The effect of ammonium and nitrate on CO2 assimilation,RuBP and PEP carboxylase activity and dry matter production in wheat

Photosynthetic¹⁴CO₂ assimilation, ribulose 1, 5-bisphosphate carboxylase (RuBPC), phosphoenol pyruvate carboxylase (PEPC) and dry matter (DM) production were examined in wheat under varying levels and forms of nitrogen.¹⁴CO₂ assimilation increased gradually after germination reaching a peak value at anthesis, followed by a sharp decline. A similar pattern was observed for both the carboxylases, RuBPC and PEPC activities. Increase in nitrogen levels, in general, brought about a significant increase over the control (zero-nitrogen) in¹⁴CO₂ assimilation, RuBPC, PEPC activities and DM production. There were no significant differences in RuBPC activity and¹⁴CO₂ assimilation with respect to the forms of nitrogen. Significantly higher PEPC activity and DM was observed in plants supplied with nitrate-nitrogen (NO₃-N), as compared to those supplied with ammonium-nitrogen (NH₄-N). The significance of PEPC activity in C₃ photosynthesis is discussed in relation to DM distribution.     

Changes in photosynthetic apparatus during dark incubation of detached leaves from control and ultraviolet-B treatedVigna seedlings

Changes in various components of photosynthetic apparatus during the 4 d dark incubation at 25°C of detached control and ultraviolet-B (UV-B) treatedVigna unguiculata L. leaves were examined. The photosynthetic apparatus was more degraded in younger control seedlings and for a longer time UV-B treated seedlings than in the older or for a shorter time UV-B treated seedlings. This was shown by determining the losses in chlorophyll (Chl) and protein contents, variable fluorescence yield, photosystem (PS) 2, PS1 and ribulose-1,5-bisphosphate carboxylase (RuBPC) activities, and photosynthetic¹⁴CO₂ fixation. In contrast, the Car/Chl ratio increased during the dark incubation due to less expressed degradation of Car.     

The Distinctive Pattern of Photosystem 2 Activity,Photosynthetic Pigment Accumulation,and Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Content of Chloroplasts along the Axis of Primary Wheat Leaf Lamina

Wheat (Triticum aestivum L. cv. Sonalika) seedlings were grown in Hoagland solution. Primary leaves were harvested at 8, 12, and 15 d and cut into five equal segments. Contents of photosynthetic pigments and proteins, and photosystem 2 (PS2) activity increased from base to apex of these leaves. Chlorophyll (Chl) content was maximum at 12 d in all the leaf segments, but PS2 activity showed a gradual decline from 8 to 15 d in all leaf segments. In sharp contrast, the CO₂ fixation ability of chloroplasts increased from 8 to 15 d. CO₂ fixation ability of chloroplasts started to decline from base to apex of 15-d-old seedlings, where the content of ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit (RuBPCO-LSU) increased acropetally. RuBPCO-LSU content was maximum in all the leaf segments in 12-d-old seedlings. This shows a distinctive pattern of PS2, Chl, CO₂ fixation ability of chloroplasts, and RuBPCO-LSU content along the axis of leaf lamina during development and senescence. RuBPCO-LSU (54 kDa) degraded to fragments of 45, 42, 37, 19, and 16 kDa products which accumulated along the leaf axis during ageing of chloroplasts. Thus the CO₂ fixation ability of chloroplasts declines earlier than PS2 activity and photosynthetic pigment contents along the leaf lamina.     

Variations in short term products of inorganic carbon fixation in exponential and stationary phase cultures of Aphanocapsa 6308

