Winter flowers for bees: reproductive biology of Trixis praestans (Asteraceae)

Abstract

The shrub Trixis praestans (Asteraceae) is widely distributed in Argentina, Brazil, Uruguay and Paraguay. It has great potential as a melliferous resource, since its abundant flowering occurs in winter (when flower offer is scarce) and spring, and it is intensely visited by honey bees. This study provides information on some aspects of the reproductive biology, which are essential for the species cultivation, conservation and breeding. Flowering phenology, floral biology and floral visitors were observed. The mechanism of fruit set was evaluated through manipulative experiments, embryological analyses and genotypic characterization of offspring using molecular DNA markers. Flower longevity was 3.5 ± 1 days and duration of the capitulum in floral state was 15 ± 6 days; peak flowering occurred in mid-August. Floral visitors belonged to nine species of the orders Hymenoptera, Diptera and Lepidoptera. Apis mellifera was the most abundant species and showed foraging behavior on pollen and nectar. Fruit set was 70% higher in open-pollination and hand cross-pollination (xenogamy) treatments than in hand self-pollination and autonomous self-pollination treatments. The embryological analysis indicated a path of sexual development which originated a reduced embryo sac (female gametophyte). Molecular analysis showed a prevalence of cross-fertilization in fruits derived from open-pollination. Based on the results, we conclude that T. praestans is a sexual species in which xenogamy predominates and that pollinators are necessary for fruit set.