Flower engineering by yeasts: reversal of nectar sugar concentration gradient within Darwin's syndrome inflorescences

Yeasts thriving in floral nectar have a proven ecological engineering capacity, as they modify the internal environment and the rewards of individual flowers in ways that influence how plants and pollinators interact. This floricentric perpective, however, produces only a partial view of the potential role of yeasts in plant-pollinator interactions, and little is known on their possible engineering effects on whole inflorescences. This study assessed yeast ability to shape features of the vertical inflorescences of Gladiolus illyricus (Iridaceae), a Darwins syndrome plant (acropetalous protandrous flowers pollinated by insects foraging from bottom to top). When yeasts were excluded from inflorescences 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 reversed in inflorescences with yeasts, where sugar concentration increased from the female-stage flowers harboring dense yeasts population to male-stage ones with few yeasts. The two species of yeasts involved (Metschnikowi reukaufii and M. gruessii) differed in their effects. Results add support to the growing consensus that specialized nectar yeasts can obfuscate intrinsic patterns of intra and interspecific variation in flowering features of plants at the level of individual flower, inflorescence, population and plant community.