Browsing by Subject "stable isotope"

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  • Björnson, E.; Packard, C. J.; Adiels, M.; Andersson, L.; Matikainen, Niina; Söderlund, S.; Kahri, J.; Sihlbom, C.; Thorsell, A.; Zhou, H.; Taskinen, M.-R.; Borén, J. (2019)
    Background Triglyceride-rich lipoproteins and their remnants have emerged as major risk factors for cardiovascular disease. New experimental approaches are required that permit simultaneous investigation of the dynamics of chylomicrons (CM) and apoB48 metabolism and of apoB100 in very low-density lipoproteins (VLDL). Methods Mass spectrometric techniques were used to determine the masses and tracer enrichments of apoB48 in the CM, VLDL1 and VLDL2 density intervals. An integrated non-steady-state multicompartmental model was constructed to describe the metabolism of apoB48- and apoB100-containing lipoproteins following a fat-rich meal, as well as during prolonged fasting. Results The kinetic model described the metabolism of apoB48 in CM, VLDL1 and VLDL2. It predicted a low level of basal apoB48 secretion and, during fat absorption, an increment in apoB48 release into not only CM but also directly into VLDL1 and VLDL2. ApoB48 particles with a long residence time were present in VLDL, and in subjects with high plasma triglycerides, these lipoproteins contributed to apoB48 measured during fasting conditions. Basal apoB48 secretion was about 50 mg day?1, and the increment during absorption was about 230 mg day?1. The fractional catabolic rates for apoB48 in VLDL1 and VLDL2 were substantially lower than for apoB48 in CM. Discussion This novel non-steady-state model integrates the metabolic properties of both apoB100 and apoB48 and the kinetics of triglyceride. The model is physiologically relevant and provides insight not only into apoB48 release in the basal and postabsorptive states but also into the contribution of the intestine to VLDL pool size and kinetics.
  • Tarvainen, Lasse; Wallin, Goran; Linder, Sune; Nasholm, Torgny; Oren, Ram; Lofvenius, Mikaell Ottosson; Rantfors, Mats; Tor-Ngern, Pantana; Marshall, John D. (2021)
    Several studies have suggested that CO2 transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused C-13-labelled solution into the xylem near the base of two 90-year-old Pinus sylvestris L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO2 efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue C-13 enrichment was also evaluated at these positions. Most of the C-13 label was lost by diffusion within a few metres of the infusion point indicating rapid CO2 loss during vertical xylem transport. No C-13 enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO2 diffused radially to the atmosphere. Our results support the notion that xylem CO2 transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO2 derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.
  • Salmon, Yann; Li, Xuefei; Yang, Bo; Ma, Keping; Siegwolf, Rolf T. W.; Schmid, Bernhard (2018)
    Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two- and four-species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L.glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant-soil-atmosphere continuum. We measured gas-exchange variables (C assimilation, transpiration and above- and belowground respiration) and C-13 in leaf biomass, phloem, soil microbial biomass, leaf- and soil-respired CO2 as well as O-18 in leaf and xylem water. The C-13 signal in phloem and respired CO2 in L.glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf-water O-18 enrichment above the xylem water in L.glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water-use efficiency. However, these increased C resources did not translate in higher growth of L.glaber in monoculture suggesting the existence of larger nongrowth-related C sinks in monoculture. These ecophysiological responses of L.glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L.glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.