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PubMed2005

Patterned cell determination in a plant tissue: the secondary phloem of trees.

The secondary vascular tissues (xylem and phloem) of woody plants originate from a vascular cambium and develop as radially oriented files of cells. The secondary phloem is composed of three or four cell types, which are organised into characteristic recurrent cellular sequences within the radial cell files of this tissue. There is a gradient of auxin (indole acetic acid) across both the cambium and the immediately postmitotic cells within the xylem and phloem domains, and it is believed that this morphogen, probably in concert with other morphogenic factors, is closely associated with the determination and differentiation of the different cells types in each tissue. A hypothesis is developed that, in conjunction with the positional values conferred by the graded radial distribution of morphogen, cell divisions at particular positions within the cambium are sufficient to determine not only each of the phloem cell types but also their recurrent pattern of differentiation within each radial cell file.

Animals↗
PubMed2006

Regeneration of the secondary vascular system in poplar as a novel system to investigate gene expression by a proteomic approach.

Wood formation is a complex process composing many biological events. To access its key developmental stages, we have established a regeneration system that can mimic the initiation and differentiation of cambium cells for Chinese white poplar. Anatomical studies showed that new cambium and xylem re-appeared in sequence within a few weeks after being debarked. This provides the opportunity to follow key stages of wood formation by sampling clonal trees at different regeneration times. We used this system in combination with a proteomic approach to analyze proteins expressed in different regeneration stages. PMFs for 244 proteins differentially displayed were obtained and queried against public databases. Putative functions of 199 of these proteins were assigned and classified. Regulatory genes for cell cycle progression, differentiation and cell fate were expressed in the formation of cambial tissue, while 27 genes involved in secondary wall formation were predominantly found in the xylem developing stage. This indicates that the change of gene expression pattern is corresponding to the progression of second vascular system regeneration when and where the key events of wood development occur. Further exploration of these interesting genes may provide insight into the molecular mechanisms of wood formation.

Electrophoresis, Gel, Two-Dimensional↗
PubMed2006

The OPT family functions in long-distance peptide and metal transport in plants.

The OPT family was first described six years ago, and much progress has been made in understanding the role these transporters play in their respective organisms. Plants are the only organisms in which both YS- and PT-type transporters have been characterized, and all of these OPTs appear to be plasma membrane-bound proteins, suggesting that they import substrates from the apoplasm or the environment. YS1 is the only OPT known to translocate substrates from the rhizosphere, whereas all the other OPTs seem to function in long-distance transport of peptides or metals. The sum of all the studies covered in this review suggest the model for OPT function in plants depicted in Figure 4. Peptides, metal-NA, and metal-MAs complexes (Strategy II plants only) are loaded into the xylem stream in the root for long-distance transport. OPTs unload the xylem by importing substrates into sink tissues such as leaves and by transloading the phloem. Peptides and metal-NA complexes exit the leaf symplasmically or by importation into the phloem from the apoplasm by OPTs. The filial tissues (endosperm and embryo) are apoplasmically separated from the maternal tissues, and OPTs may also function in loading the developing seed. Similarly, seedlings are symplasmically disconnected from the endosperm and OPTs may help move nutrients to the growing plant. Much progress has been made in the last two years toward understanding OPTs in plants, although several fundamental questions remain unanswered. Namely, what is the level of redundancy? Is there any substrate overlap between YS and PT OPTs? How crucial are their respective roles? Are there additional functions beyond peptide and metal transport? Given the recent pace of discovery, we may not have to wait long to find out the answers.

Metals↗
PubMed1996

Induction of cysteine and serine proteases during xylogenesis in Zinnia elegans.

