Carbon fixation in eucalypts in the field

Summary The rate of CO₂ assimilation at light saturation and an intercellular CO₂ concentration of 350 l l^-1 (photosynthetic capacity), measured in leaves of Eucalyptus pauciflora, E. behriana, E. delegatensis and Acacia melanoxylon, declined over the course of cloudless days under naturally varying environmental conditions as well as under constant optimal conditions for high CO₂ uptake. Since the capacity did not recover during the light period, it was different from the midday depression of gas exchange. The change appeared to be caused neither by the diurnal variation of total leaf water potential, by photoinhibition of redox-reaction centres in photosystems nor by changes in the intrinsic properties of Ribulose-bisphosphate carboxylase-oxygenase. The decline was more pronounced in winter than in summer. It was related to the duration of illumination or the cumulative carbon gain. It was reversible in the following dark phase, and it did not occur on changeable days with short peaks of high light.Despite the decline in photosynthetic capacity, the initial slope of the CO₂ response of net photosynthesis, as obtained at low intercellular CO₂ concentrations, remained constant during the day, but declined at night when photosynthetic capacity recovered. In all cases stomatal conductance varied in parallel with photosynthetic capacity. The relevance of changes in photosynthetic capacity for the intercellular CO₂ concentration is discussed.Abbreviations and symbols A CO₂ assimilation - ABA abscisic acid - A_c350 photosynthetic capacity at c_i=350l l^-1 - c_i intercellular CO₂ concentration - g leaf conductance to water vapour - I photon flux density (irradiance) - P air pressure - P_i inorganic phosphate - R_d net CO₂ release at _* - Rubisco Ribulose-bisphosphate carboxylase-oxygenase - RuBP Ribulose-bisphosphate - T leaf temperature - w leaf-to-air water vapour concentration difference - A/c_i carboxylation efficiency at low c_i - _* light-independent CO₂ compensation point - total leaf water potential     

Carbon isotope discrimination in three semi-arid woodland species along a monsoon gradient

Leaf carbonisotope discrimination () was measured for three dominant, semi-arid woodland species along a summer monsoon gradient inthe southwestern United States over a 2-year period. We tested the hypothesis that decreased humidity levels during the growing season along this gradient resulted in lower leaf values. Sites of similar elevation along the transect were selected and the range in monsoon contribution to overall annual precipitation varied from 18 to 58%, while total annual precipitation differed by a maximum of only 25% across this gradient. Leaf values in Quercus gambelii were negatively correlated with , a seasonally-weighted estimate of the evaporative humidity gradient, suggesting that stomatal conductance declined as transpiration potential increased. For two other trees that co-occurred along this gradient, Pinus edulis and Juniperus osteosperma, remained relatively constant despite large variation in . These woodland species represent the full spectrum of responses of carbon isotope discrimination to increases in evaporative potential; that of decline where c _i /c _a (ratio of internal to ambient CO₂ concentration) and presumably stomatal conductance decrease, and that of constancy where whole plant internal adjustments allow c _i /c _a to remain stable.     

Interaction between photosynthetic and respiratory electron-transfer chains in the membranes of Anabaena variabilis

The rate of CO₂- and p-benzoquione-dependent photosynthetic O₂ evolution by Anabaena variabilis cells remained unaltered and the rate of O₂ uptake observed after switching off the light (endogenous respiration) was enhanced by a factor of 6–8 when the O₂ concentration was increased from 200 to 400 M. Photosystem-I-linked O₂ uptake and respiration of the cells incubated with ascorbate and N,N,NN-tetramethyl-p-phenylenediamine was not appreciable influenced by the O₂ concentration. 2-Iodo-6-isopropyl-3-methyl-2,4,4-trinitrodiphenyl ether, blocking electron transfer at the plastoquinone level, suppressed O₂ evolution and had no influence on endogenous respiration. 2-n-Heptyl-4-hydroxyquinoline-N-oxide, an inhibitor of electron transfer between photosystems II and I, as well as the cytochrome-oxidase inhibitors N ₃ ^- , CN^- and NH₂OH, caused a 35–50% retardation of endogenous respiration and blocked photosynthetic O₂ evolution. The molar ratio of cytochromes b₆, f, c-553, aa₃ and photosystem-I reaction centers in the isolated membranes equalled approx. 2:1:2:0.7:2. It is inferred that endogenous respiration of A. variabilis cells is inhibited by the light-induced electron flow through both photosystems at the level of the plastoquinone-plastocyanin-oxidoreductase complex.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DNP-INT 2-iodo-6-isopropyl-3-methyl-2,4,4-trinitrodiphenyl ether - Hepes 4-(2-hydroxyethyl)-1-piperazine ethansulfonic acid - TMPD N,N,NN-tetramethyl-p-phenylenediamine     

