Why abaxial illumination limits photosynthetic carbon fixation in spinach leaves

Limitations of carbon fixation within spinach leaves due to light and CO2 were investigated. Under equivalent photon fluxes, carbon fixation was higher when leaves were irradiated adaxially compared to abaxially. Maximal carbon fixation occurred in the middle of the palisade mesophyll under adaxial illumination, whereas, maximal carbon fixation occurred in the spongy mesophyll under abaxial illumination. Total carbon fixation and the pattern of carbon fixation across leaves were similar, when leaves were irradiated with 800 micromol quanta m(-2) s(-1) either adaxially alone or adaxially plus abaxially (1,600 micromol quanta m(-2) s(-1)). In contrast, when both leaf surfaces were irradiated simultaneously with 200 micromol quanta m(-2) s(-1), total carbon fixation increased and carbon fixation in the middle of the leaf was higher compared to leaves irradiated unilaterally with the low light. Feeding 14CO2 through either the adaxial or abaxial leaf surface did not change the pattern of carbon fixation across the leaf. Increasing 14CO2 pulse-feeding times from 5 s to 60 s allowed more 14CO2 to be fixed but did not change the pattern of 14CO2 fixation across the leaf. We concluded that in spinach, the distribution of both light and Rubisco activity within leaves has significant effects on the patterns of carbon fixation across leaves; whereas CO2 diffusion does not appear to affect the carbon fixation pattern within spinach leaves.     

Studies on Bovine Mast Cells: I. The Effect of Formalin Fixation

Studies are reported on the effect of aqueous formalin fixation on the bovine mast cell. Paraffin sections of bovine skin were prepared from tissues fixed for 7 to 72 hr in 10% neutral formalin in saline, and from tissues which were treated with water for various times both before and after formalin fixation. Metachromatic halos, scattering of the granules, and fraying of the cell outline, were seen in the mast cells of tissues washed in water before fixation. Exposure to water after fixation did not produce these artifacts. The tendency towards orthochromatic staining, and the occurrence of perinuclear clear zones are probably effects of the formalin and not of overstaining or of exposure of the tissue to water. The majority of the metachromatic material in the bovine mast cell is water soluble, and may be removed by washing in water before fixation, whereas the granules are relatively resistant to water. The optimum time of formalin fixation of bovine skin to permit studies on the metachromatic material of the mast cells was 24 to 36 hr.     

Predicting the eye fixation locations in the gray scale images in the visual scenes with different semantic contents

In recent years, there has been considerable interest in visual attention models (saliency map of visual attention). These models can be used to predict eye fixation locations, and thus will have many applications in various fields which leads to obtain better performance in machine vision systems. Most of these models need to be improved because they are based on bottom-up computation that does not consider top-down image semantic contents and often does not match actual eye fixation locations. In this study, we recorded the eye movements (i.e., fixations) of fourteen individuals who viewed images which consist natural (e.g., landscape, animal) and man-made (e.g., building, vehicles) scenes. We extracted the fixation locations of eye movements in two image categories. After extraction of the fixation areas (a patch around each fixation location), characteristics of these areas were evaluated as compared to non-fixation areas. The extracted features in each patch included the orientation and spatial frequency. After feature extraction phase, different statistical classifiers were trained for prediction of eye fixation locations by these features. This study connects eye-tracking results to automatic prediction of saliency regions of the images. The results showed that it is possible to predict the eye fixation locations by using of the image patches around subjects’ fixation points.     

Denitrification and Nitrogen Fixation in Alaskan Continental Shelf Sediments

Rates of nitrogen fixation and denitrification were measured in Alaskan continental shelf sediments. In some regions, rates of nitrogen fixation and denitrification appeared to be equal; in other areas, rates were significantly different. Potential rates of denitrification were found to be limited primarily by the available nitrate substrate. Major regional differences in rates of denitrification were not statistically significant, but significant differences were found for nitrogen fixation rates in different regions of the Alaskan continental shelf. Estimated net losses of nitrogen from Bering Sea sediments were calculated as 1.8 × 10¹² g of N/yr. Experimental exposure of continental shelf sediments to petroleum hydrocarbons reduced rates of nitrogen fixation and denitrification in some cases but not others. Long-term exposure was necessary before a reduction in nitrogen fixation rates was observed; unamended rates of denitrification but not potential denitrification rates (NO₃⁻ added) were depressed after exposure to hydrocarbons.     

