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First published online October 14, 2009; doi:10.3732/ajb.0800362 American Journal of Botany 96: 2080-2086 (2009) © 2009 Botanical Society of America, Inc.

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Pollinators, "mustard oil" volatiles, and fruit production in flowers of the dioecious tree Drypetes natalensis (Putranjivaceae)¹

² School of Biological and Conservation Sciences, University of KwaZulu-Natal, P. Bag X01, Scottsville, Pietermaritzburg 3209, South Africa ³ School for Environmental Research, Charles Darwin University, Ellengowan Drive, Darwin 0909, NT, Australia

ABSTRACT

The Putranjivaceae is an enigmatic family, notable for being the only lineage outside the Capparales to possess the glucosinolate biochemical pathway, which forms the basis of an induced chemical defense system against herbivores (the "mustard oil bomb"). We investigated the pollination biology and floral scent chemistry of Drypetes natalensis (Putranjivaceae), a dioecious subcanopy tree with flowers borne on the stem (cauliflory). Flowering male trees were more abundant than female ones and produced about 10-fold more flowers. Flowers of both sexes produce copious amounts of nectar on disc-like nectaries accessible to short-tongued insects. The main flower visitors observed were cetoniid beetles, bees, and vespid wasps. Pollen load analysis indicated that these insects exhibit a high degree of fidelity to D. natalensis flowers. Insects effectively transfer pollen from male to female plants resulting in about 31% of female flowers developing fruits with viable seeds. Cetoniid beetles showed significant orientation toward the scent of D. natalensis flowers in a Y-maze olfactometer. The scents of male and female flowers are similar in chemical composition and dominated by fatty acid derivatives and isothiocyanates from the glucosinolate pathway. The apparent constitutive emission of isothiocyanates raises interesting new questions about their functional role in flowers.

Key Words: cauliflory • dioecy • Drypetes • floral scent • GCMS • glucosinolate • isothiocyanate • nectar • pollination syndrome • Putranjivaceae

Received for publication 23 October 2008. Accepted for publication 11 August 2009.

FOOTNOTES

¹ The authors thank R. Kaiser (Givaudan Natural Scents) for his expert analysis of the solvent scent samples and feedback on the manuscript. R. Wethered and S. Steenhuisen provided assistance in the laboratory. Mazda Wildlife Fund provided field vehicles. The National Research Foundation (South Africa) provided financial support. This study is part of a broader investigation of dune forest ecology, and we thank the iSimangaliso (greater St. Lucia) Wetland Park authority and Ezemvelo KwaZulu-Natal Wildlife for access to Cape Vidal. A. Jürgens provided helpful discussions and comments on the manuscript.

⁴ Author for correspondence (e-mail: Johnsonsd{at}ukzn.ac.za)

⁵ Present address: Department of Botany, Rhodes University, P.O. Box 94, Grahamstown, 6140, South Africa

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