Floral biology of an endangered, globose cactus from the tropical dry forest in central Mexico

Background/Question/Methods . Floral biology aims at exploring flower traits such as morphology, phenology, breeding and mating systems, as well as the interaction of the flower with other organisms (i. e. floral visitors). In endangered plant species, the knowledge of these aspects is important for in situ conservation, since sexual reproduction influences population viability and genetic variability. Flowers of globose cacti are usually hermaphroditic, diurnal, and bee-pollinated; mating systems vary from mixed (selfing and outcrossing) to strict outcrossing. In this work, we studied the floral biology of Coryphantha elephantidens, a globose cactus from the tropical dry forest in central Mexico, and threatened according to Mexican law. We conducted observations in one population to describe floral color variation, floral longevity, sexual expression, and floral visitors. We also determined breeding system by means of P/O ratios, and mating system through experimental pollinations (self, cross, and open pollination).

Results/Conclusions . Coryphantha elephantidens flowers from July to October. Flowers live two days and are diurnal; color varies widely, from pale yellow to fuchsia. Sexual expression indicates dichogamy (protandry) and herkogamy, since pistils are longer than stamens (t = 18.3, d.f. = 19, P < 0.001). P/O ratios per flower is 2108, indicating a facultative xenogamous system. However, pollination treatments indicate obligated cross-pollination, since selfed flowers did not produce fruits, and both cross and natural pollinated flowers set 62% and 70% of fruits, respectively. We found that up to 40% of plants (n = 86) produce male-sterile flowers: they present anthers without pollen, indicating that this population is functionally gynodioecius. The most frequent floral visitors are six species of native bees that pollinate flowers while collecting pollen, nectar, or both. Other floral visitors are ants and a florivorous beetle. Our results indicate that sexual reproduction of C. elephantidens depends completely on pollinator foraging, since only cross and open pollinated flowers set fruits. Male sterility is a rare condition within Cactaceae; the high incidence of male-sterile flowers in this population is a potential, negative factor for reproductive success, since P/O ratios at the population level could diminish and generate pollen limitation. Variation in floral color and its effect on reproductive success (e.g. by differential pollinator attraction) remains to be determined. Our results indicate that, in order to conserve natural populations of C. elephantidens, both native bee availability and the preservation of enough number of plants are important issues for successful sexual reproduction.