REGULATION OF CHLOROPLAST DEVELOPMENT BY RED AND BLUE LIGHT

There are specific differences between red and blue light greening of etiolated seedlings of Hordevm vulgare L. Blue light results in a different prenyl lipid composition of chloroplast as compared to red light of equal quanta density. This is documented by a much higher prenylquinone content, higher chlorophyll a/b ratios, and lower values for the ratio xanthophylls to carotenes (x/c). The photosynthetic activity of “blue light” chloroplasts (Hill reaction) is higher than that of “red light” chloroplasts. These differences in prenylquinone composition and Hill-activity are associated with a different ultrastructure of chloroplasts. “Red light” chloroplasts exhibit a much higher grana content than “blue light” chloroplasts. The difference in thylakoid composition, photosynthetic activity and chloroplast structure found between blue and red light greening are similar to those found between sun and shade leaves and those between plants grown under high and low light intensities.     

PHOTOINHIBITION OF CHLOROPLAST REACTIONS

Abstract— An attemlpt was made to localize the site of photoinhibition of photosynthesis by measuring the decay of various chloroplast reactions after exposure to very strong light. A11 substrate reductions coupled to oxygen evolution as well as photophosphorylation mediated by PMS, proved equally sensitive to photoinhibition. Reactions involving only the long wave photosystem, such as TPN reduction with ascorbate as electron donor and photooxidation of cytochromec by detergent-treated chloroplasts were sensitive to a lower degree.
Photoinhibition irreversibly annihilated the 'variable' fraction of fluorescence emission —it decreased the steady state yield 2-3 fold and abolished the slow rise of the emission at the onset of illurnination.
It is concluded that the primary site of light inactivation is in, or close to, the trapping centers of the oxygen evolving step of photosynthesis. Pre-illumination leaves these traps in a state capable of draining light from sensitizing pigments but unable to perform useful photochemistry.     

Blue light-induced chloroplast reorientations in Lemna trisulca L. (duckweed) are controlled by two separable cellular mechanisms as suggested by different sensitivity to wortmannin

Chloroplast reorientations within mesophyll cells are among the most rapid physiological responses of higher plants to blue light. At light intensities below the saturation point of photosynthesis, chloroplasts move to the cell walls perpendicular to the direction of light and maximize light absorption (low-fluence rate response [LFR]). At light intensities above the saturation point of photosynthesis, chloroplasts redistribute to cell walls parallel to the direction of light (high-fluence rate response [HFR]). The actin-based mechanism is responsible for the light-induced chloroplast movements. We have found that an inhibitor of phosphoinositide-3-kinases, wortmannin, potently and irreversibly inhibited LFR and HFR chloroplast responses to blue light in Lemna trisulca L. mesophyll cells. Microscopic observations and photometric measurement indicated that 100 nM wortmannin specifically inhibited LFR in Lemna, whereas HFR displayed no sensitivity to the inhibitor at this concentration. A complete inhibition of the HFR could be obtained by 1 microM wortmannin. These data indicate that LFR is more sensitive to wortmannin than HFR and suggest that these two responses may be under the control of different cellular mechanisms. Our results suggest that phosphoinositide kinases and other phosphoinositide cycle enzymes may play a role in the transduction of the light signal to the actin cytoskeleton in Lemna as factors specifying the direction of chloroplast movements. A hypothetical model assuming three signaling pathways regulating light-induced chloroplast reorientations in mesophyll cells is proposed.     

Conjugated Polymer Nanoparticles to Augment Photosynthesis of Chloroplasts

By coating chloroplasts with conjugated polymer nanoparticles (CPNs), a new bio‐optical hybrid photosynthesis system (chloroplast/CPNs) is developed. Since CPNs possess unique light harvesting ability, including the ultraviolet part that chloroplasts absorb less, chloroplast/CPN complexes can capture broader range of light to accelerate the electron transport rates in photosystem II (PS II), the critical protein complex in chloroplasts, and augment photosynthesis beyond natural chloroplasts. The degree of spectral overlay between emission of CPNs and absorption of chloroplasts is critical for the enhanced photosynthesis. This work exhibits good potential to explore new and facile nanoengineering strategy for reforming chloroplast with light‐harvesting nanomaterials to enhance solar energy conversion.     

