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Peter Millard

Publications and source records attributed to Peter Millard.

14 recordsLinked to original sources
PubMed2006

Collection and storage of CO2 for 13C analysis: An application to separate soil CO2 efflux into root- and soil-derived components.

Soil surface CO2 efflux is comprised of CO2 from (i) root respiration and rhizosphere microbes and (ii) heterotrophic respiration from the breakdown of soil organic matter (SOM). This efflux may be partitioned between these sources using delta13C measurements. To achieve this, continuous flow isotope ratio mass spectrometry can be used and, in conjunction with 10 mL septum-capped vials, large numbers of samples may be analysed using a Finnigan MAT Delta(plus)XP interfaced to a Gas Bench II. Here we describe a number of advances to facilitate such work, including: (i) a technique for monitoring mass spectrometer performance, (ii) improvements to sample storage, and (iii) a gas-handling system for incubating and sampling the CO2 derived from roots and soils. Mass spectrometer performance was monitored using an automated refillable vial. Compressed air analysed with this system had mean delta13C of -9.61 +/- 0.16 per thousand (+/- 1sigma, n = 28) collected over four runs. Heating the butyl rubber septa used to seal the vials at 105 degrees C for 12 h improved the sample storage. After air transportation over 12 days, the isotope composition of the CO2 at ambient concentrations was unchanged (before: -35.2 +/- 0.10 per thousand, n = 4; after: -35.3 +/- 0.10 per thousand, n = 15); without heat treatment of the septa the CO2 became slightly enriched (-35.0 +/- 0.14 per thousand, n = 15). The linearity of the Gas Bench II was found to decline above 8000 micromol CO2 mol(-1). To stay within a linear range and to allow the incubation of soil and root material we describe a gas-handling system based around a peristaltic pump. Finally, we demonstrate these methods by growing a C-4 grass (Guinea grass, Panicum maximum Jacq.) in a C-3 soil. Root respiration was found to contribute between 5 and 22% to the soil surface CO2 efflux. These methodologies will facilitate experiments aimed at measuring the isotopic composition of soil-derived CO2 across a range of ecological applications.

Journal Article↗
PubMed2006

Nitrogen availability, local light regime and leaf rank effects on the amount and sources of N allocated within the foliage of young walnut (Juglans nigra x regia) trees.

Early season leaf growth depends largely on nitrogen (N) provided by remobilization from storage, and many studies have tested the effect of N availability to roots on the amount of N provided for new leaf development by remobilization. Although it is well known that the light regime experienced by a leaf influences the amount of N per unit leaf area (LA), the effect of the local light regime on the amount of N derived either directly from root uptake or from remobilization for early season leaf growth has never been tested at an intra- canopy scale. The objective of this study was to quantify the relative importance of (1) N availability to roots, (2) local light regime experienced by the foliage (at the shoot scale) and (3) leaf rank along the shoot, on the total amount of N allocated to leaves and on the proportions of N provided by remobilization and root uptake. To quantify the importance of N uptake and remobilization as sources of leaf N, potted hybrid walnut trees (Juglans nigra L. x regia L.) were grown outdoors in sand and fed with a labeled ((15)N) nutrient solution. By removing the apical bud, the trees were manipulated to produce only two shoots. The experimental design had two factors: (1) high (HN; 8 mol N m(-3)) and low (LN; 2 mol N m(-3)) N availability; and (2) high (HL; 90% of incident photosynthetically active photon flux (PPF)) and low (LL; 10% of incident PPF) light. Total leaf N per tree was unaffected by either N availability or irradiance. The HN treatment increased the amount of leaf N derived from root uptake at the whole-tree scale (typically around 8 and 2% in the HN and LN treatments, respectively). Nitrogen allocation within foliage of individual trees was controlled by the local light regime, which strongly affected individual leaf characteristics as leaf mass per unit LA and area- based amount of leaf (N(a)). Decreasing the light availability to a branch decreased the amount of N allocated to it, benefiting the less shaded branches. In contrast, shading of the lower branch did not affect the fraction of total leaf N remobilized for either the lower, shaded branch or the upper, unshaded branch. The relevance of these findings for tree growth modeling is discussed.

