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).     

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.     

Kinetic Parameters of Denitrification in a River Continuum

Kinetic parameters for nitrate reduction in intact sediment cores were investigated by using the acetylene blockage method at five sites along the Swale-Ouse river system in northeastern England, including a highly polluted tributary, R. Wiske. The denitrification rate in sediment containing added nitrate exhibited a Michaelis-Menten-type curve. The concentration of nitrate for half-maximal activity (K_map) by denitrifying bacteria increased on passing downstream from 13.1 to 90.4 μM in the main river, but it was highest (640 μM) in the Wiske. The apparent maximal rate (V_maxap) ranged between 35.8 and 324 μmol of N m⁻² h⁻¹ in the Swale-Ouse (increasing upstream to downstream), but it was highest in the Wiske (1,194 μmol N m⁻² h⁻¹). A study of nitrous oxide (N₂O) production at the same time showed that rates ranged from below the detection limit (0.05 μmol of N₂O-N m⁻² h⁻¹) at the headwater site to 27 μmol of N₂O-N m⁻² h⁻¹ at the downstream site. In the Wiske the rate was up to 570 μmol of N₂O-N m⁻² h⁻¹, accounting for up to 80% of total N gas production.     

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.     

Kinetic Parameters of the Conversion of Methane Precursors to Methane in a Hypereutrophic Lake Sediment

The kinetic parameters K_m, V_max, T_t (turnover time), and v (natural velocity) were determined for H₂ and acetate conversion to methane by Wintergreen Lake sediment, using short-term (a few hours) methods and incubation temperatures of 10 to 14°C. Estimates of the Michaelis-Menten constant, K_m, for both the consumption of hydrogen and the conversion of hydrogen to methane by sediment microflora averaged about 0.024 μmol g⁻¹ of dry sediment. The maximal velocity, V_max, averaged 4.8 μmol of H₂ g⁻¹ h⁻¹ for hydrogen consumption and 0.64 μmol of CH₄ g⁻¹ h⁻¹ for the conversion of hydrogen to methane during the winter. Estimated natural rates of hydrogen consumption and hydrogen conversion to methane could be calculated from the Michaelis-Menten equation and estimates of K_m, V_max, and the in situ dissolved-hydrogen concentration. These results indicate that methane may not be the only fate of hydrogen in the sediment. Among several potential hydrogen donors tested, only formate stimulated the rate of sediment methanogenesis. Formate conversion to methane was so rapid that an accurate estimate of kinetic parameters was not possible. Kinetic experiments using [2-¹⁴C]acetate and sediments collected in the summer indicated that acetate was being converted to methane at or near the maximal rate. A minimum natural rate of acetate conversion to methane was estimated to be about 110 nmol of CH₄ g⁻¹ h⁻¹, which was 66% of the V_max (163 nmol of CH₄ g⁻¹ h⁻¹). A 15-min preincubation of sediment with 5.0 × 10⁻³ atm of hydrogen had a pronounced effect on the kinetic parameters for the conversion of acetate to methane. The acetate pool size, expressed as the term K_m + S_n (S_n is in situ substrate concentration), decreased by 37% and T_t decreased by 43%. The V_max remained relatively constant. A preincubation with hydrogen also caused a 37% decrease in the amount of labeled carbon dioxide produced from the metabolism of [U-¹⁴C]valine by sediment heterotrophs.     

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.     

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.     

Kinetics of Formate Metabolism in Methanobacterium formicicum and Methanospirillum hungatei

The kinetics of formate metabolism in Methanobacterium formicicum and Methanospirillum hungatei were studied with log-phase formate-grown cultures. The progress of formate degradation was followed by the formyltetrahydrofolate synthetase assay for formate and fitted to the integrated form of the Michaelis-Menten equation. The K_m and V_max values for Methanobacterium formicicum were 0.58 mM formate and 0.037 mol of formate h⁻¹ g⁻¹ (dry weight), respectively. The lowest concentration of formate metabolized by Methanobacterium formicicum was 26 μM. The K_m and V_max values for Methanospirillum hungatei were 0.22 mM and 0.044 mol of formate h⁻¹ g⁻¹ (dry weight), respectively. The lowest concentration of formate metabolized by Methanospirillum hungatei was 15 μM. The apparent K_m for formate by formate dehydrogenase in cell-free extracts of Methanospirillum hungatei was 0.11 mM. The K_m for H₂ uptake by cultures of Methanobacterium formicicum was 6 μM dissolved H₂. Formate and H₂ were equivalent electron donors for methanogenesis when both substrates were above saturation; however, H₂ uptake was severely depressed when formate was above saturation and the dissolved H₂ was below 6 μM. Formate-grown cultures of Methanobacterium formicicum that were substrate limited for 57 h showed an immediate increase in growth and methanogenesis when formate was added to above saturation.     

