Carbon allocation and fate in paddy soil depending on phosphorus fertilization and water management: results of 13C continuous labelling of rice

We grew rice in phosphorus (P) deficient subtropical paddy soil in a field study and used 13CO2 continuous labelling to investigate photosynthetic carbon (C) partitioning and allocation under FLOOD versus WET/DRY conditions, with and without P fertilization (80 mg P kg−1). The plants and soil were sampled after each of three WET/DRY cycles to determine 13C allocation in above- and belowground plant biomass, microbial biomass, the rhizosphere, and bulk soil. Irrespective of water management, P-fertilized plants had higher biomass and P content and more total 13C in the rice-soil system, especially the 13C incorporation into the shoots (51%–96%), than samples without P fertilization. Root and bulk-soil 13C were largely independent of both P fertilization and water management. However, by the third sampling, P fertilization had increased the amount of 13C and microbial biomass 13C in the rhizosphere soil (RS) by 28% (WET/DRY) and 95% (FLOOD), and by 47% (WET/DRY) and 50% (FLOOD), respectively. The WET/DRY condition had significantly higher microbial biomass and 13C contents than FLOOD condition only in the RS. These results indicate that a well-established aboveground plant biomass following P fertilization is required to increase belowground C allocation. Thus, WET/DRY conditions, like FLOOD conditions, can provide moisture sufficient for unhindered P availability in rice-paddy system.

Carbon transfer, partitioning and residence time in the plant-soil system: a comparison of two ¹³CO₂ labelling techniques

Various 13CO2 labelling approaches exist to trace carbon (C) dynamics in plant-soil systems. However, it is not clear if the different approaches yield the same results. Moreover, there is no consistent way of data analysis to date. In this study we compare with the same experimental setup the two main techniques: pulse and continuous labelling. We evaluate how these techniques perform to estimate the C transfer time, the C partitioning along time and the C residence time in different plant-soil compartments. We used identical plant-soil systems (Populus deltoides × nigra, Cambisol soil) to compare the pulse labelling approach (exposure to 99 atom % 13CO2 for three hours, traced for eight days) with a continuous labelling (exposure to 10 atom % 13CO2, traced for 14 days). The experiments were conducted in climate chambers under controlled environmental conditions. Before label addition and at four successive sampling dates, the plant-soil systems were destructively harvested, separated into leaves, petioles, stems, cuttings, roots and soil and soil microbial biomass was extracted. The soil CO2 efflux was sampled throughout the experiment. To model the C dynamics we used an exponential function to describe the 13C signal decline after pulse labelling. For the evaluation of the 13C distribution during the continuous labelling we applied a logistic function. Pulse labelling is best suited to assess the minimum C transfer time from the leaves to other compartments, while continuous labelling can be used to estimate the mean transfer time through a compartment, including short-term storage pools. The C partitioning between the plant-soil compartments obtained was similar for both techniques, but the time of sampling had a large effect: shortly after labelling the allocation into leaves was overestimated and the soil 13CO2 efflux underestimated. The results of belowground C partitioning were consistent for the two techniques only after eight days of labelling, when the 13C import and export was at equilibrium. The C mean residence times estimated by the rate constant of the exponential and logistic function were not valid here (non-steady state). However, the duration of the accumulation phase (continuous labelling) could be used to estimate the C residence time. Pulse and continuous labelling techniques are both well suited to assess C cycling. With pulse labelling, the dynamics of fresh assimilates can be traced, whereas the continuous labelling gives a more integrated result of C cycling, due to the homogeneous labelling of C pools and fluxes. The logistic model applied here, has the potential to assess different parameters of C cycling independent on the sampling date and with no disputable assumptions.

Carbon transfer, partitioning and residence time in the plant-soil system: a comparison of two ¹³CO₂ labelling techniques

Various 13CO2 labelling approaches exist to trace carbon (C) dynamics in plant-soil systems. However, it is not clear if the different approaches yield the same results. Moreover, there is no consistent way of data analysis to date. In this study we compare with the same experimental setup the two main techniques: the pulse and the continuous labelling. We evaluate how these techniques perform to estimate the C transfer velocity, the C partitioning along time and the C residence time in different plant-soil compartments. We used identical plant-soil systems (Populus deltoides x nigra, Cambisol soil) to compare the pulse labelling approach (exposure to 99 atom% 13CO2 for three hours, traced for eight days) with a continuous labelling (exposure to 10 atom% 13CO2, traced for 14 days). The experiments were conducted in climate chambers under controlled environmental conditions. Before label addition and at four successive sampling dates, the plant-soil systems were destructively harvested, separated into leaves, petioles, stems, cuttings, roots and soil and the microbial biomass was extracted from the soil. The soil CO2 efflux was sampled throughout the experiment. To model the C dynamics we used an exponential function to describe the 13C signal decline after pulse labelling. For the evaluation of the 13C distribution during the continuous labelling we suggest to use a logistic function. Pulse labelling is best suited to assess the maximum C transfer velocity from the leaves to other compartments. With continuous labelling, the mean transfer velocity through a compartment, including short-term storage pools, can be observed. The C partitioning between the plant-soil compartments was similar for both techniques, but the time of sampling had a large effect: shortly after labelling the allocation into leaves was overestimated and the soil 13CO2 efflux underestimated. The results of belowground C partitioning were consistent for the two techniques only after eight days of labelling, when the 13C import and export was at equilibrium. The C mean residence time estimated by the rate constant of the exponential and logistic function was not valid here. However, the duration of the accumulation phase (continuous labelling) could be used to estimate the C residence time. Pulse and continuous labelling techniques are both well suited to assess C cycling. With pulse labelling the dynamics of fresh assimilates can be traced, whereas the continuous labelling gives a more integrated result on C cycling, due to the homogeneous labelling of C pools and fluxes. The logistic model suggested here, has the potential to assess different parameters of C cycling independent on the sampling date and with no disputable assumptions.