Aphanocapsa 6308 metabolizes both NaHCO₃ and Na₂CO₃. The short term incorporation (5-s) metabolic pattern and the patterns of incorporation of bicarbonate for exponential versus stationary phase cultures differ, however. Cells were equilibrated for 10 min in air and distilled water prior to injection of either NaH¹⁴CO₃ at pH 8.0, or Na₂ ¹⁴CO₃ at pH 11.0. Hot ethanol extracts were analyzed via paper chromatography and autoradiography for products of CO₂ fixation. At 5 s, malate (51.5%) predominates slightly as a primary bicarbonate fixation product over 3-phosphoglycerate (40.3%); 3-phosphoglycerate is the primary product of carbonate fixation. At 60 s, the carbonate and bicarbonate labelling patterns are similar. Cells in stationary phase fix in 5 s a greater proportion of bicarbonate into malate (36% vs. 14% for 3-phosphoglycerate) than do cells in exponential growth. Likewise, 60 s incorporations show a large amount of bicarbonate fixed into aspartate (30.9%) in stationary phase cells over that of exponential phase (11.6%). These data suggest an operative C₄ pathway for purposes not related to carbohydrate synthesis but rather as compensation for the incomplete tricarboxylic acid cycle in cyanobacteria. The enhancement of both aspartate fixation and CO₂ fixation into citrulline in stationary phase correlates with an increase in cyanophycin granule production which requires both aspartate and arginine.Nonstandard Abbreviations 3-PGA 3-phosphoglyceric acid - TCA tricarboxylic acid     

Crassulacean acid metabolism (CAM) in Kalanchoë daigremontiana: Temperature response of phosphoenolpyruvate (PEP)-carboxylase in relation to allosteric effectors

Net CO₂ dark fixation of Kalanchoë daigremontiana varies with night temperature. We found an optimum of fixation at about 15° C; with increasing night temperature fixation decreased. We studied the temperature dependence of the activity of phosphoenolpyruvate (PEP)-carboxylase, the key enzyme for CO₂ dark fixation. We varied the pH, the substrate concentration (PEP), and the L-malate and glucose-6-phosphate (G-6-P) concentration in the assay. Generally, lowering the pH and reducing the amount of substrate resulted in an increase in activation by G-6-P and in an increase in malate inhibition of the enzyme. Furthermore, malate inhibition and G-6-P activation increased with increasing temperature. Activity measurements between 10° C and 45°C at a given concentration of the effectors revealed that the temperature optimum and maximum activities at that optimum varied with the effector applied. Under the influence of 5 mol m^-3 L-malate the temperature optimum and maximum activity dropped drastically, especially when the substrate level was low (at 0.5 mol m^-3 PEP from 32° C to 20° C). G-6-P raised the temperature optimum and maximum activity when the substrate level was low. If both malate and G-6-P were present, intermediate values were measured. We suggest that changes in metabolite levels in K. daigremontiana leaves can alter the temperature features of PEP-carboxylase so that the observed in vivo CO₂ dark fixation can be explained on the basis of PEP-carboxylase activity.Abbreviations PEP-c phosphoenolpyruvate carboxylase - CAM crassulacean acid metabolism - PEP phosphoenolpyruvate - G-6-P glucose-6-phosphate     

Photosynthesis and growth of Erythrina variegata as affected by water stress and triacontanol

Erythrina variegata Lam. seedlings were grown under water stress (Ψ = -3.2 MPa) and subsequently sprayed with triacontanol (Tria). Water stress significantly reduced shoot growth rate, while roots continued to grow. Content of chlorophyll (Chl) a decreased more than that of Chl b. Water stress also reduced photosynthetic activity of chloroplasts as measured by Chl fluorescence induction. Stress effect was identified at the oxidation site of photosystem (PS) 2 prior to the hydroxylamine donating site and perhaps close to or after the diphenylcarbazide donating site. The loss of O₂ evolving thylakoid polypeptides (33, 23, 17 kDa) and the large (55 kDa) and small (15 kDa) subunits of ribulose-1,5-bisphosphate carboxylase (RuBPC) were found in water stressed seedlings. The reduction in RuBPC activity was accompanied by reduction of CO₂ fixation and stomatal conductance. All photosynthetic parameters were improved by Tria. This revised version was published online in August 2006 with corrections to the Cover Date.     