Effects of Light on the Microcystin Content of Microcystis Strain PCC 7806

Many cyanobacteria produce microcystins, hepatotoxic cyclic heptapeptides that can affect animals and humans. The effects of photosynthetically active radiation (PAR) on microcystin production by Microcystis strain PCC 7806 were studied in continuous cultures. Microcystis strain PCC 7806 was grown under PAR intensities between 10 and 403 μmol of photons m⁻² s⁻¹ on a light-dark rhythm of 12 h -12 h. The microcystin concentration per cell, per unit biovolume and protein, was estimated under steady-state and transient-state conditions and on a diurnal timescale. The cellular microcystin content varied between 34.5 and 81.4 fg cell⁻¹ and was significantly positively correlated with growth rate under PAR-limited growth but not under PAR-saturated growth. Microcystin production and PAR showed a significant positive correlation under PAR-limited growth and a significant negative correlation under PAR-saturated growth. The microcystin concentration, as a ratio with respect to biovolume and protein, correlated neither with growth rate nor with PAR. Adaptation of microcystin production to a higher irradiance during transient states lasted for 5 days. During the period of illumination at a PAR of 10 and 40 μmol of photons m⁻² s⁻¹, the intracellular microcystin content increased to values 10 to 20% higher than those at the end of the dark period. Extracellular (dissolved) microcystin concentrations were 20 times higher at 40 μmol of photons m⁻² s⁻¹ than at 10 μmol of photons m⁻² s⁻¹ and did not change significantly during the light-dark cycles at both irradiances. In summary, our results showed a positive effect of PAR on microcystin production and content of Microcystis strain PCC 7806 up to the point where the maximum growth rate is reached, while at higher irradiances the microcystin production is inhibited.     

Hydrogen Metabolism by Decomposing Cyanobacterial Aggregates in Big Soda Lake, Nevada

Hydrogen production by incubated cyanobacterial epiphytes occurred only in the dark, was stimulated by C₂H₂, and was inhibited by O₂. Addition of NO₃⁻ inhibited dark, anaerobic H₂ production, whereas the addition of NH₄⁺ inhibited N₂ fixation (C₂H₂ reduction) but not dark H₂ production. Aerobically incubated cyanobacterial aggregates consumed H₂, but light-incubated rates (3.6 μmol of H₂ g⁻¹ h⁻¹) were statistically equivalent to dark uptake rates (4.8 μmol of H₂ g⁻¹ h⁻¹), which were statistically equivalent to dark, anaerobic production rates (2.5 to 10 μmol of H₂ g⁻¹ h⁻¹). Production rates of H₂ were fourfold higher for aggregates in a more advanced stage of decomposition. Enrichment cultures of H₂-producing fermentative bacteria were recovered from freshly harvested, H₂-producing cyanobacterial aggregates. Hydrogen production in these cyanobacterial communities appears to be caused by the resident bacterial flora and not by the cyanobacteria. In situ areal estimates of dark H₂ production by submerged epiphytes (6.8 μmol of H₂ m⁻² h⁻¹) were much lower than rates of light-driven N₂ fixation by the epiphytic cyanobacteria (310 μmol of C₂H₄ m⁻² h⁻¹).     

