Club-Shaped Organs as Additional Osmophores within the Sauromatum Inflorescence: Odour Analysis, Ultrastructural Changes and Pollination Aspects[1]

Abstract

Gradient odour emissions in the inflorescence of the Araceae Sauromatum guttatum aim to attract a wide range of insects into the floral chamber. The volatiles are emitted from the spadix appendix, as well as the club-shaped organs located directly above the female flowers. Volatile analysis of various regions of the appendix and the club-shaped organs led to the identification of 163 compounds emitted by the appendix top, 124 by the appendix bottom and 105 by the club-shaped organs. The dominant compounds in all investigated tissues were monoterpenes and sesquiterpenes that were accompanied by numerous aliphatic, aromatic, sulphur- and nitrogen-containing compounds of other biosynthetic origins. Within the appendix, levels of one monoterpene, β-citronellene, showed considerable variation; it constituted the major compound in the appendix top and gradually decreased in the lower regions, being undetectable in the base. The other prominent monoterpenes, α-pinene, β-pinene, limonene, α-phellandrene and β-phellandrene, showed no changes along the appendix. The club-shaped organs located at the base of the floral chamber also emitted volatiles, though of different composition: the monoterpenes α-terpinolene and linalool constituted major amounts, instead of β-pinene and β-citronellene. These qualitative differences and the absence of methanethiol in the club-shaped organs result in them having a pleasant flowery odour, in contrast to the foul-smelling appendix. The quantitative spatial variation of β-citronellene within the appendix and the existence of a second osmophore within the floral chamber, emitting a different scent, suggest that both phenomena might participate in creating an odour gradient for efficient pollinator attraction. Apart from β-citronellene, nearly all major components of the appendix have already been identified as specific attractants to a broad range of insects that have been observed to be lured to the odour-emitting inflorescence. Most likely, the club-shaped organ odour serves to induce the insects attracted by the appendix to move towards the floral chamber. A comparison of the relative emission rates demonstrated that performance of the club-shaped organs equals or even exceeds the appendix. In the club-shaped organ tissues, multivesicular bodies, originating from lipid droplets, are the most notable organelles before and during odour production. They fuse with the plasma membrane, releasing their content to the exterior of the cell. The exit route for the volatiles is an extensive intercellular channel system that extends to the surface of the club-shaped organs. Thus, chemical and ultrastructural analyses suggest, in contrast to previous studies, that the club-shaped organs function as an osmophore rather than a food source for insects.

Abbreviations

APP: appendix

CSO: club-shaped organs

ER: endoplasmic reticulum

GC: gas chromatography

MS: mass spectroscopy

MVB: multivesicular bodies

SEM: scanning electron microscopy

SPME: solid phase micro-extraction

TEM: transmission electron microscopy

TIC: total ion concentration

Araceae - qualitative odour analysis - gas chromatography - mass spectroscopy - monoterpenes - electron microscopy - multivesicular bodies

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1 Dedicated to Prof. Stefan Vogel, pioneer of osmophore analysis.

Section Editor: G. Gottsberger