Effects of specific dietary sugars on the incorporation of 13C label from dietary glucose into neutral sugars of rat intestine and serum glycoproteins

Although theoretically all glycoprotein sugars can be derived from glucose, it may be hypothesized that specific dietary sugars could be preferential substrates for glycoprotein synthesis. To test this hypothesis, groups of rats received either continuously (continuous-labelling experiment) or for a single nutritional period (pulse-labelling experiment) a 13C-rich diet containing either maize starch or artificially labelled [13C]glucose. Some groups of rats were also provided during a single nutritional period with low amounts (20–200 mg/animal) of low-13C dietary sugars (mannose, galactose, fucose or fructose). If specific dietary sugars were preferentially incorporated into glycoproteins instead nf glucose-derived labelled sugars, a decrease would be expected in the intestinal or serum glycoprotein-sugar 13C enrichment monitored by gas chromatography-isotope-ratio mass spectrometry (GC-IRMS). Contrary to this hypothesis the results showed no significant decrease with any of the specific dietary sugars. Furthermore, with dietary low-13C mannose or galactose, a significant increase in 13C enrichment of glycoprotein-sugars was observed compared with some other nutritional groups. Moreover, in the pulse-labelling experiment, dietary mannose and galactose induced similar patterns of 13C enrichment in intestinal and serum glycoprotein-sugars. Therefore, although specific dietary sugars do not appear to be preferential substrates for glycosylation under conditions and doses relevant to current concepts of nutrition, regulatory roles of some specific dietary sugars in relation to glycoprotein-sugar metabolism might be hypothesized. These findings could lead to similar studies using stable-isotope methodology in man which could have practical consequences, especially in parenteral nutrition where glucose is the only sugar provided to the metabolism.

Induction of a second neural axis by the mouse node

The anterior aspect of the mouse primitive streak resembles the organizer of Xenopus and chick in terms of its developmental fate, ability to alter pattern in the chick limb bud and with respect to the repertoire of genes that its constituent cells express. However, until now there has been no direct evidence that the mouse node organizes pattern during gastrulation, nor that the exceptionally small mouse embryonic egg cylinder can be induced to form a second axis. Grafts of transgenically marked midgastrulation mouse node, or node labelled with DiI, to a posterolateral location in a host embryo of the same developmental stage results in the induction of a second neural axis and the formation of ectopic somites. The graft gives rise predominantly to notochord and endoderm tissue whereas the neurectoderm and somites are mainly of host origin. The ectopic notochord formed is derived solely from the donor node which suggests that the node can serve as a ‘stem cell’ source of axial mesoderm. This is corroborated by the observation that labelling in situ the population of cells on the outer surface of the mid-gastrulation node with DiI results in continuous labelling of the notochord. DiI-labelled cells are present throughout the notochord from a rostral boundary in the cranial region to its most caudal extreme and the node itself always remains labelled.

The intracellular handling of insulin-related peptides in isolated pancreatic islets. Evidence for differential rates of degradation of insulin and C-peptide

Islets of Langerhans isolated from adult rats were maintained in tissue culture for 3 days in the continued presence of [3H]leucine. Labelled proinsulin, C-peptide and insulin were measured by quantitative h.p.l.c., a method which also allowed for resolution of C-peptide I and II, and of insulin I and II (the products of the two rat insulin genes). The results showed that: (1) at early times, proinsulin was the major radiolabelled product; with progressive time in culture, intra-islet levels of [3H]proinsulin decreased, despite continuous labelling with [3H]leucine, indicating that the combined rates of proinsulin conversion into insulin and of proinsulin release, exceeded the rate of synthesis; (2) insulin I levels were always greater than those of insulin II, both in the islets and for products released to the medium; (3) the molar ratio of [3H]insulin I and II to their respective 3H-labelled C-peptides increased with time for products retained within islets, reaching a value close to 3:1 by 3 days; by contrast, for products released to the medium during the culture period, the ratio was always close to unity; (4) when islets were incubated with [3H]leucine for 2 days, and then left for a further 1 day without label (chase period), the intra-islet [3H]insulin/[3H]C-peptide ratios rose to values as high as 9:1. Again, for material released to the medium, the values were close to 1:1; (5) it is concluded that C-peptide is degraded more rapidly than insulin within islet cells, thereby accounting for the elevated insulin/C-peptide ratios. The difference between the ratios observed in the islets and those for material released to the medium is taken to indicate that degradation occurs in a discrete cellular compartment and not in the secretory granule itself.