The terminal process of xylogenesis, autolysis, is essential for the formulation of a tubular system for conduction of water and solutes throughout the whole plant. Several hydrolase types are implicated in autolysis responsible for the breakdown of cytoplasm. Here, we characterize p48h-17 cDNA from in vitro tracheary elements (TEs) of Zinnia elegans which encodes a preproprotein similar to papain. The putative mature protein, a cysteine protease, has a molecular mass of 22,699 Da with a pI of 5.7. DNA gel blot analysis indicated that p48h-17 is likely encoded by one or two genes. The p48h-17 mRNA accumulated markedly in in vitro differentiating TEs, whereas it appeared not to be induced in response to senescence and wounding in the leaves or H2O2 challenge in the cultured mesophyll cells. In stems, the expression of the p48h-17 gene was preferentially associated with differentiating xylem. Activity gel assays demonstrated that a cysteine and a serine protease, which had apparent molecular masses of 20 kDa and 60 kDa, respectively, were markedly induced during in vitro TE differentiation. The cysteine protease activity was also preferentially present in the xylem of Zinnia stems. Transient expression of the p48h-17 cDNA in tobacco protoplasts resulted in the production of a 20 kDa cysteine protease. Taken together, the results indicate that the p48h-17 gene appears to be preferentially associated with xylogenesis, and both the cysteine and serine proteases might be involved in autolysis during xylogenesis.

Amino Acid Sequence↗
PubMed1976

Some patterns of herbicide and growth regulator intake, persistence, and distribution in sugarcane.

Absorption of 14C-labeled herbicides from nutrient culture solution through the roots of sugarcane plants (Saccharum spp. hybrids) resulted in three major types of distribution. Labeled residues from atrazine, ametryne, and metribuzine moved easily through the xylem to the green leaves and were deposited mainly at the leaf margins and tip. Senescence and leaf abscission removed most of the deposit from the plant. Picloram translocated rapidly into the leaves, but appeared to recycle from older to younger leaves with only small portions remaining in the senescent leaves. Asulam and pentachlorophenol (PCP) were absorbed by the roots; residues did not pass into the xylem and remained fixed in the roots. Foliar additions of labeled herbicides and growth regulators generally showed only minor absorption and translocation within the leaf. Weathering processes and volatility during a 6- to 12-week period left variable amounts of residue, characteristic of individual compounds, absorbed at the treated sites.

Absorption↗
PubMed2001

Glycine-rich proteins as structural components of plant cell walls.

Glycine-rich proteins (GRPs) have been found in the cell walls of many higher plants and form a third group of structural protein components of the wall in addition to extensins and proline-rich proteins. The primary sequences of GRPs contain more than 60% glycine. GRPs are localized mainly in the vascular tissue of the plant, and their coding genes provide an excellent system to analyze the molecular basis of vascular-specific gene expression. In French bean, the major cell wall GRP has been localized at the ultrastructural level in the modified primary cell wall of protoxylem. Immunological studies showed that it forms a major part of these highly extensible and specialized cell walls. Specific digestion of GRP1.8 from bean by collagenase suggests that it shares structural similarities with collagen. The protein is synthesized by living protoxylem cells as well as xylem parenchyma cells. After cell death, GRPs are exported from neighboring xylem parenchyma cells to the protoxylem wall, a rare example of protein transport between cells in plants. We propose that GRPs are part of a repair system of the plant during the stretching phase of protoxylem.

Amino Acid Sequence↗
PubMed2003

Physiological and morphological response patterns of Populus deltoides to alluvial groundwater pumping.