Purification and properties of a hyperthermoactive α-amylase from the archaeobacterium Pyrococcus woesei

The cultivation of the hyperthermophilic archaeobacterium Pyrococcus woesei on starch under continuous gassing (80% H₂:20% CO₂) caused the formation of 250 U/l of an extremely thermoactive and thermostable -amylase. In a complex medium without elemental sulphur under 80% N₂ and 20% CO₂ atmosphere enzyme production could be elevated up to 1000 U/l. Pyrococcus woesei grew preferentially on poly-and oligosaccharides. The amylolytic enzyme formation was constitutive. Enzyme production was also observed in continuous culture at dilution rates from 0.1 to 0.4 h^-1. A 20-fold enrichment of -amylase was achieved after adsorption of the enzyme onto starch and its desorption by preparative gel electrophoresis. The -amylase consisted of a single subunit with a molecular mass of 70 000 and was catalytically active at a temperature range between 40°C and 130°C. Enzymatic activity was detected even after autoclaving at a pressure of 2 bars at 120°C for 5 h. The purified enzyme hydrolyzed exclusively -1,4-glycosidic linkages present in glucose polymers of various sizes. Unlike many -amylases from anaerobes the enzyme from P. woesei was unable to attack short chain oligosaccharides with a chain length between 2 and 6 glucose units.     

Interaction of cytochrome c L with methanol dehydrogenase from Methylophaga marina 42:

The reaction of methanol dehydrogenase with cytochrome c _L from Methylophaga marina and the reactions of the non-physiological substrates, Wurster's blue and ascorbic acid, with both proteins were studied as a function of temperature (4–32 °C), pressure (1–2000 bar) and ionic strength using the optical high pressure stopped-flow method. The thermodynamic parameters H, S and V were determined for all reactions where electron transfers are involved. These data allowed the determination of the Maxwell relationships which proved the internal thermodynamic consistency of the system under study. A conformational change on the cytochrome c _L level was deduced from both breaks in the Arrhenius plots and the variation of the V with temperature.Abbreviations MOPS 4-morpholinepropanesulfonic acid - CHES 2-(cyclohexylamino)ethanesulfonic acid - MDH methanol dehydrogenase - EDTA ethylenedinitrilotetraacetic acid disodium salt - BTB bromothymol blue (3,3-dibromothymolsulfoneph-thalein) - PQQ 2,7,9-tricarboxy-lH-pyrrolo-[2,3f]quinoline-4,5-dione - cytochrome c _HH mammalian horse heart cytochrome c     

Photosynthetic inhibition after long-term exposure to elevated levels of atmospheric carbon dioxide

The effect of long-term exposure to elevated levels of CO₂ on biomass partitioning, net photosynthesis and starch metabolism was examined in cotton. Plants were grown under controlled conditions at 350, 675 and 1000 l l^-1 CO₂. Plants grown at 675 and 1000 l l^-1 had 72% and 115% more dry weight respectively than plants grown at 350 l l^-1. Increases in weight were partially due to corresponding increases in leaf starch. CO₂ enrichment also caused a decrease in chlorophyll concentration and a change in the chlorophyll a/b ratio. High CO₂ grown plants had lower photosynthetic capacity than 350 l l^-1 grown plants when measured at each CO₂ concentration. Reduced photosynthetic rates were correlated with high internal (non-stomatal) resistances and higher starch levels. It is suggested that carbohydrate accumulation causes a decline in photosynthesis by feedback inhibition and/or physical damage at the chloroplast level.Abbreviations Ci internal CO₂ concentration - Chl chlorophyll - DMSO dimethylsulfoxide - HSD honestly significant difference (procedure) - MCW methanolchloroform-water - Pi inorganic phosphate - S.E.M. standard error of mean     

