Seedling performance in a trioecious cactus, Pachycereus pringlei: Effectsof maternity and paternity

We studied seed germination and the growth and survivorship of seedlings of females and hermaphrodites of Pachycereus pringlei (cardon), a Mexican col umnar cactus whose geographically variable breeding system includes trioecy and gynodioecy. Results of a two-year field experiment conducted near Bahi a Kino, Sonora, Mexico and a ten-month laboratory experiment were similar a nd did not support the hypothesis that seedlings of females outperform thos e of hermaphrodites. In the field, percent seed germination and 2-yr seedli ng survivorship averaged 66% and 95%, respectively and did not differ among six treatment classes. Seedlings of hermaphrodites generally were larger t han those of females at the end of both experiments. Selfed seedlings of he rmaphrodites did not grow more slowly than outcrossed seedlings of hermaphr odites or females. Hermaphrodite seedlings performed best when pollinated w ith hermaphrodite pollen; female seedlings performed best with male pollen. We conclude that superior seedling performance cannot explain why females are able to coexist with hermaphrodites in populations of this cactus. Inst ead, we postulate that greater annual seed production, which averaged 1.6 t imes higher in females than in hermaphrodites in two years, may be sufficie nt to allow females to co-occur with hermaphrodites in this large, long-liv ed plant, especially if sex determination involves cytoplasmic-nuclear inhe ritance.