Intrapopulation heterogeneity in floral nectar attributes and foraging insects of an ecotonal Mediterranean species

A population of Buglossoides purpurocaerulea (L.) I.M. Johnst. (Boraginaceae-Lithospermeae) located in Lecceto (Siena Province, Tuscany, central Italy) has been studied to compare floral nectar attributes and forager species between sun-exposed and shaded plants. Flower anthesis and maturity of sexual organs were also investigated. Average flower anthesis lasted 3-4 days. Stigma receptivity and anther dehiscence occurred on the first day. Nectar production also began on the first day and maximum production occurred on second-third day. Significantly greater volumes and total sugars were recorded in individuals exposed to the sun. Nectar HPLC analysis showed a similar hexose-dominant sugar profile for all the individuals with percentages of sucrose, glucose and fructose around 5, 48 and 47 %, respectively. Protein amino acids represent the 90 % of the overall free amino acids profile. Significant differences between relative percentages of serine and proline were found between sun-exposed and shaded individuals. Empis pennipes and Bombilyus major were the most frequent insect visitors to shaded and sun-exposed individuals, respectively. The hexose dominance of the nectar of B. purpurocaerulea, an exception among the Mediterranean Lithospermeae, may be related to the habitat where this plant generally grows, i.e. the forest-edge, and to pollination mainly performed by dipterans.