High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

Phloem as Capacitor: Radial Transfer of Water into Xylem of Tree Stems Occurs via Symplastic Transport in Ray Parenchyma

The transfer of water from phloem into xylem is thought to mitigate increasing hydraulic tension in the vascular system of trees during the diel cycle of transpiration. Although a putative plant function, to date there is no direct evidence of such water transfer or the contributing pathways. Here, we trace the radial flow of water from the phloem into the xylem and investigate its diel variation. Introducing a fluorescent dye (0.1% [w/w] fluorescein) into the phloem water of the tree species Eucalyptus saligna allowed localization of the dye in phloem and xylem tissues using confocal laser scanning microscopy. Our results show that the majority of water transferred between the two tissues is facilitated via the symplast of horizontal ray parenchyma cells. The method also permitted assessment of the radial transfer of water during the diel cycle, where changes in water potential gradients between phloem and xylem determine the extent and direction of radial transfer. When injected during the morning, when xylem water potential rapidly declined, fluorescein was translocated, on average, farther into mature xylem (447 ± 188 µm) compared with nighttime, when xylem water potential was close to zero (155 ± 42 µm). These findings provide empirical evidence to support theoretical predictions of the role of phloem-xylem water transfer in the hydraulic functioning of plants. This method enables investigation of the role of phloem tissue as a dynamic capacitor for water storage and transfer and its contribution toward the maintenance of the functional integrity of xylem in trees.Physiological and hydraulic functioning of the two long-distance transport systems in trees, xylem and phloem, have intrigued plant researchers for more than a century. Since the pioneering work of Dixon and Joly (1895; cohesion-tension theory for xylem) and Münch (1930; pressure flow hypothesis for phloem), the majority of studies have investigated these systems independently of each other. Although the work of Stout and Hoagland (1939) as well as Biddulph and Markle (1944) laid the foundation for the physiological nexus between xylem and phloem, it is only recently that we have begun to understand the importance of the hydraulic nexus (Hölttä et al., 2006, 2009; Sevanto et al., 2011, 2014). Processes related to both nexus occur in parallel, and here the term physiological nexus covers all metabolite exchange, including the bidirectional flow of amino acids, minerals, and carbohydrates (Wardlaw, 1974; Ferrier et al., 1975; Pfautsch et al., 2009, 2015; De Schepper et al., 2013; for review, see van Bel, 1990, 2003). The term hydraulic nexus refers to the function of phloem as a capacitor, where water stored in phloem moves into xylem vessels to maintain the integrity of the transpiration stream (Zweifel et al., 2000, and refs. therein). Throughout this article, we use the term phloem collectively for cells that make up the transport phloem of woody plants (including sieve element/companion cell complexes, parenchyma cells, etc.), as opposed to collection and release phloem tissue, which differ in structure and function. Transport phloem is characterized by the retention of “high hydrostatic pressure by retrieval of leaked osmotica accompanied by water flux” (Patrick, 2013).According to the cohesion-tension theory, water in xylem vessels is constantly under tension and moves in a metastable state from roots to leaves along a hydrostatic pressure gradient. Depending on both the availability of soil moisture and the vapor pressure deficit of the atmosphere, this tension can exceed the cohesive forces that bind water molecules, resulting in the formation of a gas void that, after expanding, can lead to rupture of the water column inside individual vessels (termed cavitation; Zimmermann, 1983). Once cavitated, vessels become dysfunctional, and the transport of water and nutrients to leaves declines. However, water stored in woody tissues of trees can be mobilized to alleviate the risk of cavitation, and recent theory suggests that both water and carbohydrates from phloem may aid in the reversal of vessel embolism (i.e. air intrusion), although the evidence is indirect (Salleo et al., 2009; Brodersen et al., 2010; Nardini et al., 2011).All parts of plants have a water storage capacity (symplastic and apoplastic), and this capacitance increases with tree size and age (Phillips et al., 2003). The ability to mobilize stored water varies according to the force required to drag it out of storage (Holbrook, 1995). One-half century ago, Reynolds (1965) highlighted the importance of the volume