Stomatal responses of Argenteum — a mutant of Pisum sativum L. with readily detachable leaf epidermis

Epidermis is easily detached from both adaxial and abaxial surfaces of leaf four of the Argenteum mutant of Pisum sativum L. The isolated epidermis has stomata with large, easily-measured pores. Hairs and glands are absent. The density of stomata is high and contamination by mesophyll cells is low. In the light and in CO₂-free air, stomata in isolated adaxial epidermis of Argenteum mutant opened maximally after 4 h incubation at 25°C. The response of stomata to light was dependent on the concentration of KCl in the incubation medium and was maximal at 50 mol m^-3 KCl. Stomata did not respond to exogenous kinetin, but apertures were reduced by incubation of epidermis on solutions containing between 10^-5 and 10^-1 mol m^-3 abscisic acid (ABA). The responses of stomata of Argenteum mutant to light, exogenous KCl, ABA and kinetin were comparable with those described previously for stomata in isolated epidermis of Commelina communis. A method for preparing viable protoplasts of guard cells from isolated epidermis of Argenteum mutant is described. The response of guard cell protoplasts to light, exogenous KCl, ABA and kinetin were similar to those of stomata in isolated epidermis except that the increase in volume of the protoplasts in response to light was maximal at a lower concentration of KCl (10 mol m^-3) and that protoplasts responded more rapidly to light than stomata in isolated epidermis. The protoplasts did not respond to exogenous kinetin, but when incubated for 1 h in the light and in CO₂-free air on a solution containing 10^-3 mol m^-3 ABA, they decreased in volume by 30%. The advantages of using epidermis from Argenteum mutant for experiments on stomatal movements are discussed.Abbreviations ABA abscisic acid - MES 2-(N-morpholino)ethanesulfonic acid     

Contribution of C3 carboxylation to the circadian rhythm of carbon dioxide uptake in a Crassulacean acid metabolism plant Kalanchoë daigremontiana

During the endogenous circadian rhythm of carbon dioxide uptake in continuous light by a Crassula cean acid metabolism plant, Kalancho? daigremontiana, the two carboxylating enzymes, phosphoenolpyruvate carboxylase (PEPC) and ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco), are active simultaneously, although, until now, only the role of PEPC in generating the rhythm has been acknowledged. According to the established model, the rhythm is primarily regulated at the PEPC activity level, modulated by periodic compartmentation of its inhibitor, malate, in the vacuole and controlled by tension/relaxation of the tonoplast. However, the circadian accumulation of malic acid (the main indicator of PEPC activity) dampened significantly within the first few periods without affecting the rhythm's amplitude. Moreover, the amount of malate accumulated during a free-running oscillation was several-fold lower than the amount expected if PEPC were the key carboxylating enzyme, based on a 1:1 stoichiometry of CO(2) and malate. Together with the observation that rates of CO(2) uptake under continuous light were higher than in darkness, the evidence shows that C(3) carboxylation greatly contributes to the generation of rhythmic CO(2) uptake in continuous light in this 'obligate' CAM plant. Because the shift from predominantly CAM to predominantly C(3) carboxylation is smooth and does not distort the trajectory of the rhythm, its control probably arises from a robust network of oscillators, perhaps also involving stomata.     

Occurrence of Pyruvate Orthophosphate Dikinase in the Succulent Plant, Kalanchoë daigremontiana Hamet. et. Perr

Pyruvate orthophosphate dikinase was detected from Kalanchoë daigremontiana Hamet. et. Perr., a succulent plant with crassulacean acid metabolism. Enzyme activity was similar to that of maize extracts. Two enzymes demonstrating pyruvate orthophosphate dikinase activity from K. daigremontiana and Zea mays were found to be partially identical from enzyme-inhibition and immunoprecipitin tests with maize enzyme antiserum. A time course study demonstrated that pyruvate orthophosphate dikinase activity in leaf extracts was dependent upon exposure of leaves to light.     

