The significance of phloem transport for the speed of link between canopy photosynthesis and belowground respiration. New Phytol

University of Edinburgh, School of GeoSciences, Crew Building, West Mains Road, EH9 3JN Edinburgh, UK.
New Phytologist (Impact Factor: 7.67). 10/2009; 185(1):189-203. DOI: 10.1111/j.1469-8137.2009.03050.x
Source: PubMed


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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Available from: Maurizio Mencuccini, Oct 14, 2015
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    • "Gratani, 2014, Chastain et al., 2014, O'Brien et al., 2015). However it is also possible that R d can increase during periods of drought if substrate demand for processes such as hydraulic maintenance and repair (Brodersen & McElrone, 2013), and phloem transport regulation (Mencuccini & Hölttä, 2010) increases, or if drought conditions lead to a greater need to oxidise ROS or other redox equivalents, elevating photorespiration (Atkin & Macherel, 2009). Evidence of increasing R d during drought is limited but has been reported for both crop plants and forests (Atkin & Macherel, 2009, Miranda et al., 2005, Varone & Gratani, 2015). "
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    ABSTRACT: Determining climate change feedbacks from tropical rainforests requires an understanding of how carbon gain through photosynthesis and loss through respiration will be altered. One of the key changes that tropical rainforests may experience under future climate change scenarios is reduced soil moisture availability. In this study we examine if and how both leaf photosynthesis and leaf dark respiration acclimate following more than 12 years of experimental soil moisture deficit, via a through-fall exclusion experiment (TFE) in an eastern Amazonian rainforest. We find that experimentally drought-stressed trees and taxa maintain the same maximum leaf photosynthetic capacity as trees in corresponding control forest, independent of their susceptibility to drought-induced mortality. We hypothesise that photosynthetic capacity is maintained across all treatments and taxa to take advantage of short-lived periods of high moisture availability, when stomatal conductance (gs ) and photosynthesis can increase rapidly, potentially compensating for reduced assimilate supply at other times. Average leaf dark respiration (Rd ) was elevated in the TFE-treated forest trees relative to the control by 28.2±2.8% (mean ± one standard error). This mean Rd value was dominated by a 48.5±3.6% increase in the Rd of drought-sensitive taxa, and likely reflects the need for additional metabolic support required for stress-related repair, and hydraulic or osmotic maintenance processes. Following soil moisture deficit that is maintained for several years, our data suggest that changes in respiration drive greater shifts in the canopy carbon balance, than changes in photosynthetic capacity. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    Global Change Biology 07/2015; DOI:10.1111/gcb.13035 · 8.04 Impact Factor
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    • "The differentiation between starch and sugar pools may be important because depleted starch pools have been shown to be correlated with mortality (Marshall & Waring 1985; Adams et al., 2009). Phloem transport failure may be another component of carbon starvation through phloem unloading to refill cavitated xylem tissues and lowering carbohydrate loading (Hölttä et al., 2009) but the understanding of phloem transport and its coupling to xylem transport is still in its early stages even if important progresses have been made recently (Mencuccini & Hölttä 2009; Hölttä et al., 2011; Nikinmaa et al., 2012; Mencuccini et al., 2013). Also in our model the LAI was no able to drop under drought to mitigate evaporative losses. "
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    ABSTRACT: A simplified soil-plant-atmosphere-continuum model of carbon starvation and hydraulic failure is developed and tested against observations from a drought-manipulation experiment in a woodland dominated by piñon pine (Pinus edulis) and juniper (Juniperus monosperma) in New Mexico. The number of model parameters is reduced using allometric relationships. The model can represent more isohydric (piñon) and more anisohydric (juniper) responses. Analysis of the parameter space suggests four main controls on hydraulic failure and carbon starvation: xylem vulnerability curve, root:shoot area ratio, rooting depth, and water use efficiency. For piñon, an intermediate optimal (1.5-2 m2 m−2) tree leaf area index reduces the risk of hydraulic failure. For both piñons and junipers, hydraulic failure was relatively insensitive to root:shoot ratio across a range of tree LAI. Higher root:shoot ratios however strongly decreased the time to carbon starvation. The hydraulic safety margin of piñons is strongly diminished by large diurnal variations in xylem/leaf water potential. Diurnal drops of water potential are mitigated by high maximum hydraulic conductivity, high root:shoot ratio and stomatal regulation (more isohydric). The safety margin of junipers is not very sensitive to diurnal drops in water potential so that there is little benefit in stomatal regulation (more anisohydric). Narrower tracheid diameter and a narrower distribution of tracheid diameters, reduces the risk of hydraulic failure and carbon starvation by reducing diurnal xylem water potential drop. Simulated tree diameter-dependent mortality varies between these two species, with piñon mortality decreasing with increasing tree size, whereas juniper mortality increases with tree size. Juvenile piñons might thus be overimpacted by water stress.
    Ecohydrology 06/2015; DOI:10.1002/eco.1654 · 2.43 Impact Factor
    • "Plant Soil and (ii) a time lag between the aerial variables and its action on [CO 2 ] production (Kuzyakov and Gavrichkova 2010; Mencuccini and Holtta 2009). The parameters α 7 and α 8 in Eq. 15 have not been directly calibrated on CO 2 production measurements but their value was set to improve the representation of the temporal evolution of measured variables. "
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    ABSTRACT: Vertical profile of CO2 production (Ps) and transport, as well as their isotopic discrimination (13CO2/12CO2) should be considered to improve the soil CO2 efflux (Fs) mechanistic understanding and especially its short-term temporal variations. In this context, we propose a new methodology able to measure continuously and simultaneously Fs, the vertical soil CO2 concentration ([CO2]) profile and their respective isotopic signature (�Fs and �CO2) [1]. The Ps of the different soil layers and their isotopic signature (�Ps) can then be determined from these measurements by an approach considering diffusion as the only gas transport. A field campaign was conducted with this device at the Scots Pine Hartheim forest (Germany). The results [2] show (i) a Ps dependence on local temperature specific for each layer, (ii) an enrichment of �Ps with soil drought, (iii) Fs and [CO2] large intra-day fluctuations non explained by the soil temperature and moisture. These fluctuations can be generated by other processes creating Ps and/or transport variability. To investigate about the nature of these processes, some sensitivity analyses have been performed with a soil CO2 model simulating both production and transport. The impacts of the introduction of advection, dispersion and phloem pressure concentration wave (through dependence of Ps on vapour pressure deficit) on intra-day Fs and [CO2] variations have been quantified. We conclude that these variations are significantly better represented when the phloem pressure wave expression is included in the simulations. The study of the processes related to CO2 production seems to be a better option than an investigation about transport to explain the intra-day Fs variability.
    EGU General Assembly 2015, Vienna; 04/2015
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