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1.
A common observation in plants grown in elevated CO2 concentration is that the rate of photosynthesis is lower than expected from the dependence of photosynthesis upon CO2 concentration in single leaves of plants grown at present CO2 concentration. Furthermore, it has been suggested that this apparent down regulation of photosynthesis may be larger in leaves of plants at low nitrogen supply than at higher nitrogen supply. However, the available data are rather limited and contradictory. In this paper, particular attention is drawn to the way in which whole plant growth response to N supply constitutes a variable sink strength for carbohydrate usage and how this may affect photosynthesis. The need for further studies of the acclimation of photosynthesis at elevated CO2 in leaves of plants whose N supply has resulted in well-defined growth rate and sink activity is emphasised, and brief consideration is made of how this might be achieved.Abbreviations A rate of CO2 assimilation - Ci internal CO2 concentration - PCR photosynthetic carbon reduction - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate  相似文献   

2.
Summary The growth and photosynethetic responses to atmospheric CO2 enrichment of 4 species of C4 grasses grown at two levels of irradiance were studied. We sought to determine whether CO2 enrichment would yield proportionally greater growth enhancement in the C4 grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1-1 CO2 and 1,000 or 150 mol m-2 s-1 photosynthetic photon flux density (PPFD). An increase in CO2 concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO2. Plants grown in CO2-enriched atmosphere had lower photosynthetic capacity relative to the low CO2 grown plants when exposed to lower CO2 concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO2 compensation point for photosynthesis.  相似文献   

3.
Biochemical models of leaf photosynthesis, which are essential for understanding the impact of photosynthesis to changing environments, depend on accurate parameterizations. One such parameter, the photorespiratory CO2 compensation point can be measured from the intersection of several CO2 response curves measured under sub‐saturating illumination. However, determining the actual intersection while accounting for experimental noise can be challenging. Additionally, leaf photosynthesis model outcomes are sensitive to the diffusion paths of CO2 released from the mitochondria. This diffusion path of CO2 includes both chloroplastic as well as cell wall resistances to CO2, which are not readily measurable. Both the difficulties of determining the photorespiratory CO2 compensation point and the impact of multiple intercellular resistances to CO2 can be addressed through application of slope–intercept regression. This technical report summarizes an improved framework for implementing slope–intercept regression to evaluate measurements of the photorespiratory CO2 compensation point. This approach extends past work to include the cases of both Rubisco and Ribulose‐1,5‐bisphosphate (RuBP)‐limited photosynthesis. This report further presents two interactive graphical applications and a spreadsheet‐based tool to allow users to apply slope–intercept theory to their data.  相似文献   

4.
The C4 pathway: an efficient CO2 pump   总被引:2,自引:0,他引:2  
The C4 pathway is a complex combination of both biochemical and morphological specialisation, which provides an elevation of the CO2 concentration at the site of Rubisco. We review the key parameters necessary to make the C4 pathway function efficiently, focussing on the diffusion of CO2 out of the bundle sheath compartment. Measurements of cell wall thickness show that the thickness of bundle sheath cell walls in C4 species is similar to cell wall thickness of C3 mesophyll cells. Furthermore, NAD-ME type C4 species, which do not have suberin in their bundle sheath cell walls, do not appear to compensate for this with thicker bundle sheath cell walls. Uncertainties in the CO2 diffusion properties of membranes, such as the plasmalemma, choroplast and mitochondrial membranes make it difficult to estimate bundle sheath diffusion resistance from anatomical measurements, but the cytosol itself may account for more than half of the final calculated resistance value for CO2 leakage. We conclude that the location of the site of decarboxylation, its distance from the mesophyll interface and the physical arrangement of chloroplasts and mitochondria in the bundle sheath cell are as important to the efficiency of the process as the properties of the bundle sheath cell wall. Using a mathemathical model of C4 photosynthesis, we also examine the relationship between bundle sheath resistance to CO2 diffusion and the biochemical capacity of the C4 photosynthetic pathway and conclude that bundle sheath resistance to CO2 diffusion must vary with biochemical capacity if the efficiency of the C4 pump is to be maintained. Finally, we construct a mathematical model of single cell C4 photosynthesis in a C3 mesophyll cell and examine the theoretical efficiency of such a C4 photosynthetic CO2 pump. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