of internally stored water to support the transpiration of trees. Since then, studies have quantified the fraction of stored water in total daily transpiration for a range of tree species. This fraction varies between 2% and 20% (Tyree and Yang, 1990; Čermák et al., 2007, and refs. therein; Barnard et al., 2011; Pfautsch and Adams, 2013) and is generally smaller in angiosperms compared with gymnosperms, where a maximum fraction of 50% was reported for Pinus sylvestris (Waring et al., 1979). Given that the daily water use of large adult trees can easily reach 260 to 380 L (Čermák et al., 2007; Pfautsch et al., 2011), considerable volumes of stored water must be mobilized from and restored back into capacitors on a daily basis. Remobilization of stored water also can prolong stomatal opening and thus increase carbon gain (Goldstein et al., 1998).The volume of stored water released depends on the elasticity of the storage tissue, its connectivity to xylem vessels, and the gradient of water potential (ψ) between the storage tissue and vessels. Cells with elastic walls represent ideal capacitors because they can change their volume as a consequence of small changes in ψ. Thus, phloem, cambium, and juvenile xylem cells are well suited for water storage and release (Yang and Tyree, 1992; Zweifel et al., 2014). The magnitude of release and refill of stored water in trees can be approximated by separately measuring the change in thickness of phloem and xylem during a diel cycle using high-precision dendrometers (Zweifel et al., 2014). Whitehead and Jarvis (1981) have calculated that around 90% of the diurnal change in stem radius can be attributed to changes in the water content of cambial and phloem tissues. To date, it is commonly accepted that tree bark, independent of the wood below, swells during the night and shrinks during the day (Zweifel et al., 2000), reflecting the water flow dynamics that characterize the dynamic exchange of water between phloem and xylem.Phloem and xylem tissues are separated by rows of intermediary cambial cells. However, depending on the species, phloem and xylem are connected through uniseriate or multiseriate strands of radially aligned ray parenchyma cells, commonly termed wood rays. These rays have been shown to be capable of symplastic water transport through plasmodesmata (Höll, 1975). Based on measurements of radial conductance, Sevanto et al. (2011) suggested that aquaporins also might be involved in the radial transfer of water. Both theoretical and experimental approaches have been developed to better understand the dynamic exchange of water between xylem and phloem. Hölttä et al. (2006, 2009) developed a model based on Münch’s hypothesis and included a term that represents the hydraulic connection between the two tissue types. Through incorporating this term, model outputs suggest the occurrence of a constant exchange of water between xylem and phloem along gradients of ψ. However, some authors suggested that changes in ψ of xylem alone might be insufficient to account for the observed diurnal shrinkage and swelling of bark (Sevanto et al., 2003). Along this line of argument, loading and unloading of carbohydrates in phloem tissue has been suggested to further impact the radial transfer of water and associated changes in bark thickness (Mencuccini et al., 2013).Nevertheless, to date, all approaches remain indirect, and the routes of water transfer between phloem and xylem have yet to be determined. Here, we present a technique that enables the visualization of water transfer from phloem to xylem tissues and resolves the apoplastic and symplastic pathways and cell types. The method involves the injection of an aqueous solution that contains fluorescent dye into phloem followed by analyses of woody tissues using confocal laser scanning microscopy. We assess the effectiveness of three different dyes to stain possible transfer pathways. We also introduce dye during different time intervals of the diel transpiration cycle to test the effect of predicted dynamic changes in ψ between phloem and xylem on the transfer processes. We hypothesized that radial transfer of water would be most pronounced during periods where conditions of the hydraulic nexus between phloem and xylem differ the most. These differences are expected during high rates of transpiration that cause a steep decline in xylem ψ, commonly observed during morning hours. We use leaf water potential (ψ_L) and high-precision dendrometer measurements to identify relevant time intervals. The simultaneous assessment of ψ_L and the independent diameter fluctuation of phloem and xylem may provide empirical evidence for the role of phloem as a water storage capacitor that helps mitigate increasing tension in the transpiration stream.     

