Differences in reproductive function between terminal and lateral flowers in the capitulum of Albizia julibrissin (Fabaceae) based on female fertility, ovule number, and pistil and polyad size

Abstract

The division of labor between florets responsible for reproduction and special functions occurs widely in condensed inflorescences. The capitulum of Albizia julibrissin consists of approximately 10–20 lateral flowers and usually one nectar‐secreting terminal flower; the two flower types are morphologically distinct, but the differences in reproductive function have not been revealed. To determine the differences in reproductive function between terminal and lateral flowers, we compared the ratio of terminal and lateral flowers at the flowering and young pod stages. Additionally, ovule number, stigma size, polyad size, and the ratio of flower abscission were compared between the two flower types. Significant differences were detected only in female fertility between the two flower types. At the flowering stage, 6.8% of flowers were terminal flowers, whereas at the young pod stage, all young fruits were obtained from lateral flowers. At the final stage of blooming, based on the generalized linear model analysis, the ratio of surviving terminal flowers was lower than that of the lateral flowers in a capitulum. These results suggest that the terminal flowers of A. julibrissin had female sterility or, at least, female fertility was lower than in lateral flowers. Therefore, it is necessary to reexamine some previous studies that assumed A. julibrissin to be andromonoecious with hermaphroditic terminal flowers and male lateral flowers. Further comparisons of the quality of ovules and polyads are necessary to clarify whether there is a division of labor between the terminal and lateral flowers in A. julibrissin.