Energetic aspects of CO2 uptake in Thiobacillus neapolitanus

Uptake of inorganic carbon (C_i) in the form of CO₂ and/or HCO ₃ ^- was studied in the chemolithoautotroph Thiobacillus neapolitanus under energy (thiosulphate) or carbon (CO₂) limitation. Uptake of C₁ was found to be a metabolic energy dependent process since in the presence of uncouplers no uptake was observed. The accumulation level of C_i was higher in the CO₂-limited cells (1000-to 1500-fold) in comparison to the thiosulphate-limited cells (500-to 800-fold). The process of uptake could be influenced by addition of ionophores. Inhibition of uptake and accumulation of C_i was found after addition of valinomycin which completely dissipated the electrical potential (). After addition of nigericin an increase in the uptake and accumulation of C_i was observed with a concomitant increase of the . These results suggest that the is the main driving force for uptake of C_i. However, uptake of C_i could never be found in the absence of electron transfer, or in cells in which the electron transfer chain was blocked by potassium cyanide. Electron transfer therefore appears to be an additional requirement for C_i uptake. Kinetic experiment on the uptake of inorganic carbon at different pH values suggest that CO₂ is the carbon species taken up by T. neapolitanus.Abbreviations RuBisCO ribulose-1,5-bisphosphate carboxylase - DCCD N,N¹-dicyclohexylcarbodiimide - CCCP carbonyl cyanide m-chlorophenyl hydrazone - FCCP carbonyl cyanide p-trifluoro-methoxyphenyl hydrazone - EDTA sodium ethylene diamine tetraacetate     

Adrenergic regulation of ion transport by primary cultures of canine tracheal epithelium: Cellular electrophysiology

Summary We examined the effect of adrenergic agents on the cellular electrical properties of primary cultures of canine tracheal epithelium. Both isoproterenol and epinephrine stimulated Cl secretion, as evidenced by an increase in transepithelial voltage and a fall in transepithelial resistance. Moreover, both agents appear to increase the conductance of apical and basolateral membranes. However, the pattern of response was different. Isoproterenol initially depolarized apical voltage _a and decreased the fractional resistance of the apical membranef _R. These changes are consistent with an initial increase in apical Cl conductance. In contrast, epinephrine acutely hyperpolarized _a and increasedf _R, changes consistent with an initial increase in basolateral K conductance. Following the acute effect of epinephrine, _a depolarized andf _R decreased to values not significantly different from those observed with isoproterenol. The acute increase in basolateral K conductance produced by epinephrine appeared to result from stimulation of adrenergic receptors because it was reproduced by addition of the agonist phenylephrine, and blocked by the antagonist phentolamine. The ability of prazosin but not yohimbine to block the acute epinephrine-induced increase in K permeability indicates the presence of ₁ adrenergic receptors. The acute adrenergic-induced increase in basolateral K conductance may be mediated by an increase in cell Ca because the response was mimicked by addition of the Ca ionophore A23187. In contrast, the response to isoproterenol was similar to that observed with addition of 8-bromo-cAMP and theophylline. These results indicate that both and adrenergic agents mediate the ion transport processes in canine tracheal epithelium. adrenergic agents have their primary effect on the apical Cl conductance, probably via an increase in cAMP. adrenergic agents exert their primary effect on the basolateral K conductance, possibly via an increase in cell Ca.     