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     

维管植物木质部输导特性以及仿生应用的研究

本文从细胞壁、导管(管胞)、木质部三个不同的层次分别论述了维管植物木质部的耐压性和韧性机理,并对木质部强有力的输导性机理进行了阐述。从仿生学角度出发,分别提出了仿生耐压管道和一次性超强榆导毛细管束的仿生结构模型。其中仿生耐压管道自内向外分别由内管、纤维层、增厚层、均压层和保护层组成,具有很好的耐压性和一定的保温性;一次性超强输导毛细管束采用许多根微细的毛细管加以穿孔板组成,能最大限度地维持水的内聚张力。     

The Dawn Phenomenon Revisited: Implications for Diabetes Therapy

ObjectiveTo summarize current data on the magnitude, prevalence, variability, pathogenesis, and management of the dawn phenomenon in patients with diabetes mellitus.MethodsOn the basis of the pertinent available literature and clinical experience, we propose a quantitative definition of the dawn phenomenon, discuss potential pathogenic mechanisms, and suggest management options.ResultsThe “dawn phenomenon” is a term used to describe hyperglycemia or an increase in the amount of insulin needed to maintain normoglycemia, occurring in the absence of antecedent hypoglycemia or waning insulin levels, during the early morning hours. To be clinically relevant, the magnitude of the dawn increase in blood glucose level should be more than 10 mg/dL or the increase in insulin requirement should be at least 20% from the overnight nadir. Controversy exists regarding the frequency, reproducibility, and pathogenesis of the dawn phenomenon. Approximately 54% of patients with type 1 diabetes and 55% of patients with type 2 diabetes experience the dawn phenomenon when the foregoing quantitative definition is used. The most likely pathogenic mechanism underlying the dawn phenomenon is growth hormone-mediated impairment of insulin sensitivity at the liver and muscles. The exact biochemical pathways involved are unknown. Therapeutic decisions aimed at correcting fasting hyperglycemia should take into account the variability and magnitude of the dawn phenomenon within individual patients. Successful insulinization appears to minimize the effects of the dawn phenomenon. Currently, no subcutaneous depot preparation of insulin exists that is capable of mimicking the basal insulinsecretion of the healthy pancreas.ConclusionIncreases in the bedtime doses of hypoglycemic agents with nighttime peaks in action may correct early morning hyperglycemia but be associated with undesirable nocturnal hypoglycemia. Targeted continuous subcutaneous insulin infusion programming can facilitate the prevention of early morning hyperglycemia in selected patients. (Endocr Pract. 2005;11:55-64)     

The Theory and Practice of Measuring Transport Coefficients and Sap Flow in the Xylem of Red Maple Stems (Acer rubrum)

In this paper we report measurements concerning the conductivityof water and ions and the interaction between the two in excisedpieces of xylem of red maple stems under various conditions.We have also demonstrated that it is possible to detect theflow of solutions through the stems of maple by measuring thedegree of interaction between the flow of water and ions. Inthis technique we apply voltage pulses of ±V volts acrossa length of stem and detect the unequal current pulses resultingfrom the greater frictional drag when current (which is carriedprimarily by cations) is flowing against the water stream thanwhen flowing with the water stream. The hydraulic conductivityof recent maple sapwood ranges from 30 to 90 cm³ s^–1 cm^–2(J cm^–3)^–1 cm; in 2 mM KCl the electrical conductivityis roughly 3 x 10^–4 mho cm^–1 and the electro-kineticcross coefficient is roughly 4x10^–5 A cm^–2 (J cm^–3)^–1cm.     