Biological Transport↗
PubMed2006

Translocation of nitrogen in the xylem of field-grown cherry and poplar trees during remobilization.

Studies of small trees growing in pots have established that individual amino acids or amides are translocated in the xylem sap of a range of tree species following bud burst, as a consequence of nitrogen (N) remobilization from storage. This paper reports the first study of N translocation in the xylem of large, deciduous, field-grown trees during N remobilization in the spring. We applied 15N fertilizer to the soil around 10-year-old Prunus avium L. and Populus trichocharpa Torr. & Gray ex Hook var. Hastata (Dode) A. Henry x Populus balsamifera L. var. Michauxii (Dode) Farwell trees before bud burst to label N taken up by the roots. Recovery of unlabeled N in xylem sap and leaves was used to demonstrate that P. avium remobilizes N in both glutamine (Gln) and asparagine (Asn). Sap concentrations of both amides rose sharply after bud burst, peaking 14 days after bud burst for Gln, and remaining high some 45 days for Asn. There was no 15N enrichment of either amide until 21 days after bud burst. In the Populus trees, nearly all the N was translocated in the sap as Gln, the concentration of which peaked and then declined before the amide was enriched with 15N, 40 days after bud burst. Xylem sap of clonal P. avium trees was sampled at different positions in the crown to assess if the amino acid and amide composition of the sap varied within the crown. Sap was sampled during remobilization (when the concentration of Gln was maximal), at the end of remobilization and at the end of the experiment (68 days after bud burst). Although the date of sampling had a highly significant effect on sap composition, the effect of position of sampling was marginal. The results are discussed in relation to N translocation in adult trees and the possibility of measuring N remobilization by calculating the flux of N translocation in the xylem.

Amides↗
PubMed2006

Alteration and resilience of the soil microbial community following compost amendment: effects of compost level and compost-borne microbial community.

Compost amendment has been reported to impact soil microbial activities or community composition. However, little information is available on (i) to what extent compost amendment concurrently affects the activity, size and composition of soil microbial community, (ii) the relative effect of the addition of a material rich in organic matter versus addition of compost-borne microorganisms in explaining the effects of amendment and (iii) the resilience of community characteristics. We compared five treatments in microcosms: (i) control soil (S), (ii) soil + low level of compost (Sc), (iii) soil + high level of compost (SC), (iv) sterilized soil + high level of compost [(S)C] and (v) soil + high level of sterilized compost [S(C)]. The actual C mineralization rate, substrate-induced respiration, size of microbial community (biomass and heterotrophic cells number), and structure of total microbial (phospholipid fatty acids) and bacterial (automated ribosomal intergenic spacer analysis, A-RISA) communities were surveyed during 6 months after amendment. Our results show that (i) compost amendment affected the activity, size and composition of the soil microbial community, (ii) the effect of compost amendment was mainly due to the physicochemical characteristics of compost matrix rather than to compost-borne microorganisms and (iii) no resilience of microbial characteristics was observed 6-12 months after amendment with a high amount of compost.

Bacteria↗
PubMed2006

Sheep-urine-induced changes in soil microbial community structure.

Soil microbial communities play an important role in nutrient cycling and nutrient availability, especially in unimproved soils. In grazed pastures, sheep urine causes local changes in nutrient concentration which may be a source of heterogeneity in microbial community structure. In the present study, we investigated the effects of synthetic urine on soil microbial community structure, using physiological (community level physiological profiling, CLPP), biochemical (phospholipid fatty acid analysis, PLFA) and molecular (denaturing gradient gel electrophoresis, DGGE) fingerprinting methods. PLFA data suggested that synthetic urine treatment had no significant effect on total microbial (total PLFA), total bacterial or fungal biomass; however, significant changes in microbial community structure were observed with both PLFA and DGGE data. PLFA data suggested that synthetic urine induced a shift towards communities with higher concentrations of branched fatty acids. DGGE banding patterns derived from control and treated soils differed, due to a higher proportion of DNA sequences migrating only to the upper regions of the gel in synthetic urine-treated samples. The shifts in community structure measured by PLFA and DGGE were significantly correlated with one another, suggesting that both datasets reflected the same changes in microbial communities. Synthetic urine treatment preferentially stimulated the use of rhizosphere-C in sole-carbon-source utilisation profiles. The changes caused by synthetic urine addition accounted for only 10-15% of the total variability in community structure, suggesting that overall microbial community structure was reasonably stable and that changes were confined to a small proportion of the communities.

