitive to temperature decreasing from the maximal change at temperatures between −196 to −160 °C to no change at −100 °C. To the extent that the reaction occurred at temperatures between −160 and −100 °C the rate was largely independent of temperature. The rate of the fluorescence increase was dependent on temperature over this range being 3–4 times more rapid at −100 than at −160 °C. At −100 °C the light-induced fluorescence increase and the photoreduction of C-550 show similar kinetics. The temperature dependence of the fluorescence induction curve is attributed to the temperature dependence of the dark reduction of P680 +. The intensity dependence of the photoreduction of C-550 and of the photooxidation of cytochrome b 559 are linear at low intensities (below 200 μW/cm 2) but fall off at higher intensities. The failure of reciprocity in the photoreduction of C-550 at the higher intensities is not explained by the simple model proposed for the Photosystem II reaction centers.

Chlorophyll a fluorescence induction and changes in the electrical potential of the cellular membranes of green plant cells

1. The kinetics of light-induced changes in the near infrared chlorophyll a fluorescence were measured in intact cells and broken cell preparations of the algae Nitellopsis obtusa and Nitella translucens and of spinach and tomato leaves. 2. The part of the fluorescence induction which is associated with the transition of the pigment systems from light state 1 to state 2, i.e. the long-term decrease in fluorescence yield during the so-called P 2M 3 phase, occurring after an initial increase in the light, is not observed in broken cell preparations. 3. Simultaneous measurements of fluorescence changes and changes in the electrical potential of the two cytoplasmic membranes in Nitella translucens show a close association between the transition of the states of the pigment systems and the changes in membrane potential. 4. The results are discussed in terms of a hypothesis which emphasizes that the light and dark transitions of the states of the pigment systems occur in association with regulatory ion transport processes across the cytoplasmic and outer chloroplast membranes.