We examined the physiological and morphological response patterns of plains cottonwood [ Populus deltoides subsp. monilifera (Aiton) Eck.] to acute water stress imposed by groundwater pumping. Between 3 and 27 July 1996, four large pumps were used to withdraw alluvial groundwater from a cottonwood forest along the South Platte River, near Denver, Colorado, USA. The study was designed as a stand-level, split-plot experiment with factorial treatments including two soil types (a gravel soil and a loam topsoil over gravel), two water table drawdown depths ( approximately 0.5 m and >1.0 m), and one water table control (no drawdown) per soil type. Measurements of water table depth, soil water potential (Psi(s)), predawn and midday shoot water potential (Psi(pd) and Psi(md)), and D/H (deuterium/hydrogen) ratios of different water sources were made in each of six 600-m(2) plots prior to, during, and immediately following pumping. Two additional plots were established and measured to examine the extent to which surface irrigation could be used to mitigate the effects of deep drawdown on P. deltoides for each soil type. Recovery of tree water status following pumping was evaluated by measuring stomatal conductance ( g(s)) and xylem water potential (Psi(xp)) on approximately hourly time steps from before dawn to mid-afternoon on 11 August 1996 in watered and unwatered, deep-drawdown plots on gravel soils. P. deltoides responded to abrupt alluvial water table decline with decreased shoot water potential followed by leaf mortality. Psi(pd) and percent leaf loss were significantly related to the magnitude of water table declines. The onset and course of these responses were influenced by short-term variability in surface and ground water levels, acting in concert with physiological and morphological adjustments. Decreases in Psi(pd) corresponded with increases in Psi(md), suggesting shoot water status improved in response to stomatal closure and crown dieback. Crown dieback caused by xylem cavitation likely occurred when Psi(pd) reached -0.4 to -0.8 MPa. The application of surface irrigation allowed trees to maintain favorable water status with little or no apparent cavitation, even in deep-drawdown plots. Two weeks after the partial canopy dieback and cessation of pumping, g(s) and Psi(xp) measurements indicated that water stress persisted in unwatered P. deltoides in deep-drawdown plots.

Conservation of Natural Resources↗
PubMed2006

Colonization of maize and rice plants by strain Bacillus megaterium C4.

Bacillus megaterium C4, a nitrogen-fixing bacterium, was marked with the gfp gene. Maize and rice seedlings were inoculated with the, GFP-labeled B. megaterium C4 and then grown in gnotobiotic condition. Observation by confocal laser scanning microscope showed that the GFP-labeled bacterial cells infected the maize roots through the cracks formed at the lateral root junctions and then penetrated into cortex, xylem, and pith, and that the bacteria migrated slowly from roots to stems and leaves. The bacteria were mainly located in the intercellular spaces, although a few bacterial cells were also present within the xylem vessels, root hair cells, epidermis, cortical parenchyma, and pith cells. In addition, microscopic observation also revealed clearly that the root tip in the zone of elongation and differentiation and the junction between the primary and the lateral roots were the two sites for the bacteria entry into rice root. Therefore, we conclude that this Gram-positive nitrogen-fixer has a colonization pattern similar to those of many Gram-negative diazotrophs, such as Azospirillun brasilense Yu62 and Azoarcus sp. As far as we know, this is the first detailed report of the colonization pattern for Gram-positive diazotrophic Bacillus.

Bacillus megaterium↗
PubMed2004

Modelling the isotope enrichment of leaf water.

Farquhar and Gan have proposed a model for the spatial variation in the isotopic enrichment of H(2)(18)O across a leaf, which is specifically formulated for monocotyledoneous leaves. The model is based on the interaction between mass fluxes longitudinally within the xylem, and fluxes laterally through veinlets into the lamina mesophyll, where moisture leaves the leaf through transpiration. The lighter, more abundant, molecule H(2)(16)O escapes preferentially with the evaporating water, resulting in the enrichment of H(2)(18)O at these sites. Enriched water diffuses throughout the leaf, and it is this spatial distribution of enriched water which the model seeks to capture. In this paper we present a general formulation of the model in terms of mass flux, extending it to include variable transpiration rates across the leaf surface, as well as a tapering xylem. Solutions are developed for the general case and, since the solutions present in the form of Kummer functions, properties are established as well as methods for estimating the solutions under certain conditions relevant to the biology. The model output is compared with Gan's data collected from maize plants.

Models, Biological↗
PubMed2003

The small subunit ADP-glucose pyrophosphorylase ( ApS) promoter mediates okadaic acid-sensitive uidA expression in starch-synthesizing tissues and cells in Arabidopsis.