Glyoxylate decreases the oxygen sensitivity of nitrogenase activity and photosynthesis in the cyanobacterium Anabaena cylindrica

Raising the pO₂ reduced nitrogenase activity (C₂H₂ reduction) of Anabaena cylindrica for both glyoxylate-treated (5 mM) and untreated cells. The stimulation caused by glyoxylate, however, increased with increases of pO₂ from 2 to 99 kPa. As the pO₂ increased the net CO₂ fixation was lowered (Warburg effect) while the CO₂ compensation point increased. Glyoxylate partly relieved this sensitivity of net photosynthesis to oxygen and reduced the compensation point considerably. The cells used were preincubated in the dark to exhaust photosynthetic pools. A more pronounced reduction in sensitivity of nitrogenase to oxygen for glyoxylate-treated cells was evident when a preincubation in air with reduced pCO₂ (13 l l^-1) was used. This was, however, not evident until after a 10-h incubation in air. Before this point 2 kPa O₂ sustained the highest nitrogenase activity. Addition of 0.5 and 5 mM of HCO ₃ ^- to Anabaena cultures preincubated at low CO₂ levels (29 l l^-1) abolished the stimulatory effect of glyoxylate on the nitrogenase. Thus, the results sustain the suggestion that glyoxylate may act as an inhibitor of photorespiratory activities in cyanobacteria and can be used as a means of increasing their nitrogen and CO₂ fixation capacities.Abbreviation RuBP ribulose 1,5-bisphosphate     

Ribulose-1, 5-bisphosphate carboxylase activity,chlorophyll and protein concentrations in differentPopulus clones

Ribulose-1,5-bisphosphate carboxylase activity (RuBPC), chlorophyll (chl) and protein (prot) concentrations and chlorophyll/protein (chl/prot) ratios were determined in five differentPopulus clones together with their maximal net CO₂ uptake rates (P_max). A classic reference clone (Populus ×euramericana “Robusta” (Dode) Guinier) was compared with four recently selected euramerican and interamerican crossings. Chl/prot ratio and RuBPC activity varied among the different clones, while chl a/chlb ratio showed only a very low coefficient of variation (1.7%) for the five clones. Poplar clone “Robusta” could be distinguished from the recent faster growing clones based on the different biochemical characteristics. A significant correlation was found between both total chl concentration and chl/prot ratio with P_max for the five clones.     

Luminescence decay kinetics in relation to quenching and stimulation of dark fluorescence from high and low CO2 adapted cells of Scenedesmus obliquus and Chlamydomonas reinhardtii

Two green algal species, Chlamydomonas reinhardtii and Scenedesmus obliquus, exhibited a relative maximum during the decay of luminescence, when adapted to low CO₂ conditions that was not observed in high CO₂ adapted cells.From the kinetics of transient changes in the level of dark fluorescence, after illumination and parallel to the luminescence maxima, it was concluded that the maximum in Scenedesmus was mainly related to a decrease in nonphotochemical quenching, whereas in Chlamydomonas the maximum was mainly related to a dark reduction of the primary PS II acceptor Q_A.ATP/ADP ratios from low CO₂ adapted Scenedesmus showed transient high levels after a dark/light transition that was not observed in high CO₂ adapted cells. After 30 s of illumination the ATP/ADP ratios however stabilized at the same steady state level as in high CO₂ adapted cells.Dark addition of HCO₃ ^- to low CO₂ adapted cells of Chlamydomonas resulted in a rapid transient quenching of luminescence that was not observed in low CO₂ adapted cells of neither species.It is concluded that the luminescence maxima present in both low CO₂ adapted Scenedesmus and Chlamydomonas reflect adaptation of the cells to low CO₂ conditions. It is further suggested that the difference in mechanistic origin of luminescence maxima in the two species reflects differences in adaptation.Abbreviations ADP adenosine-diphosphate - ATP adenosine-triphosphate - C_i inorganic carbon - F_D dark fluorescence recorded under dark adapted conditions - F₀ fluorescence with all reaction centers open - FV variable fluorescence - PS I photosystem I - PS II photosystem II - Q_A the first quinone acceptor of PS II     

The effect of cadmium on CO<Subscript>2</Subscript> exchange,variable fluorescence of chlorophyll,and the level of antioxidant enzymes in pea leaves

