New aspects in floral development of Papilionoideae: initiated but suppressed bracteoles and variable initiation of sepals.

Abstract

The increase of molecular data and the resulting insights into legume systematics make the search for new morphological characters and a careful re-investigation of already stated characters necessary. Bracteoles are small, reduced leaves borne close to the base of lateral branches. Although they seem unimportant in older buds, they have an ecological function in protecting the sepal primordia. Furthermore, a morphogenetic function in mediating the onset of sepal initiation is suspected in the literature. The occurrence of bracteoles varies within Papilionoideae, and their distribution is used in legume systematics. But this is open to criticism, because there is a tendency to use 'absent' for 'caducous'. Thus attention here was paid to the initiation of bracteoles as well as to the sequence of sepal initiation. The floral development of 30 taxa out of 15 tribes of Papilionoideae was investigated using scanning electron microscopy (SEM). In five taxa the bracteoles initiated, but suppressed early. Furthermore, a broad variability of sepal initiation was found. Besides the widely stated unidirectional pattern, modified unidirectionality, tendencies towards whorled, fully whorled, bidirectional and successive initiation of sepals were all found. Initiated but suppressed bracteoles are presented as a 'new' character in Papilionoideae. Considering the presence of bracteoles as a plesiomorphy, their suppression can be seen as a step towards completely reduced bracteoles. The remarkable variability of the sequence of sepal initiation questions the widely stated unidirectionality of organ initiation in Papilionoideae. The different modes of sepal initiation are deducible from the helical pattern of some caesalpinioids, which is seen as a developmental link of the flowers of Papilionoideae and Caesalpinioideae. The bidirectional sepal initiation is possibly a consequence of the morphogenetic function of bracteoles, although bidirectionality is not found in all taxa with reduced bracteoles.