Chemical constituents from temperate and subtropical trees with reference to knotwood

Knotwood, the branch base inside the tree stem, which is undesirable in the processing of wood, constitutes an abundantly available biomass residue that is of no commercial value and remains underutilized. However, knotwood, as a rich source of various bioactive molecules, has significant importance for utilization as a renewable feedstock for the production of high-value chemicals. Knotwood in temperate climate zones has been a subject of extensive chemical studies, and a number of secondary metabolites have been characterized, including lignans, norlignans, flavonoids, stilbenoids, phenolic acids, tannins, diarylheptanoids, terpenoids, sterols, quinones, aromatic compounds, and fatty acids. Chemistry of different parts (bark, heartwood, sapwood, stemwood, wood) other than the knotwood of these tree species has also been published. Several phenolics and diarylheptanoids have been reported. These chemical constituents and their associated bioactivities have been reviewed. The structural proximity of the components of knotwood and other parts of these trees was observed. However, the concentration of the extractives was higher in knotwood than in other parts of the trees. The chemistry of knotwood of subtropical trees except Tectona grandis L.f. has not been investigated. Published information on the chemistry and bioactivities of Azadirachta indica Adr. Juss., Mangifera indica L. and Tectona grandis L.f. as representatives of subtropical trees of great economic significance was also compiled and reviewed. The constituents characterized in different parts of these trees were phenolics, terpenoids, limonoids, steroids, quinones, benzophenones and fatty acids. Based on the structural proximity of the components of knotwood and other parts of temperate trees, it is postulated that the chemical constituents characterized in different parts of tree species from subtropical regions may also occur in their hitherto uninvestigated knotwood. Our observations stimulate further chemical investigations of the knotwood of these woody perennials.