"ENERGY"-DEPENDENT QUENCHING OF CHLOROPHYLL FLUORESCENCE and THERMAL ENERGY DISSIPATION IN INTACT LEAVES DURING INDUCTION OF PHOTOSYNTHESIS

Abstract— Intact leaves, previously adapted to darkness for a prolonged period of time, were suddenly illuminated with a strong, photosynthetically saturating, white light (ca 1500 μmol m⁻² s^_1), resulting in the rapid establishment of a large energy-dependent chlorophyll fluorescence quenching (q_E) as shown by in vivo fluorescence measurements with a pulse amplitude modulation technique. Two different photothermal methods, photoacoustic spectroscopy and photothermal deflection spectroscopy, were used to monitor thermal deactivation of excited pigments during the dark-light transitions. The in vivo photothermal signals measured with both techniques were shown to remain constant during induction of photosynthesis under high light conditions, suggesting that, in contrast to current hypotheses, energy-dependent quenching q_E is not associated with significant changes in thermal dissipation of absorbed light energy in the chloroplasts. When photosynthesis was induced with a low-intensity modulated light, a noticeable decrease in the heat emission yield was observed resulting from the progressive activation of the competing photochemical processes.     

Epidermal lignin deposition in quinoa cotyledons in response to UV-B radiations

UV-B radiation (280-320 nm) is harmful to living organisms and has detrimental effects on plant growth, development and physiology. In this work we examined some mechanisms involved in plant responses to UV-B radiation. Seedlings of quinoa (Chenopodium quinoa Willd.) were exposed to variable numbers of UV-B radiation doses, and the effect on cotyledons was studied. We analyzed (1) cotyledons anatomy and chloroplasts ultrastructure; (2) peroxidase activity involved in the lignification processes; and (3) content of photosynthetic pigments, phenolic compounds and carbohydrates. Exposure to two UV-B doses induced an increase in the wall thickness of epidermal cells, which was associated with lignin deposition and higher activity of the peroxidase. The chloroplast ultrastructure showed an appearance typical of plants under shade conditions, likely in response to reduced light penetration into the mesophyll cells due to the screening effect of epidermal lignin deposition. Exposure to UV-B radiation also led to (1) enhancement in the level of phenolics, which may serve a protective function; (2) strong increase in the fructose content, a fact that might be related to higher requirement of erythrose-4P as a substrate for the synthesis of lignin and phenolics; and (3) reduction in the chlorophyll concentration, evidencing alteration in the photosynthetic system. We propose that the observed lignin deposition in epidermal tissues of quinoa is a resistance mechanism against UV-B radiation, which allows growing of this species in Andean highlands.     

Ultrafast spectroscopy studies on the mechanism of electron transfer and energy conversion in the isolated pseudo ginseng, water hyacinth and spinach chloroplasts

The spectroscopy characteristics and the fluorescence lifetime for the chloroplasts isolated from the pseudo ginseng, water hyacinth and spinach plant leaves have been studied by absorption spectra, low temperature steady-state fluorescence spectroscopy and single photon counting measurement under the same conditions and by the same methods. The similarity of the absorption spectra for the chloroplasts at room temperature suggests that different plants can efficiently absorb light of the same wavelength. The fluorescence decays in PS II measured at the natural QA state for the chloroplasts have been fitted by a three-exponential kinetic model. The three fluorescence lifetimes are 30, 274 and 805 ps for the pseudo ginseng chloroplast; 138, 521 and 1494 ps for the water hyacinth chloroplast; 197, 465 and 1459 ps for the spinach chloroplast, respectively. The slow lifetime fluorescence component is assigned to a collection of associated light harvesting Chl a/b proteins, the fast lifetime component to the react     