Redundancy of stomatal control for the circadian photosynthetic rhythm in Kalanchoë daigremontiana Hamet et Perrier

In continuous light, the Crassulacean acid metabolism plant Kalanchoe daigremontiana Hamet et Perrier has a circadian rhythm of gas exchange with peaks occurring during the subjective night. The rhythm of gas exchange is coupled to a weak, reverse phased rhythm of quantum yield of photosystem II (Phi (PSII)). To test if the rhythm of Phi (PSII) persists in the absence of stomatal control, leaves were coated with a thin layer of translucent silicone grease which prevented CO2 and H2O exchange. In spite of this treatment, the rhythm of Phi (PSII) occurred with close to normal phase timing and with a much larger amplitude than in uncoated leaves. The mechanism underlying the Phi (PSII) rhythm in coated leaves can be explained by a circadian activity of phosphoenolpyruvate carboxylase (PEPC). At peaks of PEPC activity, the small amount of CO2 contained in the coated leaf could have become depleted, preventing the carboxylase activity of Rubisco and causing decreases in electron transport rates (observed as deep troughs of Phi (PSII) at 23-h in LL and at ca. 24-h intervals afterwards). Peaks of Phi (PSII) would be caused by a downregulation of PEPC leading to improved supply of CO2 to Rubisco. Substrate limitation of photochemistry at 23 h (trough of Phi (PSII)) was also suggested by the weak response of ETR in coated leaves to stepwise light enhancement. These results show that photosynthetic rhythmicity in K. daigremontiana is independent of stomatal regulation and may originate in the mesophyll.     

ATPase activity associated with isolated vacuoles of the crassulacean acid metabolism plant Kalanchoë daigremontiana

A technique is described that allows a relatively rapid and controlled isolation of vacuoles from leaves of the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana. The method involves polybase-induced lysis of mesophyllcell protoplasts and isolation of vacuoles on a discontinuous density gradient. ATPase activity is associated with the isolated vacuoles and is not attributable to contamination by cytoplasmic constituents. It is suggested that this ATPase is responsible for the energization of malic-acid accumulation in the vacuole in CAM plants.Abbreviation CAM crassulacean acid metabolism Dedicated to Professor Dr. W. Simonis on the occasion of his 75th birthday     

Reduction State of Q and Nonradiative Energy Dissipation during Photosynthesis in Leaves of a Crassulacean Acid Metabolism Plant, Kalanchoë daigremontiana Hamet et Perr

Fluorescence was measured in leaves of the CAM plant Kalanchoë daigremontiana using a pulse modulation technique at room temperature. During a 12-h light period at 500 micromole photons per square meter per second (400-700 nanometers) in air containing 350 microbar CO₂, the component of fluorescence quenching related to the reduction state of Q, the primary electron transport acceptor of PSII, remained fairly constant and showed that only 20% of Q were in the reduced form. The reduction state was slightly increased at the onset and at the end of the light period. By contrast, the nonphotochemical component of fluorescence quenching which is a measure of the fraction of nonradiative deexcitation underwent marked diurnal changes. Nonradiative energy conversion was low during the phase of most active malic acid decarboxylation in the middle of the light period when uptake of atmospheric CO₂ was negligible, and when internal CO₂ partial pressures were higher than in air; this allowed for high rates of CO₂ reduction in the chloroplasts. Nonradiative energy conversion was high during the early and the late light period when atmospheric CO₂ was taken up and internal CO₂ partial pressures were below air level. Manipulation of the internal CO₂ partial pressure during the late light period by increasing or decreasing the external CO₂ partial pressure to 1710 and 105 microbar, respectively, led to changes in the magnitude of energy dependent fluorescence quenching which were consistent with the relationship between nonradiative energy dissipation and internal CO₂ partial pressure observed during the diurnal cycle. Again, the reduction state of Q was hardly affected by these treatments. Thus, changes in electron transport rate during the diurnal CAM cycle at a given photon flux density lead primarily to alterations in the rate of nonradiative energy dissipation, with the reduction state of Q being maintained at a relatively low and constant level. Conditions are described under which nonphotochemical dissipation of excitation energy reaches a maximum value and the reduction state of Q is increased.     

