The floral organogenesis of <Emphasis Type="Italic">Koelreuteria bipinnata</Emphasis> and its variety <Emphasis Type="Italic">K. bipinnata</Emphasis> var. <Emphasis Type="Italic">integrifolia</Emphasis> (Sapindaceae): evidence of floral constraints on the evolution of monosymmetry

The floral organogenesis and anatomy of Koelreuteria bipinnata and its variety K. bipinnata var. integrifolia (Sapindaceae) has been investigated to clarify the identity of the two taxa in relation to other species of Koelreuteria, and to understand the shift to monosymmetry in the genus. Although the floral development is highly similar, we found a number of striking differences. Flowers arise in thyrses, with lateral branches forming cincinni of 5–9 flowers. Sepals initiate in a spiral sequence. Five petals arise unidirectionally alternating to the sepals. The last formed petal and one stamen between sepals 3 and 5 are strongly delayed, appearing as a common primordium, while this petal is completely suppressed in var. integrifoliola. Eight stamens initiate sequentially, differ in size and partly precede the development of petals. The gynoecium develops as a triangular primordium on which three carpellary lobes become demarcated simultaneously. Placentation is axile. Septal slits occurring within the style are interpreted as a deep-reaching non-nectariferous extension of the stigma. The massive, oblique disk with crenate apex develops in an extrastaminal position, but is interrupted on the radius of the lost petal. Floral developmental evidence supports variety K. bipinnata var. integrifolia rather than being synonym of K. bipinnata. Floral development is compared with K. paniculata and is discussed in the context of floral evolution of Sapindaceae. Our study demonstrates the importance of developmental shifts on floral evolution. The triangular gynoecium has a strong spatial impact in obliquely reorganizing the symmetry of the flower. It is demonstrated that spatial constraints of calyx and ovary are responsible for the reduction in one of the petals, two stamens and a shift in symmetry of the flower.