Natural and artificial light-harvesting systems utilizing the functions of carotenoids

Carotenoids are essential pigments in natural photosynthesis. They absorb in the blue–green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and so expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet–singlet energy transfer and so carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. Carotenoids also act to protect photosynthetic organisms from the harmful effects of excess exposure to light. In this case, triplet–triplet energy transfer from (bacterio-)chlorophyll to carotenoid plays a key role in this photoprotective reaction. In the light-harvesting pigment–protein complexes from purple photosynthetic bacteria and chlorophytes, carotenoids have an additional role, namely the structural stabilization of those complexes. In this article we review what is currently known about how carotenoids discharge these functions. The molecular architecture of photosynthetic systems will be outlined to provide a basis from which to describe the photochemistry of carotenoids, which underlies most of their important functions in photosynthesis. Then, the possibility to utilize the functions of carotenoids in artificial photosynthetic light-harvesting systems will be discussed. Some examples of the model systems are introduced.     

Flavonoid distribution in tissues of Phillyrea latifolia L. leaves as estimated by microspectrofluorometry and multispectral fluorescence microimaging

A new method for detecting the tissue-specific distribution of flavonoids has been developed by coupling microspectrofluorometry and multispectral fluorescence microimaging techniques. Fluorescence responses of cross sections taken from 1 year old Phillyrea latifolia leaves exposed to full (sun leaves) or 15% (shade leaves) solar radiation in a coastal area of Southern Tuscany were analyzed. Fluorescence spectra of different tissue layers, each normalized at its fluorescence maximum, that were stained or not stained with Naturstoff reagent A (in ethanol), under excitation with UV light (lambdaexc = 365 nm) or blue light (lambdaexc = 436 nm) were recorded. The shape of the fluorescence spectra of tissue layers from shade and sun leaves differed only under UV excitation. The fluorescence of stained cross sections from sun and shade leaves as well as from different layers of sun leaves received a markedly different contribution from the blue (470 nm) and the yellow-red (580 nm) wavebands. Such changes in tissue fluorescence signatures were related to light-induced changes of extractable caffeic acid derivatives and flavonoid glycosides, namely quercetin 3-O-rutinoside and luteolin 7-O-glucoside. Wall-bound phenolics, i.e. hydroxycinnamic acids (p-coumaric, ferulic and caffeic acid) and flavonoids (apigenin and luteolin derivatives), did not substantially differ between sun and shade leaves. A Gaussian deconvolution analysis of fluorescence spectra was subsequently performed to estimate the contribution of flavonoids (emitting at 600 nm, F600 [red fluorescence contribution = signal integrated over a Gaussian band centered at about 600 nm]) relative to the tissue fluorescence (Ftot [total fluorescence = signal integrated over the whole fluorescence spectrum]). The F600/ Ftot ratios sharply differed between analogous tissues of sun and shade leaves, as well as among tissue layers within each leaf type. A highly resolved picture of the tissue flavonoid distribution was finally provided through a fluorescence microimaging technique by acquiring fluorescence images at the blue (fluorescence at about 470 nm [F470]) and yellow-red (fluorescence at about 580 nm [F580]) wavelengths and correcting the F580 image for the contribution of nonflavonoids to the fluorescence at 580 nm. Monochrome images were elaborated by adequate computing functions to visualize the exclusive accumulation of flavonoids in different layers of P. latifolia leaves. Our data show that in shade leaves flavonoids almost exclusively occurred in the adaxial epidermal layer. In sun leaves flavonoids largely accumulated in the adaxial epidermal and subepidermal cells and followed a steep gradient passing from the adaxial epidermis to the inner spongy layers. Flavonoids also largely occurred in the abaxial epidermal cells and constituted the exclusive class of phenylpropanoids synthesized by the cells of glandular trichomes. The proposed method also allowed for the discrimination of the relative abundance of hydroxycinnamic derivatives and flavonoids in different layers of the P. latifolia leaves.     