Effect of Varying CO(2) Partial Pressure on Photosynthesis and on Carbon Isotope Composition of Carbon-4 of Malate from the Crassulacean Acid Metabolism Plant Kalanchoë daigremontiana Hamet et Perr

Intact leaves of Kalanchoë daigremontiana were exposed to CO₂ partial pressures of 100, 300, and 1000 microbars. Malic acid was extracted, purified, and degraded in order to obtain isotopic composition of carbon-1 and carbon-4. From these data, it is possible to calculate the carbon isotope composition of newly fixed carbon in malate. In all three treatments, the isotopic composition of newly introduced carbon is the same as that of the CO₂ source and is independent of CO₂ partial pressures over the range tested. Comparison with numerical models described previously (O'Leary 1981 Phytochemistry 20: 553-567) indicates that we would expect carbon 4 of malate to be 4‰ more negative than source CO₂ if diffusion is totally limiting or 7‰ more positive than source CO₂ if carboxylation is totally limiting. Our results demonstrate that stomatal aperture adjusts to changing CO₂ partial pressures and maintains the ratio of diffusion resistance to carboxylation resistance approximately constant. In this study, carboxylation and diffusion resistances balance so that essentially no fractionation occurs during malate synthesis. Gas exchange studies of the same leaves from which malate was extracted show that the extent of malate synthesis over the whole night is nearly independent of CO₂ partial pressure, although there are small variations in CO₂ uptake rate. Both the gas exchange and the isotope studies indicate that the ratio of external to internal CO₂ partial pressure is the same in all three treatments. Inasmuch as a constant ratio will result in constant isotope fractionation, this observation may explain why plants in general have fairly invariable ¹³C contents, despite growing under a variety of environmental conditions.     

Water relation parameters of the CAM plant Kalanchoë daigremontiana in relation to diurnal malate oscillations

Summary In the CAM plant Kalanchoë daigremontiana, kept in an environmental rhythm of 12 h L: 12 h D in a growth chamber at 60% relative humidity and well watered in the root medium, decreasing water potentials and osmotic potentials of the leaves are correlated with malate accumulation in the dark. In the light increasing water and osmotic potentials ( _W and _S ) are associated with decreasing malate levels. Transpiratory H₂O loss is high in dark and low in light.In continuous light, the CAM rhythm rapidly disappears in the form of a highly damped endogenous oscillation. Malate levels, and water and osmotic potentials of the leaves remain correlated as described above. However, transpiration is very high as malate levels decrease and water and osmotic potentials increase.It can concluded, that water relation parameters like total water potential ( _W ) and osmotic potential ( _S ) change in close correlation with changes of malic acid levels. As an important osmotically active solute in CAM plants, malic acid appears to affect water relations independently of and in addition to transpiration. The question remains open, whether turgor ( _P ) is involved in CAM regulation in intact plants in a similar way as it determines malate fluxes in leaf slices.Abbreviations CAM Crassulacean Acid Metabolism - L Light - D Dark     

Identification and characterization of a rhizosphere β-galactosidase from Pisum sativum L.

