The role of colored accessory bracts in the reproductive biology of Lavandula stoechas

Abstract

This study investigates the role played by showy structures in the reproduction of Lavandula stoechas L. (Lamiaceae), a self-compatible Mediterranean shrub with compact, head-like inflorescences that are terminated by a tuft of conspicuous pink bracts. Pollinator visitation rates to plants with bracts correlated positively with that of paired, treated shrubs with all their bracts removed and, overall, bractless and control plants had equivalent visitation rates. In experimentally split shrubs, however, a strong effect on pollinator choice was evident, and distant visitor approaches were most often to that side keeping the attractive organs. Fecundity was not depressed in plants with reduced visual displays, and neither was the amount of pollen dispersed per flower or the size of stigmatic loads. There were, however, more pollen tubes, on average, at the lowermost part of the style in controls (10.8 tubes/style) than in bractless shrubs (8.3 tubes/style). Small, non-significant declines in average seed size, germinability, and seedling mass were detected following bract removal. Data support the notion that bracts are not essential to seed production in mature, relatively dense Lavandula populations where bee pollinators are often in good supply, but may be all-important f plant density is extremely low, or during population establishment. The roe of an amplified display in increasing average pollination distance and progeny quality in this species is discussed.