Successional Development of Sulfate-Reducing Bacterial Populations and Their Activities in a Wastewater Biofilm Growing under Microaerophilic Conditions

A combination of fluorescence in situ hybridization, microprofiles, denaturing gradient gel electrophoresis of PCR-amplified 16S ribosomal DNA fragments, and 16S rRNA gene cloning analysis was applied to investigate successional development of sulfate-reducing bacteria (SRB) community structure and in situ sulfide production activity within a biofilm growing under microaerophilic conditions (dissolved oxygen concentration in the bulk liquid was in the range of 0 to 100 μM) and in the presence of nitrate. Microelectrode measurements showed that oxygen penetrated 200 μm from the surface during all stages of biofilm development. The first sulfide production of 0.32 μmol of H₂S m⁻² s⁻¹ was detected below ca. 500 μm in the 3rd week and then gradually increased to 0.70 μmol H₂S m⁻² s⁻¹ in the 8th week. The most active sulfide production zone moved upward to the oxic-anoxic interface and intensified with time. This result coincided with an increase in SRB populations in the surface layer of the biofilm. The numbers of the probe SRB385- and 660-hybridized SRB populations significantly increased to 7.9 × 10⁹ cells cm⁻³ and 3.6 × 10⁹ cells cm⁻³, respectively, in the surface 400 μm during an 8-week cultivation, while those populations were relatively unchanged in the deeper part of the biofilm, probably due to substrate transport limitation. Based on 16S rRNA gene cloning analysis data, clone sequences that related to Desulfomicrobium hypogeium (99% sequence similarity) and Desulfobulbus elongatus (95% sequence similarity) were most frequently found. Different molecular analyses confirmed that Desulfobulbus, Desulfovibrio, and Desulfomicrobium were found to be the numerically important members of SRB in this wastewater biofilm.     

Distribution of Sulfate-Reducing and Methanogenic Bacteria in Anaerobic Aggregates Determined by Microsensor and Molecular Analyses

Using molecular techniques and microsensors for H₂S and CH₄, we studied the population structure of and the activity distribution in anaerobic aggregates. The aggregates originated from three different types of reactors: a methanogenic reactor, a methanogenic-sulfidogenic reactor, and a sulfidogenic reactor. Microsensor measurements in methanogenic-sulfidogenic aggregates revealed that the activity of sulfate-reducing bacteria (2 to 3 mmol of S²⁻ m⁻³ s⁻¹ or 2 × 10⁻⁹ mmol s⁻¹ per aggregate) was located in a surface layer of 50 to 100 μm thick. The sulfidogenic aggregates contained a wider sulfate-reducing zone (the first 200 to 300 μm from the aggregate surface) with a higher activity (1 to 6 mmol of S²⁻ m⁻³ s⁻¹ or 7 × 10⁻⁹ mol s⁻¹ per aggregate). The methanogenic aggregates did not show significant sulfate-reducing activity. Methanogenic activity in the methanogenic-sulfidogenic aggregates (1 to 2 mmol of CH₄ m⁻³ s⁻¹ or 10⁻⁹ mmol s⁻¹ per aggregate) and the methanogenic aggregates (2 to 4 mmol of CH₄ m⁻³ s⁻¹ or 5 × 10⁻⁹ mmol s⁻¹ per aggregate) was located more inward, starting at ca. 100 μm from the aggregate surface. The methanogenic activity was not affected by 10 mM sulfate during a 1-day incubation. The sulfidogenic and methanogenic activities were independent of the type of electron donor (acetate, propionate, ethanol, or H₂), but the substrates were metabolized in different zones. The localization of the populations corresponded to the microsensor data. A distinct layered structure was found in the methanogenic-sulfidogenic aggregates, with sulfate-reducing bacteria in the outer 50 to 100 μm, methanogens in the inner part, and Eubacteria spp. (partly syntrophic bacteria) filling the gap between sulfate-reducing and methanogenic bacteria. In methanogenic aggregates, few sulfate-reducing bacteria were detected, while methanogens were found in the core. In the sulfidogenic aggregates, sulfate-reducing bacteria were present in the outer 300 μm, and methanogens were distributed over the inner part in clusters with syntrophic bacteria.     

