Fast Fluorescence Quenching from Isolated Guard Cell Chloroplasts of Vicia faba Is Induced by Blue Light and Not by Red Light

Chlorophyll a fluorescence transients from isolated Vicia faba guard cell chloroplasts were used to probe the response of these organelles to light quality. Guard cell chloroplasts were isolated from protoplasts by passing them through a 10-μm nylon net. Intact chloroplasts were purified on a Percoll gradient. Chlorophyll a fluorescence transients induced by actinic red or blue light were measured with a fluorometer equipped with a measuring beam. Actinic red light induced a monophasic quenching, and transients induced by blue light showed biphasic kinetics having a slow and a fast component. The difference between the red and blue light-induced transients could be observed over a range of fluence rates tested (200-800 μmol m⁻² s⁻¹). The threshold fluence rate of blue light for the induction of the fast component of quenching was 200 μmol m⁻² s⁻¹, but in the presence of saturating red light, fluence rates as low as 25 μmol m⁻² s⁻¹ induced the fast quenching. These results indicate that guard cell chloroplasts have a specific response to blue light.     

Vanadate inhibits blue light-stimulated swelling of vicia guard cell protoplasts

When supplied under low chloride concentrations, vanadate inhibits the blue light-stimulated swelling of Vicia faba L. guard cell protoplasts in a dose-dependent fashion. The volume of guard cell protoplasts incubated in 10 mm K-imino-diacetic acid, 0.4 m mannitol, and 1 mm CaCl₂ remained essentially constant under 1000 μmol m⁻² s⁻¹ red light, but increased an average of 27% after 8 min of the addition of 50 μmol m⁻² s⁻¹ blue light to the background red light. At 500 μm, vanadate completely inhibits the response to blue light. Vanadate also inhibits the swelling of guard cell protoplasts stimulated by the H⁺-ATPase agonist fusicoccin. The vanadate sensitivity of the blue light-stimulated swelling implicates a proton-pumping ATPase as a component of the sensory transduction of blue light in guard cells.     

The ultraviolet action spectrum for stomatal opening in broad bean

The ultraviolet action spectrum for stomatal opening was measured using epidermal peels from leaves of broad bean (Vicia faba). The spectrum was calculated from hyperbolic fluence response curves using 11 wavelengths ranging from 275 to 459 nm. The action spectrum exhibits a major peak at approximately 280 nm and a minor peak at approximately 360 nm. The response at 280 nm is about three times greater than the response at 459 nm. Under the conditions utilized (i.e. the absence of saturating red light), stomatal opening saturated at extremely low fluence rates: <0.2 μmol m⁻² s⁻¹ at 280 nm, and approximately 1.0 μmol m⁻² s⁻¹ at 459 nm. The threshold for blue-light-induced stomatal opening was approximately 0.02 μmol m⁻² s⁻¹. In light-mixing experiments, the addition of 280 nm light to saturating 650 nm (red) light caused additional stomatal opening, which is indicative of separate photoreceptors. In contrast, adding 280 nm of light to saturating 459 nm (blue) light did not increase stomatal opening, suggesting that they both excite the same receptor. The results with white light were similar to those with blue light. We infer that ultraviolet light acts via the blue light photoreceptor rather than through photosynthesis. The additional absorbance peak at 360 nm suggests that the chromophore is either a flavin or a cis-carotenoid, both of which exhibit peaks in this region. It is proposed that the chromophore can be excited either directly by blue light or by energy transferred from the protein portion of the protein-pigment complex after it absorbs 280 nm light.     

