Flower orientation in Pachycereus pringlei

The orientation of flowers in columnar cacti has been a subject of great in terest to plant biologists. The interpretation of this pattern has invoked warmer temperatures as the underlying factor. In this paper, we describe fl ower orientation in two populations of Pachycereus pringlei (S. Watson) Bri tton & Rose and propose a hypothesis of the underlying mechanism. Stems fro m the two populations showed a significant mean direction of their flowers, with most flowers (70-77%) present between 90 and 270 degrees. Photosynthe tic photon flux density interception and stem temperature reached maximum v alues on south-facing ribs, showing concordance with flower orientation. We suggest that PFD interception, through its influence on CO2 uptake and ste m temperature, is the major factor underlying the observed orientation of f lowers in P. pringlei.