Effects of Irradiance during Growth on Adaptive Photosynthetic Characteristics of Velvetleaf and Cotton

We grew velvetleaf (Abutilon theophrasti Medic.) and cotton (Gossypium hirsutum L. var. Stoneville 213) at three irradiances and determined the photosynthetic responses of single leaves to a range of six irradiances from 90 to 2000 μeinsteins m⁻²sec⁻¹. In air containing 21% O₂, velvetleaf and cotton grown at 750 μeinsteins m⁻²sec⁻¹ had maximum photosynthetic rates of 18.4 and 21.9 mg of CO₂ dm⁻²hr⁻¹, respectively. Maximum rates for leaves grown at 320 and 90 μeinsteins m⁻²sec⁻¹ were 15.3 and 10.3 mg of CO₂ dm⁻²hr⁻¹ in velvetleaf and 12 and 6.7 mg of CO₂ dm⁻²hr⁻¹ in cotton, respectively. In 1 O₂, maximum photosynthetic rates were 1.5 to 2.3 times the rates in air containing 21% O₂, and plants grown at medium and high irradiance did not differ in rate. In both species, stomatal conductance was not significantly affected by growth irradiance. The differences in maximum photosynthetic rates were associated with differences in mesophyll conductance. Mesophyll conductance increased with growth irradiance and correlated positively with mesophyll thickness or volume per unit leaf area, chlorophyll content per unit area, and photosynthetic unit density per unit area. Thus, quantitative changes in the photosynthetic apparatus help account for photosynthetic adaptation to irradiance in both species. Net assimilation rates calculated for whole plants by mathematical growth analysis were closely correlated with single-leaf photosynthetic rates.     

Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content, Assimilatory Charge, and Mesophyll Conductance in Leaves

The content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (E_t; EC 4.1.1.39) measured in different-aged leaves of sunflower (Helianthus annuus) and other plants grown under different light intensities, varied from 2 to 75 μmol active sites m⁻². Mesophyll conductance (μ) was measured under 1.5% O₂, as well as postillumination CO₂ uptake (assimilatory charge, a gas-exchange measure of the ribulose-1,5-bisphosphate pool). The dependence of μ on E_t saturated at E_t = 30 μmol active sites m⁻² and μ = 11 mm s⁻¹ in high-light-grown leaves. In low-light-grown leaves the dependence tended toward saturation at similar E_t but reached a μ of only 6 to 8 mm s⁻¹. μ was proportional to the assimilatory charge, with the proportionality constant (specific carboxylation efficiency) between 0.04 and 0.075 μm⁻¹ s⁻¹. Our data show that the saturation of the relationship between E_t and μ is caused by three limiting components: (a) the physical diffusion resistance (a minor limitation), (b) less than full activation of Rubisco (related to Rubisco activase and the slower diffusibility of Rubisco at high protein concentrations in the stroma), and (c) chloroplast metabolites, especially 3-phosphoglyceric acid and free inorganic phosphate, which control the reaction kinetics of ribulose-1,5-bisphosphate carboxylation by competitive binding to active sites.Rubisco (EC 4.1.1.39) catalyzes the irreversible carboxylation of RuBP to form two PGA molecules (in this work the oxygenase reaction was not active since a low O₂ concentration was used). RuBP carboxylation is the major rate-determining reaction in photosynthetic CO₂ assimilation. All factors that influence the photosynthetic rate do so by influencing the activity of Rubisco and the concentration of its substrates, CO₂ and RuBP. E_t in leaves may be as high as 75 μmol m⁻², and for the extracted enzyme K_m(CO₂) = 9.4 μm (Makino et al., 1985a) and K_m(RuBP) = 30 to 40 μm (Yeoh et al., 1981). In leaves photosynthesizing under atmospheric conditions, the concentration of RuBP may increase to 10 to 15 mm (Badger et al., 1984; Sharkey et al., 1986), but the concentration of CO₂ is usually about 4 to 8 μm in leaf intercellular spaces, depending on stomatal conductance. This CO₂ concentration is well below the K_m(CO₂) of the enzyme, and it is the initial slope of the kinetic curve V_M/K_m(CO₂), termed carboxylation conductance, that becomes important.r_c limits the CO₂-fixation rate in series with the other resistances, r_g and r_md. The carboxylation rates are usually expressed in relation to C_i or C_w. C_c is usually about 20% to 30% lower than C_w because of concentration decrease generated by the carboxylation flux on r_md. Considering the above, the carboxylation conductance in intact leaves in vivo may be found as the initial slope of the A versus C_c graph at low C_c values. If C_c cannot be calculated because r_md is unknown, the closest approximation is a plot of A versus C_w or A versus C_i. The true parameters of the carboxylase can be found only from experiments carried out in nonphotorespiratory conditions (1%–2% O₂); otherwise the competing oxygenase reaction consumes a part of RuBP and partially inhibits carboxylase activity.Because of technical problems with the measurement of A versus C_w relationships, in many studies only the net photosynthetic rate under atmospheric conditions (21% O₂) was related to Rubisco activity or content. Nevertheless, good correlation has been found (Makino et al., 1983; Hudson et al., 1992; Jacob and Lawlor, 1992; Jiang and Rodermel, 1995; Nakano et al., 1997). These results indicated that the level of Rubisco protein could be a limiting factor in photosynthesis throughout the life span of the leaf under natural environmental conditions. On the other hand, when Rubisco levels in leaves exceeded 4 g m⁻² (60 μmol m⁻²), the in vivo Rubisco activity (measured as photosynthesis under pC_i = 20 to 30 Pa and 21% O₂) became curvilinearly correlated with E_t (Makino et al., 1994, 1997). When measurements were made over the whole life span of wheat leaves, the measured rates of photosynthesis were lower in young leaves, which had high protein content, than would have been expected from the amount and activity of Rubisco (Lawlor et al., 1989).During senescence the decrease in Rubisco activity was initially greater than the decrease in net photosynthesis (Hall et al., 1978). In a willow canopy, Rubisco-specific activity was higher when the apparent E_t (N content in leaves) was smaller (Vapaavuori and Vuorinen, 1989). A similar nonlinearity was found in our previous experiments (Eichelmann and Laisk, 1990), in which we obtained a saturating relationship when E_t exceeded 30 μmol m⁻². In the latter work the initial slope of the A versus C_w curves under nonphotorespiratory conditions (1.5% O₂) was assumed to represent the Rubisco activity in vivo and was compared with the E_t. We discovered that growth light had the strongest influence on the saturation of the relationship between μ and E_t. In the present work we present insight into this relationship, using not only plants grown under different light intensities but also leaves adapted to different light intensities.     

