The pollination ecology of Paraboea rufescens (Gesneriaceae): a buzz-pollinated tropical herb with mirror-image flowers

Abstract

Background and aims: Gesneriaceae is a pantropical plant family with over 3000 species. A great variety of pollination mechanisms have been reported for the neotropical members of the family, but the details of buzz-pollination and enantiostyly for the family have not been described. We investigated the floral biology and pollination ecology of Paraboea rufescens in Xishuangbanna, south-west China, considering three aspects: (1) the type of enantiostyly exhibited; (2) whether the species is self-compatible; and (3) whether pollinator behaviour could enhance the precision of pollen transfer between flowers of contrasting stylar orientation.

Methods: Flowering phenology was monitored once a month during vegetative growth, and once a week during flowering both in the field and under cultivation. Pollination manipulations and pollinator observation in the field were conducted.

Key results: Anthesis occurred early during the morning, and flowers remained open for 1-5 d, depending on weather conditions. Controlled pollinations revealed that P. rufescens is self-compatible, and exhibited inbreeding depression in seed set. Plants were pollinator limited in natural populations. The similar stylar deflection among flowers within a plant limits autonomous self-pollination as well as pollination between flowers. Two species of bumble bees (Bombus spp.), Amegila malaccensis and Nomia sp. effectively pollinated P. rufescens. These pollinators visited flowers in search of pollen with almost the same frequency. None of the pollinators appeared to discriminate between left- or right-handed flowers.

Conclusions: Paraboea rufescens exhibits monomorphic enantiostylous flowers and a buzz-pollination syndrome. Floral morphology in P. rufescens and pollinator foraging behaviour seems likely to reduce self-pollination and pollinations between flowers of the same stylar deflection.

Fig. 1.

Left- and right-styled flowers, inflorescence and visitors of Paraboea rufescens: (A) an inflorescence bearing a left- and a right-styled flower; (B) Amegila malaccensis visiting a flower; (C) Amegila malaccensis entering or exiting a flower; (D) Trigona pagdeni visiting a flower.

Fig. 2.

The mean (± standard error) number of left- and right-styled flowers produced by Paraboea rufescens on 1, 2 and 3 Jul. 2003. Each plant produces the same frequencies of left- and right-handed flowers each day. Sample size = 20 plants with a total of 169 inflorescences.

Fig. 3.

Mean (± standard error) fruit set of Paraboea rufescens inflorescences subjected to four pollination treatments: 1, self-pollination; 2, cross-pollination of first-day flowers; 3, cross-pollination of second-day flowers; and 4, open-pollination. Sample sizes are given below the bars [inflorescence number (flower number)], and statistically homogeneous groupings based on a one-way analysis of variance are indicated by the same letter (a, b or c) above the bars.

Fig. 4.

Mean (± standard error) seed number per Paraboea rufescens fruit resulting from one of three pollination treatments: 1, self-pollination; 2, cross-pollination of first-day flowers; and 3, open-pollination. Statistically homogeneous groupings based on a one-way analysis of variance are indicated by the same letter (a or b) above the bars.

Fig. 5.

Frequency of visits by bumble bees and Amegila malaccensis to flowers of Paraboea rufescens during 108 h of observation of 20 plants on 29 and 30 Jun. and 4 Jul. 2003.