Photosynthesis and ribulose 1,5-bisphosphate levels in intact chloroplasts

The response of ribulose 1,5-bisphosphate levels and CO(2) fixation rates in isolated, intact spinach chloroplasts to pyrophosphate, triose phosphates, dl-glyceraldehyde, O(2), catalase, and irradiance during photosynthesis has been studied. Within 1 minute in the light, a rapid accumulation of ribulose bisphosphate was measured in most preparations of intact chloroplasts, and this subsequently dropped as CO(2) fixation increased. Pyrophosphate, triose phosphates, and catalase increased CO(2) fixation and also the levels of ribulose bisphosphate. CO(2) fixation was inhibited by dl-glyceraldehyde and O(2) with corresponding decreases in ribulose bisphosphate. When the rate of photosynthesis decreased at limiting irradiances (low light), the level of ribulose bisphosphate in the chloroplast did not always decrease, suggesting that ribulose bisphosphate was not limiting CO(2) fixation under these conditions. When triose phosphates (fructose bisphosphate plus aldolase) were added to suspensions of chloroplasts at low irradiances, ribulose bisphosphate increased while CO(2) fixation decreased. These observations provide considerable evidence that high ribulose bisphosphate levels clearly are not solely sufficient to permit rapid rates of CO(2) fixation, but that factors other than ribulose bisphosphate concentration are overriding the control of photosynthesis.Isolated chloroplasts are capable of using carbon reserves to produce considerable ribulose bisphosphate. Upon illumination in the absence of CO(2) and O(2), intact chloroplasts produced up to 13 millimolar ribulose bisphosphate.     

Non-symbiotic nitrogen fixation in soil

Summary 1. The fixation of N¹⁵-labeled nitrogen in small vessels of California soil under various conditions of pH, substrate level, oxygen tension, and other soil conditions was observed.2. Nitrate concentrations greater than 1.0–1.5 microequivalents per gram soil were found to suppress nitrogen fixation but not the growth ofAzotobacter.3. Large amounts of nitrogen were fixed when soluble organic substrates (e.g. glucose or sucrose) were added to the soil.4. Moderate fixation also resulted from the inversion of a disc of sod.5. Fixed nitrogen appeared largely in the nitrate and ammonia-amide fractions with that in the nitrate fraction probably representing nitrification of more reduced initial products of fixation.6. Under conditions of these experiments growing grass did not enhance fixation. At higher light intensities, however, such an enhancement might be observed.7. The incorporation of grass cuttings, straw or alfalfa meal into the soil caused only a slight increase in fixation.8. The inoculation of soils with large populations ofAzotobacter did not result in increased fixation.This investigation was supported in part by a contract with the United States Atomic Energy Commission.     

切开复位钢板内固定及闭合复位外固定架固定治疗不稳定型桡骨远端骨折患者的比较研究

目的:比较切开复位钢板内固定及闭合复位外固定架固定治疗不稳定型桡骨远端骨折的临床疗效,为临床治疗方式的选择提供参考。方法:选择2014年1月至2016年1月间于我院骨科就诊的桡骨远端骨折患者102例作为研究对象。按患者的治疗方式分为内固定组(n=52)和外固定组(n=50)。内固定组患者采用切开复位钢板内固定进行治疗,外固定组患者采用闭合复位外固定架固定进行治疗。术后3个月及术后1年通过影像学测量两组患者的关节面台阶、掌倾角和尺偏角,同时测量患者腕部功能情况。随访1年,比较两组患者疗效,并观察患者并发症发生情况。结果:术后3个月及术后1年,内固定组患者关节面台阶、掌倾角和尺偏角均低于外固定组患者,差异均具有统计学意义(P0.05)。术后3个月,内固定组的旋前旋后活动度、屈伸活动度、桡尺偏活动度、捏力、握力均明显高于外固定组(P0.05),但两组的上述指标在术后1年比较无统计学差异(P0.05)。内固定组患者治疗有效率为86.54%,外固定组患者治疗有效率为86.00%,差异无统计学意义(P0.05)。内固定组并发症发生率为15.38%,外固定组并发症发生率为18.00%,差异无统计学意义(P0.05)。结论:与闭合复位外固定架固定相比,切开复位钢板内固定治疗对患者关节面台阶、掌倾角和尺偏角的恢复效果较好,同时患者能及早进行腕部功能锻炼,近期疗效更加明显。     