Biodegradation of Chloromethane by Pseudomonas aeruginosa Strain NB1 under Nitrate-Reducing and Aerobic Conditions

Pseudomonas aeruginosa strain NB1 uses chloromethane (CM) as its sole source of carbon and energy under nitrate-reducing and aerobic conditions. The observed yield of NB1 was 0.20 (±0.06) (mean ± standard deviation) and 0.28 (±0.01) mg of total suspended solids (TSS) mg of CM⁻¹ under anoxic and aerobic conditions, respectively. The stoichiometry of nitrate consumption was 0.75 (±0.10) electron equivalents (eeq) of NO₃⁻ per eeq of CM, which is consistent with the yield when it is expressed on an eeq basis. Nitrate was stoichiometrically converted to dinitrogen (0.51 ± 0.05 mol of N₂ per mol of NO₃⁻). The stoichiometry of oxygen use with CM (0.85 ± 0.21 eeq of O₂ per eeq of CM) was also consistent with the aerobic yield. Stoichiometric release of chloride and minimal accumulation of soluble metabolic products (measured as chemical oxygen demand) following CM consumption, under anoxic and aerobic conditions, indicated complete biodegradation of CM. Acetylene did not inhibit CM use under aerobic conditions, implying that a monooxygenase was not involved in initiating aerobic CM metabolism. Under anoxic conditions, the maximum specific CM utilization rate (k) for NB1 was 5.01 (±0.06) μmol of CM mg of TSS⁻¹ day⁻¹, the maximum specific growth rate (μ_max) was 0.0506 day⁻¹, and the Monod half-saturation coefficient (K_s) was 0.067 (±0.004) μM. Under aerobic conditions, the values for k, μ_max, and K_s were 10.7 (±0.11) μmol of CM mg of TSS⁻¹ day⁻¹, 0.145 day⁻¹, and 0.93 (±0.042) μM, respectively, indicating that NB1 used CM faster under aerobic conditions. Strain NB1 also grew on methanol, ethanol, and acetate under denitrifying and aerobic conditions, but not on methane, formate, or dichloromethane.     