The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO₂ gas exchange were conducted on intact leaves. Under ambient CO₂ concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed down-regulation of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (_{PS II}), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in: total linear electron transport (J₁), electron transport dedicated to carbon assimilation (J_A) and electron transport allocated to photorespiration (J_L). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (v_o/v_c), due to a higher CO₂ concentration at the carboxylation site (C_c). The relationship between _{PS II} and _{CO 2}was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J₁, J_A, J_L and v_o/v_c in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.Abbreviations C_c CO₂ concentration at the site of carboxylation - C_i intercellular CO₂ concentration - g_m mesophyll conductance to CO₂ - J₁ total linear electron flow - J_A linear electron flow allocated to CO₂ assimilation - J_c linear electron flow supporting carbon reduction and oxidation cycles - J_L linear electron flow allocated to photorespiration (RuBP oxygenation and fixation of released photorespiratory CO₂) - PRK phosphoribulokinase - q_P, q_N coefficients for photochemical and non-photochemical quenching of fluorescence respectively - Rubisco ribulose 1,5-bisphosphate carboxylase-oxygenase - S Rubisco specificity to CO₂/O₂ - v_c, v_o rates of RuBP carboxylation and RuBP oxygenation, respectively - _{CO 2} relative quantum yield of CO₂ assimilation - _C maximum _{CO 2} under non-photorespiratory conditions - _exc the efficiency of excitation capture by open PS II centres - _{PS II} relative quantum yield of PS II electron transport     

Analysis of gas exchange in seedlings of Acer saccharum: integration of field and laboratory studies

In the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 mol quanta m^-2 s^-1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 mol m^-2 s^-1 or less. At these low PPFD's there is a marked interaction of PPFD with the initial slope (CE) of the CO₂ response. At PPFD-saturation CE was 0.018 mol m^-2 s^-1/(l/l). The apparent quantum efficiency (incident PPFD) at saturating CO₂ was 0.05–0.08 mol/mol. and PPFD-saturated CO₂ exchange was 6–8 mol m^-2 s^-1. The ratio of internal CO₂ concentration to external (C _i /C _a ) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in C _i /C _a during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO₂ exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5–10%.List of symbols A measured photosynthetic rate under any set of conditions (mol m^-2 s^-1) - A _m (atm) measured photosynthetic rate at saturating PPFD, 350 l/l CO₂ and 21% (v/v) O₂ (mol m^-2 s^-1) - C constant in equation of Smith (1937, 1938) - C _a CO₂ concentration in the air (l/l) - C _i CO₂ concentration in the intercellular air space (l/l) - C _i ^/* C _i corrected for CO₂ compensation point, i.e., C _i -I ^*, (l/l) - CE initial slope of the CO₂ response of photosynthesis (mol m^-2 s^-1/(l/l)) - CEM CE at PPFD saturation - E transpiration rate (mmol m^-2 s^-1) - F predicted photosynthetic rate (mol m^-2 s^-1) - G leaf conductance to H₂O (mol m^-2 s^-1) - I photosynthetic photon flux density (mol m^-2 s^-1) - N number of data points - P _m predicted photosynthetic rate at saturating CO₂ and given PPFD (mol m^-2 s^-1) - P _ml predicted photosynthetic rate at saturating CO₂ and PPFD (mol m^-2 s^-1) - R _d residual respiratory rate (mol m^-2 s^-1) - T _a air temperature (°C) - T _l leaf temperature (°C) - V reaction velocity in equation of Smith (1937, 1938) - V _max saturated reaction velocity in equation of Smith (1937, 1938) - VPA vapor pressure of water in the air (mbar/bar) - VPD vapor pressure difference between leaf and air (mbar/bar) - X substrate concentration in equation of Smith (1937, 1938) - initial slope of the PPFD response of photosynthesis at saturating CO₂ (mol CO₂/mol quanta) - (atm) initial slope of the PPFD response of photosynthesis at 340 l/l CO₂ and 21% (v/v) O₂ (mol CO₂/mol quanta) - I ^* CO₂ compensation point after correction for residual respiration (l/l) - PPFD compensation point (mol m^-2 s^-1)     

Genetic differentiation in carbon isotope discrimination and gas exchange in Pseudotsuga menziesii