Regulation of Calcium Influx in Chara: Effects of K, pH, Metabolic Inhibition, and Calcium Channel Blockers

Measurements were made of ⁴⁵Ca influx into isolated internodal cells of Chara corallina and also into internodal cells of intact plants. ⁴⁵Ca influx was closely related to growth. In rapidly expanding internodal cells, the influx was approximately 1.4 nmol m⁻² s⁻¹ compared to the influx in mature cells from slow-growing cultures of 0.2 nmol m⁻² s⁻¹. Isolated internodal cells had influxes in the range 0.2 to 0.7 nmol m⁻² s⁻¹, but this increased to approximately 2 nmol m⁻² s⁻¹ in high calcium solutions and to 4 nmol m⁻² s⁻¹ in high potassium solutions. No significant effects on calcium influx were observed for changes in external pH or for treatments that changed internal pH, except that NH₄ was slightly inhibitory. Severe metabolic inhibition by carbonylcyanide-m-chlorophenyl-hydrazone stimulated influx, whereas dicyclohexylcarbodiimide had no effect and darkness inhibited influx. La³⁺ also inhibited influx, but the organic channel blockers nifedipine and bepridil stimulated influx. Verapamil had no effect. The results are generally consistent with voltage regulation of calcium channels as in animal cells.     

Carbon gain and photosynthetic response of chrysanthemum to photosynthetic photon flux density cycles

Most models of carbon gain as a function of photosynthetic irradiance assume an instantaneous response to increases and decreases in irradiance. High- and low-light-grown plants differ, however, in the time required to adjust to increases and decreases in irradiance. In this study the response to a series of increases and decreases in irradiance was observed in Chrysanthemum × morifolium Ramat. “Fiesta” and compared with calculated values assuming an instantaneous response. There were significant differences between high- and low-light-grown plants in their photosynthetic response to four sequential photosynthetic photon flux density (PPFD) cycles consisting of 5-minute exposures to 200 and 400 micromoles per square meter per second (μmol m⁻²s⁻¹). The CO₂ assimilation rate of high-light-grown plants at the cycle peak increased throughout the PPFD sequence, but the rate of increase was similar to the increase in CO₂ assimilation rate observed under continuous high-light conditions. Low-light leaves showed more variability in their response to light cycles with no significant increase in CO₂ assimilation rate at the cycle peak during sequential cycles. Carbon gain and deviations from actual values (percentage carbon gain over- or underestimation) based on assumptions of instantaneous response were compared under continuous and cyclic light conditions. The percentage carbon gain overestimation depended on the PPFD step size and growth light level of the leaf. When leaves were exposed to a large PPFD increase, the carbon gain was overestimated by 16 to 26%. The photosynthetic response to 100 μmol m⁻² s⁻¹ PPFD increases and decreases was rapid, and the small overestimation of the predicted carbon gain, observed during photosynthetic induction, was almost entirely negated by the carbon gain underestimation observed after a decrease. If the PPFD cycle was 200 or 400 μmol m⁻² s⁻¹, high- and low-light leaves showed a carbon gain overestimation of 25% that was not negated by the underestimation observed after a light decrease. When leaves were exposed to sequential PPFD cycles (200-400 μmol m⁻² s⁻¹), carbon gain did not differ from leaves exposed to a single PPFD cycle of identical irradiance integral that had the same step size (200-400-200 μmol m⁻² s⁻¹) or mean irradiance (200-300-200 μmol m⁻² s⁻¹).     

Phototropin Mediated Relocation of Myosins in Arabidopsis thaliana

Background

The mechanism of the light-dependent movements of chloroplasts is based on actin and myosin but its details are largely unknown. The movements are activated by blue light in terrestrial angiosperms. The aim of the present study was to determine the role of myosin associated with the chloroplast surface in the light-induced chloroplast responses in Arabidopsis thaliana. The localization of myosins was investigated under blue light intensities generating avoidance and accumulation responses of chloroplasts. The localization was compared in wild type plants and in phot2 mutant lacking the avoidance response.

Results

Wild type and phot2 mutant plants were irradiated with strong (36 µEm⁻²s⁻¹) and/or weak (0.8 µEm⁻²s⁻¹) blue light. The leaf tissue was immunolabeled with antimyosin antibodies. Different arrangements of myosins were observed in the mesophyll depending on the fluence rate in wild type plants. In tissue irradiated with weak blue light myosins were associated with chloroplast envelopes. In contrast, in tissue irradiated with strong blue light chloroplasts were almost myosin-free. The effect did not occur in red light and in the phot2 mutant.