1H HR-MAS NMR of carotenoids in aqueous samples and raw vegetables

Carotenoids are linear C40 tetraterpenoid hydrocarbons and represent a wide category of natural pigments. They are components of the pigment system of chloroplasts and are involved in the primary light absorption and the photon canalization of photosynthesis. Moreover, they also behave as quenchers of singlet oxygen, protecting cells and organisms against lipid peroxidation.Carotenoids have a strong lipophilic character and are usually analyzed in organic solvents. However, because of their biological activity, the characterization of these compounds in an aqueous environment or in the natural matrix is very important.One of the most important dietary carotenoids is beta-carotene, which has been extensively studied both in vivo and in model systems, but because of the low concentration and strong interaction with the biological matrix, beta-carotene has never been observed by NMR in solid aqueous samples.In the present work, a model system has been developed for the detection and identification of beta-carotene in solid aqueous samples by 1H HR-MAS NMR. The efficiency of the model has led to the identification of beta-carotene in a raw vegetable matrix.     

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.     

BLUE AND WHITE LIGHT EFFECTS ON CHLOROPLAST DEVELOPMENT IN A SOYBEAN MUTANT

Phenotypic difference for chloroplast development between the normal green (CL1) and the Cy₉y₉ soybean mutant was observed when the plants were grown under 18W m^?2 white or blue light. Under these conditions the mutant soybean accumulated less Chi b, neoxanthin, carotene and less total pigment than the CL1 genotype. Chloroplasts of the Cy₉y₉ line were deficient in the LHP complex relative to that of chloroplasts from the normal soybean. Specific differences were noted between chloroplasts from plants grown under blue and white light. Accumulations of a 34 kD (PSII) and a 16–17 kD (PSI) membrane polypeptide were decreased by blue light in both soybean genotypes. Blue light induced a greater accumulation of a 32 kD (PSII) polypeptide than white light. Blue light reduced granal thylakoid stacking and increased the proportion of stroma thylakoids compared to those that developed under white light. PSI electron transport activity was stimulated by the blue light treatment more than that of PSII.     

INFLUENCE OF PHOTON FLUX DENSITY IN THE 400–700 nm WAVEBAND ON INHIBITION OF PHOTOSYNTHESIS BY UV-B (280–320 nm) IRRADIATION IN SOYBEAN LEAVES: SEPARATION OF INDIRECT AND IMMEDIATE EFFECTS

Abstract— Visible radiation can substantially influence the degree to which plant photosynthesis is inhibited by UV-B radiation. This study was designed to separate the immediate effects of visible radiation on UV-B photosynthetic inhibition from the indirect influence of visible irradiation on morphological and physiological properties of leaves during leaf development. Soybean plants were pretreated in growth chambers with either high or low visible irradiance (750 and 70 μmol m^-2s^-1 quantum flux in the 400–700 nm waveband, respectively) during the development of leaves used subsequently for UV irradiation. Test leaves still attached to the plant were exposed to 5 h of polychromatic UV-B irradiation and the photosynthetic capacity (net CO₂ exchange) was determined before and after the UV irradiation. During the UV irradiation, plants from both pretreatment groups received either high or low visible flux. Development of leaves in the high visible flux pretreatment conditions resulted in thicker leaves, higher chlorophyll a/b ratios, more UV-absorbing pigments, and reduced sensitivity to the UV-B irradiation. However, higher visible flux during the UV-B irradiation resulted in greater depression of photosynthesis by the UV-B irradiation. The relative magnitude of photosynthetic depression under these treatment combinations was the same when photosynthesis was measured under either light-limited or light-saturated conditions.     

