The reproductive biology of Voyria (Gentianaceae) species in French Guiana

Abstract

AbstractThe reproductive biology of the mycoheterotrophic Voyria caerulea, V. clavata and V. rosea (Gentianaceae) was studied in French Guiana. Floral characteristics indicated adaptations to long–tongued flower visitors. The three species possess large, conspicuously colored flowers that produced sucrose–rich nectar (concentrations: 16.4%–23.9%) and emit a rosy floral perfume with a citrus–related aspect, dominated by terpenoids. Principal scent compounds of V. caerulea and V. rosea were (E)–nerolidol and geraniol, and (E)–8–oxo–β–farnesene [(E)–2,6–dimethyl–10–methylenedodeca–2,6,11–trien–5–one, a new natural product] and (E,E)–farnesyl acetate in V. clavata. Germinating pollen grains were observed in the anthers of all species, forming large interwoven clumps that were transferred by the pollinators as a unit. In V. caerulea and V. rosea, stamens and stigmas were positioned close together, enabling the plants to self–pollinate. In contrast, male and female organs are widely separated in V. clavata. Pollen to ovule ratios of all species were low (33–78), indicating that pollen transfer (either by selfing or by outcrossing) is very efficient. Flower visitors were butterflies (V. caerulea, V. rosea) and/or long–tongued bees of the genus Euglossa (V. clavata, V. rosea). Nevertheless, Voyria caerulea and V. rosea were visited only rarely and irregularly. We suggest that these two species follow a mixed selfing–outcrossing strategy to ensure seed production when pollen transfer by visitors fails. In contrast, V. clavata showed a high degree of specialization towards euglossine–pollinators, which were frequent visitors. The close plant–pollinator interaction increases the probability of outcrossings and might have enabled this species to completely rely on pollinators for reproduction. Average seed set in the studied species ranged between 36% and 49% and was very variable among individuals, probably due to resource–limitation. The capsules of V. caerulea and V. clavata showed characteristics of endozoochorous dispersal and were frequently found damaged by possible rodent dispersal agents.