AOBPreview originally published online on April 23, 2008
Annals of Botany 2008 102(1):31-37; doi:10.1093/aob/mcn058
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Characteristic Thickened Cell Walls of the Bracts of the ‘Eternal Flower’ Helichrysum bracteatum
1 Vegetable and Ornamental Horticulture, Division of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Kyoto 606-8502, Japan
2 AJINOMOTO Integrative Research for Advanced Dieting, Graduate School of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Kyoto 606-8502, Japan
3 Tree Cell Biology, Division of Forest and Biomaterials Science, Kyoto University, Oiwake-cho, Kitashirakawa, Kyoto 606-8502, Japan
* For correspondence. E-mail hiroaki{at}kais.kyoto-u.ac.jp
Received: 31 January 2008 Returned for revision: 20 February 2008 Accepted: 20 March 2008 Published electronically: 23 April 2008
Background and Aims: Helichrysum bracteatum is called an ‘eternal flower’ and has large, coloured, scarious bracts. These maintain their aesthetic value without wilting or discoloration for many years. There have been no research studies of cell death or cell morphology of the scarious bract, and hence the aim of this work was to elucidate these characteristics for the bract of H. bracteatum.
Methods: DAPI (4'6-diamidino-2-phenylindol dihydrochloride) staining and fluorescence microscopy were used for observation of cell nuclei. Light microscopy (LM), transmission electron microscopy (TEM) and polarized light microscopy were used for observation of cells, including cell wall morphology.
Key Results: Cell death occurred at the bract tip during the early stage of flower development. The cell wall was the most prominent characteristic of H. bracteatum bract cells. Characteristic thickened secondary cell walls on the inside of the primary cell walls were observed in both epidermal and inner cells. In addition, the walls of all cells exhibited birefringence. Characteristic thickened secondary cell walls have orientated cellulose microfibrils as well as general secondary cell walls of the tracheary elements. For comparison, these characters were not observed in the petal and bract tissues of Chrysanthemum morifolium.
Conclusions: Bracts at anthesis are composed of dead cells. Helichrysum bracteatum bracts have characteristic thickened secondary cell walls that have not been observed in the parenchyma of any other flowers or leaves. The cells of the H. bracteatum bract differ from other tissues with secondary cell walls, suggesting that they may be a new cell type.
Key words: Helichrysum bracteatum, scarious bract, secondary cell wall, primary cell wall, cell morphology, birefringence, orientated cellulose microfibrils, cell death, DAPI, transmission electron microscopy, polarized light microscopy