CO₂ exchange, variable chlorophyll fluorescence, the intensity of lipid peroxidation (POL), and the activity of antioxidant enzymes in leaves of two-week-old pea seedlings (Pisum sativum L.) exposed to 0.01, 0.1, and 1 mM aqueous solutions of Cd(NO₃)₂ for 2 h were studied. Incubation with Cd²⁺ ions resulted in a reduction of the maximum quantum efficiency of photosynthesis, CO₂ uptake rate, and photosystem II (PSII) activity, as assessed by F _v/F ₀ ratio. The intensity of POL in leaves of all treated seedlings was below or close to the control level. The activity of superoxide dismutase (SOD) and glutathione reductase (GR) increased in all treatments; the activity of ascorbate peroxidase (AP) exceeded the control level only in seedlings exposed to the high Cd²⁺ concentration (1 mM), and the activity of peroxidase increased at the low concentration (0.01 mM). We found that the reduction in the activity of the photosynthetic apparatus under conditions of 2-h-long Cd²⁺-induced stress was not related to an intensification of POL processes. It was concluded that stimulation of the activity of antioxidant enzymes—SOD, GR, AP, and peroxidase—is a pathway for pea plant adaptation to toxic effect of cadmium.Translated from Fiziologiya Rastenii, Vol. 52, No. 1, 2005, pp. 21–26.Original Russian Text Copyright © 2005 by Balakhnina, Kosobryukhov, Ivanov, Kreslavskii.     

Chlorophyll a Fluorescence Emission,Xanthophyll Cycle Activity,and Net Photosynthetic Rate Responses to Ozone in Some Foliose and Fruticose Lichen Species

The lichens Parmelia quercina, Parmelia sulcata, Evernia prunastri, Hypogymnia physodes, and Anaptychia ciliaris were exposed to ozone (O₃) in controlled environment cuvettes designed to maintain the lichens at optimal physiological activity during exposure. Measurements of gas exchange, modulated chlorophyll (Chl) fluorescence, and pigment analysis were conducted before and after exposure to 300 mm³ (O₃) m^–3, 4 h per d for 14 d. No changes in the efficiency of photosystem 2 (PS2) photochemistry, the reduction state of Q_A, or the electron flow through PS2, measured by Chl fluorescence, were detected in any of the five lichen species studied. Additionally, neither photosynthetic CO₂ assimilation nor xanthophyll cycle activity or photosynthetic pigment concentration were affected by high O₃ concentrations. Thus the studied lichen species have significant capacities to withstand oxidative stresses induced by high concentration of O₃.     

Nitrogenase activity and dark CO2 fixation in the lichen Peltigera aphthosa Willd

The lichen Peltigera aphthosa consists of a fungus and green alga (Coccomyxa) in the main thallus and of a Nostoc located in superficial packets, intermixed with fungus, called cephalodia. Dark nitrogenase activity (acetylene reduction) of lichen discs (of alga, fungus and Nostoc) and of excised cephalodia was sustained at higher rates and for longer than was the dark nitrogenase activity of the isolated Nostoc growing exponentially. Dark nitrogenase activity of the symbiotic Nostoc was supported by the catabolism of polyglucose accumulated in the ligh and which in darkness served to supply ATP and reductant. The decrease in glucose content of the cephalodia paralleled the decline in dark nitrogenase activity in the presence of CO₂; in the absence of CO₂ dark nitrogenase activity declined faster although the rate of glucose loss was similar in the presence and absence of CO₂. Dark CO₂ fixation, which after 30 min in darkness represented 17 and 20% of the light rates of discs and cephalodia, respectively, also facilitated dark nitrogenase activity. The isolated Nostoc, the Coccomyxa and the excised fungus all fixed CO₂ in the dark; in the lichen most dark CO₂ fixation was probably due to the fungus. Kinetic studies using discs or cephalodia showed highest initial incorporation of ¹⁴CO₂ in the dark in to oxaloacetate, aspartate, malate and fumarate; incorporation in to alanine and citrulline was low; incorporation in to sugar phosphates, phosphoglyceric acid and sugar alcohols was not significant. Substantial activities of the enzymes phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and carbamoyl-phosphate synthase (EC 2.7.2.5 and 2.7.2.9) were detected but the activities of PEP carboxykinase (EC 4.1.1.49) and PEP carboxyphosphotransferase (EC 4.1.1.38) were negligible. In the dark nitrogenase activity by the cephalodia, but not by the free-living Nostoc, declined more rapidly in the absence than in the presence of CO₂ in the gas phase. Exogenous NH ₄ ⁺ inhibited nitrogenase activity by cephalodia in the dark especially in the absence of CO₂ but had no effect in the light. The overall data suggest that in the lichen dark CO₂ fixation by the fungus may provide carbon skeletons which accept NH ₄ ⁺ released by the cyanobacterium and that in the absence of CO₂, NH ₄ ⁺ directly, or indirectly via a mechanism which involves glutamine synthetase, inhibits nitrogenase activity.Abbreviations CP carbamoyl phosphate - EDTA ethylenedi-amine tetraacetic acid - PEP phosphoenolpyruvate - RuBP ribulose 1,5 bisphosphate     