Animals↗
PubMed2004

Unravelling rhizosphere-microbial interactions: opportunities and limitations.

The rhizosphere is a biologically active zone of the soil around plant roots that contains soil-borne microbes including bacteria and fungi. Plant-microbe interactions in the rhizosphere can be beneficial to the plant, the microbes or to neither of them. One of the major difficulties that plant biologists and microbiologists face when studying these interactions is that many groups of microbes that inhabit this zone are not cultivable in the laboratory. Recent developments in molecular biology methods are shedding some light on rhizospheric microbial diversity. This review discusses recent findings and future challenges in the study of plant-microbe interactions in the rhizosphere.

Bacteria↗
PubMed2003

Recycling of nitrogen in the xylem of Prunus avium trees starts when spring remobilization of internal reserves declines.

Nitrogen (N) storage capacity of cherry (Prunus avium L.) trees grown in sand culture was preconditioned by applying contrasting N supplies for one year. During the spring of the following year, a constant amount of 15N was supplied and the dynamics of N remobilization and root uptake were characterized as a function of internal N status of the trees. To calculate the flux of N through xylem, both xylem sap N concentration and whole-tree transpiration rates were measured. By comparing the cumulative flux of N through the xylem with the amount of N recovered in the new above ground growth, we indirectly evaluated the recycling of N in the xylem, i.e., the amount of N derived from shoot-root translocation that was subsequently reloaded into the xylem. The contrasting N storage capacities imposed during the first year affected both N remobilization and uptake from roots in the following year. Recycling of N in the xylem apparently did not occur during the remobilization of internal reserves (i.e., during the first 6-8 weeks after bud burst). However, when remobilization declined, measurement of the cumulative flux of N through the xylem overestimated the amount of N recovered in the new biomass, allowing the extent of N recycling to be evaluated. The amount of N recycling in the xylem was greater in high-N trees, which also took up less N through their roots than trees preconditioned to have a lower internal N status. This suggests that recycling of N in the xylem is a mechanism by which plants regulate N uptake by roots.

Nitrogen↗
PubMed2002

Hypoxia-inducible factors HIF-1alpha and HIF-2alpha in head and neck cancer: relationship to tumor biology and treatment outcome in surgically resected patients.

Hypoxia within head and neck squamous cell carcinoma (HNSCC) predicts a poor response to radiotherapy and poor prognosis. Hypoxia-inducible factor (HIF)-1 and HIF-2 are nuclear transcription factors that regulate the cellular response to hypoxia and are important for solid tumor growth and survival. Overexpression of HIF-1alpha and HIF-2alpha was demonstrated in three HNSCC cell lines under hypoxia and tumor tissue versus normal tissue (n = 20, HIF-1alpha, P = 0.023; HIF-2alpha, P = 0.013). On immunostaining, HIF-1alpha and HIF-2alpha expression were localized to tumor nuclei; HIF-2alpha expression was also seen in tumor-associated macrophages. Expression of HIF-1alpha in surgically treated patients with HNSCC (n = 79) was associated with improved disease-free survival (P = 0.016) and overall survival (P = 0.027).

Adolescent↗
PubMed2002

Spatial distribution of leaf nitrogen and photosynthetic capacity within the foliage of individual trees: disentangling the effects of local light quality, leaf irradiance, and transpiration.

