Flower biology and pollinators of Trichocereus pasacana (Cactaceae) in LosCardones National Park, Argentina

Many columnar cacti are bat pollinated. It has been suggested that this kin d of pollination would be more important in tropical than in temperate regi ons where flowers are open only one night. Thrichocereus pasacana produces big and resistant white flowers. We analyzed flower characteristics, floral cycle, stigmatic receptivity, nectar production, pollen presence and flora l visitors in a T pasacana population at National Park Los Cardones (Salta, Argentina) in November 1997. Flower features were constant between individ uals of the population. Flowers start opening at evening and anthesis time is from 18 to 40 hs. The estigma was receptive throughout the floral cycle. Anther dehiscence occurs with flower opening. Nectar production was highes t between 18 to 24hs. Although T pasacana are open during the night, floral visitors are diurnal. The most frequent was Xylocopa sp. In the study area , nectarivorous bats were not detected. The morphological features of T pas acana flowers were similar but bigger compared to other columnar cacti. Ant hesis time was also longer while nectar production was lower. T pasacana po llination at National Park Los Cardones is done by bees.