Vascular Regeneration and Long Distance Transport of Indole-3-acetic Acid in Coleus Stems

Excision of all leaves and buds of Coleus blumei Benth. plants reduced xylem cell and sieve tube regeneration a highly significant amount around a wound in internode number 5 when compared with regeneration in intact (wounded) plants. Application of indoleacetic acid (IAA-¹⁴C) to the cut surface of internode number 2 restores regenerative activity around the wound in internode number 5. Radioactivity applied as IAA-¹⁴C reaches the wound area when applied at the cut surface of inter-node number 2 showing a logarithmic decrease with distance from the point of application. Chromatography showed that radioactivity was located close to the R_F of IAA as well as near the solvent front.     

The Transport and Affinity of Substituted Benzenes in Soybean Stems

The sorption of non-ionized substituted benzenes in the xylemtissue of excised soybean stems was studied. A positive pressureperfusion technique was used to force solutions containing chemicalsand tritiated water through 50-mm stem segments. The stem effluentwas collected at timed intervals and analysed for each chemicaland tritium activity. A theoretical mass transport model wasdeveloped and the experimental data were analysed to calculatethe flux of water, chemical sorption, and first order rate constants.Sorption of hydrophobic chemicals in the xylem tissue appearedto be the dominant interaction responsible for impeding solutemovement. Linear relationships between sorption and accumulationof the chemicals in the xylem tissue, and the chemical octanol/waterpartition coefficients were demonstrated. The mathematical derivationof the mass transport model is described. Key words: Mass transport, adsorption, partition coefficients     

Cadmium-Induced Structural and Ultrastructural Changes in the Vascular System of Bush Bean Stems

The effects of a phytotoxic cadmium concentration (4.45 . 10^?5 M) on the structure and ultrastructure of bean plant stems were analysed by light (LM), transmission electron (TEM) and scanning electron microscopy (SEM). Cadmium significantly reduced both the number and the size of tracheary elements. Cadmium-induced electron dense depositions, which seemed to obstruct partially some xylem vessels, were found only in the later maturing tracheary elements (helical, scalariform or reticulate structure), but not in the annular structured early differentiated ones. Plants exposed to Cd showed less fiber development than the control plants. In the plants treated with Cd abnormally high amounts of calcium oxalate crystals were found in the paratracheary parenchyma cells of the bottom of the stems. The levels of soluble Ca²⁺ in the expressed stem sap of Cd-treated plants was significantly decreased, while substantial amounts of soluble Cd were detected. The probable mechanisms of the structural alterations observed are discussed.     

The Transport of Potassium to the Xylem Exudate of Ryegrass: II. EXUDATION

The membrane potentials of ryegrass root cells (E_v0) were foundto be linearly related to the logarithm of the external KClconcentration ([KCl]_o), over the range 0.1 to 20.0 mM. Exudationwas studied over the same concentration range. The concentrationof potassium in the exudate did not vary significantly with[KCl]₀ but the rates of movement of water and potassium to theexudate (f_H2O and f_K respectively) and the electrical potentialand electrochemical potential for potassium in the exudate (E_xoand _x0,K respectively) all tended to decreaseas [KCl]₀ increased. There was a very highly significant correlationbetween f_K and f_H2O. By rapidly increasing [KCl]E₀ and following the depolarization,two components of E_x0 were observed. The first of these wasinstantaneous and was attributed to E_v0 of the epidermal cells.The second component, a gradual repolarisation which commencedabout 9 min later, was attributed to E_v0 of the stelar cells.With an additional contribution from electro-osmosis, thesetwo components quantitatively account for E_x0. The implications of these data for the mechanism of radial iontransport in roots are discussed and it is concluded that thestelar cells are not exclusively specialized for transportingpotassium into the xylem vessels.     

