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
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    • "The isotope signal fluctuation has proven to be useful to estimate lags between photosynthetic carbon assimilation and the use of the assimilated carbon for metabolic processes in trees ( Ekblad and Högberg 2001). It was shown that isotopic mixing of recent photosynthates into large carbon pools such as stored carbohydrates can delay the recovery of the isotope signals into respiration ( Mencuccini and Hölttä 2010, Wingate et al. 2010). While stable carbon isotope composition and its temporal variations in latex have not yet been addressed, short lags of a few days duration may be expected if the carbohydrate used for latex production is derived from sucrose directly carried by the phloem sap. "
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    ABSTRACT: Latex, the cytoplasm of laticiferous cells localized in the inner bark of rubber trees (Hevea brasiliensis Müll. Arg.), is collected by tapping the bark. Following tapping, latex flows out of the trunk and is regenerated, whereas in untapped trees, there is no natural exudation. It is still unknown whether the carbohydrates used for latex regeneration in tapped trees is coming from recent photosynthates or from stored carbohydrates, and in the former case, it is expected that latex carbon isotope composition of tapped trees will vary seasonally, whereas latex isotope composition of untapped trees will be more stable. Temporal variations of carbon isotope composition of trunk latex (δ13C-L), leaf soluble compounds (δ13C-S) and bulk leaf material (δ13C-B) collected from tapped and untapped 20-year-old trees were compared. A marked difference in δ13C-L was observed between tapped and untapped trees whatever the season. Trunk latex from tapped trees was more depleted (1.6‰ on average) with more variable δ13C values than those of untapped trees. δ13C-L was higher and more stable across seasons than δ13C-S and δ13C-B, with a maximum seasonal difference of 0.7‰ for tapped trees and 0.3‰ for untapped trees. δ13C-B was lower in tapped than in untapped trees, increasing from August (middle of the rainy season) to April (end of the dry season). Differences in δ13C-L and δ13C-B between tapped and untapped trees indicated that tapping affects the metabolism of both laticiferous cells and leaves. The lack of correlation between δ13C-L and δ13C-S suggests that recent photosynthates are mixed in the large pool of stored carbohydrates that are involved in latex regeneration after tapping.
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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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    • "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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