Use of the 15N natural abundance technique to quantify biological nitrogen fixation by woody perennials

Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands to plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N₂ fixed. Some of the problems involved in measuring N₂ fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N₂-fixing species, especially in natural ecosystems; (b) long-term, perennial nature of growth and the seasonal or year-to-year changes in patterns of N assimilation; and (c) logistical limitations of working with mature trees which are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N₂ fixation to trees is the so-called `¹⁵N natural abundance' technique which exploits naturally occurring differences in ¹⁵N composition between plant-available N sources in the soil and that of atmospheric N₂. In this review we discuss probable explanations for the origin of the small differences in ¹⁵N abundance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine the potential advantages and limitations inherent in the application of the technique to study N₂ fixation by woody perennials. Calculation of the proportion of the plant N derived from atmospheric N₂ (%Ndfa) using the natural abundance procedure requires that both the ¹⁵N natural abundance of the N derived from BNF and that derived from the soil by the target N₂-fixing species be determined. It is then assumed that the ¹⁵N abundance of the N₂-fixing species reflects the relative contributions of the N derived from these two sources. The ¹⁵N abundance of the N derived from BNF (B) can vary with micro-symbiont, plant species/provenance and growth stage, all of which create considerable difficulties for its precise evaluation. If the%Ndfa is large and the ¹⁵N abundance of the N acquired from other sources is not several ¹⁵N units higher or lower than B, then this can be a major source of error. Further difficulties can arise in determining the ¹⁵N abundance of the N derived from soil (and plant litter, etc.) by the target plant as it is usually impossible to predict which, if any, non-N₂-fixing reference species will obtain N from the same N sources in the same proportions with the same temporal and spatial patterns as the N₂-fixing perennial. The compromise solution is to evaluate the ¹⁵N abundance of a diverse range of neighbouring non-N₂-fixing plants and to compare these values with that of the N₂-fixing species and the estimate of B. Only then can it be determined whether the contribution of BNF to the target species can be quantified with any degree of confidence. This review of the literature suggests that while the natural abundance technique appears to provide quantitative measures of BNF in tree plantation and agroforestry systems, particular difficulties may arise which can often limit its application in natural ecosystems.     

数据恢复技术在计算机犯罪侦查中的应用

在计算机犯罪案件中,犯罪分子往往破坏现场、毁灭证据,以逃脱罪责。数据恢复技术具有将被破坏的数据还原为原始数据的功能。把数据恢复技术应用于计算机犯罪侦查中,将为我们有效地打击计算机犯罪开辟一条新的途径。     

A novel human homologue of the Drosophila frizzled wnt receptor gene binds wingless protein and is in the Williams syndrome deletion at 7q11.23

Prenylated Rab GTPases occur in the cytosol in their GDP-bound conformations bound to a cytosolic protein termed GDP-dissociation inhibitor (GDI). Rab-GDI complexes represent a pool of active, recycling Rab proteins that can deliver Rabs to specific and distinct membrane-bound compartments. Rab delivery to cellular membranes involves release of GDI, and the membrane-associated Rab protein then exchanges its bound GDP for GTP. We report here the identification of a novel, membrane-associated protein factor that can release prenylated Rab proteins from GDI. This GDI-displacement factor (GDF) is not a guanine nucleotide exchange factor because it did not influence the intrinsic rates of nucleotide exchange by Rabs 5, 7 or 9. Rather, GDF caused the release of each of these endosomal Rabs from GDI, permitting them to exchange nucleotide at their intrinsic rates. GDF displayed the greatest catalytic rate enhancement on Rab9-GDI complexes. However, catalytic rate enhancement paralleled the potency of GDI in blocking nucleotide exchange: GDI was shown to be most potent in blocking nucleotide exchange by Rab9. The failure of GDF to act on Rab1-GDI complexes suggests that it may be specific for endosomal Rab proteins. This novel, membrane-associated activity may be part of the machinery used to localize Rabs to their correct intracellular compartments.     

