Patterns of petal and stamen reduction in australian species of Lepidium l. (Brassicaceae)

The floral ground plan in the Brassicaceae is highly conserved. However, organ reduction is common in the genus Lepidium (pepper cresses). In many cases, petals are reportedly reduced or absent, and stamen numbers are reduced from six to either four or two in more than half of all species. To gain an understanding of possible mechanisms of reduction, we have observed flower organ development by SEM in six Australian species showing a range of reduction patterns. We conclude that reduction occurs by three independent mechanisms. Petal primordia are always formed, but their growth is suppressed in species in which they are reduced. This occurs at different times in the different species, although all except Lepidium oxytrichum show some differentiation of mature tissues. On the other hand, stamens are reduced either by apparent loss of primordia (in the two lateral positions) or by fusion of two primordia into one (in the two medial positions). Overall, Lepidium species show one of four different stamen ground plans: no reduction (six stamens), lateral loss alone (four stamens), medial fusion alone (four stamens), or both reductions (two stamens), suggesting that the two mechanisms of stamen reduction are different. Petal suppression is limited to species lacking lateral stamens, although it does not occur in all such species. Nectaries also vary between species, but they arise late 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.