Abstract: Aims Sex allocation in plants is often plastic, enabling individuals to adjust to variable environments. However, the predicted male-biased sex allocation in response to low resource conditions has rarely been experimentally tested in hermaphroditic plants. In particular, it is unknown whether distal flowers in linear inflorescences show a larger shift to male allocation relative to basal flowers when resources are reduced. In this study, we measure position-dependent plasticity of floral sex allocation within racemes of Aconitum gymnandrum in response to reduced resource availability.
Methods Using a defoliation treatment in the field applied to potted plants from a nested half-sibling design, we examined the effects of the treatment, flower position, family and their interactions.
Important findings Allocation to male function increased with more distal flower position, while female allocation either did not change with position or declined at the most distal flowers. Defoliation significantly reduced the mass of both the androecium and gynoecium, but not anther number or carpel number. Gynoecial mass declined more strongly with defoliation than did androecial mass, resulting in a significant increase in the androecium/gynoecium ratio as predicted by sex allocation theory. Plastic responses of androecium mass and gynoecium mass were affected by flower position, with less mass lost in basal flowers, but similar plastic magnitude in both sexual traits across flower position lead to consistent variation in the androecium/gynoecium ratio along the inflorescence. A significant treatment*paternal family interaction for the androecium/gynoecium ratio is evidence for additive genetic variation for plastic floral sex allocation, which means that further evolution of allocation can occur.

Key words: Aconitum gymnandrum, sex allocation, plasticity, inflorescence architecture