PATTERNS OF PETAL AND STAMEN REDUCTION IN AUSTRALIAN SPECIES OF LEPIDIUM L. (BRASSICACEAE)

The floral ground plan in the Brassicaceae is highly conserved. Howeve r, organ reduction is common in the genus Lepidium (pepper creases). I n many cases, petals are reportedly reduced or absent, and stamen numb ers are reduced from six to either foul or two in more than half of al l species. To gain an understanding of possible mechanisms of reductio n, we have observed flower organ development by SEM in six Australian species showing a range of reduction patterns. We conclude that reduct ion occurs by three independent mechanisms. Petal primordia are always formed, but their growth is suppressed in species in which they are r educed. This occurs at different times in the different species, altho ugh all except Lepidium oxytrichum show some differentiation of mature tissues. On the other hand, stamens are reduced either by apparent lo ss of primordia (in the two lateral positions) or by fusion of two pri mordia into one fin the two medial positions). Overall, Lepidium speci es show one of four different stamen ground plans: no reduction (six s tamens), lateral loss alone (four stamens), medial fusion alone (four stamens), or both reductions (two stamens), suggesting that the two me chanisms of stamen reduction are different. Petal suppression is limit ed to species lacking lateral stamens, although it does not occur in a ll such species. Nectaries also vary between species, but they arise l ate in flower development and are not part of the floral ground plan. The possible involvement of homologues of genes known from Arabidopsis thaliana, also in the Brassicaceae, in the three mechanisms of floral organ reduction is discussed.