Conclusions

Myosin displacement is blue light specific, i.e., it is associated with the activation of a specific blue-light photoreceptor. We suggest that the reorganization of myosins is essential for chloroplast movement. Myosins appear to be the final step of the signal transduction pathway starting with phototropin2 and leading to chloroplast movements.Key Words: Arabidopsis, blue light, chloroplast movements, myosins, phototropins     

Intra- specific variation in response of Jatropha (Jatropha curcas L.) to elevated CO2 conditions

Twenty genotypes of Jatropha collected from diverse eco-geographic regions from the states of Chhattisgarh (3), Andhra Pradesh (12), Rajasthan (4) and Uttarakhand (1) of India were subjected to elevated CO₂ conditions. All the genotypes showed significant difference (p < 0.05 and 0.01) in the phenotypic traits in both the environments (elevated and ambient) and genotype x environment interaction. Among the physiological traits recorded, maximum photosynthetic rate was observed in IC565048 (48.8 μmol m⁻² s⁻¹) under ambient controlled conditions while under elevated conditions maximum photosynthetic rate was observed in IC544678 (41.3 μmol m⁻² s⁻¹), and there was no significant difference in the genotype x environment interaction. Stomatal conductance (Gs) emerged as the key factor as it recorded significant difference among the genotypes, between the environments and also genotype x environment interaction. The Gs and transpiration (E) recorded a significant decline in the genotypes under the elevated CO₂ condition over the ambient control. Under elevated CO₂ conditions, the minimum values recorded for Gs and E were 0.03 mmol m⁻² s⁻¹ and 0.59 mmol m⁻² s⁻¹ respectively in accession IC565039, while the maximum values for Gs and E were 1.8 mmol m⁻² s⁻¹ and 11.5 mmol m⁻² s⁻¹ as recorded in accession IC544678. The study resulted in the identification of potential climate ready genotypes viz. IC471314, IC544654, IC541634, IC544313, and IC471333 for future use.     

Toxin Production by Microcystis aeruginosa as a Function of Light in Continuous Cultures and Its Ecological Significance

The effects of light intensity and light quality on toxin production by Microcystis aeruginosa were examined in continuous cultures. Light intensity had a pronounced effect on toxicity and the toxin production rate. Toxicity and the toxin production rate increased with light intensity up to an intensity of about 40 microeinsteins m^-2 s^-1 and decreased at higher light intensities, while the ratio of toxin to protein was constant at intensities of more than 40 microeinsteins m^-2 s^-1. Light quality had only slight effects on toxicity. The results of our laboratory experiments were supported by the results of field work in which we examined toxin production at different depths in a lake. Our observations explain the mixed pattern of high and low toxicity found in a surface bloom of M. aeruginosa. Our findings also indicate that production of the peptide toxin can be uncoupled from general protein synthesis.     

Light-Modulated Responses of Growth and Photosynthetic Performance to Ocean Acidification in the Model Diatom Phaeodactylum tricornutum

Ocean acidification (OA) due to atmospheric CO₂ rise is expected to influence marine primary productivity. In order to investigate the interactive effects of OA and light changes on diatoms, we grew Phaeodactylum tricornutum, under ambient (390 ppmv; LC) and elevated CO₂ (1000 ppmv; HC) conditions for 80 generations, and measured its physiological performance under different light levels (60 µmol m⁻² s⁻¹, LL; 200 µmol m⁻² s⁻¹, ML; 460 µmol m⁻² s⁻¹, HL) for another 25 generations. The specific growth rate of the HC-grown cells was higher (about 12–18%) than that of the LC-grown ones, with the highest under the ML level. With increasing light levels, the effective photochemical yield of PSII (F_v′/F_m′) decreased, but was enhanced by the elevated CO₂, especially under the HL level. The cells acclimated to the HC condition showed a higher recovery rate of their photochemical yield of PSII compared to the LC-grown cells. For the HC-grown cells, dissolved inorganic carbon or CO₂ levels for half saturation of photosynthesis (K_1/2 DIC or K_1/2 CO₂) increased by 11, 55 and 32%, under the LL, ML and HL levels, reflecting a light dependent down-regulation of carbon concentrating mechanisms (CCMs). The linkage between higher level of the CCMs down-regulation and higher growth rate at ML under OA supports the theory that the saved energy from CCMs down-regulation adds on to enhance the growth of the diatom.     

