POLLINATION BIOLOGY OF ABRONIA-MACROCARPA (NYCTAGINACEAE), AN ENDANGERED TEXAS SPECIES

Floral maturation, the pollination system, extent of selfing and outcr ossing, and fruit set of Abronia macrocarpa Galloway, a federally and state endangered species, were examined. The perianth consists of five pink sepals fused around the ovary and extending to form a narrow flo ral tube averaging 2.0 cm in length. Nectar is secreted at the base of the floral tube. Anthers occur positioned approximately 1.0 cm above the stigma. Flowers open at 1500-1600 h. A sweet floral odor is presen t once the flowers begin opening, increases in intensity towards dusk and remains strong during the night. Flowers close at approximately 09 00-1000 h. Correlations of floral morphology and observed floral visit or activity indicate that hawkmoths and noctuid moths are pollinators. Plants were experimentally crossed autogamously, geitonogamously and xenogamously. Fruit resulted only from xenogamous crosses, indicating that A. macrocarpa is self-incompatible. The percentage of viable poll en is relatively high (ca. 94%) yet achene set is low (28%-66%), sugge sting a reduced fecundity.