Porcine Intestinal Mast Cells. Evaluation of Different Fixatives for Histochemical Staining Techniques Considering Tissue Shrinkage

Staining of mast cells (MCs), including porcine ones, is critically dependent upon the fixation and staining technique. In the pig, mucosal and submucosal MCs do not stain or stain only faintly after formalin fixation. Some fixation methods are particularly recommended for MC staining, for example the fixation with Carnoy or lead salts. Zinc salt fixation (ZSF) has been reported to work excellently for the preservation of fixation-sensitive antigens. The aim of this study was to establish a reliable histological method for counting of MCs in the porcine intestinum. For this purpose, different tissue fixation and staining methods that also allow potential subsequent immunohistochemical investigations were evaluated in the porcine mucosa, as well as submucosa of small and large intestine. Tissues were fixed in Carnoy, lead acetate, lead nitrate, Zamboni and ZSF and stained subsequently with either polychromatic methylene blue, alcian blue or toluidine blue. For the first time our study reveals that ZSF, a heavy metal fixative, preserves metachromatic staining of porcine MCs. Zamboni fixation was not suitable for histochemical visualization of MCs in the pig intestine. All other tested fixatives were suitable. Alcian blue and toluidine blue co-stained intestinal goblet cells which made a prima facie identification of MCs difficult. The polychromatic methylene blue proved to be the optimal staining. In order to compare MC counting results of the different fixation methods, tissue shrinkage was taken into account. As even the same fixation caused shrinkagedifferences between tissue from small and large intestine, different factors for each single fixation and intestinal localization had to be calculated. Tissue shrinkage varied between 19% and 57%, the highest tissue shrinkage was found after fixation with ZSF in the large intestine, the lowest one in the small intestine after lead acetate fixation. Our study emphasizes that MC counting results from data using different fixation techniques can only be compared if the respective studyimmanent shrinkage factor has been determined and quantification results are adjusted accordingly.Key words: mast cell, swine, fixation, tissue shrinkage factor     

The role of pH in the regulation of carbon fixation in the chloroplast stroma. Studies on CO2 fixation in the light and dark.

1. The pH in the stroma and in the thylakoid space has been measured in a number of chloroplast preparations in the dark and in the light at 20 degrees C. Illumination causes a decrease of the pH in the thylakoid space by 1.5 and an increase of the pH in the stroma by almost 1 pH unit. 2. CO2 fixation is shown to be strongly dependent on the pH in the stroma. The pH optimum was 8.1, with almost zero activity below pH 7.3.Phosphoglycerate reduction, which is a partial reaction of CO2 fixation, shows very little pH dependency. 3. Low concentrations of the uncoupler m-chlorocarbonylcyanide phenylhydrazone (CCCP) inhibit CO2 fixation without affecting phosphoglycerate reduction. This inhibition of CO2 fixation appears to be caused by reversal of light induced alkalisation in the stroma by CCCP. 4. Methylamine has a very different effect compared to CCCP. Increasing concentrations of methylamine inhibit CO2 fixation and phosphoglycerate reduction to the same extent. The light induced alkalisation of the stroma appears not to be significantly inhibited by methylamine, but the protons in the thylakoid space are neutralized. The inhibition of CO2 fixation by higher concentrations of methylamine is explained by an inhibition of photophosphorylation. It appears that methylamine does not abolish proton transport. 5. It is shown that intact chloroplasts are able to fix CO2 in the dark, yielding 3-phosphoglycerate. This requires the addition of dihydroxyacetone phosphate as precursor of ribulosemonophosphate and also to supply ATP, and the addition of oxaloacetate for reoxidation of the NADPH in the stroma. 6. Dark CO2 fixation in the presence of dihydroxyacetone phosphate and oxaloacetate has the same pH dependency as CO2 fixation in the light. This demonstrates that CO2 fixation in the dark is not possible, unless the pH in the medium is artificially raised to pH 8.8.     

