How are nitrogen availability,fine‐root mass,and nitrogen uptake related empirically? Implications for models and theory

Understanding the effects of global change in terrestrial communities requires an understanding of how limiting resources interact with plant traits to affect productivity. Here, we focus on nitrogen and ask whether plant community nitrogen uptake rate is determined (a) by nitrogen availability alone or (b) by the product of nitrogen availability and fine‐root mass. Surprisingly, this is not empirically resolved. We performed controlled microcosm experiments and reanalyzed published pot experiments and field data to determine the relationship between community‐level nitrogen uptake rate, nitrogen availability, and fine‐root mass for 46 unique combinations of species, nitrogen levels, and growing conditions. We found that plant community nitrogen uptake rate was unaffected by fine‐root mass in 63% of cases and saturated with fine‐root mass in 29% of cases (92% in total). In contrast, plant community nitrogen uptake rate was clearly affected by nitrogen availability. The results support the idea that although plants may over‐proliferate fine roots for individual‐level competition, it comes without an increase in community‐level nitrogen uptake. The results have implications for the mechanisms included in coupled carbon‐nitrogen terrestrial biosphere models (CN‐TBMs) and are consistent with CN‐TBMs that operate above the individual scale and omit fine‐root mass in equations of nitrogen uptake rate but inconsistent with the majority of CN‐TBMs, which operate above the individual scale and include fine‐root mass in equations of nitrogen uptake rate. For the much smaller number of CN‐TBMs that explicitly model individual‐based belowground competition for nitrogen, the results suggest that the relative (not absolute) fine‐root mass of competing individuals should be included in the equations that determine individual‐level nitrogen uptake rates. By providing empirical data to support the assumptions used in CN‐TBMs, we put their global climate change predictions on firmer ground.     

Cloning and gastrointestinal expression of rat hephaestin: relationship to other iron transport proteins

The membrane-bound ceruloplasmin homolog hephaestin plays a critical role in intestinal iron absorption. The aims of this study were to clone the rat hephaestin gene and to examine its expression in the gastrointestinal tract in relation to other genes encoding iron transport proteins. The rat hephaestin gene was isolated from intestinal mRNA and was found to encode a protein 96% identical to mouse hephaestin. Analysis by ribonuclease protection assay and Western blotting showed that hephaestin was expressed at high levels throughout the small intestine and colon. Immunofluorescence localized the hephaestin protein to the mature villus enterocytes with little or no expression in the crypts. Variations in iron status had a small but nonsignificant effect on hephaestin expression in the duodenum. The high sequence conservation between rat and mouse hephaestin is consistent with this protein playing a central role in intestinal iron absorption, although its precise function remains to be determined.     

A new hammer to crack an old nut: interspecific competitive resource capture by plants is regulated by nutrient supply, not climate

Although rarely acknowledged, our understanding of how competition is modulated by environmental drivers is severely hampered by our dependence on indirect measurements of outcomes, rather than the process of competition. To overcome this, we made direct measurements of plant competition for soil nitrogen (N). Using isotope pool-dilution, we examined the interactive effects of soil resource limitation and climatic severity between two common grassland species. Pool-dilution estimates the uptake of total N over a defined time period, rather than simply the uptake of ¹⁵N label, as used in most other tracer experiments. Competitive uptake of N was determined by its available form (NO₃ ⁻ or NH₄ ⁺). Soil N availability had a greater effect than the climatic conditions (location) under which plants grew. The results did not entirely support either of the main current theories relating the role of competition to environmental conditions. We found no evidence for Tilman''s theory that competition for soil nutrients is stronger at low, compared with high nutrient levels and partial support for Grime''s theory that competition for soil nutrients is greater under potentially more productive conditions. These results provide novel insights by demonstrating the dynamic nature of plant resource competition.     

DNA barcoding the native flowering plants and conifers of Wales

We present the first national DNA barcode resource that covers the native flowering plants and conifers for the nation of Wales (1143 species). Using the plant DNA barcode markers rbcL and matK, we have assembled 97.7% coverage for rbcL, 90.2% for matK, and a dual-locus barcode for 89.7% of the native Welsh flora. We have sampled multiple individuals for each species, resulting in 3304 rbcL and 2419 matK sequences. The majority of our samples (85%) are from DNA extracted from herbarium specimens. Recoverability of DNA barcodes is lower using herbarium specimens, compared to freshly collected material, mostly due to lower amplification success, but this is balanced by the increased efficiency of sampling species that have already been collected, identified, and verified by taxonomic experts. The effectiveness of the DNA barcodes for identification (level of discrimination) is assessed using four approaches: the presence of a barcode gap (using pairwise and multiple alignments), formation of monophyletic groups using Neighbour-Joining trees, and sequence similarity in BLASTn searches. These approaches yield similar results, providing relative discrimination levels of 69.4 to 74.9% of all species and 98.6 to 99.8% of genera using both markers. Species discrimination can be further improved using spatially explicit sampling. Mean species discrimination using barcode gap analysis (with a multiple alignment) is 81.6% within 10×10 km squares and 93.3% for 2×2 km squares. Our database of DNA barcodes for Welsh native flowering plants and conifers represents the most complete coverage of any national flora, and offers a valuable platform for a wide range of applications that require accurate species identification.     

Day-to-day alteration of 24-hour sleep pattern immediately before and after giving birth

Sleep and Biological Rhythms - The current literature suggests that nighttime sleep is compromised during late pregnancy and early postpartum periods, but little is known about the 24-hour sleep...     

