Caroline Plain

Carbon Transfer from the Host to Tuber melanosporum Mycorrhizas and Ascocarps Followed Using a 13C Pulse-Labeling Technique

Truffles ascocarps need carbon to grow, but it is not known whether this carbon comes directly from the tree (heterotrophy) or from soil organic matter (saprotrophy). The objective of this work was to investigate the heterotrophic side of the ascocarp nutrition by assessing the allocation of carbon by the host to Tuber melanosporum mycorrhizas and ascocarps. In 2010, a single hazel tree selected for its high truffle (Tuber melanosporum) production and situated in the west part of the Vosges, France, was labeled with 13CO2. The transfer of 13C from the leaves to the fine roots and T. melanosporum mycorrhizas was very slow compared with the results found in the literature for herbaceous plants or other tree species. The fine roots primarily acted as a carbon conduit; they accumulated little 13C and transferred it slowly to the mycorrhizas. The mycorrhizas first formed a carbon sink and accumulated 13C prior to ascocarp development. Then, the mycorrhizas transferred 13C to the ascocarps to provide constitutive carbon (1.7 mg of 13C per day). The ascocarps accumulated host carbon until reaching complete maturity, 200 days after the first labeling and 150 days after the second labeling event. This role of the Tuber ascocarps as a carbon sink occurred several months after the end of carbon assimilation by the host and at low temperature. This finding suggests that carbon allocated to the ascocarps during winter was provided by reserve compounds stored in the wood and hydrolyzed during a period of frost. Almost all of the constitutive carbon allocated to the truffles (1% of the total carbon assimilated by the tree during the growing season) came from the host.

Study of nitrogen and carbon transfer from soil organic matter to Tuber melanosporum mycorrhizas and ascocarps using 15N and 13C soil labelling and whole-genome oligoarrays

Background and aimsWe previously showed by 13CO2 host labelling that almost all of the constitutive carbon allocated to the truffles originated from the host. The objective of this present work was to determine the putative capacity of T. melanosporum ectomycorrhizas and ascocarps to use soil carbon and to uptake or assimilate soil nitrate.MethodsThe current investigation involved 13C and 15N soil labelling by incorporating labelled leaf litter and expression of genes involved in carbon and nitrogen metabolism in ascocarps and ectomycorrhizas.ResultsThe ascocarps harvested in the labelled plots were highly enriched in 15N but were almost never enriched in 13C. The main source of soil mineral nitrogen was nitrate. A nitrate transporter, one nitrate reductase and a nitrite reductase were well expressed in ectomycorrhizas. Several genes involved in aminoacid synthesis or in transamination processes were also well expressed in ectomycorrhizas. No nitrate transporter was expressed in ascocarps where the CAZyme genes upregulated were mainly Glycosyltransferases involved in saccharide transfer.ConclusionAscocarps did not exhibit saprotrophic capacity for C, supporting previous results from 13CO2 host labelling showing that C is provided by the host tree. The 15N present in the ascocarps after soil labelling is supplied as ammonium or aminoacids by the ectomycorrhizas, which are able to uptake, reduce and metabolize nitrate.

The impact of prolonged drought on phloem anatomy and phloem transport in young beech trees

Phloem failure has recently been recognized as one of the mechanisms causing tree mortality under drought, though direct evidence is still lacking. We combined 13C pulse-labelling of 8-year-old beech trees (Fagus sylvatica L.) growing outdoors in a nursery with an anatomical study of the phloem tissue in their stems to examine how drought alters carbon transport and phloem transport capacity. For the six trees under drought, predawn leaf water potential ranged from -0.7 to -2.4 MPa, compared with an average of -0.2 MPa in five control trees with no water stress. We also observed a longer residence time of excess 13C in the foliage and the phloem sap in trees under drought compared with controls. Compared with controls, excess 13C in trunk respiration peaked later in trees under moderate drought conditions and showed no decline even after 4 days under more severe drought conditions. We estimated higher phloem sap viscosity in trees under drought. We also observed much smaller sieve-tube radii in all drought-stressed trees, which led to lower sieve-tube conductivity and lower phloem conductance in the tree stem. We concluded that prolonged drought affected phloem transport capacity through a change in anatomy and that the slowdown of phloem transport under drought likely resulted from a reduced driving force due to lower hydrostatic pressure between the source and sink organs.

Les sols forestiers, puits de méthane : un service écosystémique méconnu

Le méthane (CH4) est le second gaz à effet de serre, derrière le gaz carbonique (CO2), dont la concentration dans l’atmosphère augmente de manière exponentielle depuis deux siècles du fait des activités humaines, passant de 0,72 partie par million (ppm) en 1750 à 1,84 ppm en 2016. Bien qu’en quantité beaucoup plus faible que le CO2 (402 ppm en 2016), son pouvoir de réchauffement global, calculé sur un siècle, est 28 fois supérieur à celui du CO2 (IPCC, 2013). Il est responsable à hauteur de 20 % du réchauffement global. Le méthane contribue également à la production d’ozone dans la troposphère, dont les impacts négatifs sur les écosystèmes forestiers ne seront pas traités ici.