Light Enhances Survival of Dinoroseobacter shibae during Long-Term Starvation

Aerobic anoxygenic phototrophs (AAPs) as being photoheterotrophs require organic substrates for growth and use light as a supplementary energy source under oxic conditions. We hypothesized that AAPs benefit from light particularly under carbon and electron donor limitation. The effect of light was determined in long-term starvation experiments with Dinoroseobacter shibae DFL 12^T in both complex marine broth and defined minimal medium with succinate as the sole carbon source. The cells were starved over six months under three conditions: continuous darkness (DD), continuous light (LL), and light/dark cycle (LD, 12 h/12 h, 12 µmol photons m⁻² s⁻¹). LD starvation at low light intensity resulted in 10-fold higher total cell and viable counts, and higher bacteriochlorophyll a and polyhydroxyalkanoate contents. This coincided with better physiological fitness as determined by respiration rates, proton translocation and ATP concentrations. In contrast, LD starvation at high light intensity (>22 µmol photons m⁻² s⁻¹, LD conditions) resulted in decreasing cell survival rates but increasing carotenoid concentrations, indicating a photo-protective response. Cells grown in complex medium survived longer starvation (more than 20 weeks) than those grown in minimal medium. Our experiments show that D. shibae benefits from the light and dark cycle, particularly during starvation.     

How and why do plant nuclei move in response to light?

We recently found that nuclei take different intracellular positions depending upon dark and light conditions in Arabidopsis thaliana leaf cells. Under dark conditions, nuclei in both epidermal and mesophyll cells are distributed baso-centrally within the cell (dark position). Under light conditions, in contrast, nuclei are distributed along the anticlinal walls (light position). Nuclear repositioning from the dark to light positions is induced specifically by blue light at >50 µmol m⁻² s⁻¹ in a reversible manner. Using analysis of mutant plants, it was demonstrated that the response is mediated by the blue-light photoreceptor phototropin2. Intriguingly, phototropin2 also seems to play an important role in the proper positioning of nuclei and chloroplasts under dark conditions. Light-dependent nuclear positioning is one of the organelle movements regulated by phototropin2. However, the mechanisms of organelle motility, physiological significance, and generality of the phenomenon are poorly understood. In this addendum, we discussed how and why nuclei move depending on light, together with future perspectives.Key words: actin, Arabidopsis, blue light, cytoskeleton, nuclear positioning, nucleus, phototropin     

Kinesin-2 KIF3AB Exhibits Novel ATPase Characteristics

KIF3AB is an N-terminal processive kinesin-2 family member best known for its role in intraflagellar transport. There has been significant interest in KIF3AB in defining the key principles that underlie the processivity of KIF3AB in comparison with homodimeric processive kinesins. To define the ATPase mechanism and coordination of KIF3A and KIF3B stepping, a presteady-state kinetic analysis was pursued. For these studies, a truncated murine KIF3AB was generated. The results presented show that microtubule association was fast at 5.7 μm⁻¹ s⁻¹, followed by rate-limiting ADP release at 12.8 s⁻¹. ATP binding at 7.5 μm⁻¹ s⁻¹ was followed by an ATP-promoted isomerization at 84 s⁻¹ to form the intermediate poised for ATP hydrolysis, which then occurred at 33 s⁻¹. ATP hydrolysis was required for dissociation of the microtubule·KIF3AB complex, which was observed at 22 s⁻¹. The dissociation step showed an apparent affinity for ATP that was very weak (K_½,ATP at 133 μm). Moreover, the linear fit of the initial ATP concentration dependence of the dissociation kinetics revealed an apparent second-order rate constant at 0.09 μm⁻¹ s⁻¹, which is inconsistent with fast ATP binding at 7.5 μm⁻¹ s⁻¹ and a K_d,ATP at 6.1 μm. These results suggest that ATP binding per se cannot account for the apparent weak K_½,ATP at 133 μm. The steady-state ATPase K_m,ATP, as well as the dissociation kinetics, reveal an unusual property of KIF3AB that is not yet well understood and also suggests that the mechanochemistry of KIF3AB is tuned somewhat differently from homodimeric processive kinesins.     