Plant enzyme activities in the rhizosphere potentially are a resource for improved plant nutrition, soil fertility, bioremediation, and disease resistance. Here we report that a border cell specific β-galactosidase is secreted into the acidic extracellular environment surrounding root tips of pea, as well as bean, alfalfa, barrel medic, sorghum, and maize. No enzyme activity was detected in radish and Arabidopsis, species that do not produce viable border cells. The secreted enzyme activity was inhibited by galactose and 2-phenylethyl 1-thio-β-d-galactopyranoside (PETG) at concentrations that altered root growth without causing cell death. A tomato galactanase encoding gene was used as a probe to isolate a full length pea cDNA clone (BRDgal1) from a root cap-border cell cDNA library. Southern blot analysis using full length BRDgal1 as a probe revealed 1–2 related sequences within the pea genome. BRDgal1 mRNA expression was analysed by whole mount in situ hybridization (WISH) and found to occur in the outermost peripheral layer of the cap and in suspensions of detached border cells. No expression was detected within the body of the root cap. Repeated efforts to develop viable hairy root clones expressing BRDgal1 antisense mRNA under the control of the CaMV35S promoter, whose expression in the root cap is limited to cells at the root cap periphery only during root emergence, were unsuccessful. These data suggest that altered expression of this enzyme is deleterious to early root development. The first two authors contributed equally to the completion of this project.     

Characteristics,partial purification and reconstitution of the vacuolar malate transporter of the CAM plant Kalanchoë daigremontiana Hamet et Perrier de la Bâthie

When native tonoplast vesicles of Kalanchoë daigremontiana Hamet et Perrier de la Bâthie were energized by an artificial K⁺ gradient establishing only an inside-positive electrical membrane potential (), it was shown that was sufficient as the sole driving force and that a proton gradient (pH) is not required for malate uptake. Following [¹⁴C]malate uptake, K _m-malate of the malate transporter was estimated as 2.7–3.0 mM, a value that would allow malate synthesis via phosphoenolpyruvate carboxylase and malate accumulation in vivo in view of the feed-back inhibition of cytosolic phosphoenolpyruvate carboxylase by malate. The maximum reaction velocity (V _max) was found to be between 30 and 85 nmol malate·min^–1·mg _protein ^–1 , a value that would explain nocturnal malate accumulation in K. daigremontiana even if the transporter were operating below substrate saturation. Citrate (50 mM at pH 7) inhibited transport by 78%. The malate-transport protein of the tonoplast of K. daigremontiana may be a carboxylate uniporter with strong affinities for malate and citrate. From total tonoplast proteins solubilized from native tonoplast vesicles the malate transporter was functionally reconstituted into phospholipid liposomes. The malate transporter was purified and separated from the tonoplast H⁺-ATPase by hydroxyapatite chromatography, but not from the tonoplast H⁺-pyrophosphatase. The partially purified malate-transport protein was functionally reconstituted into phospholipid liposomes. In these final proteoliposomes, 0.6% of the protein of the initial tonoplast-vesicle preparation used for solubilization of membrane proteins was recovered. Using the specific rates of malate transport as a reference, i.e. rates of transport related to protein in the preparations, enrichment of the malate transporter in the final proteoliposomes obtained with the reconstitution of the hydroxyapatite eluate was 44-fold compared to the initial native tonoplast vesicles and 2000-fold compared to the liposomes reconstituted from solubilized tonoplast proteins. Sodium dodecyl sulfate polyacrylamide gel electrophoresis of the peptides from the final proteoliposomes, which were functional in malate transport, showed only a few polypeptide bands among which the malate transporter must be found.Abbreviations and Symbols CAM Crassulacean acid metabolism - DIDS 4,4-diisothiocyanatostilbene-2,2-disulfonic acid - Triton X-100 polyoxyethylene(9,10)p-t-octylphenol - pH proton gradient at the tonoplast - membrane potential at the tonoplast This work was supported by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie and is now funded in SFB 199 (Teilprojekt B2) of the Deutsche Forschungsgemeinschaft. We thank Dr. Elke Fischer-Schliebs for valuable discussions and Dr. E. Martinoia for making us acquainted with his experimental approaches in his laboratory in Zürich, Switzerland, and for much valuable exchange. Dr. D.P.S. Verma, Ohio, USA, kindly provided Nod-26 antibodies, and the tonoplast H⁺-pyrophosphatase antibodies were a generous gift of Dr. M. Maeshima, Sapporo, Japan.     

