Nectar regulation in Euphorbia tithymaloides L., a hummingbird‐pollinated Euphorbiaceae

Abstract

Floral sexual phases can differ in nectar production and might be under selective pressure by pollinators. We studied Euphorbia tithymaloides, which has inflorescences that are initially female and then hermaphroditic. Volume and concentration of nectar were measured in both stages. Nectar production and the effect of extractions were determined using sets of bagged inflorescences; inflorescences in the hermaphroditic phase had higher values of nectar concentration, volume and sugar mass than inflorescences in the female phase. Nectar resorption was detected in senescent inflorescences. To test for homeostatic nectar regulation, artificial nectar was added and the response assessed after 24 h. The experiments showed that concentration and sugar mass are regulated within a narrow range, and the homeostatic points differ between the two sexual phases. These differences in nectar can be detected by hummingbirds, which prefer the female stage. Resorption and secretion seem to be part of a homeostatic mechanism by which nectar attributes are maintained to optimise sugar recovery.

Salvage of floral resources through re-absorption before flower abscission

“…3), at the same time that volume decreased, which indicates that hygroscopic dilution of nectar did not occur, and the flowers must have separately and preferentially reabsorbed the nectar sugar. Separate reabsorption of nectar water and nectar sugar has similarly been found in other studies 11,[28][29][30] .…”

“…However, the presence of UV-absorbing pigment was as expected, since the flowers are white to human eyes, but visited and pollinated by bees 21 . Further information in relation to pollination syndromes involving plants and their pollinators should Similarly high levels of nectar reabsorption as flowers senesce have been reported for some other plant species e.g., 5,11,13 and implied to be a common phenomenon 27 (Fig. 3), at the same time that volume decreased, which indicates that hygroscopic dilution of nectar did not occur, and the flowers must have separately and preferentially reabsorbed the nectar sugar.…”

Convergent evolution in floral morphology in a plant ring species, the Caribbean Euphorbia tithymaloides

“…One potential explanation of convergence toward relatively shortened cyathia relies on the relative importance of insect versus hummingbird pollination. Euphorbia tithymaloides , as is typical of the Pedilanthus clade, is reported to be pollinated by hummingbirds (Dressler, ; Cacho et al., ; Veiga Blanco et al., ), in an otherwise typically insect‐pollinated Euphorbia (Croizat, ; Webster, ). In at least one species of the Pedilanthus clade, E. diazlunana , reversal to bee pollination has been documented.…”

“…B). Although clade‐wide studies are lacking, available data and observations support that members of the Pedilanthus clade are primarily pollinated by hummingbirds (Dressler, ; Cacho et al., ; Veiga Blanco et al., ). The bilateral cyathia characteristic of the clade resemble the single flowers typical of hummingbird‐pollinated groups in possessing large amounts of nectar, reddish coloration, and a tubular structure (Dressler, ; Cacho et al., ; Veiga Blanco et al., ).…”

“…Although clade‐wide studies are lacking, available data and observations support that members of the Pedilanthus clade are primarily pollinated by hummingbirds (Dressler, ; Cacho et al., ; Veiga Blanco et al., ). The bilateral cyathia characteristic of the clade resemble the single flowers typical of hummingbird‐pollinated groups in possessing large amounts of nectar, reddish coloration, and a tubular structure (Dressler, ; Cacho et al., ; Veiga Blanco et al., ). In at least one member of the clade, E. diazlunana (Lomelí & Sahagún) Steinm., reversion to the more typical hymenopteran pollination of Euphorbia has been documented (Sahagún‐Godínez and Lomelí‐Sención, ), a change that is accompanied by shortened cyathia with yellowish rather than reddish coloration.…”