Transgenic plants of Arabidopsis thaliana Heynh., transformed with a bacterial beta-glucuronidase (GUS) gene under the control of the promoter of the small subunit (ApS) of ADP-glucose pyrophosphorylase (AGPase), exhibited GUS staining in leaves (including stomata), stems, roots and flowers. Cross-sections of stems revealed GUS staining in protoxylem parenchyma, primary phloem and cortex. In young roots, the staining was found in the root tips, including the root cap, and in vascular tissue, while the older root-hypocotyl axis showed prominent staining in the secondary phloem and paratracheary parenchyma of secondary xylem. The GUS staining co-localized with ApS protein, as found by tissue printing using antibodies against ApS. Starch was found only in cell and tissue types exhibiting GUS staining and ApS labelling, but not in all of them. For example, starch was lacking in the xylem parenchyma and secondary phloem of the root-hypocotyl axis. Sucrose potently activated ApS gene expression in leaves of wild-type (wt) plants, and in transgenic seedlings grown on sucrose medium where GUS activity was quantified with 4-methylumbelliferyl-beta-glucuronide as substrate. Okadaic acid, an inhibitor of protein phosphatases 1 and 2A, completely blocked expression of ApS in mature leaves of wt plants and prevented GUS staining in root tips and flowers of the transgenic plants, suggesting a similar signal transduction mechanism for ApS expression in various tissues. The data support the key role of AGPase in starch synthesis, but they also underlie the ubiquitous importance of the ApS gene for AGPase function in all organs/tissues of Arabidopsis.

Arabidopsis↗
PubMed2004

Systemic induction of traumatic resin ducts and resin flow in Austrian pine by wounding and inoculation with Sphaeropsis sapinea and Diplodia scrobiculata.

The potential role of the resin system in the response of Austrian pine (Pinus nigra Arn.) seedlings to mechanical injury and fungal infection was studied in greenhouse experiments. Anatomical observations were performed on 2-year-old plants wounded at collar level and inoculated with Sphaeropsis sapinea (Fr.: Fr.) Dyko & Sutton in Sutton or Diplodia scrobiculata (J. de Wet, B. Slippers & M. J. Wingfield, sp. nov.; sensu de Wet et al. 2003), two fungal pathogens that cause shoot blight and canker on conifers, and that are characterized by different levels of aggressiveness. Histological examination of host tissue taken from the stem at 0, 8, and 12 cm above the treatment site revealed significant treatment- and time-dependent effects on the course of locally and systemically induced traumatic resin duct (TRD) development. Occurrence of TRDs was observed after 4 days only in seedlings inoculated with D. scrobiculata. At 12 days, TRDs were present also in mock-inoculated controls. No TRDs appeared in seedlings inoculated with S. sapinea. However, S. sapinea caused loss of vacuolar phenolics, severe disruption of cambial tissue and invaded the host xylem quickly and apparently unimpeded, whereas D. scrobiculata was never detected in the host xylem. Five-year-old Austrian pines subjected to the same stem base treatments were used to determine the resin mass flowing from the stem 30 cm above the treatment sites. Wounding and/or inoculation induced a significant, 8.3-fold average increase in systemic resin flow over the untreated trees 3 weeks after basal treatment, suggesting that wounding is the sole prerequisite for systemic induction of resin flow. The results are discussed in the context of current disease resistance models.

Ascomycota↗
PubMed2006

Taproot promoters cause tissue specific gene expression within the storage root of sugar beet.

The storage root (taproot) of sugar beet (Beta vulgaris L.) originates from hypocotyl and primary root and contains many different tissues such as central xylem, primary and secondary cambium, secondary xylem and phloem, and parenchyma. It was the aim of this work to characterize the promoters of three taproot-expressed genes with respect to their tissue specificity. To investigate this, promoters for the genes Tlp, His1-r, and Mll were cloned from sugar beet, linked to reporter genes and transformed into sugar beet and tobacco. Reporter gene expression analysis in transgenic sugar beet plants revealed that all three promoters are active in the storage root. Expression in storage root tissues is either restricted to the vascular zone (Tlp, His1-r) or is observed in the whole organ (Mll). The Mll gene is highly organ specific throughout different developmental stages of the sugar beet. In tobacco, the Tlp and Mll promoters drive reporter gene expression preferentially in hypocotyl and roots. The properties of the Mll promoter may be advantageous for the modification of sucrose metabolism in storage roots.