A comparative study on the regulation of C3 and C4 carboxylation processes in the constitutive crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana and the C3-CAM intermediate Clusia minor

A comparison of carbon metabolism in the constitutive crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. and the C₃-CAM intermediate Clusia minor L. was undertaken under controlled environmental conditions where plants experience gradual changes in light intensity, temperature and humidity at the start and end of the photoperiod. The magnitude of CAM activity was manipulated by maintaining plants in ambient air or by enclosing leaves overnight in an atmosphere of N₂ to suppress C₄ carboxylation. Measurements of diel changes in carbonisotope discrimination and organic acid content were used to quantify the activities of C₃ and C₄ carboxylases in vivo and to indicate the extent to which the activities of phosphoenolpyruvate carboxylase (PEPCase), ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and decarboxylation processes overlap at the start and end of the photoperiod. These measurements in vivo were compared with measurements in vitro of changes in the diel sensitivity of PEPCase to malate inhibition. The results demonstrate fundamental differences in the down-regulation of PEPCase during the day in the two species. While PEPCase is inactivated within the first 30 min of the photoperiod in K. daigremontiana, the enzyme is active for 4 h at the start and 3 h at the end of the photoperiod in C. minor. Enclosing leaves in N₂ overnight resulted in a two-to threefold increase in PEPCase-mediated CO₂ uptake during Phase II of CAM in both species. However, futile cycling of CO₂ between malate synthesis and decarboxylation does not occur during Phase II in either species. In terms of overall carbon balance, C₄ carboxylation accounted for ≈ 20% of net daytime assimilation in both species under control conditions, increasing to 30–34% after a night in N₂. Although N₂-treated leaves of K. daigremontiana took up 25% more CO₂ than control leaves during the day this was insufficient to compensate for the loss of CO₂ taken up by CAM the previous night. In contrast, in N₂-treated leaves of C. minor, the twofold increase in daytime PEPCase activity and the increase in net CO₂ uptake by Rubisco during Phase III compensated for the inhibition of C₄ carboxylation at night in terms of diel carbon balance.     

Cellular localization and cytochemical probing of resistance reactions to arbuscular mycorrhizal fungi in a ‘locus a’ myc− mutant of Pisum sativum L.

Pisum sativum L. myc^– mutants which fail to form arbuscular mycorrhiza have recently been identified amongst nod^– mutants (Duc et al., 1989, Plant Sci. 60, 215–222). The reason for this resistance to symbiotic fungi has been investigated in the case of a locus a mutant (P2) inoculated with Glomus mosseae (Nicol. and Gerd.) Gerd, and Trappe. The fungal symbiont formed viable appressoria in contact with the root surface but its development was stopped at the root epidermis. Abundant material was deposited on the inner face of root cell walls adjacent to the appressoria in the P2 mutant, but not in the wild-genotype parent cultivar (Frisson) forming a symbiotic mycorrhizal infection. Fluorescence, histochemical, cytochemical and immunocytological approaches were used to characterize the paramural deposits in epidermal and hypodermal cells of the mutant. Strong fluorescence under blue light indicated the accumulation of phenolic compounds although polymers like lignin or suberin were not localized. Proteins and glycoproteins were homogeneously distributed within the paramural deposits. In the latter, the periodic acid-thiocarbohydrazide-silver proteinate (PATAg) reaction for 1,4-polysaccharide detection showed a heterogeneous composition with electron-dense points surrounded by non-reactive material, but cytological tests for cellulose and pectin gave weak responses as compared to epidermal and hypodermal walls of the wild genotype. -1,3-Glucans indicative of callose were detected by in-situ immunolocalization in the paramural deposits below appressoria on mutant roots, but not in walls of the wild genotype. Thus, appressorium formation by G. mosseae on roots of the locus a P. sativum mutant elicits wall modifications usually associated with activation of defence responses to pathogens. It is proposed that this locus must be involved in a key event in symbiotic infection processes in P. sativum, and the possible role of complex regulatory interactions between symbiosis and defence genes in endomycorrhiza development is discussed.Abbreviations DAPI 4,6-diamino-2-phenylindole - FDA fluo-rescein diacetate - PATAg periodic acid-thiocarbohydrazide-silver proteinate The authors are grateful to C. Arnould for technical assistance, K. Niehaus for the purified Sirofluor, K. Roberts for the AFRC JIM5 antibody and J. Lherminier (INRA, Dijon, France), for useful discussion. This collaborative research programme was financially supported by MRT, INRA, EPR-Bourgogne (grant to A.G., Contrat de Plan project 3060A), EEC COST ACTION 8.10 (Endomycorrhizas) and the National Research Council of Italy, Special Project RAISA, Sub-project N.2, Paper N. 801     