Reduction of UV-B radiation causes an enhancement of photoinhibition in high light stressed aquatic plants from New Zealand lakes

Anthropogenic stratospheric ozone depletion causes an increase of UV-B radiation impinging on the earth surface, which is a threat to plants not adapted to higher UV-B irradiances. Investigations were undertaken with aquatic plants from New Zealand, where UV-irradiances are naturally higher due to the southern latitude, to compare with former results of polar species. The experiments reported in this study were undertaken with plants collected from different lakes of the South Island, with different UV transparencies. Photoinhibition was induced under controlled conditions using a sun simulator, which mimicked the natural underwater radiation spectrum. Photosynthetic activity during high light stress, and during recovery in dim light, was determined in vivo by measuring fluorescence changes, using a PAM fluorometer device. A comparison of different species showed that the extent to which UV causes an additional decrease of photosynthetic performance during high light stress varies according to the depth of growth and UV transparency of the water body. This observation fits with previous studies. However, a new finding was that some species were even more strongly inhibited when UV-B was filtered out of the simulated sun spectrum, indicating a supporting effect of the short UVR wavelength range against photoinhibition. These results were also confirmed by field experiments under natural radiation conditions. Thus, UV-B does not solely cause negative effects on photosynthesis, but it may even support recovery processes in aquatic plants adapted to a high UV-radiation environment. The latter is in contrast to earlier studies, in which UV-B radiation was considered causing only harmful effects on photosynthesis of aquatic plants.     

A LINEAR DICHROISM STUDY USING CHLOROPLASTS OF VARIOUS STRUCTURE and PIGMENT COMPOSITION

Abstract— The orientation of pigments was studied in maize chloroplasts of different granum content and pigment composition. Comparison of the linear dichroism (LD)t of granal and agranal chloroplasts exhibited differences in the chl a forms, Ca 670 and Ca 691. The chl b content of the chloroplasts appeared to have little effect on the LD spectra.
In the LD spectra of carotenoid deficient mutant chloroplasts, the red band corresponding to oriented Ca 678 was considerably smaller than in those of normal chloroplasts. LD spectra of lycopenic chloroplasts revealed a band at 740 nm not present in the spectra of normal chloroplasts. Various signals of carotenoids observed in the LD spectra of carotenoid deficient mutant chloroplasts imply that the different carotenoids can be oriented in a different manner in the photosynthetic membrane.     

THE SUSCEPTIBILITY OF MUNG BEAN CHLOROPLASTS TO PHOTOINHIBITION IS INCREASED BY AN EXCESS SUPPLY OF IRON TO PLANTS: A PHOTOBIOLOGICAL ASPECT OF IRON TOXICITY IN PLANT LEAVES

In an attempt to elucidate the underlying mechanisms for iron toxicity in plants, the combined effects of iron overload and light intensities on the photosynthetic capacity of leaves were particularly focussed upon in this study, using mung bean seedlings grown under varied conditions regarding the supply of light and iron. The seedlings, when supplied with excess iron (up to 1.0 m M ) and low light (40 W/m²), did not suffer any loss of photosynthesis; further, the typical symptoms of iron toxicity, as shown in the leaves grown in sunlight at ca 450 W/m² on an average, were not seen in those. Nonetheless, excess iron supply resulted in a marked increase in photosensitivity of the low light-adapted seedlings. A large portion of iron accumulated in chloroplasts by the supply of excess iron was found to be incorporated into thylakoids as nonheme iron (NHI), which acts as a potent sensitizer, photogenerating singlet oxygen (¹O₂). The generation rate of ¹O₂ from thylakoids linearly increased with increasing content of NHI; this was in parallel with the NHI content dependence of photoinactivation rates of photosynthetic electron transport and key enzymes of the Calvin cycle in chloroplasts. The results suggest that Fe-dependent photosensitization reactions, occurring via the ¹O₂ mechanism, may be deeply involved in cellular processes leading to developing iron toxicity symptoms in plants.     

Carotenoid dependence of the protochlorophyllide to chlorophyllide phototransformation in dark-grown wheat seedlings.

