Photosynthetic responses to slowly decreasing leaf water potentials in Encelia frutescens

The importance of reduced leaf conductance (stomatal and boundary layer) in limiting photosynthetic rates during water stress was studied in Encelia frutescens, a drought-deciduous leaved subshrub of the Mohave and Sonoran Deserts. Light-saturated CO₂ assimilation rates of greenhouse grown plants decreased from 42.6±1.6mol CO₂ m^-2 s^-1 (x±s.e.) to 1.7±1.7 mol CO₂ m^-2s^-1 as leaf water potential decreased from-1.5 MPa to-4.0 MPa. The dependence of light saturated, CO₂ assimilation rate on leaf intercellular CO₂ concentrations between 60 and 335 l l^-1 was also determined as leaf water potential decline. This enabled us to compare the effects of leaf water potentials on limitations to carbon assimilation imposed by leaf conductance and by intrinsic photosynthetic capacity. Both leaf conductance and intrinsic photosynthetic capacity decreased with decreasing leaf water potential, but the decrease in leaf conductance was proportionately greater. The relative stomatal limitation, defined as the percent limitation in photosynthetic rate due to the presence of gas-phase diffusional barriers, increased from (x±s.e.) to 41±3% as water potentials became more negative. Since both leaf conductance and intrinsic photosynthetic capacity were severely reduced in an absolute sense, however, high photosynthetic rates could not have been restored at low leaf water potentials without simultaneous increases in both components.     

Photosynthetic characteristics of photomixotrophic and photoautotrophic cell suspension cultures ofChenopodium rubrum

Summary Cell suspension cultures ofChenopodium rubrum have been grown for more than 2 years photoautotrophically with CO₂ as sole carbon source. Average increase in fresh weight is appr. 600% within 14 days. The chlorophyll content of photoautotrophic cells (200 g/g fresh weight) is much higher than of photomixotrophic cells (50 g/g fresh weight). The photosynthetic activity of the cells (190 moles CO₂×mg^–1 chlorophyllXh^–1) is comparable to the values found with intact leaves. As shown by short-term¹⁴CO₂ photosynthesis, both, the photomixotrophic and the photoautotrophic cell suspension cultures assimilate CO₂ predominantly via the Calvin pathway.Major differences were found with cells from either exponential or stationary phase of growth with regard to differential labelling of 3-phosphoglyceric acid, malate, sucrose and glucose/fructose.In vitro measurements of carboxylation reactions only partially corroborate our findings with¹⁴CO₂ incorporation. The ratio of ribulosebisphosphate to phosphoenolpyruvate carboxylase activity is 4.7 for leaves of C.rubrum, 1.2 for photoautotrophic cells during stationary growth and 0.5 for cells during exponential growth phase, however, 0.18 was found for photomixotrophic cells. Though the¹⁴CO₂ incorporation into 3-phosphoglyceric acid is clearly higher than into malate, thein vitro activity of phosphoenolpyruvatecarboxylase is 2–6 fold higher than that of ribulosebisphosphate carboxylase. We postulate that anaplerotic reactions of the tricarboxylic acid cycle are involved in the regulation of phosphoenolpyruvate carboxylase.Abbreviations 2,4-D didilorophenoxyacetic acid - EDTA ethylene-diamine-tetraacetic acid - fr. w. fresh weight - HEPES N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - PGA 3-phosphoglyceric acid - PPO 2,5-diphenyloxazole - PEP phosphoenolpyruvate - RuBP nbulosebisphosphate     

Intercellular CO2 concentration and water-use efficiency of temperate plants with different life-forms and from different microhabitats

Summary Photosynthesis and transpiration were measured simultaneously, under near-optimum and constant environmental conditions, in intact leaves of plants native to the temperate forest region. A linear relationship between photosynthetic rate and stomatal conductance was found in every species tested irrespective of leaf age or season, indicating that the calculated intercellular CO₂ concentration and water-use efficiency were fairly constant within a species. The values of intercellular CO₂ concentration and water-use efficiency ranged from 221 to 271 l l^–1 and 4.46 to 8.20 mol CO₂ mmolH₂O^–1 (6.24±0.90 mol CO₂ mmolH₂O^–1), respectively. The variations in intercellular CO₂ concentration and water-use efficiency were not directly related to photosynthetic capacities, life-forms, or microhabitat preferences. The intercellular CO₂ concentrations found in this study were close to values reported from cultivated plants and plants native to more arid regions, suggesting a common mechanism to maintain the stomatal conductance proportional to photosynthetic capacity over a wide variety of C₃ plants.     

