Wood anatomy of the crown lineages in Proteoideae (Proteaceae): implications for evolution and adaptive value of bordered pits in imperforate tracheary elements

Abstract

As Proteaceae subfamily Proteoideae are richly represented in the Cape Floristic Region of southern Africa and in south-western Australia, it is a convenient group for comparative studies of evolution and adaptation of plants in widely separated areas with a Mediterranean-type climate. Although species of Proteoideae attract considerable attention of researchers, the structural diversity of wood in this group remains under-explored. The wood structure of 51 species of 14 African and Australian genera of Proteoideae (Proteaceae) has been studied. These taxa are uniform in their wood structure; the diversity patterns are more related to plant stature, climatic factors and fire-survival strategies than to the taxonomy or phylogeny of the subfamily. Increases in length and diameter of fibres and diameter of pits in fibre walls are associated with a shift from a semi-arid winter-dry climate to a milder climate with winter rainfall. These trends may imply the transition from non-conducting imperforate tracheary elements to conducting elements in Mediterranean-type climatic regions. This presumable gain of hydraulic function of imperforate tracheary elements could be explained by adaptation to freeze–thaw stress during the late Quaternary, at least in the Cape Region. Vessel grouping shows no correlations with climatic factors. The seeders have a higher vessel frequency and narrower and lower rays than sprouters. We hypothesize that the larger rays in overground stems of sprouters serve as the sites of epicormic buds that provide for resprouting after fire or other damage. The occurrence of sclerified cells and/or irregularly arranged (occasionally circular) tracheary elements in broad rays of some species are thought to be responses to excessive sucrose transport in secondary phloem.