Flower orientation in Pachycereus weberi (Cactaceae): Effects on ovule production, seed production and seed weight

Few studies have determined the importance of orientation on flower reproductive success. Flower orientation may affect reproduction of Cactaceae; the interception of photosynthetic active radiation (PAR) differs along the different sides of cacti stems, and translocation of resources between cacti ribs does not appear to occur. Therefore, PAR received by each cacti stem face may determine the resources available for reproduction. We examined the reproductive success of Pachycereus weberi flowers with contrasting orientations. Ovule production per ovary, number of seeds per fruit, seed set per fruit and seed weight were used as indicators of reproductive success for flowers facing southeast and northeast. PAR received on opposite stem sides was recorded as an estimator of resource availability for each side. Results showed that flowers facing southwards produced more ovules and seeds and produced heavier seeds. Seed set was not significantly different between flowers with contrasting orientations. The southern stem faces received more PAR than the northern faces. Reproductive success was different for flowers with contrasting orientations, suggesting that there is an association between PAR received on different P. weberi stem faces and floral reproductive success.