Studies on reproductive strategies of Vitex negundo L. var. heterophylla (Franch.) Rehder (Lamiaceae) based on morphological characteristics and SSR markers

Vitex negundo L. var. heterophylla (Franch.) Rehder (Lamiaceae) is an
important tree species for soil and water conservation, yet the
reproductive ecology of this species remains to be elucidated. To
investigate the reproductive traits of V. negundo var. heterophylla, the
phenology, morphological characteristics (a suite of characters was
assessed: floral morphology, nectar production, pollen viability, and
stigma receptivity), and mating system of this species were systematically
revealed for the first time in this study. Phenological observations,
morphological measurements, and nectar production analysis were conducted
during anthesis. Pollen viability and stigma receptivity at different
flowering stages were measured by biochemical methods. Finally, genetic
analysis based on SSR markers was used to reveal the mating system,
outcrossing index and pollen-ovule ratio were also calculated to help
analysis. V. negundo var. heterophylla showed several obvious
characteristics of outcrossing, such as abundant and attractive flowers,
secreting nectar, and emitting scent. In addition, mechanisms such as
homogamy and a short anther-stigma distance that can promote
self-fertilization were also identified in this species. The coexistence
of selfing and outcrossing characteristics demonstrates a predominantly
outcrossed mixed-mating system (outcrossing rate, t = 95%). The scientific
information provided by this study may contribute to conservation of V.
negundo var. heterophylla from a reproductive perspective.