Are Mistletoes Shade Plants? CO2Assimilation and Chlorophyll Fluorescence of Temperate Mistletoes and their Hosts

Mistletoes usually have slower rates of photosynthesis thantheir hosts. This study examines CO₂assimilation, chlorophyllfluorescence and the chlorophyll content of temperate host–parasitepairs (nine hosts parasitized by Ileostylus micranthus and Carpodetusserratus parasitized by Tupeia antarctica). The hosts of I.micranthus had higher mean annual CO₂assimilation (3.59 ±0.41 µmol m^-2 s^-1) than I. micranthus(2.42 ± 0.20µmol m^-2 s^-1), and C. serratus(2.41 ± 0.43 µmolm^-2 s^-1) showed higher CO₂assimilation than T. antarctica(0.67± 0.64 µmol m^-2 s^-1). Hosts saturated at significantlyhigher electron transport rates (ETR) and light levels thanmistletoes. The positive relationship between CO₂assimilationand electron transport suggests that the lower CO₂assimilationrates in mistletoes are a consequence of lower electron transportrates. When photosynthetic rates, ETR and chlorophyll a /b ratioswere adjusted for photosynthetically active radiation, hostsdid not have significantly higher CO₂assimilation (3.21 ±0.37 µmol m^-2 s^-1) than mistletoes (2.54 ± 0.41µmol m^-2 s^-1), but still had significantly higher ETRand chlorophyll a / b ratios. The electron transport rates,saturating light and chlorophyll a / b ratios of sun leavesfrom mistletoes were similar to host shade leaves. These responsesindicate that in comparison with their hosts, mistletoe leaveshave the photosynthetic characteristics of the leaves of shadeplants. Copyright 2000 Annals of Botany Company CO₂assimilation, photosynthetic active radiation (PAR), chlorophyll fluorescence, electron transport rate (ETR), photochemical quenching (q_p), non-photochemical quenching (q_n), sun and shade leaves, chlorophyll content, Ileostylus micranthus, Tupeia antarctica, New Zealand     

Transport of Aromatic Amino Acids by Pseudomonas aeruginosa

Kinetic studies of the transport of aromatic amino acids by Pseudomonas aeruginosa revealed the existence of two high-affinity transport systems which recognized the three aromatic amino acids. From competition data and studies on the exchange of preformed aromatic amino acid pools, the first transport system was found to be functional with phenylalanine, tyrosine, and tryptophan (in order of decreasing activity), whereas the second system was active with tryptophan, phenylalanine, and tyrosine. The two systems also transported a number of aromatic amino acid analogues but not other amino acids. Mutants defective in each of the two and in both transport systems were isolated and described. When the amino acids were added at low external concentrations to cells growing logarithmically in glucose minimal medium, the tryptophan pool very quickly became saturated. Under identical conditions, phenylalanine and tyrosine each accumulated in the intracellular pool of P. aeruginosa at a concentration which was 10 times greater than that of tryptophan.     

The Influence of Nitrogen Nutrition on the Accumulation of Free Amino Acids in Root Tissue of Urtica dioica and their Apical Transport in Xylem Sap

Urtica dioica plants were grown on a nitrogen supply of 3, 15and 22 mM with nitrate and ammonium as nitrogen source. In contrastto nitrate reductions amino acid synthesis occurred in roottissue. At 3 mM ammonium obviously the amino acids were rathertransported via xylem upwards to the shoots than stored in theroots. Particularly increased ammonium supply led to stimulatedstorage of free amino acids in the roots, mainly as asparagineand arginine. In xylem asparagine was the dominant nitrogentransporting compound, while arginine was hardly translocated.With the enhancement of nitrogen supply, the second amide, glutamine,became more and more important with respect to the transportof nitrogen. (Received September 3, 1984; Accepted November 2, 1984)     

Secondary Transport of Amino Acids in Prokaryotes

Amino acid transport is a ubiquitous phenomenon and serves a variety of functions in prokaryotes, including supply of carbon and nitrogen for catabolic and anabolic processes, pH homeostasis, osmoprotection, virulence, detoxification, signal transduction and generation of electrochemical ion gradients. Many of the participating proteins have eukaryotic relatives and are successfully used as model systems for exploration of transporter structure and function. Distribution, physiological roles, functional properties, and structure-function relationships of prokaryotic α-amino acid transporters are discussed.     

Phloem Transport of Amino Acids in Relation to their Cytosolic Levels in Barley Leaves

A comparison of barley (Hordeum vulgare L.) leaves was made between the cytosolic content of amino acids and sucrose as determined by subcellular fractionation and the corresponding concentration in phloem sap, which was collected continuously for up to 6 days from severed aphid stylets. Because amino acids were found to be almost absent from the vacuoles, and because the amino acid patterns in the stroma and cytosol are similar, whole leaf contents could be taken as a measure of cytosolic amino acid levels for a comparison of data during a diurnal cycle. The results show that the pattern of amino acids in the phloem sap was very similar to the pattern in the cytosol. Therefore, we concluded that the overall process of transfer of amino acids from the cytosol of the source cells into the sieve tubes, although carrier mediated, may be a passive process and that the translocation of amino acids via the sieve tubes requires the mass flow of sucrose driven by the active sucrose transport involved by the phloem loading.     

