Phenolic extractives in Salix caprea wood and knots Suvi P. Pohjamo*, Jarl E. Hemming, Stefan M. Willfo¨r, Markku H.T. Reunanen, Bjarne R. Holmbom Process Chemistry Group, A ˚ bo Akademi University, Porthansgatan 3, FIN-20500 Turku/A ˚ bo, Finland Received 1 July 2002; received in revised form 7 January 2003 Abstract Salix caprea stemwood and knots were found to contain the phenolic extractives vanillic acid, 3-p-coumaryl alcohol, coniferyl alcohol, sinapylaldehyde, dihydrokaempferol, catechin, naringenin, gallocatechin, dihydromyrcetin and taxifolin. The knots con- tained larger quantities of flavonoids than did stemwood of the same tree. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Salicaceae; Salix caprea; Wood; Knots; Phenolic extractives 1. Introduction A knot is the part of a branch that is embedded in the tree stem. When a branch is broken close to the stem, an open wound is formed, making the knot susceptible to attack by fungi and other micro-organisms. In recent studies, knots of softwoods (Picea abies and Pinus sylvestris) have been found to contain exceptionally large amounts of various phenolic sub- stances, especially lignans and stilbenes (Willfo¨r et al., 2003a,b; Ekman et al., 2002). The concentration of lignans in spruce knots can be 100–500 times higher than in the stemwood (Willfo¨r et al., 2003a). Lignans have antimicrobial and antioxidative properties and they are involved in the defence against fungi and other micro-organisms. However, no studies have so far been reported on extractives in the knots of any hardwoods. Salix caprea is a common tree in the agricultural landscape of northern Europe and has been used as a wind shield bush growing in ditches between fields. The economic value of S. caprea is however small. As a fast- growing tree it may be of interest for bioenergy production. Willows are one of the most taxonomically diverse plant genera in the Northern Hemisphere (Argus, 1997). The extractives in bark, wood, leaves and pollen of dif- ferent Salix species have been studied to some extent. According to Lo¨nnberg (1975), gravimetrical amounts of hydrophilic extractives in S. caprea stem- wood vary between 2.0 and 3.1 mg g 1 and the amounts of lipophilic extractives between 1.5 and 1.7 mg g 1 . Larger amounts were found in young stems than in older ones. Flavonoids present in S. caprea wood were reported to have antifungal properties (Malterud et al., 1985). Of six identified flavonoids [dihydrokaempferide, nar- ingenin, aromadendrin, taxifolin, prunin and (+)-cate- chin], naringenin was found to be the most effective one against both fungi and microbes. Naringenin-5-glucoside has been found in the bark of Salix-species (Rowe, 1989). Catechin is a common constituent in plants and trees (Rowe, 1989). It has been shown to have antioxidant (Choi et al., 2001), as well as antifungal and anti- microbial (Mantani et al., 2001) properties. Astralgin, quercimeritrin and quercitin-3,7-di-O-glu- coside were found in pollen of S. caprea (Gorobets et al., 1982), while salicin, saligenin, (  )-gallocatechin, rutin, cynaroside, quercitin and luteolin were found in the leaves (Sagareishvili et al., 1990). Here we present the results from analysis of the hydrophilic extractives of stemwood and knots from two S. caprea trees. 0031-9422/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0031-9422(03)00050-5 Phytochemistry (2003) 165–169 www.elsevier.com/locate/phytochem * Corresponding author. Tel.: +358-2-215-4887; fax: +358-2-215- 4868. E-mail address: spohjamo@abo.fi (S.P. Pohjamo).