5.
The effects on photosynthesis of CO2 and desiccation in Porphyra haitanensis were investigated to establish the effects of increased atmospheric CO2 on this alga during emersion at low tides. With enhanced desiccation, net photosynthesis, dark respiration, photosynthetic efficiency, apparent carboxylating efficiency and light saturation point decreased, while the light compensation point and CO2 compensation point increased. Emersed net photosynthesis was not saturated by the present atmospheric CO2 level (about 350?ml?m?3), and doubling the CO2 concentration (700?ml?m?3) increased photosynthesis by between 31% and 89% at moderate levels of desiccation. The relative enhancement of emersed net photosynthesis at 700?ml?m?3 CO2 was greater at higher temperatures and higher levels of desiccation. The photosynthetic production of Porphyra haitanensis may benefit from increasing atmospheric CO2 concentration during emersion.  相似文献   

6.
揭示作物光合作用、蒸腾作用和水分利用效率(WUE)对大气CO2浓度变化的响应, 对预测未来大气CO2浓度升高条件下作物生产力与需水规律的变化具有重要意义。在自然CO2浓度、CO2倍增和倍增后恢复到自然CO2浓度3种情况下, 对大豆(Glycine max)、甘薯(Ipomoea batatas)、花生(Arachis hypogaea)、水稻(Oryza sativa)、棉花(Gossypium hirsutum)、玉米(Zea mays)、高粱(Sorghum vulgare)和谷子(Setaria italica) 8种作物的气体交换参数进行了研究。结果表明: CO2浓度倍增可以提高光合速率, 降低蒸腾速率, 从而提高WUE, 其中光合速率提高的贡献更大; C3比C4作物的光合速率、WUE增幅大, C3作物光合速率提高对WUE的贡献大于C4作物; 通过对比倍增后恢复到自然CO2浓度时气体交换参数随环境条件变化的响应确定了其内在调控机制; 倍增后恢复到自然CO2浓度时作物光合速率低于自然CO2浓度下的光合速率, 而蒸腾速率无明显差异。由此判断: CO2浓度倍增下存在光合下调现象, 这可能是由于Rubisco酶蛋白含量、活化水平和比活性降低等“非气孔因素”造成的, 并非由气孔导度的降低引起的。  相似文献   

7.
Kellomäki  Seppo  Wang  Kai-Yun 《Plant Ecology》1998,136(2):229-248
Starting in early spring of 1994, naturally regenerated, 30-year-old Scots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O3,doubled ambient CO2 and a combination of O3 and CO2 from 15 April to 15 September. To investigate daily and seasonal responses of CO2 exchange to elevated O3 and CO2, the CO2 exchange of shoots was measured continuously by an automatic system for measuring gas exchange during the course of one year (from 1 Januray to 31 December 1996). A process-based model of shoot photosynthesis was constructed to quantify modifications in the intrinsic capacity of photosynthesis and stomatal conductance by simulating the daily CO2 exchange data from the field. Results showed that on most days of the year the model simulated well the daily course of shoot photosynthesis. Elevated O3 significantly decreased photosynthetic capacity and stomatal conductance during the whole photosynthetic period. Elevated O3 also led to a delay in onset of photosynthetic recovery in early spring and an increase in the sensitivity of photosynthesis to environmental stress conditions. The combination of elevated O3 and CO2 had an effect on photosynthesis and stomatal conductance similar to that of elevated O3 alone, but significantly reduced the O3-induced depression of photosynthesis. Elevated CO2 significantly increased the photosynthetic capacity of Scots pine during the main growing season but slightly decreased it in early spring and late autumn. The model calculation showed that, compared to the control treatment, elevated O3 alone and the combination of elevated O3 and CO2 decreased the annual total of net photosynthesis per unit leaf area by 55% and 38%, respectively. Elevated CO2 increased the annual total of net photosynthesis by 13%.  相似文献   

