Intraplant variation in nectar traits in Helleborus foetidus (Ranunculaceae) as related to floral phase, environmental conditions and pollinator exposure

Factors that contribute to variation in nectar sugar composition, nectar concentration and volume have been a central concern in studies of pollinator assemblages in angiosperms. In an effort to better understand the mechanisms underlying variation in nectar traits, we designed a series of experiments with flowering Helleborus foetidus individuals under natural and glasshouse conditions, to identify intraplant variation in nectar traits which depend on both intrinsic (sexual phases of individual flowers) and external (pollinator visits and plant growth conditions) factors. The results showed that nectar volume, sugar composition and concentration in Helleborus foetidus varied between floral sexual phases, environmental growing conditions, and levels of flower exposure to pollinator visits. Processes of mate-limitation in male reproductive success or pollen-limitation in female success, as well as flower protogyny and holocrine secretion of nectaries may be involved in nectar variability between floral phases. By comparing different environments we observed that nectar volume and concentration at the nectary and flower level were plastic traits sensitive to external conditions, emphasizing responsiveness to environmental changes and a consequent plasticity in nectar traits such as sugar concentration and volume. Nectar sugar composition did not respond to different growing conditions, suggesting that this is an intrinsic characteristic of this species, but pollinator exposure produced significant changes in the nectar of single nectaries, particularly in the sucrose–fructose balance. Future research on nectar ecology and nectar chemistry will need to consider that nectar traits exhibit different kinds of variation at the intraplant level and under different environmental conditions.