葡萄糖与氨基酸的跨膜转运机制

葡萄糖是体内最主要的供能物质,因其为极性物质,不能自由通过细胞膜,故其跨膜转运需由细胞膜上特殊的蛋白质（载体）协助。但在机体内的不同部位．其转运机制有所不同,下面分别介绍机体内不同部位葡萄糖的跨膜转运机制。[第一段]     

Blood-Brain Barrier Carrier-Mediated Transport and Brain Metabolism of Amino Acids

The transport of neutral amino acids through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is an important control point for the overall regulation of cerebral metabolism, including protein synthesis and neurotransmitter production. The Michaelis-Menten kinetics of BBB amino acid transport have been investigated in vivo with the brain uptake index (BUI) technique, and in vitro with the isolated human brain capillary preparation. The only amino acid that is albumin-bound is tryptophan, and the majority of albumin-bound tryptophan in the plasma is available for transport through the BBB via an enhanced dissociation mechanism that operates at the surface of the brain capillary endothelium. The availability in brain of amino acids is predicted from the BBB Km values to be sharply influenced by supra-physiological concentrations of phenyalanine in the 200–500 M range. Moreover, the measurement of cerebral protein synthesis with an internal carotid artery perfusion technique and HPLC-based measurements of aminoacyl-transfer RNA specific activities shows an inverse relationship between cerebral protein synthesis and plasma phenyalanine concentrations in the 200–500 M range. These findings indicate the neurotoxicity of hyperphenylalninemia is not restricted to the phenylketonuria range of approximately 2000 M, but is exerted in the supra-physiological range of 200–500 M.     

Xylem Transport and the Negative Pressures Sustainable by Water

Experimental measurements of water's ability to sustain negativepressures are reviewed with special emphasis on the relevanceof the results to xylem transport. Results vary over severalorders of magnitude and depend on the conditions of testing.The greatest negative pressures are measured using heated, pressurizedwater or purified, degassed water. When conditions more closelyapproximating those found in biological systems are used, wateris considerably weaker. From these measurements, one can predictthe most likely range of negative pressures that can be sustainedin the xylem. This range is between -0·1 and -0·6MPa (absolute pressure). Negative pressures near -1 MPa arepossible, but require stringent conditions. Negative pressuresgreater than -2 MPa are also possible, but unlikely, based onthe experimental evidence.Copyright 1994, 1999 Academic Press Cavitation, embolism, negative pressure, xylem transport, Z-tube     

Regulation of Membrane Transport Sites for Amino Acids in Myogenic Cells

Abstract. Myoblasts from 12-day chick embryos in cell culture transport the nonmetabolizable amino acid α-aminoisobutyric acid (AIB) two to three-fold more rapidly than multinucleated myotubes which form from them. This decrease in transport is due to a relative decrease in the number of transport sites per unit area of cell surface suggesting a compositional change in the plasma membrane during myogenesis. In studies reported here, AIB transport was monitored throughout myogenesis and correlated with other aspects of differentiation. During myogenesis the number of amino acid transport sites remains constant per myotube nucleus. As myogenesis proceeds, there is a marked increase in cellular protein and cell surface without a commensurate increase in amino acid transport sites. The net consequence of the surface area change is fewer amino acid transport sites per unit area of myotube membrane surface. The decrease in membrane transport sites for AIB per unit area of membrane is not a result of length of time in culture per se, medium depletion, or cell density, but is a result of differentiation, since blocking myoblast fusion by deprivation of calcium delays the decrease in AIB transport sites per unit cell surface area while reversal of the calcium deprivation block is accompanied by a rapid decrease in the number of AIB transport sites per unit cell surface area. Thus, the decrease in AIB transport sites is an aspect of differentiation which accompanies the marked elaboration of surface membrane during myogenesis.     

液膜体系中氨基酸迁移的研究

用Ａｌｉｑｕａｔ３３６－Ｓｐａｎ８０－甲苯制成液体表面活性剂的膜体系,对氨基酸的迁移行为进行了研究,确定了苯丙氨酸完全及快速迁移的制乳和适宜条件,２ｍｉｎ的迁移率可达９０％以上。在苯丙氨酸的最佳迁移条件下,其它氨基酸如天冬氨酸、精氨酸、甘氨酸等都能有较高的迁移率。此法适用于微量氨基酸的提取和分离。     

Carrier Protein-mediated Transport of Neutral Amino Acids into Mung Bean Mitochondria

The transport of neutral amino acids into mitochondria isolated from the hypocotyl of mung bean (Roxb.) was studied by the swelling technique. Isolated mitochondria swelled when added to an isosmotic solution of proline, serine, methionine, threonine, alanine, and glycine. The swelling was stereospecific in that it was faster in the l-amino acid than in the corresponding d-amino acid. Preincubation of the mitochondria with the sulfhydryl modifying reagents, p-mercuribenzoate and mersalyl, resulted in an inhibition of the swelling caused by proline, serine, threonine, and glycine. The swelling induced by alanine was inhibited only by mersalyl, whereas that by methionine was inhibited only by p-mercuribenzoate. In all cases, the inhibition caused by the sulfhydryl modifying reagents was readily reversible by the subsequent treatment of the mitochondria with dithiothreitol. N-Ethylmaleimide, another sulfhydryl-modifying reagent, did not cause any inhibition of the swelling. The findings indicate the existence of a protein mediated mechanism for the transport of neutral amino acids into plant mitochondria.     