8.
The relationship between CO2 exchange and relative electron-transport rate through photosystem II (ETR, measured using chlorophyll a fluorescence) was determined for a moss and a green algal lichen, photobiont probably Trebouxia sp., in the field in Antarctica. Net photosynthesis (NP) and dark respiration (DR) were measured over temperatures from zero to 25 °C and gross photosynthesis (GP) calculated (GP = NP + DR). The strong response of DR to temperature in these organisms resulted in substantial changes in CO2 exchange rates. The moss Bryum argenteum Hedw. showed a strong, linear relationship between GP and ETR. This was an unexpected result since mosses are C3 plants and, in higher plants, this group normally has a curvilinear GP versus ETR relationship. It is suggested that suppression of DR in the light might be involved. The lichen, Umbilicaria aprina Nyl., had nonlinear relationships between ETR and GP that were different at each measurement temperature. In some cases the lowest ETR was at the higher CO2 exchange rates. It is suggested that these relationships are the result of strong quenching mechanisms that are inversely proportional to GP. The results support a growing impression that the relationships between ETR and CO2 exchange are complex in these organisms and different from those found for higher plants. Received: 24 November 1997 / Accepted 2 May 1998  相似文献   

9.
Open-top chambers were used to study the effects of CO2 enrichment on leaf-level photosynthetic rates of the C4 grass Andropogon gerardii in the native tallgrass prairie ecosystem near Manhattan, Kansas. Measurements were made during a year with abundant rainfall (1993) and a year with below-normal rainfall (1994). Treatments included: No chamber, ambient CO2 (A); chamber with ambient CO2 (CA); and chamber with twice-ambient CO2 (CE). Measurements of photosynthesis were made at 2-hour intervals, or at midday, on cloudless days throughout the growing season using an open-flow gas-exchange system. No significant differences in midday rates of photosynthesis or in daily carbon accumulation as a result of CO2 enrichment were found in the year with abundant precipitation. In the dry year, midday rates of photosynthesis were significantly higher in the CE treatment than in the CA or A treatments throughout the season. Estimates of daily carbon accumulation also indicated that CO2 enrichment allowed plants to maximize carbon acquisition on a diurnal basis. The increased carbon accumulation was accounted for by greater rates of photosynthesis in the CE plots during midday. During the wet year, CO2 enrichment decreased stomatal conductance, which allowed plants to decrease transpiration while still photosynthesizing at rates similar to plants in ambient conditions. During the dry year, CO2 enrichment allowed plants to maintain photosynthetic rates even though stomatal conductance and transpiration had been reduced in all treatments due to stress. Estimates of instantaneous water-use efficiency were reduced under CO2 enrichment for both years. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

10.
Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO2 that accumulates within their leaf cavities. Leaf cavity CO2 concentrations ranged from 2250 L L–1 near the leaf base to below atmospheric (<350 L L–1) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO2 concentrations with minimum values near midday and maximum values at night. Conductance to CO2 from the leaf cavity ranged from 24 to 202 mol m–2 s–1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO2 was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO2 concentrations and conductance values or as measured directly by 14CO2 labeling experiments. The photosynthetic responses to CO2 and O2 were measured to determine whether onion leaves exhibited a typical C3-type response. A linear increase in CO2 uptake was observed in intact leaves up to 315 L L–1 of external CO2 and, at this external CO2 concentration, uptake was inhibited 35.4±0.9% by 210 mL L–1 O2 compared to 20 mL L–1 O2. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO2 and greatly restrict the refixation of leaf cavity CO2 by photosynthetic tissue.Abbreviations Ca external CO2 concentration - Ci intercellular CO2 concentration - CO2 compensation concentration - PPFR photosynthetic photon fluence rate  相似文献   