Patterns of genetic variation in gas-exchange physiology were analyzed in a 15-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation that contains 25 populations grown from seed collected from across the natural distribution of the species. Seed was collected from 33°30 to 53°12 north latitude and from 170 m to 2930 m above sea level, and from the coastal and interior (Rocky Mountain) varieties of the species. Carbon isotope discrimination () ranged from 19.70() to 22.43() and was closely related to geographic location of the seed source. The coastal variety (20.50 (SE=0.21)) was not significantly different from the interior variety (20.91 (0.15)). Instead, most variation was found within the interior variety; populations from the southern Rockies had the highest discrimination (21.53 (0.20)) (lowest water-use efficiency). Carbon isotope discrimination (), stomatal conductance to water vapor (g), the ratio of intercellular to ambient CO₂ concentration (c_i/c_a), and intrinsic water-use efficiency (A/g) were all correlated with altitude of origin (r=0.76, 0.73, 0.74, and –0.63 respectively); all were statistically significant at the 0.01 level. The same variables were correlated with both height and diameter at age 15 (all at P0.0005). Observed patterns in the common garden did not conform to our expectation of higher WUE, measured by both A/g and , in trees from the drier habitats of the interior, nor did they agree with published in situ observations of decreasing g and with altitude. The genetic effect opposes the altitudinal one, leading to some degree of homeostasis in physiological characteri tics in situ.     

Calculation of leaf photosynthetic parameters from light-response curves for ecophysiological applications

Measurement of the light response of photosynthetic CO₂ uptake is often used as an implement in ecophysiological studies. A method is described to calculate photosynthetic parameters, such as the maximum rate of whole electron transport and dissimilative respiration in the light, from the light response of CO₂ uptake. Examples of the light-response curves of flag leaves and ears of wheat (Triticum aestivum cv. ARKAS) are shown.Abbreviations and symbols A net photosynthesis rate - D ¹ rate of dissimilative respiration occurring in the light - f loss factor - I incident PPFD - I effective absorbed PPFD - J rate of whole electron transport - J _m maximum rate of whole electron transport - p ^c intercellular CO₂ partial pressure - PPFD photosynthetic photon flux density - q effectivity factor for the use of light (electrons/quanta) - absorption coefficient - I ^* CO₂ compensation point in the absence of dissimilative respiration (bar) - II conversion factor for calculation of CO₂ uptake from the rate of whole electron transport - convexity factor Gas-exchange rates relate to the projective area and are given in mol·m^-2·s^-1. Electron-transport rates are given in mol electrons·m^-2·s^-1; PPFD is given in mol quanta·m^-2·s^-1.     

Growth,acetylene reduction activity and localization of nitrogenase in relation to vesicle formation in Frankia strains Cc1.17 and Cp1.2

A comparative study was conducted on the effect of NH₄Cl on growth, vesicle formation and formation of nitrogenase of Frankia strains Cc1.17 and Cp1.2, derived from root nodules of Colletia cruciata and Comptonia peregrina, respectively. On a medium without combined nitrogen (P-N), both strains formed spherical cells, called vesicles, like many other Frankia strains. Data are presented on the number of vesicles per mg protein, after cultivation in media with sodium propionate as C-source without combined nitrogen (P-N) or with 0.2 g NH₄Cl/l (P+N). Strain Cp1.2 as may other Frankia strains, showed on P+N medium a very strong reduction of vesicle formation of 99% relative to the number of vesicles formed on P-N medium, after 11 days growth. However, in strain Cc11.17 this reduction was only 70%. The occurence of relatively large numbers of vesicles in P+N media has not yet been reported for other Frankia strains. No acetylene reduction activity was found in NH ₄ ⁺ -grown cells. The regulation of induction of nitrogenase in Frankia by NH₄Cl was tested by immuno-gelectrophoresis using antisera against nitrogenase of Rhizobium leguminosarum PRE. The component I of the enzyme showed crossreactivity while the component II had only a weak crossreaction. The experiments indicated that no nitrogenase was detectable in the NH ₄ ⁺ -grown cells. For the localization of nitrogenase, relative amounts of the enzyme were compared in whole cells and vesicle-enriched fractions. Western blots showed a significant enrichment of nitrogenase in the vesicle fractions, which indicated that most of the nitrogenase was localized in the vesicle.     

