How far does pollinator behaviour -conditioned by floral rewards- determine geitonogamy rate?

The quality and quantity of floral rewards, together with pattern of presentation, “drive” pollinator visits within and among plants. In multi-flowering species, besides floral traits and display, the degree of geitonogamous pollination is determined by intra-plant pollinator movements. We considered two case-studies, in order to analyze the role of pollinator behaviour in relation to plant sexual reproduction performance. In particular, we try to answer the following questions: how far does the incidence of pollinator behaviour on geitonogamy depend on the type of inflorescence? Does it have the same significance regardless of the degree of specialization of pollination system? Investigations were performed on natural populations of Dictamnus albus L. and Gentiana lutea L.. Phenology was studied during flower lifespan and inflorescence development. Visitors activity on flowers was monitored in the field, followed by pollen loads analysis, to evaluate the potential pollinators capacity. We investigated patterns of nectar production within the single flower and within the inflorescence; nectar quality (concentration and composition) was also considered. Results highlighted potential high inbreeding depression and different degree of self-compatibility in the studied species. Intra-flower selfing is prevented or strongly limited by dichogamy and herkogamy. Nevertheless, considering the pollination unit, Gentiana lutea’s “anthium” is represented by the single pseudo-whorl of flowers, where inter-floral self-pollination can in fact occur. Pollination is rather generalized in G. lutea, where visitors, belonging to several orders, behave in a “sedentary” or “dynamic” way to collect pollen and/or nectar. These behaviours are determined by insects’ intrinsic traits, but our observations suggest that nectar quality could also play a role. Depending on the frequency of “dynamic” vs “sedentary” pollinators, geitonogamy results to be more or less limited, and cross-pollination increased or not. However, the low levels of self-compatibility reduce the success of self-pollination. The relationship with pollinators is more specialized in the highly self-compatible Dictamnus albus. Major pollinators are represented by few bee species: their nectar-induced bottom-to-top movements along the raceme contribute to avoid geitonogamy and maximize pollen export, promoting outcrossing. Our findings show that the level of geitonogamy is strongly dependent on pollinator behaviour, although inflorescence structure and pollination system specialization contribute to determine the extent of this effect.