New Insights Into Heartwood And Heartwood Formation

The formation of true or obligate heartwood in the innermost living xylem tissues of many tree species is one of the most important ecological and economical secondary differentiation processes. Natural durability, biological, technological, and esthetic parameters of wood and its products depend on the presence, quality, and quantity of heartwood extractives. This chapter presents the actual knowledge about heartwood formation. The main focus is on the biochemical and molecular basis of heartwood formation as well as on new developments in using heartwood extractives as biocides or as pharmaceuticals. Biochemical studies, corroborated by gene expression studies, prove that (1) season of heartwood formation starts in summer, reaches a maximum in early fall, and ceases during dormancy; (2) the transient shift in metabolism towards an enhanced secondary metabolism is regulated by gene expression and protein de novo synthesis; and (3) programmed cell death (PCD) of parenchyma cells during heartwood formation shares some similarities with PCD during xylem formation. However, more work is needed (1) to find more similarities and/or differences between the two types of heartwood formation (Robinia-Type and Juglans-Type), (2) to elucidate the involvement of axial and ray parenchyma cells, and (3) to learn more about regulation of heartwood formation, such as the role of transcription factors or phytohormones.