Unstable patterns of flower structure and development in the almost dioecious species Acer negundo (Sapindaceae, Sapindales)

Abstract Acer negundo L. is a wind-pollinated, dioecious tree that occasionally produces flowers with rudiments of the opposite sex. Both the male (staminate) and female (pistillate) flowers possess only two whorls: sepals and stamens or carpels, the arrangement of which is disputed. Here we present data on inflorescence and floral development, pollen fertility in staminodes and the diversity of male and female flowers. We found that the merism of male flowers is unstable, and the numbers of stamens and sepals vary independently. The different floral groundplans always occur within the inflorescences of the same generative shoot. The development of a flower begins with the initiation of sepals, but in female flowers, sepals are initiated sequentially and remain of different sizes, whereas in male flowers, sepals are initiated almost simultaneously and are equal. After the appearance of stamen primordia or carpel primordia, a part of the undifferentiated meristem remains. This unused meristem can be involved in producing staminodes or pistillodia. Both unisexual and (partly) bisexual flowers are found in the same inflorescence. Staminodes are either completely sterile or produce some amount of viable pollen. These features indicate the possibility of functional bisexuality in A. negundo.