Water Transport during Acid-Induced Growth of Excised Hypocotyl Sections of Vigna unguiculata under Xylem Perfusion

Elongation growth of abraded hypocotyl sections of Vigna unguiculataunder xylem perfusion was markedly promoted a few minutes afterthe application of an acid aerosol generated from a solutionof HCl. At the beginning of the acid-induced growth, intracellularpressure (Pⁱ) began to decrease and the membrane potential betweenthe symplast and the xylem apoplast (V^px) began to depolarize.Subsequently, Pⁱ and V^px remained at a reduced level and a depolarizedlevel, respectively, while the promotion of elongation growthcontinued for more than 4 hours. The electrogenic componentof the xylem membrane potential (V^px_act) gradually increasedto about twice that before acid treatment. There was a closecorrelation between the enhanced growth and the decrease inintracellular pressure within 30 min after application of acidbut little correltion after 60 min. By contrast, there was littlecorrelation between the promotion of growth and the activityof the xylem pump after 30 min while a close correlation wasobserved after 60 min. It is inferred that the acid-induced activation of water uptakeconsists of two major processes, in series, that are drivenby different forces: the rapid uptake of water for more than30 min, driven by hydrostatic force generated by loosening ofcell walls; and a long-lasting enhancement of water uptake forat least 4 h, which is driven by osmotic force that is generatedby the canal system within the xylem. (Received October 17, 1994; Accepted January 23, 1995)     

Water Transport Properties of the Grape Pedicel during Fruit Development: Insights into Xylem Anatomy and Function Using Microtomography

Xylem flow of water into fruits declines during fruit development, and the literature indicates a corresponding increase in hydraulic resistance in the pedicel. However, it is unknown how pedicel hydraulics change developmentally in relation to xylem anatomy and function. In this study on grape (Vitis vinifera), we determined pedicel hydraulic conductivity (k_h) from pressure-flow relationships using hydrostatic and osmotic forces and investigated xylem anatomy and function using fluorescent light microscopy and x-ray computed microtomography. Hydrostatic k_h (xylem pathway) was consistently 4 orders of magnitude greater than osmotic k_h (intracellular pathway), but both declined before veraison by approximately 40% and substantially over fruit development. Hydrostatic k_h declined most gradually for low (less than 0.08 MPa) pressures and for water inflow and outflow conditions. Specific k_h (per xylem area) decreased in a similar fashion to k_h despite substantial increases in xylem area. X-ray computed microtomography images provided direct evidence that losses in pedicel k_h were associated with blockages in vessel elements, whereas air embolisms were negligible. However, vessel elements were interconnected and some remained continuous postveraison, suggesting that across the grape pedicel, a xylem pathway of reduced k_h remains functional late into berry ripening.In grape (Vitis vinifera), fruit growth by water accumulation follows a double sigmoid pattern and is influenced by the diurnal and developmental changes in water flow between fruit and the parent plant (Matthews and Shackel, 2005). Until the onset of fruit ripening (i.e. veraison), water enters the fruit predominantly via the xylem and thereafter mainly through the phloem (Greenspan et al., 1994, 1996). Choat et al. (2009) showed that the hydraulic conductance (i.e. 1/resistance) of the grape berry and pedicel declines substantially at later ripening stages predominantly due to a decline in pedicel conductance. Significant developmental changes in pedicel hydraulic properties were also reported for tomato (Solanum lycopersicum) and were found to be associated with xylem anatomical changes (Lee 1989; Van Ieperen et al., 2003; Rancić et al., 2008, 2010). Due to its position along the vascular transport pathway between fruit and the parent plant, the pedicel can play an important role in affecting fruit growth, as in kiwi (Actinidia deliciosa; Mazzeo et al., 2013). However, for grape, it needs to be elucidated how pedicel hydraulic properties change developmentally in relation to xylem anatomy and function.The location and nature of the loss in hydraulic conductance between the parent plant and the fruit is unclear and may differ among fruits. For tomato, Malone and Andrews (2001) showed that most of the loss of hydraulic conductance occurs in the fruit per se, but Van Ieperen et al. (2003) reported important and decreasing hydraulic conductance in the pedicel abscission zone over fruit development. For Citrus spp., Garcia-Luis et al. (2002) reported that xylem vessels in the pedicel remain largely functional late into fruit ripening. For grape, although vessel breakage in the berry was thought to lead to xylem dysfunction (Coombe and McCarthy 2000), several studies and methods have shown that xylem vessels in the fruit remain functional (Rogiers et al., 2001; Bondada et al., 2005; Chatelet et al., 2008a, 2008b). In line with these findings, data by Keller et al. (2006) suggest that the pedicel xylem also remains at least partially functional in ripening grape berries and can conduct water to and from the parent plant. Nevertheless, a reduction in the ability to transport water during ripening has been reported for grape (Tyerman et al., 2004; Choat et al., 2009) and other fleshy fruits, such as apple (Malus domestica; Lang and Ryan, 1994) and kiwi (Mazzeo et al., 2013), and it still remains unclear what causes this loss in xylem hydraulic conductance. For the grape pedicel, Choat et al. (2009) detected higher concentrations of xylem solutes postveraison and proposed that this is related to the deposition of gels into the xylem vessel lumen. However, direct evidence for the presence of xylem blockage and/or embolism formation in the grape pedicel is missing.This study of the grape ‘Cabernet Sauvignon’ pedicel was conducted with the goal to obtain a comprehensive understanding of how changes in hydraulic properties relate to changes in xylem structure and function over fruit development. Over the course of fruit development from 20 to 90 d after anthesis (DAA), water transport properties of pedicels were investigated under osmotic and hydrostatic driving forces using a modified pressure-probe system. This was combined with analyses of spatial and temporal changes in pedicel xylem anatomy and function using fluorescent light microscopy and x-ray computed microtomography (microCT; Brodersen et al., 2010, 2013; Rancić et al., 2010).     