In vitro and in vivo studies to assess the effectiveness of cholestyramine as a binding agent for fumonisins

Several adsorbent materials were tested at 1 mg/ml for their in vitrocapacity to adsorb fumonisin B₁ (FB₁) from aqueous solutions. Cholestyramine showed the best adsorption capacity (85% from a solution containing 200 g/ml FB₁) followed by activated carbon (62% FB₁). Bentonite adsorbed only 12% of the toxin from a solution containing 13 g/ml FB₁, while celite was not effective even at the lowest tested FB₁ concentration (3.2 g/ml). Cholestyramine was tested in vivoto evaluate its capacity to reduce the bioavailability of fumonisins (FBs) in rats fed diet contaminated with toxigenic Fusarium verticillioidesculture material. Rats were exposed for one week to FBs-free diet, FBs-contaminated diet containing 6 or 20 g/g FB₁ + FB₂ and the same FBs-contaminated diet added of 20 mg/g cholestyramine. The increase of sphinganine/sphingosine (SA/SO) ratio in urine and kidney of treated rats was used as specific and sensitive biomarker of fumonisin exposure.The addition of cholestyramine to the FBs-contaminated diets consistently reduced the effect of FBs by reducing significantly (P < 0.05) both urinary and renal SA/SO ratios.This revised version was published online in October 2005 with corrections to the Cover Date.     

Regulation of antibiotic production by iron and oxygen during defined medium fermentations of Streptomyces clavuligerus

Summary When grown in a chemically defined medium, Streptomyces clavuligerus excreted cephamycin C, in addition to other components, throughout most of the growth phase. Ferrous iron and oxygen are required for the biosynthesis of this antibiotic and the concentration of these cofactors was manipulated to maximize cephamycin C production. The iron content of the chemically defined medium was shown to be sub-optimal for antibiotic production and the addition of 130 g/ml ferrous iron almost doubled the cephamycin C levels to 200 g/ml. When dissolved oxygen was maintained at saturation levels, only 60–80 g/ml cephamycin C was produced, and the intermediate penicillin N accumulated to high levels (50 g/ml). This suggests that the high concentration of dissolved oxygen had a greater effect on the enzymes catalysing the conversion of penicillin N to cephamycin C, than on those involved in the earlier steps of the pathway leading to the formation of penicillin N.     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

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)     

Growth,photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O2 concentrations and photon flux density

Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO₂ enrichment up to 6000 l CO₂ l^–1. This high CO₂ optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO₂ was less or absent following four days growth in 2% O₂. The Leaf Area Ratio decreased in response to CO₂ enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO₂ enrichment up to 1500 l CO₂ l^–1 during measurement, showing only small increases with further CO₂ enrichment up to 5000 l CO₂ l^–1 at a photon flux density of 210 mol m^–2 s^–1 and small decreases at 2000 mol m^–1 s^–1. The actual rate of photosynthesis of those plants cultivated at high CO₂ levels, however, was less than the air grown plants. The response of photosynthesis to O₂ indicated that the enhancement of growth and photosynthesis by CO₂ enrichment was a result of decreased photorespiration. Plants cultivated in low O₂ produced abnormal morphological features and after a short time showed a reduction in growth.     

Image processing technique for measuring specific growth rates of Gibberella fujikuroi

Summary Two methods were compared for estimating of Gibberella fujikuroi grown with different proportions of glucose and starch. They were, =Ln2 (V_r/Le) on Petri dish (V_r= rate of tip extension and L_e= mean hyphal length) and, ₂=d (LnX)/dt in stirred fermenter. Values of ₁ and ₂ were in close agreement with each other.     