THE FREE ENERGY OF NITROGEN FIXATION BY LIVING FORMS

Fixation of nitrogen even with liberation of energy or free energy, will take place if either oxygen gas or hydrogen gas, or other substances, especially gases, whose standard free energies are close to zero, are involved to form either nitrates, ammonia, or cyanide, not to speak of still other compounds. It has been pointed out that there are two and only two general conditions where nitrogen fixation can require energy. These are, first, if nitrogen reacts with some compound like water with an already high negative free energy of formation and where negligible oxidation of nitrogen would occur; second, if the plant does not take advantage of working at concentrations where the process would yield free energy. If nitrogen fixation is exothermic and free energy-yielding, how is the carbohydrate requirement of nitrogen-fixing organisms to be interpreted? Are the experimental determinations of the carbon to nitrogen ratio purely circumstantial? Is further hope given to those who may experimentally try to narrow this ratio to where the carbon used is only for the carbon requirements of general metabolism, exclusive of fixation? Do not hypotheses concerning the fixation of nitrogen in the evolutionary process, which are based on the conception that energy is required, lose some of their significance? Does it not suggest that perhaps fixation is far more universal than is supposed among living forms, particularly among the higher green plants, and thereby give encouragement to those who may wish to demonstrate this experimentally? Does it not indicate that perhaps the function of fixation is often to obtain energy for use in general metabolism? Is the general carbohydrate metabolism of the fixation forms to be regarded as being merely extremely inefficient? Or most suggestive of all, is the carbohydrate serving some unobserved function?     

Biomechanical effects of posterior pedicle fixation techniques on the adjacent segment for the treatment of thoracolumbar burst fractures: a biomechanical analysis

Abstract

Posterior pedicle fixation technique is a common method for treating thoracolumbar burst fractures, but the effect of different fixation techniques on the postoperative spinal mechanical properties has not been clearly defined, especially on adjacent segments. A finite element model of T10-L2 with moderate T12 vertebra burst fracture was constructed to investigate biomechanical behavior of three posterior pedicle screw fixation techniques. Compared with traditional short-segment 4 pedicle screw fixation (TS-4) and intermediate long-segment 6 pedicle screw fixation (IL-6), mono-segment 4 pedicle screw fixation (MS-4) provides a safer surgical selection to prevent the secondary degeneration of adjacent segments in the long-term.     

Fixation requirements for the immunohistochemical reactivity of PCNA antibody PC10 on cryostat sections

Summary In contrast to that in paraffin-embedded tissue, the reactivity of monoclonal PCNA antibody PC10 on cryostat sections requires a special fixation procedure as the target epitope is seemingly not accessible to its antibody. A panel of 18 fixation protocols was investigated. Chilled methanol or acetone, or PLP (paraformaldehyde-lysine-periodate) was found to be unsuitable for skin preparations. A two-step fixation protocol was developed for normal skin and basal cell carcinomas. They were fixed first in 3.4% buffered formaldehyde, followed by fixation in 2:1 v/v ethanol-acetic acid. Following this fixation regime, cryostat sections displayed the same PCNA/PC10 labelling pattern as paraffin sections of formalin-fixed tissue.     

Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N₂ fixation in the Subarctic. We hypothesised a contrast in N₂ fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied ¹⁵N-labelled N₂ gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N₂ fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N₂ fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N₂ fixation rates across species and functional types. Liverworts did not fix detectable amounts of N₂. Despite the very high rates of N₂ fixation associated with cyanolichens, large cover of mosses per unit area at the landscape scale compensates for their lower fixation rates, thereby probably making them the primary regional atmospheric nitrogen sink.     

