Annals of Botany

Death and glory

When UK rock group The Rolling Stones sang ‘. . .you can send me dead flowers every morning. . .’, floral decoration was very far from their minds. However, dead and dried flowers used as decorations often last several years. One plant used in this way is Helichrysum bracteatum, one of the so-called ‘eternal’ flowers. In this species, the colour is provided by several rows of large, scarious (but otherwise corolla-like) bracts, as described by Nishikawa et al. (Kyoto, Japan, pp. 31–37). Bract water content is low, well below 50 % of the water content of growing tissues and much more typical of tissues containing high proportions of dead cells. So, at what stage do the bracts of H. bracteatum actually die? The authors used DAPI staining combined with fluorescence microscopy to examine cell nuclei; other aspects of cell structure and morphology were investigated with a range of microscopic techniques. Even in the unopened flower bud, loss of nuclei had started in the bract tip and this loss worked its way down towards the base of the bract as the bud expanded and then the flower opened. By the anthesis stage there were no nuclei in the upper half of the bracts (although cells at the base of the bract had retained their nuclei) and cells lacked all other organelles. Both the epidermal and the inner cells were secondarily thickened (in contrast to bracts and petals of a conventional flower) and, further, the cell walls showed birefringence, indicative of orientated cellulose microfibrils. The cells thus have some features of tracheary elements. However, in the bracts these features are present in epidermal and parenchymatous cells, leading the authors to postulate that they represent a new cell type. Here, then, is a specific form of programmed cell death associated with floral function and initiated very early in the life of the flower.