Partial purification and characterization of the soluble DNA polymerase (polymerase-α) from seedlings of Pisum sativum L.

Radish seedlings (Raphanus sativus L. Saxa Treib) were grown in the dark with or without added kinetin (2 mg/l=9.29 M). Low-temperature (77°K) fluorescence emission and absorption spectra of etiolated cotyledons were registered at increasing seedling age before and immediately, 30 s and 30 min after one 1-ms flash. Kinetin was found to induce a higher accumulation of the phototransformable protochlorophyll(ide) P_657–650 in the etiolated cotyledons, especially from day 6 to day 10 after germination. The amount of the P_657–650 protochlorophyll(ide) resynthesized during a 30-min dark period after a 1-ms flash decreased with seedling age. It was smaller in cotyledons from kinetin-treated seedlings at day 6 after germination and at that age only. The ability to perform the Shibata shift decreased with increasing seedling age. In cotyledons from 10- and 13-day-old seedlings, the shift was accomplished to a greater extent when the plants were grown in the presence of kinetin.     

δ13C values of an herbivore and the ratio of C3 to C4 plant carbon in its diet

Summary The ¹³C values of whole body samples of the beetle Tribolium castaneum are closely correlated with the ¹³C values of the plant carbon in its diet. The correlation is always high for diets ranging from 100% C₄ to 100% C₃ plant material. The degree of correlation is independent of the growth rate of the animals.     

Oxidations of various substrates and effects of the inhibitors on purified mitochondria isolated from <Emphasis Type="Italic">Kalanchoë pinnata</Emphasis>

Kalanchoë pinnata mitochondria readily oxidized succinate, malate, NADH, and NADPH at high rates and coupling. The highest respiration rates usually were observed in the presence of succinate. The high rate of malate oxidation was observed at pH 6.8 with thiamine pyrophosphate where both malic enzyme (ME) and pyruvate dehydrogenase were activated. In CAM phase III of K. pinnata mitochondria, both ME and malate dehydrogenase (MDH) simultaneously contributed to metabolism of malate. However, ME played a main function: malate was oxidized via ME to produce pyruvate and CO₂ rather than via MDH to produce oxalacetate (OAA). Cooperative oxidation of two or three substrates was accompanied with the dramatic increase in the total respiration rates. Our results showed that the alternative (Alt) pathway was more active in malate oxidation at pH 6.8 with CoA and NAD⁺ where ME operated and was stimulated, indicating that both ME and Alt pathway were related to malate decarboxylation during the light. In K. pinnata mitochondria, NADH and NADPH oxidations were more sensitive with KCN than that with succinate and malate oxidations, suggesting that these oxidations were engaged to cytochrome pathway rather than to Alt pathway and these capacities would be desirable to supply enough energy for cytosol pyruvate orthophosphate dikinase activity.     

