Floral rewards and pollinator performance in the generalist Gentiana lutea L.

We studied the pollination ecology of G. lutea L., an incompletely compatible species (Kozuharova 1998, Rossi 2012), assessing the spectrum of visitors and their pollinating role in relation to floral rewards, and specifically to nectar quality. Floral visitors behaviour and frequency were recorded in three natural populations of G. lutea belonging to subspecies lutea, vardjanii and symphyandra, during two consecutive years. We analysed insect pollen load and calculated an index of Pollinator Importance based on visitor frequency, fidelity and behavior (Galloni et al. 2008, Gibson et al. 2006). The occurrence of pollen limitation on reproductive success and nectar chromatographic analysis were performed on each studied population. The spectrum of pollinators shows a generalized system, since four orders of insects were observed, confirming previous results by Kozuharova (1999). However, not all pollinators play the same role: insects with sedentary activity would mainly contribute to geitonogamy while a dynamic behaviour would mainly increase out-crossing. Bumblebees, common wasps, bees, halictid bees and syrphid flies are example of taxa with a dynamic behaviour; ants are regarded as nectar thieves. Flies and Coleoptera show a sedentary activity. The best pollinator performance was found in subsp. symphyandra population, followed by subsp. lutea (where sedentary pollinators show a limited impact) and subsp. vardjanii, where the importance of sedentary pollinators exceeds that of dynamics. In this population pollen limitation was observed in both study years, while in the other ones in just one year. Reduced seed set can result as a consequence of low pollen quantity or quality. The high pollinator activity observed suggests that pollen limitation could be mainly explained by the quality of pollen (cross vs. self). Moreover multi-flowering species, characterized by a scarce level of compatibility may be particularly prone to pollen limitation by quality, primarily due to stigma clogging. Chromatographic analysis reveals that G. lutea nectar is rich in hexose and shows a significant presence of B-alanine and proline. From field surveys we observed two different behaviours of bumblebees: individuals collecting pollen showed a dynamic behaviour, while individuals foraging for nectar showed a tendency to become sluggish and sedentary. Pollen vectors are critical for the successful reproduction of this mainly xenogamous species, which moreover shows a very high inbreeding depression (Rossi 2012). Among pollinators is possible to discriminate “the good” (dynamic) – “the bad” (sedentary) – and “the null” (no contribution to pollination). Pollen limitation seems to be related with the degree of geitonogamy, enhanced by the “bad” pollinators. Pollinator behaviour seems to be affected by nectar quality (see also Nocentini et al. 2012). In particular, the role of amino acids needs further investigations, with special regard to B-alanine which is a non protein amino acid.