Adaptational changes in the lipids and fatty acid profile of the cell and thylakoid membrane of rice plants exposed to sunlight

Adaptational changes occurring in the lipids and fatty acids of the cell and the thylakoid membrane in response to high light treatment, was studied in 30 days old rice (Oryza sativa L. cv. Jyothi) plants grown under low (150–200 μmol m⁻² s⁻¹) or moderate (600–800 μmol m⁻² s⁻¹) light conditions. Results were compared with rice plants grown in high (1200–2200 μmol m⁻² s⁻¹) light conditions. Exposure of rice plants and isolated chloroplast to high light, resulted in an increase in the amount of malonaldehyde, indicating oxidation of membrane lipids. Qualitative and quantitative changes in the phosphoglycolipids and quantitative changes in neutral lipids were observed in rice plants grown under the different growth conditions. A few of the phosphoglycolipids and neutral lipids were present exclusively in plants grown at low or moderate or high light, indicating requirement of different type of lipid composition of rice plants in response to their different growth irradiances. However, no significant quantitative changes were observed in the different saturated and unsaturated fatty acid groups of total lipids in low, moderate and high light grown rice plants, as a result of exposure to high light. No qualitative changes in the fatty acid composition due to difference in growth irradiance or high light treatment were seen. The changes observed in the phosphoglycolipids and neutral lipid composition of cell and thylakoid membrane of low, moderate and high light grown rice plants in response to high light, are probably the result of physiological changes in the rice plants, to sustain optimum structure and function of the cell and thylakoid membrane to maintain active physiological functions to endure high light conditions.     

Influence of Pretreatment and Experimental Conditions on Electrophoretic Mobility and Hydrophobicity of Cryptosporidium parvum Oocysts

Surface properties of Cryptosporidium parvum oocysts were investigated by using electrophoretic mobility and hydrophobicity measurements. Oocysts purified from calf feces by several sucrose flotation steps and deionized water (DI) washes (DIS method) had an electrophoretic mobility (neutral surface charge) near 0.0 m² V⁻¹ s⁻¹ over a pH range of 2 to 10. The mean electrophoretic mobility of oocysts stored in DI containing a mixture of antibiotics had a lower standard deviation (ς = 0.36) than that of oocysts stored in DI without antibiotics (ς = 0.53); their electrophoretic mobility remained unchanged up to 121 days after collection. The electrophoretic mobility of oocysts purified on a cold Percoll-sucrose gradient after the feces was defatted with ethyl acetate (EAPS method) varied linearly with pH from 0.0 m² V⁻¹ s⁻¹ at pH 2.4 to −3.2 × 10⁻⁸ m² V⁻¹ s⁻¹ at pH 10 (ς = 0.52), thus displaying the negative surface charge at neutral pH observed by other researchers. The hydrophobicity of oocysts and two types of polystyrene beads was measured as a function of ionic strength by adhesion to polystyrene. Oocysts were purified by the DIS method. The ionic strength of the suspending solution was varied from 0 to 95 mmol liter⁻¹. Two-week-old oocysts exhibited strong adhesion (~85%) at ionic strengths of 0 to 10 mmol liter⁻¹ and moderate adhesion (~20%) at ionic strengths of 20 to 95 mmol liter⁻¹. Two-month-old oocysts exhibited high adhesion (~60 to 80%) at all ionic strengths. These results show that adhesion properties governed by the electrophoretic mobility of purified C. parvum oocysts can be altered by the method of purification and that hydrophobicity can change as oocysts age.     