Micropropagation of garlic through in vitro bulblet formation

We developed a novel micropropagation method for garlic (Allium sativum L.) by the combination of initial shoot-tip culture, shoot multiplication and in vitro bulblet formation. Garlic shoot-tips were cultured on LS medium containing 1 M indole-3-acetic acid (IAA) and 1 M 6-benzyladenine (BA) to regenerate proliferative shoots. These shoot-tips produced multiple shoots when transferred to modified LS medium containing 5 M 1-naphthaleneacetic acid (NAA) and 10 M BA, and cultured at 20°C under 12-h light conditions. Higher ratios of KNO₃/NH₄Cl in the media promoted multiple shoot formation, together with suppressing vitrification of these shoots. The proliferated shoots of early maturing cultivars produced bulblets by culture on LS growth regulator-free medium at 25°C under 16-h light. On the other hand, the late maturing cultivar, Howaito-roppen, formed bulblets after a low temperature treatment of the proliferated shoots for 6 months followed by culture on LS medium containing 6 to 12% sucrose for two months. The dormancy of the bulblets of cv. Howaito-roppen was broken by successive treatments at a high (35°C), a middle (20°C), and then a low (5°C) temperature.Abbreviations IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - BA 6-benzyladenine - LS Linsmaier and Skoog macro- and microelements     

A gel-sandwich technique for the qualitative and quantitative determination of dehydrogenases in the enzyme histochemistry

Summary A gel-sandwich technique for the histochemical demonstration of dehydrogenases is introduced with LDH set up as an example. Especially suitable, of the gels examined, for this technique is 1.5% W/V agar-agar low gel strength. In it several reaction ingredients for the histochemical reaction are dissolved. Considering LDH the following gel composition showed good results: 1.5% W/V agar-agar low gel strength, 5 mM TNBT in 150 l DMF, 120 mM L-lactate, 3–5 mM NAD⁺, 10 mM amytal, 22,4–32×10^–5 M Meldola Blue, 160 mM soldium phosphate buffer pH 7.6 (total solution of 1 ml). After the solidification of the gel, gel-bars were frozen with CO₂-snow. The 40–80 m thick gel slices were gained in the cryostat. Of the three different arrangement possibilities of the gel slices and the tissue-sections a sandwich arrangement (cover-gel slice — tissue section — ground-gel slice) produced the best results. The enzyme reaction is started by thawing of the gel slices (together with the tissue sections) and by putting them between the hotplate and the evaporator-head-piece, especially developed for this technique. The gel slices also remain in combination with the tissue sections after the reaction.The influence of the gel in combination with the electron carrier Meldola Blue on the spontaneous reduction rates of ditetrazolium salts in day light, were examined as well as the diffusion rates of TNBT and NADH out of gel slices and the influence of DMF and DMSO on the LDH activity.This technique prevents both, the loss of enzymes and the loss of reduction equivalents. There are given presuppositions for qualitative and quantitative histochemical investigations as well. The advantages of the new gel technique are discussed.     

Induction of fertile male flowers in genetically female Cannabis sativa plants by silver nitrate and silver thiosulphate anionic complex

Summary Apical application of silver nitrate (AgNO₃; 50 and 100 g per plant) and silver thiosulphate anionic complex (Ag(S₂O₃) ₂ ^3– ; STS; 25, 50 and 100 g per plant) to female plants of Cannabis sativa induced the formation of reduced male, intersexual and fully altered male flowers on the newly formed primary lateral branches (PLBs); 10 g per plant of AgNO₃ was ineffective and 150 g treatment proved inhibitory. A maximum number of fully altered male flowers were formed in response to 100 g STS. The induced male flowers produced pollen grains that germinated on stigmas and effected seed set. Silver ion applied as STS was more effective than AgNO₃ in inducing flowers of altered sex. The induction of male flowers on female plants demonstrated in this work is useful for producing seeds that give rise to only female plants. This technique is also useful for maintaining gynoecious lines.     