植物对氨基酸的吸收及其生理效应

<正> 在古代的农业系统中,所用的肥料都是有机肥,人们并不知道植物依赖于何种物质得以生长繁衍。自1840年,德国化学家李比希(J.U.Liebig)发表了《化学在农业和植物生理学上应用》一书后,人们对植物的营养有了新的认识,认识到矿物质对植物的重要性。李比希在书中指出:“土壤中矿物质是一切绿色植物唯一的养料。植物可以完全依赖于无机物质而生长发育。这一观点即为“植物的矿质营养学说”。该学说后来为生产实践所充分证明,并成为农业化学     

外源氨基酸胁迫下植物体内游离氨基酸的测定

目的：通过检测外源氨基酸胁迫下植物体内的游离氨基酸含量,探讨氨基酸胁迫下植物生长受抑制的机制。方法：小麦胚接种在加入D-丙氨酸、D-丝氨酸和L-缬氨酸等3种外源氨基酸的培养基中,其发芽和生长会受到强烈抑制。取材胁迫植株和对照植株,提取并用HPLC方法检测游离氨基酸含量,分析主要氨基酸特征的改变。结果：作为处理的胁迫氨基酸在体内有数倍到数十倍的增高,其他氨基酸,尤其是同族及相近族氨基酸的量也出现较大改变,有增加也有减少,有些氨基酸甚至检测不出。结论：在外源氨基酸胁迫下,植物可直接吸收这些氨基酸,胁迫氨基酸在体内的积累。至少影响到了部分氨基酸的合成,使细胞内正常游离态氨基酸的数量增加或减少,这些变化引起代谢失调,进而引起生长抑制。     

Effect of Chlorpromazine on Active Transport of Amino Acids in Tetrahymena*

SYNOPSIS. Low concentrations of chlorpromazine (～0.01 mM) inhibit growth and nucleic acid synthesis in the ciliate Tetrahymena pyriformis. Brief exposure of the cells to, e.g. 0.018 mM chlorpromazine, had very little effect on ¹⁴CO₂ production or on label incorporation into glycogen from [1-¹⁴C]glucetate, [6–14C]glucose, or [1-¹⁴C]leucine, but 17-h exposure of stationary phase cultures to this drug caused marked alterations in metabolism, including an almost complete loss of ability to decarboxylate L-[1-¹⁴C]leucine and L-[1-¹⁴C]tyrosine. It was shown that loss of ability to decarboxylate these amino acids results from loss of ability to transport them.     

Inhibition of Glutamine Transport in Rat Brain Mitochondria by Some Amino Acids and Tricarboxylic Acid Cycle Intermediates

Glutamine transport into rat brain synaptic and non-synaptic mitochondria has been monitored by the uptake of [³H]glutamine and by mitochondrial swelling. The concentration of glutamate in brain mitochondria is calculated to be high, 5–10 mM, indicating that phosphate activated glutaminase localized inside the mitochondria is likely to be dormant and the glutamine taken up not hydrolyzed. The uptake of [³H]glutamine is largely stereospecific. It is inhibited by glutamate, asparagine, aspartate, 2-oxoglutarate and succinate. Glutamate inhibits this uptake into synaptic and non-synaptic mitochondria by 95 and 85%, respectively. The inhibition by glutamate, asparagine, aspartate and succinate can be explained by binding to an inhibitory site whereas the inhibition by 2-oxoglutarate is counteracted by aminooxyacetic acid, which indicates that it is dependent on transamination. The glutamine-induced swelling, a measure of a very low affinity uptake, is inhibited by glutamate at a glutamine concentration of 100 mM, but this inhibition is abolished when the glutamine concentration is raised to 200 mM. This suggests that the very low affinity glutamine uptake is competitively inhibited by glutamate. Furthermore, glutamine-induced swelling is inhibited by 2-oxoglutarate, succinate and malate, similarly to that of the [³H]glutamine uptake. The properties of the mitochondrial glutamine transport are not identical with those of a recently purified renal glutamine carrier.     

含硫和含硒氨基酸及其光学异构体的气相色谱分离和分析

<正> 这篇综述的写作构思有三:一是时至今天,含硫特别是含硒氨基酸的准确分析仍然面临不少难题;二是硒蛋白及组成它的含硒氨基酸是当前生物化学的热门课题之一,例如有人提出硒半胱氨酸可能是构成生命遗传密码的第21种氨基酸;三是近10多年来国内外还未见有这方面的专门综述。     

Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.