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First published online July 6, 2009; doi:10.3732/ajb.0800355 American Journal of Botany 96: 1399-1408 (2009) © 2009 Botanical Society of America, Inc.

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What is disjunctive xylem parenchyma? A case study of the African tropical hardwood Okoubaka aubrevillei (Santalaceae)¹

² Laboratory for Wood Biology and Xylarium, Royal Museum for Central Africa, Tervuren B-3080, Belgium ³ Forestry and Forest Products Research Institute, Tsukuba Science City, 305-8687 Japan ⁴ Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu-Tokyo 183-8509, Japan

ABSTRACT

The morphological variation and structure–function relationships of xylem parenchyma still remain open to discussion. We analyzed the three-dimensional structure of a poorly known type of xylem parenchyma with disjunctive walls in the tropical hardwood Okoubaka aubrevillei (Santalaceae). Disjunctive cells occurred among the apotracheal parenchyma cells and at connections between axial and ray parenchyma cells. The disjunctive cells were partly detached one from another, but their tubular structures connected them into a continuous network of axial and ray parenchyma. The connecting tubules had thick secondary walls and simple pits with plasmodesmata at the points where one cell contacted a tubule of another cell. The imperforate tracheary elements of the ground tissue were seven times longer than the axial parenchyma strands, a fact that supports a hypothesis that parenchyma cells develop disjunctive walls because they are pulled apart and partly separated during the intrusive growth of fibers. We discuss unresolved details of the formation of disjunctive cell walls and the possible biomechanical advantage of the wood with disjunctive parenchyma: the proportion of tissue that improves mechanical strength is increased by the intrusive elongation of fibers (thick-walled tracheids), whereas the symplastic continuum of the parenchyma is maintained through formation of disjunctive cells.

Key Words: disjunctive cell wall • Okoubaka aubrevillei • Santalaceae • wood formation • wood structure • xylem parenchyma

Received for publication 21 October 2008. Accepted for publication 9 March 2009.

FOOTNOTES

¹ The authors thank the late professor Jun Ohtani for his valuable advice on the SEM work and Dr. Barbara Lachenbruch for discussion and critical reading of the manuscript. This work was supported by a grant from the Japanese Society for the Promotion of Science (JSPS) and a Marie Curie Outgoing International Fellowship of the European Commission (MOIF 040400 to P.K.).

⁵ Author for correspondence (e-mail: peter.kitin{at}oregonstate.edu); present address: Department of Wood Science and Engineering, Oregon State University, Corvallis, Oregon 97331 USA

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