Fixation techniques for fine needle aspiration biopsy smears prepared off site

OBJECTIVE: To identify a simple, cost-effective, reliable fixation method for fine needle aspiration biopsy (FNAB) yielding a specimen suitable for mail transport. STUDY DESIGN: Smears prepared from 59 FNABs of surgical specimens were fixed by continuous fixation in 95% ethanol, spray fixation, air drying, ethanol fixation for either 5 minutes or 4 hours followed by spray fixation, or fixation in 95% ethanol for either 30 minutes or 4 hours followed by air drying. Fixation was graded as unsatisfactory, suboptimal, average, good or excellent. RESULTS: Of smears continuously fixed in ethanol, 96.6% were graded as excellent. Of smears fixed in ethanol followed by spray fixation, 93.2% were excellent irrespective of fixation time; 64.4% of spray-fixed smears were excellent and 27.1% good. Of air dried smears, 93.2% were unsatisfactory or suboptimal; 83.0% of smears fixed in ethanol for 30 minutes and 74.6% of smears fixed for 4 hours prior to air drying were unsatisfactory or suboptimal. CONCLUSION: Fixation of smears in 95% ethanol followed by spray fixation produces excellent results, comparable to those with continuous fixation in ethanol. Spray fixation is generally good but not consistently excellent. Air drying or fixation in ethanol followed by air drying yields unsatisfactory or suboptimal results in most cases.     

Exotic grass invasion alters potential rates of N fixation in Hawaiian woodlands

Exotic grass invasion promotes fire which drives the conversion of native woodlands to exotic grasslands in the seasonally dry submontane forests of the island of Hawai'i. We compared potential rates of N fixation in an unburned forest site and a converted grassland site using the acetylene reduction assay. In addition to measuring rates of N fixation on separate and mixed substrates in each site, we tested the effect of abiotic factors on rates of N fixation of specific substrates. We hypothesized that rates of N fixation would be higher in the converted grassland site. N fixation estimates were 4.9 kg N ha⁻¹ year⁻¹ for the unburned forest, and 0.10 kg N ha⁻¹ year⁻¹ for the grassland site, so our hypothesis was rejected. The N fixation in the unburned forest occurs mostly on the leaf litter of native woody species. These substrates are absent from the grassland site, except for wood debris which was not consumed during the fires. No nitrogenase activity was detected in the rhizosphere and litter of grasses, the rhizospheres of shrubs or in soil. Although wood debris is not a significant contributor to the N fixed in the unburned forest, it contributes the majority of N fixed in the grassland. The response of nitrogenase activity to varying conditions of moisture and temperature suggests that microclimatic differences between sites do not control differences in N fixation activity; rather, these differences are due to the abundance of N-fixing substrates. The substantial decrease in N fixation activity after the conversion from woodland to grassland implies that ecosystem-level rates of N accretion are decreased by fire in these sites so much that the N lost during volatilization due to fire is not replenished over the long term by N fixation. Received: 10 January 1997 / Accepted: 7 August 1997     

Legume species identity and soil nitrogen supply determine symbiotic nitrogen-fixation responses to elevated atmospheric [CO2

In nitrogen (N)-limited systems, the response of symbiotic N fixation to elevated atmospheric [CO2] may be an important determinant of ecosystem responses to this global change. Experimental tests of the effects of elevated [CO2] have not been consistent. Although rarely tested, differences among legume species and N supply may be important. In a field free-air CO2 enrichment (FACE) experiment, we determined, for four legume species, whether the effects of elevated atmospheric [CO2] on symbiotic N fixation depended on soil N availability or species identity. Natural abundance and pool-dilution 15N methods were used to estimate N fixation. Although N addition did, in general, decrease N fixation, contrary to theoretical predictions, elevated [CO2] did not universally increase N fixation. Rather, the effect of elevated [CO2] on N fixation was positive, neutral or negative, depending on the species and N addition. Our results suggest that legume species identity and N supply are critical factors in determining symbiotic N-fixation responses to increased atmospheric [CO2].     

