Stigma form and surface in the tribe Boragineae (Boraginaceae): Micromorphological diversity, relationships with pollen, and systematic relevance

A comparative analysis of stigma form in 62 species representing 15 genera of the tribe Boragineae by means of light and electron (transmission and scanning) microscopy highlighted the structural diversity, peculiarity, and functional specialization of the receptive surface of the boraginaceous stigma. Cluster analysis identified 13 stigma morphotypes, 6 of which were peculiar to the genera Anchusella, Brunnera, Cynoglottis, Gastrocotyle, Pentaglottis, and Trachystemon. Nonea showed the broadest intrageneric stigma polymorphism, with 6 types differing in terms of stigma form and papillar structure. Stigma and pollen variations were poorly associated in this complex genus, while in other groups, the pollen-stigma typological correspondence indicated the taxonomic distinctiveness of some critical species, such as Gastrocotyle hispida (Forssk.) C.B. Clarke and Anchusella spp. Positive correlations were found between pollen and papillar size, between interpapillar distance and pollen equatorial diameter, and between pollen volume and style length. Qualitative and quantitative pollen-stigma relationships suggested functional implications of an adaptive nature that were interpreted through the recognition of three distinct stages of early interaction, i.e., capture, lodging, and hydration of the grains.