Pollination ecology of Magnolia ovata may explain the overall large flower size of the genus

Flowering and fruiting biology of Magnolia ovata was studied in Atlantic forests in the interior of São Paulo State, Brazil. The large, bisexual flowers are protogynous, nocturnal, thermogenic and emit a strong scent in two consecutive evenings. In the first night of anthesis, the flowers are in the pistillate stage and thermogenesis starts at about sunset and lasts about 3 h. In the second night, the flowers enter the staminate stage and produce heat for 4 h. Heat is generated by the petals, gynoecium and anthers. Temperatures measured inside the petals reach 26.7 °C and 31.9 °C in the pistillate and staminate stages, 6.0 and 10.6 °C above ambient air, respectively. In the pistillate stage, the perianth opens after sunset and closes tightly a few hours later, and remains closed until the next evening. The initial opening and closing, however, is not synchronous for all flowers during the night. In the following evening, flowers in the staminate stage again open and remain so until the petals drop. Scent compounds, analyzed by GC–MS, contain C5-branched chain compounds, aliphatics, benzenoids and monoterpenoids. Emission of the most prominent compound, C5-branched methyl 2-methyl butyrate, commences before flower opening and continues throughout anthesis, but is accentuated in the thermogenic pistillate and staminate stages. Female and male individuals of only one beetle species, the dynastid scarab Cyclocephala literata, are attracted to the scented flowers in both pistillate and staminate stages. Once inside the flowers they feed on the petals and mate. Tests with synthetic methyl 2-methyl butyrate indicate that this compound is a strong attractant for the beetles. Because this scent compound is strongly emitted in both pistillate and staminate stages, the beetles fly indiscriminately between flowers of both stages. This behavior enhances pollen mixing and effective cross-pollination of the self-compatible species. The evolutionary history of Magnolia appears to be influenced by an ancestral condition of dynastid scarab beetle pollination. Large magnolia flowers are best explained as an archaic structure resulting from the initial association of tropical American species of section Talauma with large and voracious dynastid beetles.