Pollination ecology of Agave macroacantha (Agavaceae) in a Mexican tropical desert. I. Floral biology and pollination mechanisms

In a study of sexual reproduction in long-lived semelparous plants, we obse rved Agave macroacantha in the tropical desert of Tehuacan-Cuicatlan, Mexic o, describing duration of flowering, flower phenology, and nectar productio n patterns. We also performed two manipulative experiments evaluating (a) t he seed production efficiency of different crossing systems (selfing, cross -pollination. apomixis. and control), and (b) the effect of different polli nators (diurnal exposure to pollinators, nocturnal exposure, exclusion, and control) on the seeds produced. Flowering occurred from early May to late July and had a mean duration of 29 days in the individual rosettes. The flo wers were protandrous; anthesis occurred in the afternoon of the third day after floral opening, and the pistils matured in the afternoon of the fifth day. The stigmas remained receptive from dusk to the following morning. Po llination was mostly allogamous. Nectar was produced principally during the night, from the first stages of floral aperture until the stigmas wilted a nd flowering ceased. The flowers were visited during the day by hymenoptera , butterflies, and hummingbirds and during the night by bats and moths. Onl y the nocturnal visitors, however, were successful pollinators. Agave macro acanthus is extremely dependent on nocturnal pollinators for its reproducti ve success.