Trumpet flowers of the sonoran desert: Floral biology of Peniocereus cacti and sacred Datura

The floral biology of night-blooming Peniocereus cacti and Datura plants was studied in North America's Sonoran Desert. In populations of two rare cactus species (Peniocereus greggii and Peniocereus striatus), individual plants bloom synchronously on less than five nights per year and are self-incompatible. In contrast, the abundant Datura discolor and Datura wrightii bloom nearly continuously from spring to autumn and are self-compatible. Flowers of all species studied are visited by hawk moths at dusk and by honeybees and native bees the following morning. Hawk moths have the appropriate behavior and body dimensions to pollinate Peniocereus effectively, but visits are rare. Nonnative honeybees also visit P. greggii and may contribute incrementally to fruit set. Peniocereus and Datura flowers are highly reflective at all wavelengths above 400 nm but lack UV reflectance or contrast. All species studied secrete 10-80 μL of sucrose-rich nectar within flared corollas so deep that moths must land within them, ensuring pollen carriage. These flowers provide rich energetic resources for hawk moths because the caloric content of a single flower would support from 3 to 20 min of hovering flight. Floral scents were more species specific than visual cues because D. discolor and D. wrightii emit complex blends of terpenoid, benzenoid, aliphatic, and nitrogenous scent compounds, whereas flowers of P. greggii produce only eight benzenoid compounds (all of which are present in Datura species) and Peniocereus striatus is scentless. Peniocereus cacti may benefit from sequential mutualism with the more abundant Datura species by way of pollinators nurtured as larvae by Datura foliage and as adults by Datura floral nectar.