Effects of Abiotic Factors on Acetylene Reduction by Cyanobacteria Epiphytic on Moss at a Subantarctic Island

Acetylene reduction (AR) rates by cyanobacteria epiphytic on a moss at Marion Island (46°54′ S, 37°45′ E) increased from −5°C to a maximum at 25 to 27°C. Q₁₀ values between 0 and 25°C were between 2.3 and 2.9, depending on photosynthetic photon flux density. AR rates declined sharply at temperatures above the optimum and were lower at 35°C than at 0°C. Photosynthetic photon flux density at low levels markedly influenced AR, and half of the maximum rate occurred at 84 μmol m⁻² s⁻¹, saturation occurring at ca. 1,000 μmol m⁻² s⁻¹. Higher photosynthetic photon flux density levels decreased AR rates. AR increased up to the highest sample moisture content investigated (3,405%), and the pH optimum was between 5.9 and 6.2. The addition of P, Co, and Mo, individually or together, depressed AR.     

Growth and Nitrogen Uptake Kinetics in Cultured Prorocentrum donghaiense

We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5–500 μmol N L⁻¹ for NO₃ ⁻ and NH₄ ⁺, 0.5–50 μmol N L⁻¹ for urea, 0.5–100 μmol N L⁻¹ for glu and cyanate, and 0.5–200 μmol N L⁻¹ for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (μ_m, 1.51±0.06 d⁻¹ and 1.50±0.05 d⁻¹, respectively). However, cultures grown on cyanate, NO₃ ⁻, and NH₄ ⁺ had lower half saturation constants (K_μ, 0.28–0.51 μmol N L⁻¹). N uptake kinetics were measured in NO₃ ⁻-deplete and -replete batch cultures of P. donghaiense. In NO₃ ⁻-deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO₃ ⁻, NH₄ ⁺, urea and algal amino acids; uptake was saturated at or below 50 μmol N L⁻¹. In NO₃ ⁻-replete batch cultures, NH₄ ⁺, urea, and algal amino acid uptake kinetics were similar to those measured in NO₃ ⁻-deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth.     

Altered Growth of Transgenic Tobacco Lacking Leaf Cytosolic Pyruvate Kinase

Previously, we reported that transformation of tobacco (Nicotiana tabacum L.) with a vector containing a potato cytosolic pyruvate kinase (PK_c) cDNA generated two plant lines specifically lacking leaf PK_c (PK_c−) as a result of co-suppression. PK_c deficiency in these primary transformants did not appear to alter plant development, although root growth was not examined. Here we report a striking reduction in root growth of homozygous progeny of both PK_c− lines throughout development under moderate (600 μE m⁻² s⁻¹) or low (100 μE m⁻² s⁻¹) light intensities. When both PK_c− lines were cultivated under low light, shoot and flower development were also delayed and leaf indentations were apparent. Leaf PK activity in the transformants was significantly decreased at all time points examined, whereas root activities were unaffected. Polypeptides corresponding to PK_c were undetectable on immunoblots of PK_c− leaf extracts, except in 6-week-old low-light-grown PK_c− plants, in which leaf PK_c expression appeared to be greatly reduced. The metabolic implications of the kinetic characteristics of partially purified PK_c from wild-type tobacco leaves are discussed. Overall, the results suggest that leaf PK_c deficiency leads to a perturbation in source-sink relationships.     

Thermotolerance of Leaf Discs from Four Isoprene-Emitting Species Is Not Enhanced by Exposure to Exogenous Isoprene

The effects of exogenously supplied isoprene on chlorophyll fluorescence characteristics were examined in leaf discs of four isoprene-emitting plant species, kudzu (Pueraria lobata [Willd.] Ohwi.), velvet bean (Mucuna sp.), quaking aspen (Populus tremuloides Michx.), and pussy willow (Salix discolor Muhl). Isoprene, supplied to the leaves at either 18 μL L⁻¹ in compressed air or 21 μL L⁻¹ in N₂, had no effect on the temperature at which minimal fluorescence exhibited an upward inflection during controlled increases in leaf-disc temperature. During exposure to 1008 μmol photons m⁻² s⁻¹ in an N₂ atmosphere, 21 μL L⁻¹ isoprene had no effect on the thermally induced inflection of steady-state fluorescence. The maximum quantum efficiency of photosystem II photochemistry decreased sharply as leaf-disc temperature was increased; however, this decrease was unaffected by exposure of leaf discs to 21 μL L⁻¹ isoprene. Therefore, there were no discernible effects of isoprene on the occurrence of symptoms of high-temperature damage to thylakoid membranes. Our data do not support the hypothesis that isoprene enhances leaf thermotolerance.     

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.