11.
Biochemistry of C3-photosynthesis in high CO2   总被引:3,自引:0,他引:3  
The short-term responses of C3 photosynthesis to high CO2 are described first. Regulation of photosynthesis in the short term is determined by interaction among the capacities of light harvesting, electron transport, ribulose-1, 5-bisphosphate carboxylase (Rubisco) and orthophosphate (Pi) regeneration during starch and sucrose synthesis. Photosynthesis under high CO2 conditions is limited by either electron transport or Pi regeneration capacities, and Rubisco is deactivated to maintain a balance between each step in the photosynthetic pathway. Subsequently, the long-term effects on, photosynthesis are discussed. Long-term CO2 enhancement leads to carbohydrate accumulation. Accumulation of carbohydrates is not associated with a Pi-regeneration limitation on photosynthesis, and this limitation is apparently removed during long-term exposure to high CO2. Enhanced CO2 does not affect Rubisco content and electron transport capacity for a given leaf-nitrogen content. In addition, the deactivated Rubisco immediately after exposure to high CO2 does not recover during the subsequent prolonged exposure. Such evidence may indicate that plants do not necessarily have an ideal acclimation response to high CO2 at the biochemical level.  相似文献   

12.
A computer model comprising light reactions in PS II and PS I, electron-proton transport reactions in mesophyll and bundle sheath chloroplasts, all enzymatic reactions and most of the known regulatory functions of NADP-ME type C4 photosynthesis has been developed as a system of differential budget equations for intermediate compounds. Rate-equations were designed on principles of multisubstrate-multiproduct enzyme kinetics. Some of the 275 constants needed (ΔG0′ and K m values) were available from literature and others (V m) were estimated from reported rates and pool sizes. The model provided good simulations for rates of photosynthesis and pool sizes of intermediates under varying light, CO2 and O2. A basic novelty of the model is coupling of NADPH production via NADP-ME with ATP production and regulation of the C3 cycle in bundle sheath chloroplasts. The functional range of the ATP/NADPH ratio in bundle sheath chloroplasts extends from 1.5 to 2.1, being energetically most efficient around 2. In the presence of such stoichiometry, the CO2 concentrating function can be explained on the basis of two processes: (a) extra ATP consumption for starch and protein synthesis in bundle sheath leads to a faster NADPH and CO2 import compared with CO2 fixation in bundle sheath, and (b) the residual photorespiratory activity consumes RuBP by oxygenation, NADPH and ATP and causes the imported CO2 to accumulate in bundle sheath cells. As a wider application, the model may be used for predicting results of genetic engineering of plants. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

13.
H. Fock  K. Klug  D. T. Canvin 《Planta》1979,145(3):219-223
Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 l/l external CO2. The ratio of PR/TPS and the solubility ratio of O2/CO2 in the intercellular spaces both decreased with increasing CO2. The rate of PR was not affected by the CO2 concentration in the leaves and was independent of the solubility ratio of oxygen and CO2 in the leaf cell. At photosynthesis-limiting concentrations of CO2, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO2 dm-2 h-1 at 18°C to 8.6 mg CO2 dm-2 h-1 at 30°C. The specific activity of photorespired CO2 was CO2-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O2 and CO2 around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.Abbreviations APS apparent photosynthesis (net CO2 uptake) - PR photorespiration (CO2 evolution in light) - RuBP ribulose-1,5-bisphosphate - TPS true photosynthesis (true CO2 uptake)  相似文献   

14.
Gas exchanges of wheat (Triticum aestivum L. cv. Courtot) shoots were measured before and during a water stress. While photosynthesis, transpiration and dark respiration decreased because of the stress, photorespiration increased initially, up to a maximum of 50% above its initial value. The CO2 concentration in the intercellular space was calculated from gas-diffusion resistances, and remained approximately constant before and during the stress. On the other hand, the CO2 concentration in the chloroplast, in the vicinity of Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco), was evaluated from the ratio of CO2 to O2 uptake, using the known kinetic constants of the oxygenation and carboxylation reactions which compete for Rubisco. In the well-watered plants, the calculated chloroplastic concentration was slightly smaller than the substomatal concentration. During water stress, this concentration decreased while the substomatal CO2 concentration remained constant. Hypotheses to explain this difference between substomatal and chloroplastic CO2 concentrations are discussed.  相似文献   

15.
The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a standard time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO2 uptake. Midday depression of CO2 uptake is the result of i) a decrease in CO2-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.Abbreviations and symbols CE carboxylation efficiency - NP net photosynthesis rate - PAR photosynthetically active radiation - Pi leaf internal CO2 partial pressure - W water vapor mole fraction difference between leaf and air - T CO2 compensation pressure Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday  相似文献   