Can mushrooms fix atmospheric nitrogen?

It is generally reported that fungi likePleurotus spp. can fix nitrogen (N₂). The way they do it is still not clear. The present study hypothesized that only associations of fungi and diazotrophs can fix N₂. This was testedin vitro. Pleurotus ostreatus was inoculated with a bradyrhizobial strain nodulating soybean andP. ostreatus with no inoculation was maintained as a control. At maximum mycelial colonization by the bradyrhizobial strain and biofilm formation, the cultures were subjected to acetylene reduction assay (ARA). Another set of the cultures was evaluated for growth and nitrogen accumulation. Nitrogenase activity was present in the biofilm, but not when the fungus or the bradyrhizobial strain was alone. A significant reduction in mycelial dry weight and a significant increase in nitrogen concentration were observed in the inoculated cultures compared to the controls. The mycelial weight reduction could be attributed to C transfer from the fungus to the bradyrhizobial strain, because of high C cost of biological N₂ fixation. This needs further investigations using¹⁴C isotopic tracers. It is clear from the present study that mushrooms alone cannot fix atmospheric N₂. But when they are in association with diazotrophs, nitrogenase activity is detected because of the diazotrophic N₂ fixation. It is not the fungus that fixes N₂ as reported earlier. Effective N₂ fixing systems, such as the present one, may be used to increase protein content of mushrooms. Our study has implications for future identification of as yet unidentified N₂ systems occurring in the environment.     

Dark fixation of CO2 during gluconeogenesis by the cotyledons of Cucurbita pepo L.

We did this work to discover the pathway of CO₂ fixation into sugars in the dark during gluconeogenesis by the cotyledons of 5-day-old seedlings of Cucurbita pepo L. We paid particular attention to the possibility of a contribution from ribulosebisphosphate carboxylase. The detailed distribution of ¹⁴C after exposure of excised cotyledons to ¹⁴CO₂ in the dark was determined in a series of pulse and chase experiments. After 4s in ¹⁴CO₂, 89% of the ¹⁴C fixed was in malate and aspartate. In longer exposures, and in chases in ¹²CO₂, label appeared in alanine, phosphoenolpyruvate, 3-phosphoglycerate and sugar phosphates, and accumulated in sugars. The transfer of label from C-4 acids to sugars was restricted by inhibition of phosphoenolpyruvate carboxykinase in vivo by 3-mercaptopicolinic acid. We conclude as follows. Initial fixation of CO₂ in the dark is almost entirely into phosphoenolpyruvate, probably via phosphoenolpyruvate carboxylase (EC 4.1.1.31) which we showed to be present in appreciable amounts. Incorporation into sugars occurs chiefly, if not completely, as a result of randomization of the carboxyl groups of the C-4 acids and subsequent conversion of the oxaloacetate to sugars via the accepted sequence for gluconeogenesis. Ribulosebisphosphate carboxylase appears to make very little contribution to sugar synthesis from fat.     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