Beta vulgaris↗
PubMed1999

Purification, characterization, and immunolocalization of hydroxycinnamoyl-CoA: tyramine N-(hydroxycinnamoyl)transferase from opium poppy.

A development-specific and elicitor-inducible acyltransferase [hydroxycinnamoyl-CoA: tyramine N-(hydroxycinnamoyl)transferase (THT; EC 2.3.1.110)] that catalyzes the transfer of hydroxycinnamic acids from hydroxycinnamoyl-CoA esters to hydroxyphenethylamines was purified 988-fold to apparent homogeneity from opium poppy (Papaver somniferum L.) cell-suspension cultures. The purification procedure, which resulted in a 6.8% yield, involved hydrophobic interaction and anion-exchange chromatography, followed by affinity chromatography on Reactive Yellow-3-Agarose using the acyl donor (feruloyl-CoA) as eluent. Purified THT had an isoelectric point of 5.2, a native molecular mass of approximately 50 kDa, and consisted of two apparently identical 25-kDa subunits as determined by two-dimensional polyacrylamide gel electrophoresis. The purified enzyme was able to synthesize a variety of amides due to a relatively low specificity for cinnamoyl-CoA derivatives and hydroxyphenethylamines. The best substrates were feruloyl-CoA (VK(m)(-1)13.4 mkat g(-1) M(-1)) and tyramine (VK(m)(-1)6.57 mkat g(-1) M(-1)). The THT activity increased during development of opium poppy seedlings, occurred at high levels in roots and stems of mature plants, and was induced in cell-suspension cultures after treatment with a pathogen-derived elicitor. Immunoblot analysis using THT mouse polyclonal antibodies did not always show a correlation between THT polypeptide and enzyme activity levels. For example, despite low THT activity in leaves, an abundant 25-kDa immunoreactive polypeptide was detected. Immunohistochemical localization showed that THT polypeptides occur in cortical and xylem parenchyma, immature xylem vessel elements, root periderm, anthers, ovules, and the inner layer of the seed coat, but are most abundant in phloem sieve-tube members in roots, stems, leaves, and anther filaments.

Acyltransferases↗
PubMed2000

Different pH-dependences of K+ channel activity in bundle sheath and mesophyll cells of maize leaves.

The isolation of bundle sheath protoplasts from leaves of Zea mays L. for patch clamp whole-cell experiments presents special problems caused by the suberin layer surrounding these cells. These problems were overcome by the isolation technique described here. Two different types of whole-cell response were found: a small response caused by MB-1 (maize bundle sheath conductance type 1) which was instantaneously activated, and another caused by MB-2 (maize bundle sheath conductance type 2) consisting of an instantaneous response (maize bundle sheath K+ instantaneous current type 2; MB-KI2) similar to but stronger than the current through MB-1 plus a small time-dependent outward rectifying component (maize bundle sheath activated outward rectifying current; MB-AOR) with voltage-dependent delayed activation. The occurrence of MB-AOR was often accompanied by a smaller contribution from an inward rectifying channel at negative potentials. Activation of MB-2 required ATP. It is suggested that MB-1 and MB-2 are related to bundle sheath cells with and without direct contact with the xylem vessels. In mesophyll cells, only one type of response caused by MM-2 (maize mesophyll conductance type 2) was found with an instantaneous (maize mesophyll K+ instantaneous current type 2, MM-KI2) and a voltage-dependent delayed component (maize mesophyll activated outward rectifying current, MM-AOR). The most striking difference between bundle sheath and mesophyll cells was the pH dependence of K+ uptake. At pH 7.2, uptake of K+ by MB-2 was identical to that by MM-2 over the whole voltage range. However, acidification stimulated K+ conductance in bundle sheath cells, whereas a decrease was found for MM-2. At pH 6.15, the bundle sheath channel MB-2 had more than a 10-fold higher K+ uptake at positive and negative potentials than MM-2. The channel MB-1, too, was stimulated by low pH. This seems to indicate a putative role for MB-1 and MB-2 in charge balance during uptake of nutrients via cotransport from the xylem into the symplasm.