Sphingosylphosphocholine reduces the calcium ion requirement for activating tissue transglutaminase

Tissue transglutaminase (tTG) catalyzes a Ca2+-dependent transglutaminase reaction resulting in the formation of gamma-glutamyl-epsilon-lysine bonds and is activated during apoptosis to catalyze the formation of apoptotic body. We investigate whether lipids that are membrane components and involved in cell signaling could modify the Ca2+-dependent activation of tTG. We found that sphingosylphosphocholine (lyso-SM) was the only lipid to activate transglutaminase at low Ca2+ concentrations. In the presence of lyso-SM (125 microM), transglutaminase was detectable at 10 microM Ca2+, whereas in the absence of lyso-SM, similar activity was obtained at 160 microM Ca2+. Furthermore, in the presence of lipid vesicles lyso-SM retained the ability to enhance the Ca2+-dependent activation of tTG. Lyso-SM did not significantly change the Km for the glutamyl and primary amine substrates. However, the Kact for Ca2+ was reduced from 300 microM to 90 microM. Structure-function studies of lyso-SM analogs indicate that phosphocholine group on C1, the free amino group at C2 and a C4-C5 double bond are critical for the activation of transglutaminase activity. This is the first demonstration that a specific sphingolipid could enhance the activity of tTG and could play a role in vivo in activation of the tTG at physiologic Ca2+ levels.     

Acid pH augments excitatory action of ATP on a dissociated mammalian sensory neuron

ATP stimulates nociceptive neurons via an action on ligand-gated ion channels. Since tissue injury and inflammation result in both localized acidosis and release of ATP, we studied the effect of acid pH on ATP-gated ion channels in rat nodose ganglion neurons. Lowering pH dramatically increased membrane depolarization and action potential firing elicited by ATP. ATP-activated current was enhanced by acid pH and suppressed by alkaline pH. A pH of 7.2 produced the half-maximal effect. Acidification increased the apparent affinity of the receptor for ATP, as evidenced by a parallel shift of the ATP concentration-response curve to the left. The observations suggest that the localized acidosis associated with tissue injury may enhance pain perception via an action on ATP-gated ion channels on mammalian sensory neurons.     

Quality without paper

Quality management is often associated with a huge increase in paper consumption within a factory or company. It does not have to be that way, as the author describes based the case study of reorganisation at the Pilkington PE Ltd. They have succeeded in evolving to an interactive, paperless quality management system     

THE RECTANGULAR HYSTERESIS LOOP OF GRAIN ORIENTED NICKEL-IRON ALLOY

用直流和交流方法试测合镍与含铁各约50％的镍-铁合金的磁性。剩磁感与饱和磁感的比值Br/B_r,与矫顽力H_c作为评比材料的指标。 本实验所用样品于热轧后经过三种冷轧压延率;即(Ⅰ)94％,(Ⅱ)98％,与(Ⅲ)99％。第(Ⅰ)类与第(Ⅱ)类样品分别在5个温度保持1小时;即(1)900°,(2)1000°,(3)1100°,(4)1200°,与(5)1300℃。 第(Ⅲ)类样品在1100℃分3个保温时间熟炼;即(1)30分,(2)1小时与(3)11/2小时。这三类经过再结晶的样品的磁滞回线均经分别测出。 合金的立方织构的形成曾用X-射线极图方法进行研究。结果将见另一篇报告。 对于这种用于磁放大器等的合金材料的发展,最后作了讨论。     

Preparation of poly(ethylene-co-dicyclopentadiene) copolymers and a study on their post-polymerization epoxidation

The copolymerization of ethylene with dicyclopentadiene (DCP) using the metallocene catalyst rac-dimethylsilylbis(indenyl)zirconium dichloride (Me₂Si(Ind)₂ZrCl₂) proceeds with high activity producing materials with DCP incorporations of 0.5–2.7 mol%. The residual olefin moiety of the DCP comonomer is still available for reaction following polymerization and was epoxidized using H₂O₂ and formic acid. This reaction was optimized and proceeds with good conversion and the resulting materials show increased physical properties compared to the untreated copolymers.