CO2 exchange characteristics during dark-light transitions in wild-type and mutant Chlamydomonas reinhardii cells

A burst of net CO₂ uptake was observed during the first 3–4 min after the onset of illumination in both wild-type Chlamydomonas reinhardii in which carbonic anhydrase was chemically inhibited with ethoxyzolamide and in a mutant of C. reinhardii (ca-1-12-1C) deficient in carbonic anhydrase activity. The burst was followed by a rapid decrease in the CO₂ uptake rate so that net evolution often occurred. After a 2–3 min period of CO₂ evolution, net CO₂ uptake again increased and ultimately reached a steady-state, positive rate. From [¹⁴CO₂]-tracer studies it was determined that CO₂ fixation proceeded at a nearly linear rate throughout the period of illumination. Thus, prior to reaching a steady state, there was a rapid accumulation of inorganic carbon inside the cells which apparently reached a supercritical concentration and the excess was excreted, causing a subsequent efflux of CO₂. A post illumination burst of net CO₂ efflux was also observed in ethoxyzolamide-inhibited wild type and ca-1 mutant cells, but not in the unihibited wild type. [¹⁴CO₂]-tracer experiments revealed that this burst was the result of a collapse of a large internal inorganic carbon pool at the onset of darkness rather than a photorespiratory post-illumination burst. These results indicate that upon illumination, chemical or genetic inhibition of carbonic anhydrase initially causes an accumulation of excess inroganic carbon in C. reinhardii cells, and that unknown regulatory mechanisms correct for this imbalance by first excreting the excess inorganic carbon and then, after several dampened oscillations, achieving an equilibrium between bicarbonate uptake, bicarbonate dehydration, and CO₂ fixation.     

Photosynthetic rate,chlorophyll fluorescence parameters,and lipid peroxidation of maize leaves as affected by zinc deficiency

Pot trial in greenhouse was conducted using cumulic cinnamon soil from North China to study the effects of zinc deficiency on CO₂ exchange, chlorophyll fluorescence, the intensity of lipid peroxidation, and the activity of superoxide dismutase (SOD) in leaves of maize seedlings. Zn deficiency resulted in a reduction of net photosynthetic rate and stomatal conductance to H₂O. The maximum quantum efficiency of photosystem 2 (PS2) and the PS2 activity were depressed, while the pool size of the plastoquinone molecules was not affected by Zn deficiency. The content of super oxygen anion radical (O₂ ^·−) and the intensity of lipid peroxidation as assessed by malonyldialdehyde content in Zn-deficient leaves were higher than those in Zn-sufficient leaves. The activity of SOD increased with Zn application. The adverse influence of Zn-deficiency on the light stage of photosynthesis is probably one of possible reasons for the limitation of photosynthetic capacity in maize leaves.     

Non-phototrophic CO 2 fixation by soil microorganisms

Although soils are generally known to be a net source of CO₂ due to microbial respiration, CO₂ fixation may also be an important process. The non-phototrophic fixation of CO₂ was investigated in a tracer experiment with ¹⁴CO₂ in order to obtain information about the extent and the mechanisms of this process. Soils were incubated for up to 91 days in the dark. In three independent incubation experiments, a significant transfer of radioactivity from ¹⁴CO₂ to soil organic matter was observed. The process was related to microbial activity and could be enhanced by the addition of readily available substrates such as acetate. CO₂ fixation exhibited biphasic kinetics and was linearly related to respiration during the first phase of incubation (about 20–40 days). The fixation amounted to 3–5% of the net respiration. After this phase, the CO₂ fixation decreased to 1–2% of the respiration. The amount of carbon fixed by an agricultural soil corresponded to 0.05% of the organic carbon present in the soil at the beginning of the experiment, and virtually all of the fixed CO₂ was converted to organic compounds. Many autotrophic and heterotrophic biochemical processes result in the fixation of CO₂. However, the enhancement of the fixation by addition of readily available substrates and the linear correlation with respiration suggested that the process is mainly driven by aerobic heterotrophic microorganisms. We conclude that heterotrophic CO₂ fixation represents a significant factor of microbial activity in soils.     

