Patterns of Petal and Stamen Reduction in Australian Species of Lepidium L. (Brassicaceae)

Abstract

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 gro...