A comparative study on the reproductive success of two rewarding Habenaria species (Orchidaceae) occurring in roadside verge habitats

Abstract

Background: Most orchid species have been shown to be severely pollination limited, and the factors affecting reproductive success have been widely studied. However, the factors determining the reproductive success vary from species to species. Habenaria species typically produce nectar but exhibit variable fruit set and reproductive success among species. Here, we investigated the influence of the flowering plant density, inflorescence size, breeding system, and pollinator behaviour on the reproductive success of two rewarding Habenaria species.

Results: Our observations indicated that Habenaria limprichtii and H. petelotii co-occur in roadside verge habitats and present overlapping flowering periods. Both species were pollination limited, although H. limprichtii produced more fruits than H. petelotii under natural conditions during the 3-year investigation. H. petelotii individuals formed distinct patches along roadsides, while nearly all H. limprichtii individuals clustered together. The bigger floral display and higher nectar sugar concentration in H. limprichtii resulted in increased attraction and visits from pollinators. Three species of effective moths pollinated for H. limprichtii, while Thinopteryx delectans (Geometridae) was the exclusive pollinator of H. petelotii. The percentage of viable seeds was significantly lower for hand geitonogamy than for hand cross-pollination in both species. However, H. limprichtii may often be geitonogamously pollinated based on the behaviours of the pollinators and viable embryo assessment.

Conclusions: In anthropogenic interference habitats, the behaviours and abundance of pollinators influence the fruit set of the two studied species. The different pollinator assemblages in H. limprichtii can alleviate pollinator specificity and ensure reproductive success, whereas the more viable embryos of natural fruit seeds in H. petelotii suggested reducing geitonogamy by pollinators in the field. Our results indicate that a quantity-quality trade-off must occur between species with different breeding strategies so that they can fully exploit the existing given resources.

Keywords: Geitonogamy; Habenaria; Plant density; Reproductive success; Viable embryo.

Fig. 1

Population distribution patterns of Habenaria petelotii and H. limprichtii at the study sites in 2018. a Population distribution patterns of H. petelotii along the road near Daxiechang. b Population distribution patterns of H. limprichtii along the road near Shangchuandong

Fig. 2

Flowering phenology (the proportion of flowering plants per day) of Habenaria petelotii and H. limprichtii. One census was taken per day from 23 July to 27 August 2014

Fig. 3

Plant, inflorescences, flowers and pollinators of Habenaria petelotii and H. limprichtii. a Plant and habitat of H. petelotii. b Single flower of H. petelotii. c Thinopteryx delectans visiting flowers of H. petelotii with pollinia attached to the base of its proboscis and legs. d Thinopteryx delectans with pollinia of H. petelotii attached to its head. e Plant and habitat of H. limprichtii. f Single flower of H. limprichtii. g Deilephila elpenor visiting flowers of H. limprichtii. h Thysanoplusia intermixta visiting flowers of H. limprichtii. i Cucullia fraterna visiting flowers of H. limprichtii with pollinia attached to the lateral-ventral side of its thorax

Fig. 4

Comparisons between the two rewarding species, Habenaria petelotii and H. limprichtii. a Inflorescence size comparison between H. limprichtii and H. petelotii. b Overall natural fruit set during the 3 investigated years. c Correlation coefficient between the inflorescence size and fruit set in H. petelotii, and fruit set measured as the number of capsules produced (solid line) and proportion of fruits produced (dotted line). d The correlation coefficient between the inflorescence size and fruit set in H. limprichtii, and fruit set measured as the number of capsules produced (solid line) and proportion of fruits produced (dotted line). e Pollinia removal in 2018. f Pollinator-mediated geitonogamy