Physiology of Cambial Growth, Storage of Reserves and Heartwood Formation

Abstract

Wood constitutes a renewable bio-product of huge ecological and economical value. Within axes of trees, wood formation starts during cambial growth and inwards produced cells which differentiate and mature during the first growing season forming functional units. In a standing tree, living, mature wood parts serve three functions: water and mineral transport, storage of food reserves and mechanical stability. Wood is thus composed of different cell types which enable the different functional purposes and which show a highly ordered arrangement. The dominant reserve substances starch, triacylglycerids, and storage proteins, are accumulated in living parenchyma cells during favorable periods and consumed at times of demand. However, they behave differently with respect to the deposition period and the pool sizes within the cells, but show similar behavior with respect to mobilization in spring. In most tree species, the final step in the life cycle of living xylem cells, is a genetically determined, programmed cell death which is characterized by the activation of hydrolytic enzymes, gene expression and de novo protein synthesis. The activation of metabolic pathways leads to the formation of phenolic heartwood extractives, which are responsible for the biological, chemical and physical features of heartwood.