Nectar sugar gradient within Darwin's syndrome inflorescence shaped by yeasts rather than intrinsic plant features

Abstract

Features of floral rewards, particularly the energy content and distribution of nectar, have been traditionally considered intrinsic plant traits. Since specialized floricolous yeasts thriving in floral nectar can modify features of single flowers, they could also alter whole inflorescence traits which impinge on pollinator behavior and components of plant pollination success such as the proportion of outcrossed progeny. This study assessed yeast ability to shape variation in nectar quality within vertical inflorescences of Gladiolus illyricus (Iridaceae), a Darwin's syndrome plant (flowers functionally male at first, that open and are visited by pollinators from bottom to top). In inflorescences with yeasts experimentally excluded a declining gradient in nectar sugar concentration built up from the oldest, female-stage flowers at bottom up to the youngest, male-stage ones at top. This plant-intrinsic gradient was fully reversed in inflorescences naturally exposed to yeasts. The two yeast species involved (Metschnikowia reukaufii and M. gruessii) differed in their effects on the sugar gradient. Results show that floral variation within inflorescences was shaped by flower yeast composition and abundance rather than intrinsic plant features, hence supporting the notion that some inflorescence traits typically attributed to plant genotype and pollinator-mediated selection can actually reflect the metabolism of flower microbes.