Wood anatomy of Bupleurum L. (Apioideae, Apiaceae) in relation to habit, phylogenetic relationships, and infrageneric taxonomy

摘要

The structure of secondary xylem in perennial shoots of 20 species of Bupleurum (Apioideae, Apiaceae) representing different habits, viz. shrubs (B. fruticosum L., B. foliosum Salzm, B. gibraltaricum Lam.), a dwarf shrub (B. dianthifolium Guss.), semishrubs (B. acutifolium Boiss., B. atlanticum Murb. B. fruticescens L.), a cushion plant (B. spinosum Gonan), and perennial herbs (B. americanum Coult. et Rose, B. angulosum L., B. candollei Wall., B. commelynoideum Boiss., B. exaltatum M.B., B. longifolium L., B. longiradiatum Turch., B. marginatum Wall. ex DC, B. mundtii Cham. and Schldl., B. petraeum L., B. rigidum L., B. stellatum L.) is examined. The results confirm relationships of Bupleurum to Heteromorpha and related genera belonging to basal clades within Apioideae revealed by molecular phylogenetics (Downie & Katz-Downie, 1999). Presence of helical thickenings on vessel walls and septate fibres can be considered as ancestral conditions for Bupleurum. The following wood anatomical trends appear to have evolved in Bupleurum species during their specialisation towards herbaceousness: (1) the development of ray cells with unlignified walls, (2) the loss of distinct growth ring boundaries, (3) the replacement of alternate by scalariform intervessel pitting, (4) enlargement of intervessel pits, (5) a higher frequency of vessels, and (6) the replacement of imperforate tracheary elements by axial parenchyma. Within the herbaceous species, the members of the series Falcata (subsection Nervosa, section Eubupleura of Wolff's (1910) system of Bupleurum) are distinctive from others in several wood characters, as distinctness of growth rings, type of intervessel pitting, type and abundance of axial parenchyma, etc. Close relationships of the perennial herb B. rigidum to the shrubby species B. gibraltaricum and B. fruticosum suggested by molecular phylogenetics (Neves & Watson, 2004) can be confirmed by wood anatomical data. Therefore, the secondary xylem features of herbaceous species appear to be promising for the taxonomy of Bupleurum.