The Willey And Phillips System Revisited:

Abstract

The purpose of this paper is to evaluate the logical structure of the Willey and Phillips system of culture historic-integration and propose the expansion of that logical structure to its inherent limits. The dimensions and features of this expanded paradigm are discussed and the resulting classes defined. Previous evaluations of the Willey and Phillips system by Plains archeologists are reviewed in light of the original and expanded paradigm.     

Polarity of IAA Effect on Sieve-Tube and Xylem Regeneration in Coleus and Tomato Stems

A technique is described for the processing of regenerated xylem and sieve tubes from the same wound area for microscopic and quantitative comparison.

Regeneration was examined in internodes of 2 developmental stages in Coleus: internode 2, elongating, characteristic of primary growth; and internode 5, non-elongating, characteristic of secondary growth.

Transport of indoleacetic acid (IAA) in excised number 5 internodes of Coleus is strictly polar, in a basipetal direction, judging by a regeneration bioassay involving both sieve tube strands and xylem cells. Similar results were obtained with tomato.

If isolated number 5 Coleus internodes are not treated with hormone, they regenerate no xylem cells and a small number of sieve tube strands. With increasing concentrations of IAA added apically, the number of regenerated sieve tube strands (and, with higher concentrations, of xylem cells) increases progressively up to 1% IAA, the highest concentration applied.

Internode 2 of Coleus regenerates fewer xylem cells or sieve tube strands than internode 5, whether on the otherwise intact plant or with a given concentration of IAA added apically. The amount of regenerated xylem increases with added apical IAA, except that the highest concentration gives no further increase. The number of xylem cells regenerated in intact plants occurs at the same interpolated IAA concentration as in number 5 internodes. No concentration of IAA tried provided replacement of intact number of sieve tube strands in internode 2.

IAA can exert a regenerative stimulus on both xylem and sieve tubes in the area immediately adjacent to the site of its application.

