Structural and histochemical approach to the fruit and seed diversity of Cyperaceae in an evolutionary context.

Abstract

Key message: The Cyperaceae fruit consistency depends on the mesocarp. Seed structure is diverse and related to the evolutionary history of their species. A new storage tissue is described for Cyperaceae. Anatomy and histochemistry of Cyperaceae fruits (including seeds) are poorly known due to their hard, isolating tissues that prevent anatomical techniques. We performed the first, most comprehensive structural diversity characterisation of fruit and seed in Cyperaceae, accompanied by an unprecedented histochemical characterisation of seeds for this family. We analysed fruits of 29 species, included in 19 genera and 12 tribes within the subfamilies Cyperoideae and Mapanioideae, using light microscopy. Cyperaceae fruits have a pericarp with a one-cell-layered exocarp and endocarp, and a multi-cell-layered mesocarp. The mesocarp of the Mapanioideae has a spongy-fleshy outer region and a hard inner region. The mesocarp of the Cyperoideae has only a hard region. The pericarp is free from the seed coat. Cyperaceae seeds have a three-layered seed coat, an embryo with haustorial function of its scutellum, and two storage tissues: the endosperm and a putative perisperm. Nine seed morphotypes and four seed subtypes were observed among the studied species. Our results suggested that the fruit consistency is determined by the mesocarp. Both the terms "nut" and "achene" should be accepted to refer to the dry fruit of the Cyperaceae until a widely accepted fruit classification for angiosperms is proposed. The Cyperaceae seed structural diversity is high and related to the evolutionary history of the species. The "perisperm" is a new tissue proposed for sedge seeds, and is here characterized for the first time. The seed coat has a different structure than the one described so far for the family.