Cellular distribution of ferric iron, ferritin, transferrin and divalent metal transporter 1 (DMT1) in substantia nigra and basal ganglia of normal and beta2-microglobulin deficient mouse brain.

We examined whether high levels of circulatory iron may cause iron accumulation in the brain. In particular, we focussed on the substantia nigra and basal ganglia as several papers have indicated that iron may accumulate here and cause death of dopaminergic neurons. Normal mice and a mouse model of hereditary haemochromatosis, the beta2-microglobulin (beta2m) knock out [beta2m (-/-)] mouse, which has high levels of circulating iron due to increased iron absorption, were examined. The iron concentration in livers were: 170+/-15 microg/g (mean +/- SD) in controls and 1010+/-50 microg/g in beta2m (-/-) mice (p<0.001), whereas in the brain the respective values were 47 +/-1 microg/g and 53+/-2 microg/g (p<0.02). Hence, the difference between cerebral iron levels of normal and beta2m (-/-) mice was small. Histological examination of the brains revealed an unequivocal distribution of ferric iron, ferritin, transferrin and divalent metal transporter 1 (DMT1), which were indistinguishable when normal and beta2m (-/-) mice were compared. In the substantia nigra and basal ganglia, ferric iron and the iron-binding proteins were present in identical cell types, which mainly comprised oligodendrocytes and microglia. Neurons were lightly labelled with transferrin and DMT1. The virtual lack of an increase in cerebral iron in beta2m (-/-) mice clearly shows that the blood-brain barrier (BBB) is capable of restricting the transport of excess plasma iron into the brain.     

Transferrin receptor 2: a new molecule in iron metabolism

Transferrin receptor 1 (TfR1) which mediates uptake of transferrin-bound iron, is essential for life in mammals. Recently, a close homologue of human transferrin receptor 1 was cloned and called transferrin receptor 2 (TfR2). A similar molecule has been identified in the mouse. Human transferrin receptor 2 is 45% identical with transferrin receptor 1 in the extracellular domain, but contains no iron responsive element in its mRNA and is apparently not regulated by intracellular iron concentration nor by interaction with HFE. Transferrin receptor 2, like transferrin receptor 1, binds transferrin in a pH-dependent manner (but with 25 times lower affinity) and delivers iron to cells. However, transferrin receptor 2 distribution differs from transferrin receptor 1, increasing in differentiating hepatocytes and decreasing in differentiating erythroblasts. Expression of both receptors is cell cycle dependent. Mutations in the human transferrin receptor 2 gene cause iron overload disease, suggesting it has a role in iron homeostasis.     

Interaction of chemical and state effects on ventilation during sleep onset

Dunai, Judith, Mal Wilkinson, and John Trinder.Interaction of chemical and state effects on ventilation duringsleep onset. J. Appl. Physiol. 81(5):2235-2243, 1996.Ventilation varies as a function of state, beinghigher during wakefulness (as indicated by alpha electroencephalogramactivity) than during sleep (theta activity). A recent experimentobserved a progressive increase in the magnitude of these state-relatedfluctuations in ventilation over the sleep-onset period (28). The aimof the present experiment was to test the hypothesis that this effectresulted from chemical (feedback-related) amplification of stateeffects on ventilation. A hyperoxic condition was used to eliminateperipheral chemoreceptor activity. It was hypothesized that hyperoxiawould reduce the amplification of changes in ventilation associatedwith electroencephalogram state transitions. Ventilation was measuredover the sleep-onset period under both hyperoxic and normoxicconditions in 10 young healthy male subjects. Sleep onsets were dividedinto three phases. Phase 1 corresponded to presleep wakefulness; andphases 2 and 3 corresponded to early and late sleep onset,respectively. The magnitudes of state-related changes in ventilationduring phases 2 and 3, and under hyperoxic and normoxic conditions werecompared using a phase by condition analysis of variance. Resultsrevealed a significant phase by condition interaction, confirming that hyperoxia reduced the amplification of state-related changes in ventilation by selectively decreasing the magnitude of phase 3 statechanges in ventilation. However, some degree of amplification wasevident during hyperoxia, thus the results demonstrated that peripheralchemoreceptor activity contributed to the amplification ofstate-related changes in ventilation but that additional factors mayalso be involved.

     

Correlation between ventilation and EEG-defined arousal during sleep onset in young subjects

Trinder, John, John A. Van Beveren, Philip Smith, JanKleiman, and Amanda Kay. Correlation between ventilation and EEG-defined arousal during sleep onset in young subjects.J. Appl. Physiol. 83(6):2005-2011, 1997.In studies of elderly individuals, ventilationand EEG-defined arousal have been shown to vary periodically andsynchronously. Such results have been interpreted as indicating theprimacy of sleep/wake state in causing ventilatory instability duringsleep onset. However, because the elderly individuals studied wereperiodic breathers, the results do not unequivocally support thisconclusion. In this study the relationship between ventilation andEEG-defined arousal was assessed in a group of 21 young, healthy men inwhom ventilatory instability during sleep onset was not periodic.Ventilation and EEG(O₁-A₂)recordings were collected, and the longest uncontaminated periods fromearly and late in sleep onset were selected for subsequent analysis.The 84 time series (21 subjects, 2 variables, and 2 occasions in sleeponset) were subjected to spectral analysis to identify periodicity, and the relationship between the two variables was determined bycross-correlational methods. The results indicated that the time serieswere nonperiodic, yet significant correlations were observed betweenthe two variables. The data support the view that during sleep onsetventilatory instability is driven primarily by variations in sleep/wakearousal level.