The significance of phloem transport for the speed with which canopy photosynthesis and belowground respiration are linked.

University of Edinburgh, School of GeoSciences, Crew Building, West Mains Road, EH9 3JN Edinburgh, UK.
New Phytologist (Impact Factor: 6.55). 10/2009; 185(1):189-203. DOI: 10.1111/j.1469-8137.2009.03050.x
Source: PubMed

ABSTRACT Ecosystem respiration is known to vary following changes in canopy photosynthesis. However, the timing of this coupling is not well understood. Here, we summarize the literature on soil and ecosystem respiration where the speed of transfer of photosynthetic sugars from the plant canopy via the phloem to the roots was determined. Estimates of the transfer speed can be grouped according to whether the study employed isotopic or canopy/soil flux-based techniques. These two groups should provide different estimates of transfer times because transport of sucrose molecules, and pressure-concentration waves, in phloem differ. A steady-state and a dynamic photosynthesis/phloem-transport/soil gas diffusion model were employed to interpret our results. Starch storage and partly soil gas diffusion affected transfer times, but phloem path-length strongly controlled molecule transfer times. Successful modelling required substantially different phloem properties (higher specific conductivity and turgor pressure difference) in tall compared with small plants, which is significant for our understanding of tall trees' physiology. Finally, we compared isotopic and flux-based approaches for the determination of the link between canopy photosynthesis and ecosystem respiration. We conclude that isotopic approaches are not well suited to document whether changes in photosynthesis of tall trees can rapidly affect soil respiration.

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    ABSTRACT: Plants rapidly release photoassimilated carbon (C) to the soil via direct root exudation and associated mycorrhizal fungi, with both pathways promoting plant nutrient availability. This study aimed to explore these pathways from the root's vascular bundle to soil microbial communities. Using nanoscale secondary ion mass spectrometry (NanoSIMS) imaging and 13C-phospho- and neutral lipid fatty acids, we traced in-situ flows of recently photoassimilated C of 13CO2-exposed wheat (Triticum aestivum) through arbuscular mycorrhiza (AM) into root- and hyphae-associated soil microbial communities. Intraradical hyphae of AM fungi were significantly 13C-enriched compared to other root-cortex areas after 8 h of labelling. Immature fine root areas close to the root tip, where AM features were absent, showed signs of passive C loss and co-location of photoassimilates with nitrogen taken up from the soil solution. A significant and exclusively fresh proportion of 13C-photosynthates was delivered through the AM pathway and was utilised by different microbial groups compared to C directly released by roots. Our results indicate that a major release of recent photosynthates into soil leave plant roots via AM intraradical hyphae already upstream of passive root exudations. AM fungi may act as a rapid hub for translocating fresh plant C to soil microbes.
    New Phytologist 11/2014; 205(4). DOI:10.1111/nph.13138 · 6.55 Impact Factor
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    ABSTRACT: Carbon cycling has become of major interest for the understanding and mitigation of global climatic change. Terrestrial ecosystems have a large carbon sequestration potential, but many processes and fluxes of organic matter cycling within the plant-soil system are not yet well understood [1]. Recent studies have shown that aboveground vegetation and soil organic matter (SOM) processes are closely linked [2], that roots and root-derived carbon play a major role in SOM stabilization [3] and that plants can increase the SOM turnover by stimulating microbial populations (priming effect) [4]. The investigation of these complex interactions between plants and soil is challenging and is asking for new methodological approaches. We developed a multi-labelling technique to label new assimilates with 13C, 18O and 2H. In order to trace organic molecules from the leaf to the soil and within the SOM the label is added continuously in the gaseous phase (CO2, water vapour) to the plants atmosphere. The multi-labelling technique has been successfully tested in preliminary experiments and a complex chamber system (MICE) has been developed to apply this technique. The MICE facility consists of two labelling chambers. The upper parts (shoot) of the plant-soil system are separated hermetically from the lower parts (roots, soil) to prevent the diffusion of the label into the soil. Each chamber carries 15 plants in single pots, which can be sampled at 5 sampling dates (3 replicates) with minor disturbance to the labelling atmosphere. Each chamber has a separate upper gas circuits by which the label added is recycled and the CO2 concentration and air humidity within the chambers is automatically regulated and monitored. Thus the two labelling chambers can be used to conduct multi-labelling experiments with controls (background measurements, optimal environmental conditions) for each sampling date. In the lower part of the labelling chamber each pot is aerated to prevent anaerobic conditions. With a combined gas sampling system the total CO2 efflux is monitored and the isotopic signature of the soil respiration can be assessed frequently. The new multi-labelling approach and the labelling facility developed represent a powerful tool to address still open questions in plant and soil research such as the allocation of organic molecules within the plant-soil system under changing environmental conditions or the influence of plants on soil organic matter stabilization and destabilization processes.. The significance of phloem transport for the speed with which canopy photosynthesis and belowground respiration are linked.
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    ABSTRACT: Aims Our aim was to study the effect of potential biotic drivers, including evapotranspiration (ET) and gross primary production (GPP), on the soil CO2 production and efflux on the diel time scale. Methods Eddy covariance, soil respiration and soil CO2 gradient systems were used to measure the CO2 and H2O fluxes in a dry, sandy grassland in Hungary. The contribution of CO2 production from three soil layers to plot-scale soil respiration was quantified. CO2 production and efflux residuals after subtracting the effects of the main abiotic and biotic drivers were analysed. Results Soil CO2 production showed a strong negative correlation with ET rates with a time lag of 0.5 h in the two upper layers, whereas less strong, but still significant time-lagged and positive correlations were found between GPP and soil CO2 production. Our results suggest a rapid negative response of soil CO2 production rates to transpiration changes, and a delayed positive response to GPP. Conclusions We found evidence for a combined effect of soil temperature and transpiration that influenced the diel changes in soil CO2 production. A possible explanation for this pattern could be that a significant part of CO2 produced in the soil may be transported across soil layers via the xylem.
    Plant and Soil 11/2014; DOI:10.1007/s11104-014-2314-3 · 3.24 Impact Factor

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