The influence of carotenoids on partial protochlorophyllide (Pchlide) photoreduction and the successive formation of long-wavelength chlorophyllide (Chlide) forms was studied by low-temperature fluorescence spectroscopy (77 K). Wheat leaves with a decreased content of carotenoids obtained from norflurazon-treated seedlings (10 and 100 micromol l(-1)) were compared with leaves containing normal amounts of these pigments. Partial photoreduction of Pchlide was achieved by irradiation of the leaves with one light flash in combination with a number of neutral gray and/or red Perspex filters. There were significant differences between the fluorescence emission spectra (the position and height of the peaks) of dark-grown normal and carotenoid-deficient leaves irradiated with non-saturating white light of increasing intensity. The long-wavelength Chlide forms appeared first in the leaves nearly devoid of carotenoids (treated with 100 micromol l(-1) norflurazon), then in the leaves with carotenoid deficiency (treated with 10 micromol l(-1) norflurazon), and finally in normal leaves. After irradiation with non-saturating light of the same intensity, the ratio Chlide/Pchlide(657) was always the highest in the leaves nearly deficient of carotenoids, medium in the leaves with carotenoid deficiency and lowest in the normal leaves. Similarly to white light, red light of low intensity induced faster formation of long-wavelength Chlide species in the leaves with carotenoid deficiency in comparison to the normal leaves. We propose that, in leaves with reduced carotenoid content, a greater number of Pchlide molecules transform to Chlide per light flash than in normal leaves. The results are discussed in relation to the involvement of carotenoids in competitive absorption and light screening, as well as to their influence on Pchlide-Chlide interactions.     

Color of illumination during growth affects LHCII chiral macroaggregates in pea plant leaves

To determine whether the color of illumination under which plants are grown, affects the structure of photosynthetic antennae, pea plants were grown under either blue-enriched, red-enriched, or white light. Carotenoid content of isolated chloroplasts was found to be insensitive to the color of illumination during growth, while chlorophyll a/b ratio in chloroplasts isolated from young illuminated leaves showed susceptibility to color. Color of illumination affects the LHCII chiral macroaggregates in intact leaves and isolated chloroplasts, providing light-induced alteration of the handedness of the LHCII chiral macroaggregate, as measured with circular dichroism and circularly polarized luminescence. The susceptibility of handedness to current illumination (red light excitation of chlorophyll fluorescence) is dependent on the color under which the plants were grown, and was maximal for the red-enriched illumination. We propose the existence of a long-term (growth period) color memory, which influences the susceptibility of the handedness of LHCII chiral macroaggregates to current light.     

Photobiological Interactions of Blue Light and Photosynthetic Photon Flux: Effects of Monochromatic and Broad‐Spectrum Light Sources

Photosynthesis (Pn) and photomorphogenesis (Pm) are affected by light quality, light intensity and photoperiod. Although blue light (BL) is necessary for normal development, it is less efficient in driving Pn than other wavelengths of photosynthetically active radiation. The effects of BL on Pm are highly species dependent. Here we report the interacting effects of BL and photosynthetic photon flux (PPF) on growth and development of lettuce, radish and pepper. We used light‐emitting diode (LED) arrays to provide BL fractions from 11% to 28% under broad‐spectrum white LEDs, and from 0.3% to 92% under monochromatic LEDs. All treatments were replicated three times at each of two PPFs (200 and 500 μmol m^?2 s^?1). Other than light quality, environmental conditions were uniformly maintained across chambers. Regardless of PPF, BL was necessary to prevent shade‐avoidance responses in radish and lettuce. For lettuce and radish, increasing BL reduced stem length, and for both species, there were significant interactions of BL with PPF for leaf expansion. Increasing BL reduced petiole length in radish and flower number in pepper. BL minimally affected pepper growth and other developmental parameters. Pepper seedlings were more photobiologically sensitive than older plants. Surprisingly, there were few interactions between monochromatic and broad‐spectrum light sources.