Adenosine Triphosphate↗
PubMed2001

The cytoskeleton facilitates a three-dimensional symplasmic continuum in the long-lived ray and axial parenchyma cells of angiosperm trees.

The microtubule (MT), microfilament (MF) and myosin components of the cytoskeleton were studied in the long-lived ray and axial parenchyma cells of the secondary xylem (wood) and secondary phloem of two angiosperm trees, Aesculus hippocastanum L. (horse-chestnut) and Populus tremula L. x P. tremuloides Michx. (hybrid aspen), using indirect immunofluorescence localisation and transmission electron microscopy. MTs and MFs were bundled and oriented axially (parallel to the cell's long axis) within all parenchyma cell types after they had fully differentiated. Additionally, actin and myosin were immunolocalised at the thin-walled membranes of the pits, which linked cells in neighbouring files of both ray and axial parenchyma, and at the pits between axial and ray parenchyma cells themselves. Anti-callose antibody immunolocated the plasmodesmata at the pit membranes, and in the same pattern as that of anti-myosin. Ray cells are important symplasmic pathways between the xylem and the phloem throughout the life of trees. We hypothesise that the MT and MF components of the cytoskeleton in the ray and axial parenchyma cells are involved in the transport of materials within those cells, and, in association with the acto-myosin of plasmodesmata at pit fields, are also important in intercellular transport. Thus, the symplasmic coupling between ray cells, between axial parenchyma cells, and between axial parenchyma and ray cells represents an extensive three-dimensional communication pathway permeating the tree from the phloem through the cambium into the wood. We suggest that this cytoskeletal pathway has an important role in delivery of photosynthate, and mobilised reserves, to the actively dividing cambium, and in the movement of materials to sites of reserve deposition, principally within the wood. This pathway could also have an important role in co-ordinating developmental processes throughout the tree.

Actin Cytoskeleton↗
PubMed2001

Analysis of the distribution of copper amine oxidase in cell walls of legume seedlings.

Copper-containing amine oxidase (CuAO) has been proposed to play a role in H2O2 production in plant cell walls during cell development and in response to pathogen attack. We have compared the localisation of CuAO in pea (Pisum sativum L.), lentil (Lens culinaris M.) and chick pea (Cicer arietinum L.) grown under different light conditions, using both immuno- and histochemical techniques. The enzyme was detected by indirect immunofluorescence in the cell walls of parenchyma tissues of etiolated pea and lentil plants and was particularly abundant at intercellular spaces. Upon de-etiolation, CuAO largely disappeared from cortical cell walls except in the region of intercellular spaces. In the apical internode of light-grown seedlings, CuAO occurred mainly in cortical cell walls and, to some extent, in cell walls of xylem vessels. In both the elongation zone and mature regions of roots, CuAO was restricted to cortical cell walls and some cell junctions close to the meristem. Extensin epitopes co-localised to intercellular spaces of the cortex in de-etiolated pea, indicating that CuAO may have a role in cell wall strengthening at intercellular spaces. In chick pea, the localisation of the enzyme varied between different cultivars that have differing susceptibility to the fungus Ascochyta rabiei. In a susceptible cultivar Calia, immunogold labelling localised CuAO to cell walls of the cortex, as in lentil and pea, while in a resistant cultivar Sultano, it was most abundant in xylem vessels and, in light-grown plants, in the epidermis. These expression patterns are discussed with regard to the possible functions of amine oxidase in cell growth, cell differentiation and pathogen resistance.

Amine Oxidase (Copper-Containing)↗
PubMed2002

Myosin, microtubules, and microfilaments: co-operation between cytoskeletal components during cambial cell division and secondary vascular differentiation in trees.

