Pollinator responses to floral colour change, nectar, and scent promote reproductive fitness in Quisqualis indica (Combretaceae)

Abstract

Floral colour change is visual signals for pollinators to avoid old flowers and increase pollination efficiency. Quisqualis indica flowers change colour from white to pink to red may be associated with a shift from moth to butterfly pollination. To test this hypothesis, we investigated Q. indica populations in Southwest China. Flowers secreted nectar continuously from the evening of anthesis until the following morning, then decreased gradually with floral colour change. The scent compounds in the three floral colour stages were similar; however, the scent composition was different, and the scent emission rate decreased from the white to red stage. Dichogamy in Q. indica prevents self-pollination and interference of male and female functions. Controlled pollinations demonstrated that this species is self-incompatible and needs pollinators for seed production. Different pollinators were attracted in each floral colour stage; mainly moths at night and bees and butterflies during the day. Observations of open-pollinated inflorescences showed that white flowers had a higher fruit set than pink or red flowers, indicating the high contribution of moths to reproductive success. We concluded that the nectar and scent secretion are related to floral colour change in Q. indica, in order to attract different pollinators and promote reproductive fitness.

Figure 1. Floral colour change and some visitors in Quisqualis indica.

(a) Blooming inflorescences showing floral colour change from pink to red (Image taken by Mr. Guangyu Liu, Xishuangbanna Tropical Botanical Garden); (b) A moth visiting white flowers at night (Image taken by Dr. John Kress, National Museum of Natural History, USA); (c) A butterfly visiting pink and red flowers in the day; (d) A bee visiting pink flowers.

Figure 2. Changes in pollen viability and frequency of stigma receptivity in Quisqualis indica.

Data are expressed as means ± standard error.

Figure 3. Nectar volume and sucrose concentration of Quisqualis indica flowers in relation to temperature and humidity.

Data are expressed as means ± standard error.

Figure 4. Visiting frequency of pollinators to Quisqualis indica flowers at anthesis.

Figure 5. Effects of pollination treatments on fruit set rate in Quisqualis indica.

Data are expressed as means ± standard error. XTBG, Xishuangbanna Tropical Botanical Garden; MLD, Menglun Nature Reserve.

Figure 6. Nonmetric multidimensional scaling (NMDS) ordination of scent profiles emitted by Quisqualis indica white flowers (W), white and red flowers (WR), pink flowers (P), pink and red flowers (PR), and red flowers (R) compared with the control (CK) based on Bray-Curtis distance.

Data were rotated by principal component to maximize the first dimension. Stress = 0.11.