There is presently no consensus about the factor(s) driving photosynthetic acclimation and the intra-canopy distribution of leaf characteristics under natural conditions. The impact was tested of local (i) light quality (red/far red ratio), (ii) leaf irradiance (PPFD(i)), and (iii) transpiration rate (E) on total non-structural carbohydrates per leaf area (TNC(a)), TNC-free leaf mass-to-area ratio (LMA), total leaf nitrogen per leaf area (N(a)), photosynthetic capacity (maximum carboxylation rate and light-saturated electron transport rate), and leaf N partitioning between carboxylation and bioenergetics within the foliage of young walnut trees grown outdoors. Light environment (quantity and quality) was controlled by placing individual branches under neutral or green screens during spring growth, and air vapour pressure deficit (VPD) was prescribed and leaf transpiration and photosynthesis measured at branch level by a branch bag technique. Under similar levels of leaf irradiance, low air vapour pressure deficit decreased transpiration rate but did not influence leaf characteristics. Close linear relationships were detected between leaf irradiance and leaf N(a), LMA or photosynthetic capacity, and low R/FR ratio decreased leaf N(a), LMA and photosynthetic capacity. Irradiance and R/FR also influenced the partitioning of leaf nitrogen into carboxylation and electron light transport. Thus, local light level and quality are the major factors driving photosynthetic acclimation and intra-canopy distribution of leaf characteristics, whereas local transpiration rate is of less importance.

Acclimatization↗
PubMed2002

Coupling sap flow velocity and amino acid concentrations as an alternative method to (15)N labeling for quantifying nitrogen remobilization by walnut trees.

The temporal dynamics of N remobilization was studied in walnut (Juglans nigra x regia) trees growing in sand culture. Trees were fed with labeled N ((15)N) during 1999 and unlabeled N in 2000. Total N and (15)N contents in different tree compartments were measured during 80 d after bud burst and were used to estimate N remobilization for spring growth. The seasonal (and occasionally diurnal) dynamics of the concentration and (15)N enrichment of the major amino acids in xylem sap were determined concurrently. Sap flow velocity was also measured for sample trees. A new approach coupling amino acid concentrations to sap flow velocity for quantifying N remobilization was tested. A decrease of the labeled N contents of medium roots, tap roots, and trunk was observed concurrently to the increase in the labeled N content of new shoots. Remobilized N represented from previous year storage 54% of N recovered in new shoots. Arginine, citruline, gamma-amino butyric acid, glutamic acid, and aspartic acid always represented around 80% of total amino acid and amide N in xylem sap and exhibited specific seasonal trends and significant diurnal trends. N translocation was mainly insured by arginine during the first 15 d after bud burst, and then by glutamic acid and citruline. The pattern of N remobilization estimated by the new approach was consistent with that measured by the classical labeling technique. Implications for quantifying N remobilization for large, field-growing trees are discussed.

Amino Acids↗
PubMedDate not supplied

Impacts of water and nitrogen supplies on the physiology, leaf demography and nitrogen dynamics of Betula pendula.

We determined the response of Betula pendula Roth. trees to a restricted water supply, and quantified the interactions between tree N and water status on leaf demography and internal N cycling. In April 1993, 3-year-old trees were planted in sand culture and four treatments applied: high-N supply (56 mg tree(-1) week(-1)) with either 2 dm(3) water week(-1) (HN+) or 0.9 dm(3) water week(-1) (HN-), or low-N supply (14 mg tree(-1) week(-1)) with 2 dm(3) (LN+) or 0.9 dm(3) (LN-) water week(-1). Until 1994, the N supplied to trees was enriched with (15)N to 5.4 atom %. During 1993, there were few differences in the growth or leaf demography of trees in the LN+ and LN- treatments, but the high-N treatment increased tree growth. Leaf mass and area were initially similar in trees in the HN+ and HN- treatments, but the trees in the HN- treatment had a smaller root system. Net assimilation rate under saturating light was higher in trees in the HN+ treatment than in trees in the LN+ treatment. There was an N x water supply interaction as a result of trees in the HN- treatment closing their stomata by the beginning of August. However, there was no difference in gas exchange characteristics of leaves in the LN+ and LN- treatments. Although leaf senescence and abscission started in the HN- treatment by mid-August and continued for about 90 days, whereas leaf abscission in the other treatments did not start until the beginning of October and only lasted 25-30 days, the trees in the HN+ and HN- treatments remobilized similar amounts of (15)N for leaf growth in the spring of 1994. There were no differences in predawn water potential among treatments and no evidence of osmotic adjustment. We conclude that B. pendula trees avoid rather than tolerate drought. The interaction between the effects of nitrogen and water supplies on leaf demography and internal cycling of N are discussed.

Journal Article↗