On the unusual flowering of plagiotropic shoots in the seagrass Posidonia oceanica

The seagrass Posidonia oceanica (L.) Delile can produce plagiotropic (horizontally oriented) and orthotropic (vertically oriented) axes. These axes differ in various morphological and growth characteristics and ecological functions. Only orthotropic shoots are likely to support flowering and seed production. To asses whether flowering may occur on plagiotropic shoots, we examined plagiotropic axes that grew at the edge of a meadow during the flowering of 2003. We identified and analysed 10 rhizome fragments that bore at least one reproductive plagiotropic shoot. We then investigated whether reproductive plagiotropic shoots differed from orthotropic ones in morphological characteristics and investment in flowering (i.e., number of hermaphroditic flowers per inflorescence). Multivariate analysis did not show any significant difference between plagiotropic and orthotropic shoots with regard to the characteristics examined and investment in flowering. Further studies are necessary to ascertain if the observed pattern of flowering is a common or an exceptional event in P. oceanica.