Down to the wire: late season changes in sex expression in a sexually labile tree species, Acer pensylvanicum (Sapindaceae)

KEY MESSAGE: In sexually plastic Acer pensylvanicum, determination of sex can occur extremely late, within three weeks of spring flowering. Physical damage causing complete vascular tissue severance results in increased female expression. Species with environmental sex determination are rare amongst angiosperms but widely distributed across taxa. The timing of floral development in species that change sex based on environmental cues is unexplored. We investigated the timing of differentiation of sexual organs in buds of Acer pensylvanicum, an understory tree in eastern North America with environmental sex determination. We collected branches from individuals at three collection times in the early spring of 2016 and kept them in a warm greenhouse until anthesis. All individuals exhibited complete or partial female inflorescences in the greenhouse in one or more collection. However, none of these same individuals produced only female flowers in the field. Unlike many other woody species that differentiate bud sexual primordia 9–12 months prior to flowering, A. pensylvanicum may differentiate the sexual organs in its flower buds as late as three weeks prior to anthesis. In a separate series of branch collections in 2017, we found that the stress response to cutting leads to increased female sex expression in branches, while earlier warm temperatures (e.g., those caused by growing in a protected greenhouse environment) or increased carbohydrate availability does not. Given the labile sex determination system of A. pensylvanicum, the ability to delay differentiation of buds into male or female until shortly before spring flowering would allow individual trees to respond to sex-determining damage cues as late as mid-spring. This supports the hypothesis that A. pensylvanicum may not exhibit the lag-time characteristic of temperate spring and early-summer flowering woody species and may change sex expression in response to stress.