Temperature effects on the photosynthetic response of C3 plants to long-term CO2 enrichment

To assess the long-term effect of increased CO₂ and temperature on plants possessing the C₃ photosynthetic pathway, Chenopodium album plants were grown at one of three treatment conditions: (1) 23 °C mean day temperature and a mean ambient partial pressure of CO₂ equal to 350 bar; (2) 34 °C and 350 bar CO₂; and (3) 34 °C and 750 bar CO₂. No effect of the growth treatments was observed on the CO₂ reponse of photosynthesis, the temperature response of photosynthesis, the content of Ribulose-1,5-bisphosphate carboxylase (Rubisco), or the activity of whole chain electron transport when measurements were made under identical conditions. This indicated a lack of photosynthetic acclimation in C. album to the range of temperature and CO₂ used in the growth treatments. Plants from every treatment exhibited similar interactions between temperature and CO₂ on photosynthetic activity. At low CO₂ (< 300 bar), an increase in temperature from 25 to 35 °C was inhibitory for photosynthesis, while at elevated CO₂ (> 400 bar), the same increase in temperature enhanced photosynthesis by up to 40%. In turn, the stimulation of photosynthesis by CO₂ enrichment increased as temperature increased. Rubisco capacity was the primary limitation on photosynthetic activity at low CO₂ (195 bar). As a consequence, the temperature response of A was relatively flat, reflecting a low temperature response of Rubisco at CO₂ levels below its k_m for CO₂. At elevated CO₂ (750 bar), the temperature response of electron transport appeared to control the temperature dependency of photosynthesis above 18 °C. These results indicate that increasing CO₂ and temperature could substantially enhance the carbon gain potential in tropical and subtropical habitats, unless feedbacks at the whole plant or ecosystem level limit the long-term response of photosynthesis to an increase in CO₂ and temperature.Abbreviations A net CO₂ assimilation rate - C _a ambient partial pressure of CO₂ - C _i intercellular partial pressure of CO₂ - Rubisco Ribulose-1,5-bisphosphate carboxylase - VPD vapor pressure difference between leaf and air     

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

Summary Gas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO₂ pressure remained near 220 bar during diurnal courses in the spring. On midsummer days, leaf internal CO₂ decreased from a late morning value of 200 bar to a late afternoon value of approximately 150 bar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO₂ supply due to stomatal closure reduced net CO₂ uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO₂ compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO₂ saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar^-1.     

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     

Phytotoxic activity of selected water-soluble metabolites of Fusarium against Lemna minor L. (Duckweed)

Phytotoxicity and inhibitory effects of the fusarial toxins fumonisin B₁ (FB₁) [m.p. 103–105 °C], fusaric acid [m.p. 106–107 °C], butenolide (4-acetamido-4-hydroxy-2-butenoic acid lactone) [116–117 °C], 9, 10-dihydroxyfusaric acid [m.p. 150–155 ° C], and moniliformin on chlorophyll synthesis in the aquatic macrophyte Lemna minor (duckweed) were examined. FB₁ proved to be most active, reducing the growth of L. minor fronds and their ability to synthesize chlorophyll by 53% and 59%, respectively, at 0.7 g/ml. The growth rate of L. minor was reduced 59% by 6.7 g/ml fusaric acid, 62% by 66.7 g/ml butenolide, and 22% by 66.7 g/ml 9,10-dihydroxyfusaric acid. Moniliformin was the least phytotoxic to L. minor, with only a 16% suppression of growth rate and a 54% reduction in chlorophyll at 66.7 g/ml.The mention of firm names or trade products does not imply that they are endorsed or recommended by the US Department of Agriculture over other firms or similar products not mentioned.     

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.     

Regulation of C4-phosphoenolpyruvate carboxylase activity by ambient CO2

Light activation of phosphoenolpyruvate carboxylase from the leaves of the C₄ plant Setaria verticillata (L.) is more pronounced at low CO₂ levels. The 2-fold activation observed at physiological ambient CO₂ becomes 3.64-fold at 5 L/L and completely abolished above 700 L/L. When the stomata close under the influence of abscisic acid at 330 L/L CO₂, the extent of light activation is high (3.59-fold), probably because the increased diffusive resistance keeps the internal CO₂ at much lower levels. Under darkness. CO₂ and absicisic acid do not affect the extractable phosphoenolpyruvate carboxylase activity. Internal CO₂ levels may determine phosphoenolpyruvate concentratio in the cytoplasm through the control of its utilization by phosphoenolpyruvate carboxylase. We have recently proposed (Samaras et al. 1988) that photosynthetically produced phosphoenolpyruvate could be an activator of the enzyme. It is therefore suggested that CO₂ indirectly affects the activation state of phosphoenolpyruvate carboxylase by controlling the levels of phosphoenolpyruvate which may act as an activator.Abbreviations PEPCase phosphoenolpyruvate carboxylase - PEP phosphoenolpyruvate - PAR photosynthetically active radiation - G6P glucose-6-phosphate - ABA abscisic acid - MDH malate dehydrogenase - PPDK pyruvate, Pi, dikinase - CAM Crassulacean Acid Metabolism     