Controls of Evapotranspiration and CO2 Fluxes from Scots Pine by Surface Conductance and Abiotic Factors

Evapotranspiration (E) and CO₂ flux (F_c) in the growing season of an unusual dry year were measured continuously over a Scots pine forest in eastern Finland, by eddy covariance techniques. The aims were to gain an understanding of their biological and environmental control processes. As a result, there were obvious diurnal and seasonal changes in E, F_c, surface conductance (g_c), and decoupling coefficient (Ω), showing similar trends to those in radiation (PAR) and vapour pressure deficit (δ). The maximum mean daily values (24-h average) for E, F_c, g_c, and Ω were 1.78 mmol m⁻² s⁻¹, −11.18 µmol m⁻² s⁻¹, 6.27 mm s⁻¹, and 0.31, respectively, with seasonal averages of 0.71 mmol m⁻² s⁻¹, −4.61 µmol m⁻² s⁻¹, 3.3 mm s⁻¹, and 0.16. E and F_c were controlled by combined biological and environmental variables. There was curvilinear dependence of E on g_c and F_c on g_c. Among the environmental variables, PAR was the most important factor having a positive linear relationship to E and curvilinear relationship to F_c, while vapour pressure deficit was the most important environmental factor affecting g_c. Water use efficiency was slightly higher in the dry season, with mean monthly values ranging from 6.67 to 7.48 μmol CO₂ (mmol H₂O)⁻¹ and a seasonal average of 7.06 μmol CO₂ (μmol H₂O)⁻¹. Low Ω and its close positive relationship with g_c indicate that evapotranspiration was sensitive to surface conductance. Mid summer drought reduced surface conductance and decoupling coefficient, suggesting a more biotic control of evapotranspiration and a physiological acclimation to dry air. Surface conductance remained low and constant under dry condition, supporting that a constant value of surface constant can be used for modelling transpiration under drought condition.     

Stimulation of growth and ion uptake in bean leaves by red and blue light

Red and blue light both stimulate growth and ion accumulation in bean (Phaseolus vulgaris L.) leaves, and previous studies showed that the growth response is mediated by phytochrome and a blue-light receptor. Results of this study confirm that there is an additional photosynthetic contribution from the growing cells that supports ion uptake and growth. Disc expansion in the light was enhanced by exogenous K⁺ and Rb⁺, but was not specific for anions. Light increased K⁺ accumulation and the rate of ⁸⁶Rb⁺ uptake by discs, over darkness, with no effect of light quality. The photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, inhibited light-driven ⁸⁶Rb⁺ uptake by 75%. Light quality caused differences in short-term kinetics of growth and acidification of the leaf surface. At comparable fluence rates (50 μmol m⁻² s⁻¹), continuous exposure to blue light increased the growth rate 3-fold after a 2-min lag, whereas red light caused a smaller growth response after a lag of 12 min. In contrast, the acidification of the leaf surface normally associated with growth was stimulated 3-fold by red light but only slightly (1.3-fold) by blue light. This result shows that, in addition to acidification caused by red light, a second mechanism specifically stimulated by blue light is normally functioning in light-driven leaf growth.     

Phosphorylation of NONPHOTOTROPIC HYPOCOTYL3 affects photosensory adaptation during the phototropic response

Photosensory adaptation, which can be classified as sensor or effector adaptation, optimizes the light sensing of living organisms by tuning their sensitivity to changing light conditions. During the phototropic response in Arabidopsis (Arabidopsis thaliana), the light-dependent expression controls of blue-light (BL) photoreceptor phototropin 1 (phot1) and its modulator ROOT PHOTOTROPISM2 (RPT2) are known as the molecular mechanisms underlying sensor adaptation. However, little is known about effector adaption in plant phototropism. Here, we show that control of the phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 (NPH3) leads to effector adaptation in hypocotyl phototropism. We generated unphosphorable and phosphomimetic NPH3 proteins on seven phosphorylation sites in the etiolated seedlings of Arabidopsis. Unphosphorable NPH3 showed a shortening of its retention time in the cytosol and caused an inability to adapt to very low fluence rates of BL (∼10⁻⁵ µmol m⁻² s⁻¹) during the phototropic response. In contrast, the phosphomimetic NPH3 proteins had a lengthened retention time in the cytosol and could not enable the adaptation to BL at fluence rates of 10⁻³ µmol m⁻² s⁻¹ or more. Our results indicate that the activation level of phot1 and the corresponding phosphorylation level of NPH3 determine the dissociation rate and the reassociation rate of NPH3 on the plasma membrane, respectively. These mechanisms may moderately maintain the active state of phot1 signaling across a broad range of BL intensities and contribute to the photosensory adaptation of phot1 signaling during the phototropic response in hypocotyls.

The phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 pr     

Photosynthetic Electron Transport System Promotes Synthesis of Au-Nanoparticles

In this communication, a novel, green, efficient and economically viable light mediated protocol for generation of Au-nanoparticles using most vital organelle, chloroplasts, of the plant system is portrayed. Thylakoids/chloroplasts isolated from Potamogeton nodosus (an aquatic plant) and Spinacia oleracea (a terrestrial plant) turned Au³⁺ solutions purple in presence of light of 600 µmol m⁻² s⁻¹ photon flux density (PFD) and the purple coloration intensified with time. UV-Vis spectra of these purple colored solutions showed absorption peak at ∼545 nm which is known to arise due to surface plasmon oscillations specific to Au-nanoparticles. However, thylakoids/chloroplasts did not alter color of Au³⁺ solutions in dark. These results clearly demonstrated that photosynthetic electron transport can reduce Au³⁺ to Au⁰ which nucleate to form Au-nanoparticles in presence of light. Transmission electron microscopic studies revealed that Au-nanoparticles generated by light driven photosynthetic electron transport system of thylakoids/chloroplasts were in range of 5–20 nm. Selected area electron diffraction and powder X-ray diffraction indicated crystalline nature of these nanoparticles. Energy dispersive X-ray confirmed that these nanoparticles were composed of Au. To confirm the potential of light driven photosynthetic electron transport in generation of Au-nanoparticles, thylakoids/chloroplasts were tested for their efficacy to generate Au-nanoparticles in presence of light of PFD ranging from 60 to 600 µmol m⁻² s⁻¹. The capacity of thylakoids/chloroplasts to generate Au-nanoparticles increased remarkably with increase in PFD, which further clearly demonstrated potential of light driven photosynthetic electron transport in reduction of Au³⁺ to Au⁰ to form nanoparticles. The light driven donation of electrons to metal ions by thylakoids/chloroplasts can be exploited for large scale production of nanoparticles.     

Microcystin Production by Microcystis aeruginosa in a Phosphorus-Limited Chemostat

The production of microcystins (MC) from Microcystis aeruginosa UTEX 2388 was investigated in a P-limited continuous culture. MC (MC-LR, MC-RR, and MC-YR) from lyophilized M. aeruginosa were extracted with 5% acetic acid, purified by a Sep-Pak C₁₈ cartridge, and then analyzed by high-performance liquid chromatography with a UV detector and Nucleosil C₁₈ reverse-phase column. The specific growth rate (μ) of M. aeruginosa was within the range of 0.1 to 0.8/day and was a function of the cellular P content under a P limitation. The N/P atomic ratio of steady-state cells in a P-limited medium varied from 24 to 15 with an increasing μ. The MC-LR and MC-RR contents on a dry weight basis were highest at μ of 0.1/day at 339 and 774 μg g⁻¹, respectively, while MC-YR was not detected. The MC content of M. aeruginosa was higher at a lower μ, whereas the MC-producing rate was linearly proportional to μ. The C fixation rate at an ambient irradiance (160 microeinsteins m⁻² s⁻¹) increased with μ. The ratios of the MC-producing rate to the C fixation rate were higher at a lower μ. Accordingly, the growth of M. aeruginosa was reduced under a P limitation due to a low C fixation rate, whereas the MC content was higher. Consequently, increases in the MC content per dry weight along with the production of the more toxic form, MC-LR, were observed under more P-limited conditions.     

Spinach Leaf Chloroplast CO(2) and NO(2) Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations

Potential competition between CO₂ and NO₂⁻ photoassimilation for photogenerated reductant (e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature `source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO₂ and/or NO₂⁻ to produce the highest demand for reductant by CO₂ and/or NO₂⁻ assimilatory processes (enzymes). Even in the presence of CO₂ fixation, NO₂⁻ reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO₂ assimilation. Regardless of the magnitude of the incident light energy, CO₂ assimilation did not repress NO₂⁻ reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO₂ as well as with inorganic nitrogen photoassimilation.