Net CO2 assimilation of cacao seedlings during periods of plant water deficit

The effect of leaf water potential () on net CO₂ assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 mol CO₂ m^-2 s^-1. The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55–65 mmol H₂O m^-2 s^-1. Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 mol CO₂ mmol^-1 H₂O throughout the range of stomatal conductances observed. C _i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C _i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO₂ and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.Abbreviations ABA Abscisic Acid     

Homeostatic gas-exchange parameters inferred from 13C/12C in tree rings of conifers

The CO₂ concentration of the atmosphere has increased by almost 30% in the past two centuries, with most of the increase (>5 Pa) during the past 60 years. Controlled environment studies of crop plants dependent on the C₃ photosynthetic pathway indicate that an increase of this magnitude would enhance net photosynthesis, reduce stomatal conductance, and increase the difference in CO₂ concentration across the stomata, i.e., CO₂ concentration outside the leaf to that within (c _a-c _i). Here we report evidence, based on stable isotope composition of tree rings from three species of field-grown, native conifer trees, that the trees have indeed responded. However, rather than increasing c _a-c _i, intercellular CO₂ concentrations have shifted upward to match the rise in atmospheric concentrations, holding c _a-c _i constant. No differences were detected among Douglas-fir (Pseudotsuga menziesii), ponderosa pine (Pinus ponderosa), or western white pine (Pinus monticola). The values of c _a-c _i were inferred from stable carbon isotope ratio (¹³C) of tree ring holocellulose adjusted for the 0.6–2.6 difference between holocellulose and whole sapwood. The cellulose extraction removed contaminants deposited in the tree ring after it formed and the adjustment corrected for the enrichment of cellulose relative to whole tissue. The whole sapwood values were then adjusted for bublished estimates of past atmospheric ¹³CO₂ and CO₂ concentrations. To avoid confounding tree age with CO₂, cellulose deposited by saplings in the 1980s was compared to cellulose deposited in the inner rings of nature trees when the mature trees were saplings, between 1910–1929 and 1941–1970; thus saplings were compared to saplings. In a separate analysis, the juvenile effect, which describes the tendency for ¹³C to increase in the first decades of a tree's life, was quantified independent of source CO₂ effects. This study provides evidence that conifers have undergone adjustments in the intercellular CO₂ concentration that have maintained c _a-c _i constant. Based on these results and others, we suggest that c _a-c _i, which has also been referred to as the intrinsic water-use efficiency, should be considered a homeostatic gas-exchange set point for these conifer species.     

Effect of CO2 enrichment and nitrogen availability on resource acquisition and resource allocation in a grass,Bromus mollis

Summary The effects of CO₂ enrichment on the growth, biomass partitioning, photosynthetic rates, and leaf nitrogen concentration of a grass, Bromus mollis (C₃), were investigated at a favorable and a low level of nitrogen availability. Despite increases in root: shoot ratios, leaf nitrogen concentrations were decreased under CO₂ enrichment at both nitrogen levels. For the low-nitrogen treatment, this resulted in lower photosynthetic rates measured at 650 l/l for the CO₂-enriched plants, compared to photosynthetic rates measured at 350 l/l for the non-enriched plants. At higher nitrogen availability, photosynthetic rates of plants grown and measured at 650 l/l were greater than photosynthetic rates of the non-enriched plants measured at 350 l/l. Water use efficiency, however, was increased in enriched plants at both nitrogen levels. CO₂ enrichment stimulated vegetative growth at both high and low nitrogen during most of the vegetative growth phase but, at the end of the experiment, total biomass of the high and low CO₂ treatments did not differ for plants grown at low nitrogen availability. While not statistically significant, CO₂ tended to stimulate seed production at high nitrogen and to decrease it at low nitrogen.     

Effects of CO2 enrichment and water stress on gas exchange of Liquidambar styraciflua and Pinus taeda seedlings grown under different irradiance levels