Use of human reconstructed epidermis to analyze the regulation of β-defensin hBD-1, hBD-2, and hBD-3 expression in response to LPS

Defensins have been identified as key elements of innate immunity against microbial infections. In the present study, human beta-defensin-2 (hBD-2) mRNA and peptide expression were evaluated by RT-PCR and Western blotting in normal human keratinocytes, in function of their stage of differentiation. In proliferating, non-differentiating keratinocytes generated in serum-free, low-calcium medium, a very low hBD-2 mRNA expression was found. A significantly higher expression was detected in high-calcium cultivated keratinocytes grown either as monolayers or as multilayers under submerged conditions. In an air-liquid interface culture of keratinocytes, allowing epidermis to be reconstructed, hBD-2 mRNA expression level was significantly higher than in the other conditions and displayed inter-individual variability as observed in native epidermis. The peptide was detected only in reconstructed epidermis. These results indicate that hBD-2 gene expression in normal human keratinocytes is dependent upon their stage of differentiation. The level of expression of hBD-1 mRNA was lower and that of hBD-3 was higher than that of hBD-2 in reconstructed epidermis. Exposure of reconstructed epidermis to bacterial lipopolysaccharide (LPS) resulted in an average 4-fold increase in hBD-2 mRNA 18 h after challenge, but not of hBD-1 and hBD-3 gene expression. These results show the selective regulation of hBD-2-encoding gene in an organotypic epidermal model, in response to LPS. They also provide evidence that in vitro reconstructed epidermis represents a useful model for studying regulation of expression of beta-defensins after skin challenge with pathogenic microorganisms in conditions as close as possible to the in vivo situation.     

Identification of a Methioninase Inhibitor,Myrsinoic Acid B,from Myrsine seguinii Lév., and Its Inhibitory Activities

A methioninase inhibitor from Myrsine seguinii was purified and identified as myrsinoic acid B. Its inhibitory activities as to crude methioninase from periodontal bacteria such as Fusobacterium nucleatum, Porphyromonas gingivalis, and Treponema denticola were determined. The IC₅₀ values were 10.5, 82.4, and 30.3 μM respectively.     

Effluxes of nitrous oxide,methane and carbon dioxide and their responses to increasing nitrogen deposition in the Gurbantünggüt Desert of Xinjiang,China北大核心CSCD

Aims Desert soils play an important role in the exchange of major greenhouse gas (GHG) between atmosphere and soil. However, many uncertainties existed in understanding of desert soil role, especially in efflux evaluation under a changing environment. Methods We conducted plot-based field study in center of the Gurbantünggüt Desert, Xinjiang, and applied six rates of simulated nitrogen (N) deposition on the plots, i.e. 0 (N0), 0.5 (N0.5), 1.0 (N1), 3.0 (N3), 6.0 (N6) and 24.0 (N24) g·m-2·a-1. The exchange rates of N2O, CH4 and CO2 during two growing seasons were measured for two years after N applications. Important findings The average efflux of two growing seasons from control plots (N0) were 4.8 μg·m-2·h-1, -30.5 μg·m-2·h-1 and 46.7 mg·m-2·h-1 for N2O, CH4 and CO2, respectively. The effluxes varied significantly among seasons. N0, N0.5 and N1 showed similar exchange of N2O in spring and summer, which was relatively higher than in autumn, while the rates of N2O in N6 and N24 were controled by time points of N applications. The uptake of CH4 was relatively higher in both spring and summer, and lower in autumn. Emission of CO2 changed minor from spring to summer, and greatly decreased in autumn in the first measured year. In the second year, the emission patterns were changed by rates of N added. N additions generally stimulated the emission of N2O, while the effects varied in different seasons and years. In addition, no obvious trends were found in the emission factor of N2O. The uptake of CH4 was not significantly affected by N additions. N additions did not change CO2 emissions in the first year, while high N significantly reduced the CO2 emissions in spring and summer of the second year, without affected in autumn. Structure equation model analysis on the factors suggested that N2O, CH4 and CO2 were dominantly affected by the N application rates, soil temperature or moisture and plant density, respectively. Over the growing seasons, both the net efflux and the global warming potential caused by N additions were small.