Microbial community shifts influence patterns in tropical forest nitrogen fixation

The role of biodiversity in ecosystem function receives substantial attention, yet despite the diversity and functional relevance of microorganisms, relationships between microbial community structure and ecosystem processes remain largely unknown. We used tropical rain forest fertilization plots to directly compare the relative abundance, composition and diversity of free-living nitrogen (N)-fixer communities to in situ leaf litter N fixation rates. N fixation rates varied greatly within the landscape, and ‘hotspots’ of high N fixation activity were observed in both control and phosphorus (P)-fertilized plots. Compared with zones of average activity, the N fixation ‘hotspots’ in unfertilized plots were characterized by marked differences in N-fixer community composition and had substantially higher overall diversity. P additions increased the efficiency of N-fixer communities, resulting in elevated rates of fixation per nifH gene. Furthermore, P fertilization increased N fixation rates and N-fixer abundance, eliminated a highly novel group of N-fixers, and increased N-fixer diversity. Yet the relationships between diversity and function were not simple, and coupling rate measurements to indicators of community structure revealed a biological dynamism not apparent from process measurements alone. Taken together, these data suggest that the rain forest litter layer maintains high N fixation rates and unique N-fixing organisms and that, as observed in plant community ecology, structural shifts in N-fixing communities may partially explain significant differences in system-scale N fixation rates.     

Effects of inorganic nitrogen compounds on the activity and synthesis of nitrogenase in Gloeothece (Nägeli) sp. ATCC 27152

Addition of 2 mM nitrite or ammonium to aerobically incubated cultures of Gloeothece rapidly inhibited N₂ fixation (measured as acetylene reduction). In contrast, 2 mM nitrate inhibited N₂ fixation less rapidly and less extensively, and often temporarily stimulated nitrogenase activity. The inhibitory effects of both nitrate and ammonium could be prevented by addition of 3 mM L-methionine-DL-sulphoximine, suggesting that the true inhibitor of N₂ fixation was an assimilatory product of ammonium rather than either ammonium or nitrate itself. The inhibition of N₂ fixation by nitrite could not, however, be prevented by addition of L-methionine-DL- sulphoximine. On the other hand, nitrite (unlike nitrate and ammonium) did not inhibit N₂ fixation in cultures incubated under a gas phase lacking oxygen. These findings suggest that the mechanism whereby nitrite inhibits N₂ fixation in Gloeothece differs from that of either nitrate or ammonium. The inhibitory effect of nitrite on N₂ fixation did not involve reduction of nitrite to nitric oxide, though nitric oxide was a potent inhibitor of nitrogenase activity in Gloeothece . Nitrate and nitrite inhibited the synthesis of nitrogenase in Gloeothece , while ammonium not only inhibited nitrogenase synthesis but also stimulated degradation of the enzyme. In addition, all three compounds favoured the appearance of the Fe-protein of nitrogenase in its larger, presumed inactive, form.     

Paraformaldehyde fixation of LPS-stimulated human monocytes: technical parameters permitting the study of membrane IL-1 activity

The existence of IL-1 activity on the cell surface of stimulated mononuclear phagocytes is a matter of controversy. In particular, fixation of IL-1-expressing cells for 15 min in 1% paraformaldehyde (PFA) is commonly used to evidence such "membrane-associated" IL-1 activity but other authors have attributed this to passive leakage of IL-1 alpha from the cells and report no activity with longer fixation times. Using specific IL-1 alpha and IL-1 beta assays, we found that after the mild standard PFA fixation procedure, not only IL-1 alpha but also IL-1 beta were released into the supernatants for up to 96 h following fixation; membrane IL-1 activity cannot thus be measured in these conditions. However, using conditions in which neither immunoreactive IL-1 molecules nor IL-1 activity are found in the supernatants (i.e. assay at 144 h, increased fixation time), we were still able to detect IL-1 activity on LPS-stimulated, PFA-fixed monocytes. This activity was independent of the duration of PFA fixation and was inhibited by anti-IL-1 alpha but not anti-IL-1 beta antibodies. Our data thus underline the importance of technical conditions in the study of membrane-associated IL-1 activity.