Inflorescence and floral development of Dipteronia sinensis (Sapindaceae): Demonstration of developmental constraints in the shaping of the flower

Abstract

The inflorescence, floral structure and ontogeny, and breeding system of the relict woody species Dipteronia sinensis was investigated to understand the developmental basis for its floral Bauplan, more particularly the octandrous androecium, dimerous gynoecium and the occurring flower symmetry in comparison with Sapindaceae.The functionally unisexual flowers arise in thyrses. Structural floral dimorphism was clearly observed in the different lengths of style and filaments and in the presence or absence of fructifications. Functionally pistillate flowers are found in a terminal position of each cincinnus maturing before the functionally staminate flowers. Petals are initiated unidirectionally and there is a time overlap with stamens. No common petal-stamen primordium was observed. There are two empty stamen positions, always opposite the first initiated petal, between sepals 1 and 3, and opposite the second petal between sepals 2 and 4; subsequently stamens are displaced to accommodate to the available space. Two median carpels are almost apocarpous in early stages and produce two parallel ovules each that become later superposed. Floral dimorphism develops as a late event when pollen and ovules are formed.Empty stamen positions are inversely correlated with the sequence of initiation and delayed growth of the petals, suggesting that a subtle heterochronic shift is responsible for the balance between petal and stamen initiation. Although the flower is initially polysymmetric to weakly monosymmetric, the bicarpellate gynoecium causes a shift to disymmetry during the mid-developmental stage as in the sister genus Acer, differently from tricarpellate Sapindaceae. Floral development demonstrates the flexibility in building symmetry in Sapindaceae, which is linked with available space and subtle changes in the sequence of initiation, rate of development and reduction of floral organs. This study underlines the importance of analyzing developmental constraints responsible for the floral diversity of Sapindaceae and provides further support for a monosymmetric ancestry in Sapindaceae.