16.
17.
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.  相似文献   

18.
When cells of Chlorococcum littorale that had been grown in air (air-grown cells) were transferred to extremely high CO2 concentrations (>20%), active photosynthesis resumed after a lag period which lasted for 1–4 days. In contrast, C. littorale cells which had been grown in 5% CO2 (5% CO2-grown cells) could grow in 40% CO2 without any lag period. When air-grown cells were transferred to 40% CO2, the quantum efficiency of PS II (ΦII) decreased greatly, while no decrease in ΦII was apparent when the 5% CO2-grown cells were transferred to 40% CO2. In contrast to air-grown cells, 5% CO2-grown cells showed neither extracellular nor intracellular carbonic anhydrase (CA) activity. Upon the acclimation of 5% CO2-grown cells to air, photosynthetic susceptibility to 40% CO2 was induced. This change was associated with the induction of CA. In addition, neither suppression of photosynthesis nor arrest of growth was apparent when ethoxyzolamide (EZA), a membrane-permeable inhibitor of CA, had been added before transferring air-grown cells of C. littorale to 40% CO2. The intracellular pH value (pHi) decreased from 7.0 to 6.4 when air-grown C. littorale cells were exposed to 40% CO2 for 1–2 h, but no such decrease in pHi was apparent in the presence of EZA. Both air- and 5% CO2-grown cells of Chlorella sp. UK001, which was also resistant to extremely high CO2 concentrations, grew in 40% CO2 without any lag period. The activity of CA was much lower in air-grown cells of this alga than those in air-grown C. littorale cells. These results prompt us to conclude that intracellular CA caused intracellular acidification and hence inhibition of photosynthetic carbon fixation when air-grown C. littorale cells were exposed to excess concentrations of CO2. No such harmful effect of intracellular CA was observed in Chlorella sp. UK001 cells. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

19.
Current models of C3 photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO2 and O2, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO2 and O2, i.e., increasing O2 stimulates assimilation and increasing CO2 inhibits assimilation. We propose a model which explains reversed sensitivity to CO2 and O2 by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO2, O2, and light is demonstrated.Abbreviations PFD photon flux density - PGA 3-phosphoglycerate - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - TPU triose phosphate utilization  相似文献   

20.
Photosynthesis controls of CO2 efflux from maize rhizosphere   总被引:4,自引:0,他引:4  
The effects of different shading periods of maize plants on rhizosphere respiration and soil organic matter decomposition were investigated by using a 13C natural abundance and 14C pulse labeling simultaneously. 13C was a tracer for total C assimilated by maize during the whole growth period, and 14C was a tracer for recently assimilated C. CO2 efflux from bare soil was 4 times less than the total CO2 efflux from planted soil under normal lighting. Comparing to the normal lighting control (12/12 h day/night), eight days with reduced photosynthesis (12/36 h day/night period) and strongly reduced photosynthesis (12/84 h day/night period) resulted in 39% and 68% decrease of the total CO2 efflux from soil, respectively. The analysis of 13C natural abundance showed that root-derived CO2 efflux accounted for 82%, 68% and 56% of total CO2 efflux from the planted soil with normal, prolonged and strongly prolonged night periods, respectively. Clear diurnal dynamics of the total CO2 efflux from soil with normal day-night period as well as its strong reduction by prolonged night period indicated tight coupling with plant photosynthetic activity. The light-on events after prolonged dark periods led to increases of root-derived and therefore of total CO2 efflux from soil. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO2 efflux, and thereby, total CO2 efflux from soils. 14C labeling of plants before the first light treatment did not show any significant differences in the 14CO2 respired in the rhizosphere between different dark periods because the assimilate level in the plants was high. Second labeling, conducted after prolonged night phases, showed higher contribution of recently assimilated C (14C) to the root-derived CO2 efflux by shaded plants. Results from 13C natural abundance showed that the cultivation of maize on Chromic Luvisol decreased soil organic matter (SOM) mineralization compared to unplanted soil (negative priming effect). A more important finding is the observed tight coupling of the negative rhizosphere effect on SOM decomposition with photosynthesis.  相似文献   

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