Effects of elevated CO2 and nitrogen on nutrient uptake in ponderosa pine seedlings

This paper reports on the results of a controlled-environment study on the effects of CO₂ (370, 525, and 700 mol mol^-1) and N [0, 200, and 400 g N g soil^-1 as (NH₄)SO₄] on ponderosa pine (Pinus ponderosa) seedlings. Based upon a review of the literature, we hypothesized that N limitations would not prevent a growth response to elevated CO₂. The hypothesis was not supported under conditions of extreme N deficiency (no fertilizer added to a very poor soil), but was supported when N limitations were less severe but still suboptimal (lower rate of fertilization). The growth increases in N-fertilized seedlings occurred mainly between 36 and 58 weeks without any additional N uptake. Thus, it appeared that elevated CO₂ allowed more efficient use of internal N reserves in the previously-fertilized seedlings, whereas internal N reserves in the unfertilized seedlings were insufficient to allow this response. Uptake rates of other nutrients were generally proportional to growth. Nitrogen treatment caused reductions in soil exchangeable K⁺, Ca²⁺, and Mg²⁺ (presumably because of nitrification and NO₃ ^- leaching) but increases in extractable P (presumably due to stimulation of phosphatase activity).The results of this and other seedling studies show that elevated CO₂ causes a reduction in tissue N concentration, even under N-rich conditions. The unique response of N is consistent with the hypothesis that the efficiency of Rubisco increases with elevated CO₂. These results collectively have significant implications for the response of mature, N-deficient forests to evevated CO₂.     

Inhibition of photosynthesis by chilling in moderate light: a comparison of plants sensitive and insensitive to chilling

Photosynthetic activity, in leaf slices and isolated thylakoids, was examined at 25° C after preincubation of the slices at either 25° C or 4° C at a moderate photon flux density (PFD) of 450 mol·m^–2·s^–1, or at 4° C in the dark. The plants used wereSpinacia oleracea L.,Cucumis sativus L. andNerium oleander L. which was acclimated to growth at 20° C or 45° C. The plants were grown at a PFD of 550 mol·m^–2·s^–1. Photosynthesis, measured as CO₂-dependent O₂ evolution, was not inhibited in leaf slices from any plant after preincubation at 25° C at a moderate PFD or at 4° C in the dark. However, exposure to 4° C at a moderate PFD induced an inhibition of CO₂-dependent O₂ evolution within 1 h inC. sativus, a chilling-sensitive plant, and in 45° C-grownN. oleander. The inhibition in these plants after 5 h reached 80% and 40%, respectively, and was independent of the CO₂ concentration but was reduced at O₂ concentrations of less than 3%. Methyl-viologen-dependent O₂ exchange in leaf slices from these plants was not inhibited. There was no photoxidation of chlorophyll, in isolated thylakoids, or any inhibition of electron transport at photosystem (PS)II, PSI or through both photosystems which would account for the inhibition of photosynthesis. The conditions which inhibit photosynthesis in chilling-sensitive plants do not cause inhibition inS. oleracea, a chilling-insensitive plant, or in 20° C-grownN. oleander. The CO₂-dependent photosynthesis, measured at 5° C, was reduced to about 3% of that recorded at 25° C in chilling-sensitive plants but only to about 30% in the chilling-insensitive plants. Methyl-viologen-dependent O₂ exchange, measured at 5° C, was greater than 25% of the activity at 25° C in all the plants. The results indicate that the mechanism of the chilling-induced inhibition of photosynthesis does not involve damage to PSII. That inhibition of photosynthesis is observed only in the chilling-sensitive plants indicates it is related, in some way, to the disproportionate decrease in photosynthetic activity in these plants at chilling temperatures.Abbreviations Chl chlorophyll - DPIPH reduced form of 2,6-dichlorophenol-indophenol - DMQ 2,5-dimethyl-p-benzoquinone - MV methyl viologen - 20°-oleander Nerium oleander grown at 20° C - 45°-oleander N. oleander grown at 45° C - PFD photon flux density (photon fluence rate) - PSI and PSII photosystem I and II, respectively