The immunolocalisation of unconventional myosin VIII ('myosin') in the cells of the secondary vascular tissues of angiosperm (Populus tremula L. x P. tremuloides Michx. and Aesculus hippocastanum L.) and gymnosperm (Pinus pinea L.) trees is described for the first time and related to other cytoskeletal elements, as well as to callose. Both myosin and callose are located at the cell plate in dividing cambial cells, whereas actin microfilaments are found alongside the cell plate; actin and tubulin are both associated with the phragmoplast. Myosin and callose also localise to the plasmodesmata-rich pit fields in the walls of living cells, which are particularly abundant within the common walls between ray cells and between ray cells and axial parenchyma cells in the phloem and xylem. In those xylem ray cells that contact developing vessel elements and tracheids, myosin, tubulin, actin and callose are localised at the periphery of developing contact and cross-field pits; the respective antibodies also highlight the bordered pits between vessels and between tracheids. The aperture of the bordered pits, whose diameter diminishes as the over-arching border of these pits develops, also houses myosin, actin and tubulin. Myosin, actin and callose are also found together around the sieve pores of sieve elements and sieve cells. We suggest that an acto-myosin contractile system (a 'plant muscle') is present at the cell plate, the sieve pores, the plasmodesmata within the walls of long-lived parenchyma cells, and at the apertures of bordered pits during their development.

Actin Cytoskeleton↗
PubMed2003

Delta15N values of tropical savanna and monsoon forest species reflect root specialisations and soil nitrogen status.

A large number of herbaceous and woody plants from tropical woodland, savanna, and monsoon forest were analysed to determine the impact of environmental factors (nutrient and water availability, fire) and biological factors (microbial associations, systematics) on plant delta(15)N values. Foliar delta(15)N values of herbaceous and woody species were not related to growth form or phenology, but a strong relationship existed between mycorrhizal status and plant delta(15)N. In woodland and savanna, woody species with ectomycorrhizal (ECM) associations and putative N(2)-fixing species with ECM/arbuscular (AM) associations had lowest foliar delta(15)N values (1.0-0.6 per thousand ), AM species had mostly intermediate delta(15)N values (average +0.6 per thousand ), while non-mycorrhizal Proteaceae had highest delta(15)N values (+2.9 to +4.1 per thousand ). Similar differences in foliar delta(15)N were observed between AM (average 0.1 and 0.2 per thousand ) and non-mycorrhizal (average +0.8 and +0.3 per thousand ) herbaceous species in woodland and savanna. Leguminous savanna species had significantly higher leaf N contents (1.8-2.5% N) than non-fixing species (0.9-1.2% N) indicating substantial N acquisition via N(2) fixation. Monsoon forest species had similar leaf N contents (average 2.4% N) and positive delta(15)N values (+0.9 to +2.4 per thousand ). Soil nitrification and plant NO(3)(-) use was substantially higher in monsoon forest than in woodland or savanna. In the studied communities, higher soil N content and nitrification rates were associated with more positive soil delta(15)N and plant delta(15)N. In support of this notion, Ficus, a high NO(3)(-) using taxa associated with NO(3)(-) rich sites in the savanna, had the highest delta(15)N values of all AM species in the savanna. delta(15)N of xylem sap was examined as a tool for studying plant delta(15)N relations. delta(15)N of xylem sap varied seasonally and between differently aged Acacia and other savanna species. Plants from annually burnt savanna had significantly higher delta(15)N values compared to plants from less frequently burnt savanna, suggesting that foliar (15)N natural abundance could be used as marker for assessing historic fire regimes. Australian woodland and savanna species had low leaf delta(15)N and N content compared to species from equivalent African communities indicating that Australian biota are the more N depauperate. The largest differences in leaf delta(15)N occurred between the dominant ECM Australian and African savanna (miombo) species, which were depleted and enriched in (15)N, respectively. While the depleted delta(15)N of Australian ECM species are similar to those of previous reports on ECM species in natural plant communities, the (15)N-enriched delta(15)N of African ECM species represent an anomaly.

Environmental Monitoring↗