Nectar, nectaries, flower visitors, and breeding system in five terrestrial Orchidaceae from central Argentina

Floral nectar sugar composition, nectary anatomy, and visitors are studied in five Argentine Orchidaceae, from 18 populations. Hand-pollinations were performed to evaluate their breeding system. We found two different types of perigonal nectaries located either in the spur (Habenaria gourlieana, H. hieronymi, Habenariinae), or in the basal lateral parts of the labellum (Beadlea dutraei, Pelexia bonariensis, Stenorrhynchos orchioides, Spiranthinae). The spur of Habenaria is a nonvascularised and nonstructural nectary. The inner epidermis bears one-celled long papillae. In bud stage, the papillae are filled with starch grains, but when the flower opens and nectar secretion starts, they show no starch grains. This fact may indicate that starch is a source for some of the secreted nectar. In the remainder genera, the lateral basal parts of the labellum are secretory. The two glands are located in the adaxial basal lateral laces of the labellum. These nectaries are structural and nonvascularised. Stenorrhynchos produces abundant, concentrated nectar (40-50%). Habenaria gourlieana accumulates copious nectar in a lower concentration (20%), whereas the other species produce small quantities of concentrated nectar (ca. 50%). Three of the studied species have sucrose predominant nectar (Beadlea dutraei, Habenaria gourlieana, and Pelexia bonariensis) while H. hieronymi, Stenorrhynchos orchioides have hexose predominant ones. Nectar removal and/or pollination induce flower senescence. H. gourlieana is visited by sphingids, S. orchioides by hummingbirds, and B. dutraei by bees. For the two other species we did not record flower visitors. Pelexia bonariensis, B. dutraei, and S. orchiodes are self-compatible species but a pollinator is needed