Soil Biology & Biochemistry 39 (2007) 1655–1663 Relation between oak tree phenology and the secretion of organic matter degrading enzymes by Lactarius quietus ectomycorrhizas before and during bud break Pierre-Emmanuel Courty a,Ã , Nathalie Bre´da b , Jean Garbaye a a INRA Nancy, Laboratoire Interactions Arbres/Microorganismes, UMR 1136 INRA/UHP Route d’amance, F-54280 Champenoux, France b INRA Nancy, UMR 1137 INRA/UHP Ecologie et Ecophysiologie Forestie`res, F-54280 Champenoux, France Received 26 August 2006; received in revised form 11 January 2007; accepted 17 January 2007 Available online 26 February 2007 Abstract The ectomycorrhizas (ECM) formed by Lactarius quietus, an ECM fungus specifically associated with Quercus spp., are abundant all the year long. Root and stem growth, beginning before bud break in oak, are strong carbon sinks partially fulfilled with carbohydrate reserves. We hypothesized that L. quietus contributes to providing trees with carbon at bud break through enzymatic activities before photosynthesis begins. Activities of eight secreted enzymes (xylosidase, glucuronidase, cellobiohydrolase, b-glucosidase, N-acetyl-glucosamine, leucine aminopeptidase, acid phosphatase and laccase) relevant to carbon cycling and the release of phophorus and nitrogen from soil organic matter were measured on L. quietus ECMs before, during and after the bud break. Phenological, climatic and pedoclimatic parameters were also measured. Laccase, glucuronidase, cellobiohydrolase and b-glucosidase activities proved to be significantly related to tree reactivation and climate. All these activities can help the formation of new tissues by supplying carbon. L. quietus can behave saprotrophically, using soil organic matter as substrate. This is consistent with the hypothesis that it provides the oak trees with carbon when demand is high and photoassimilates are not yet available. r 2007 Elsevier Ltd. All rights reserved. Keywords: Lactarius quietus; Ectomycorrhizal fungi; Bud break; Stem growth; Secreted enzymes; Organic matter; Carbon; Mixotrophy; Mycoheterotrophy 1. Introduction In Central Europe, pedunculate (Quercus robur L.) and sessile (Quercus petraea Matt. Liebl.) oaks are the most important deciduous forest tree species, ecologically and economically. Tree phenology changes seasonally due to both climate and endogenous rhythms. In oak, a ring- porous species, the early wood including large vessels and a part of the annual radial stem growth is achieved before leaf expansion in spring (Dougherty et al., 1979; Hinckley and Lassoie, 1981; Bre´da and Granier, 1996). Oak stem growth starts 10 days before bud burst (Bre´ da and Granier, 1996; Barbaroux and Bre´da, 2002) and about 30% of the total annual stem increment is added before bud burst (Hinckley and Lassoie, 1981). Primary root elongation, related to changes in external climatic variables (soil temperature and soil water potential) (Teskey and Hinckley, 1981) and endogenous factors including water and hormone transfer (Lachaud, 1989), begin before bud swell and last until bud burst when trees are leafless. Root and stem growth, as well as bud burst, are strong carbon sinks (Ashworth et al., 1993). Total nonstructural carbohydrate reserves (TNC: starch and sugars) provide the most important part of the energy needed for tree reactivation (Hinckley and Lassoie, 1981). However, measurements of carbohydrate reserves show that TNC are depleted after bud burst (Barbaroux ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2007.01.017 Ã Corresponding author. Tel.:+33 3 83 39 40 41 (station 42-20); fax: +33 3 83 39 40 69. E-mail address: courty@nancy.inra.fr (P.-E. Courty).