Summary The effects of CO₂ enrichment and water stress on gas exchange of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) seedlings were examined for individuals grown from seed under high (1000 mol·m^-2·s^-1) and low (250 mol·m^-2·s^-1) photosynthetic photon flux density at 350, 675 and 1000 l·l^-1 CO₂. At 8 weeks of age, half the seedlings in each CO₂-irradiance treatment were subjected to a drying cycle which reduced plant water potential to about -2.5 MPa in the most stressed plants, while control plants remained well-watered (water potentials of -0.3 and -0.7 MPa for sweetgum and loblolly pine, respectively). During this stress cycle, whole seedling net photosynthesis, transpiration and stomatal conductance of plants from each CO₂-irradiance-water treatment were measured under respective growth conditions.For both species, water stress effects on gas exchange were greatest under high irradiance conditions. Waterstressed plants had significantly lower photosynthesis rates than well-watered controls throughout most of the drying cycle, with the most severe inhibition occurring for low CO₂, high irradiance-grown sweetgum seedlings. Carbon dioxide enrichment had little effect on gas exchange rates of either water-stressed or well-watered loblolly pine seedlings. In contrast, water stress effects were delayed for sweetgum seedlings grown at elevated CO₂, particularly in the 1000 l·l^-1 CO₂, high irradiance treatment where net photosynthesis, transpiration and conductance of stressed plants were 60, 36 and 33% of respective control values at the end of the drying cycle. Development of internal plant water deficits was slower for stressed sweetgum seedlings grown at elevated CO₂. As a result, these seedlings maintained higher photosynthetic rates over the drying cycle than stressed sweetgum seedlings grown at 350 l·l^-1 CO₂ and stressed loblolly pine seedlings grown at ambient and enriched CO₂ levels. In addition, water-stressed sweetgum seedlings grown at elevated CO₂ exhibited a substantial increase in water use efficiency.The results suggest that with the future increase in atmospheric CO₂ concentration, sweetgum seedlings should tolerate longer exposure to low soil moisture, resulting in greater first year survival of seedlings on drier sites of abandoned fields in the North Carolina piedmont.     

Conformational Change of the Rieske [2Fe–2S] Protein inCytochrome bc 1 Complex

Structures of mitochondrial bc ₁ complex have been reported based on four different crystalforms by three different groups. In these structures, the extrinsic domain of the Rieske [2Fe–2S]protein, surprisingly, appeared at three different positions: the c ₁ position, where the [2Fe–2S]cluster exists in close proximity to the heme c ₁; the b position, where the [2Fe–2S] clusterexist in close proximity to the cytochrome b; and the intermediate position where the[2Fe–2S] cluster exists in between c ₁ and b positions. The conformational changes betweenthese three positions can be explained by a combination of two rotations; (1) a rotation of theentire extrinsic domain and (2) a relative rotation between the cluster-binding fold and thebase fold within the extrinsic domain. The hydroquinone oxidation and the electron bifurcationmechanism at the Q_P binding pocket of the bc ₁ complex is well explained using theseconformational changes of the Rieske [2Fe–2S] protein.     

Comparison of Some Photosynthetic Characters Between Two Hybrid Rice Combinations Differing in Yield Potential

Photosynthetic characteristics of two hybrid rice combinations, Peiai 64S/E32 and Shanyou 63, were compared at the panicle differentiation stage. As compared with Shanyou 63, the new combination Peiai 64S/E32 showed a significantly higher net photosynthetic rate (P _N), apparent quantum yield of carbon assimilation (_c), carboxylation efficiency (CE), and photorespiratory rate (R _P) as well as leaf chlorophyll content, but a significantly lower dark respiration rate (R _D) and compensation irradiance (I _c). It also showed a slightly higher photochemical efficiency (F_v/F_m and F/F_m) of photosystem 2, a lower non-photochemical quenching (q_N), and a similar CO₂ compensation concentration () as compared to Shanyou 63.     

Thermodynamics of Mn2+-binding to goat α-lactalbumin

By means of reaction calorimetry we measured the apparent enthalpy change, H^app, of the binding of Mn²⁺-ions to goat -lactalbumin as a function of temperature. The observed H^app can be written as the sum of contributions resulting from a conformational and a binding process. In combination with the thermal unfolding curve of goat -lactalbumin, we succeeded in separating the complete set of thermodynamic parameters (H, G, S, C_p) into the binding and conformational contributions. By circular dichroism we showed that NH ₄ ⁺ -ions, upon binding to bovine a-lactalbumin, induce the same conformational change as do Na⁺ and K⁺: the binding constant equals 98 ± 9 M^–1.Abbreviations BLA bovine -lactalbumin - GLA goat -lactalbumin - HLA human -lactalbumin - CD circular dichroism Offprint requests to: H. Van DaelDeceased