Responses of nitrogen-fixing and nitrate-supplied alfalfa (Medicago sativa L.) to iron chelates in an alkaline hydroponic medium

Ferric ethylenediamine di-(o-hydroxyphenylacetate) (FeEDDHA) and ferric hydroxyethylethylenediaminetriacetic acid (FeHEDTA) were evaluated as Fe sources for hydroponic growth of alfalfa (Medicago sativa L., cv. Mesilla), either dependent on N₂ fixation or supplied with NO₃. The hydroponic medium was maintained at pH 7.5 by addition of CaCO₃. Nitrogen-fixing cultures were inoculated with Rhizobium meliloti 102 F51 and grown in medium without added nitrogen. After five to seven weeks of growth under greenhouse conditions, plants were harvested. Nitrogen fixation was measured by the acetylene reduction method.When FeEDDHA was supplied, growth of alfalfa, whether dependent on N₂ fixation or supplied with NO₃, was severely limited at concentrations typically used in hydroponic medium (10 or 20 M). Maximum yield of NO₃-supplied alfalfa was obtained at 100 M while maximum yield of N₂-fixing alfalfa was obtained in the range of 33 to 200 M FeEDDHA. Nodule fresh weights and N₂ fixation rates increased with FeEDDHA concentration up to 33 M and remained essentially constant up to 200 M. With FeHEDTA, maximum yields of both NO₃-grown and N₂-fixing alfalfa were obtained at 10 M. Growth of NO₃-supplied plants was inhibited at 200 M FeHEDTA while growth of N₂-fixing plants was inhibited at 100 M FeHEDTA. The numbers of nodules per plant increased between 3.3 and 10 M FeHEDTA; however, inhibition of nodule formation occurred at a concentration of 33 M or higher. Nodule weights per plant and N₂ fixation rates were depressed at 3.3 M as well as at 100 M FeHEDTA. The results suggest that alfalfa dependent on N₂ fixation is more sensitive to limited Fe availability than alfalfa supplied with NO₃.     

Interactive effects of soil nitrogen and atmospheric carbon dioxide on root/rhizosphere carbon dioxide efflux from loblolly and ponderosa pine seedlings

We measured CO₂ efflux from intact root/rhizosphere systems of 155 day old loblolly (Pinus taeda L.) and ponderosa (Pinus ponderosa Dougl. ex Laws.) pine seedlings in order to study the effects of elevated atmospheric CO₂ on the below-ground carbon balance of coniferous tree seedlings. Seedlings were grown in sterilized sand culture, watered daily with either 1, 3.5 or 7 mt M NH ₄ ⁺ , and maintained in an atmosphere of either 35 or 70 Pa CO₂. Carbon dioxide efflux (mol CO₂ plant^–1 s^–1) from the root/rhizosphere system of both species significantly increased when seedlings were grown in elevated CO₂, primarily due to large increases in root mass. Specific CO₂ efflux (mol CO₂ g root^–1 s^–1) responded to CO₂ only under conditions of adequate soil nitrogen availability (3.5 mt M). Under these conditions, CO₂ efflux rates from loblolly pine increased 70% from 0.0089 to 0.0151 mol g^–1 s^–1 with elevated CO₂ while ponderosa pine responded with a 59% decrease, from 0.0187 to 0.0077 mol g^–1 s^–1. Although below ground CO₂ efflux from seedlings grown in either sub-optimal (1 mt M) or supra-optimal (7 mt M) nitrogen availability did not respond to CO₂, there was a significant nitrogen treatment effect. Seedlings grown in supra-optimal soil nitrogen had significantly increased specific CO₂ efflux rates, and significantly lower total biomass compared to either of the other two nitrogen treatments. These results indicate that carbon losses from the root/rhizosphere systems are responsive to environmental resource availability, that the magnitude and direction of these responses are species dependent, and may lead to significantly different effects on whole plant carbon balance of these two forest tree species.