     

泡桐维管形成层及木质部区的转录组分析

采用Illumina HiSeqTM2500高通量测序技术,获得泡桐维管形成层及木质部区组织转录组的109021918条clean reads(16.35 Gb)。将clean reads从头组装得到104432个单基因簇(Unigene),平均长度662 nt。将组装得到的Unigenes与公共数据库进行序列比对,分别有40789(Nr:39.05%)、31675(NT:30.33%)、15539(COG:14.87%)、29168(GO:27.93%)、16316(KEGG:15.62%)、30499(SwissProt:29.20%)以及28828(Pfam:27.6%)个Unigenes获得功能注释。通过与GO数据库的比对分析,注释的29168个Unigenes归于生物过程、细胞组分及分子功能三大类的55个功能组;15539条Unigenes注释到COG数据库,被分为25个类别中;基于KEGG数据库可将16316个Unigenes归于130个代谢途径。此外,在泡桐的维管形成层及木质部区转录组中共检测出16118个简单序列重复(SSR)位点。为进一步挖掘泡桐重要功能基因提供了大量数据。     

Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

The amino acid and protein metabolism of roots of maize has been studied. The important role of the free amino acids and proteins of the roots as active agents in nitrogen assimilation is pointed out. Nitrogen supplied as nitrate is preferably incorporated into α-ketoglutaric acid, and then by trans-aminases transferred to other ketoacids. In the case of ammonia supply the function of a nitrogen-accumulating assimilation system leading to the formation of Arg, Glu-NH₂ and Asp-NH₂ is shown.     

Proline Metabolism and Transport in Maize Seedlings at Low Water Potential

The growing zone of maize seedling primary roots accumulatesproline at low water potential. Endosperm removal and excisionof root tips rapidly decreased the proline pool and greatlyreduced proline accumulation in root tips at low water potential.Proline accumulation was not restored by exogenous amino acids.Labelling root tips with [¹⁴C]glutamate and [¹⁴C]proline showedthat the rate of proline utilization (oxidation and proteinsynthesis) exceeded the rate of biosynthesis by five-fold athigh and low water potentials. This explains the reduction inthe proline pool following root and endosperm excision and theinability to accumulate proline at low water potential. Theendosperm is therefore the source of the proline that accumulatesin the root tips of intact seedlings. Proline constituted 10% of the amino acids released from the endosperm. [¹⁴C]Prolinewas transported from the scutellum to other parts of the seedlingand reached the highest concentration in the root tip. Less[¹⁴C]proline was transported at low water potential but becauseof the lower rate of protein synthesis and oxidation, more accumulatedas proline in the root tip. Despite the low biosynthesis capacityof the roots, the extent of proline accumulation in relationto water potential is precisely controlled by transport andutilization rate.     

The Effect of Coumarin on Young's Modulus in Sunflower Stems and Maize Roots and on Water Permeability in Potato Parenchyma

By means of the resonance frequency method Young's, modulus has been determined after coumarin treatment of growing segments of etiolated sunflower hypocotyl segments and in maize roots. Coumarin caused a decrease in Young's modulus in both shoot and root tissue. The response was very rapid; in sunflower hypocotyls the decrease in elastic modulus appeared 3 min after application of coumarin. The effects produced by coumarin were similar to those found by auxin. Coumarin increased the rate of water efflux out of potato parenchyma by about 20%. The increase in water permeability was evident within 3 min.     

Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

Xylem sap was collected from nitrogen-starved maize plants and investigations were made on the nitrogen transported. It appears from the results that several pools for different amino acids exist, which have different relations to the transport of nitrogen taken up. While in maize roots Glu, Glu-NH₂ and Arg are transported directly from the place of their synthesis, the transported Asp and Ala predominantly derive from the pool of amino acids synthetized before application of nitrogen. An explanation of this observation is offered.     

Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

The uptake and assimilation of nitrate and ammonia have been studied in Zea mays. Nitrogen-starved maize roots are capable of accumulating a potential capacity for nitrogen uptake and assimilation. Reestablishment of nitrogen supply leads to intense uptake, reaching 154 % of the reference variant level after 24 hours when nitrate is supplied, and 121 % when ammonia is supplied. After 24 hours the insoluble nitrogen fraction accounts for 80, 54 and 55 % of the total taken up in the PK + NO₃^-, PK + NH₄⁺ and NPK variants respectively.