Nectar sugar composition in angiosperms from Chaco and Patagonia (Argentina): an animal visitor's matter?

We relate nectar sugar composition with floral visitors in samples from two biogeographic regions from Argentina: Chaco (99 spp. from its southern region) and Patagonia (48 spp. from its central region), using our own data published in earlier papers. The variables to be compared were sugar ratio and sucrose percentage. Differences in the sugar composition were observed when comparing both regions. Bee- and butterfly-visited Patagonian species showed lower sucrose percentages and sugar ratios than Chaquean species; i.e., a convergence in sugar composition was not verified. Moth-visited species showed a wide range of sucrose proportions and sugar ratios in both regions with no significant differences between them. As the differences found in sugar ratio between Chaco and Patagonia, according to the predominant pollinator guilds, may have been influenced by the sample size, we searched for similarities in sugar ratios between plants with the same visitor type in a comparative way, comparing our data pooled together against data by Baker and Baker's (1983a). Our data do not agree with theirs, i.e., no trends can be drawn for bees, moths, and butterflies sugar preferences. The exceptions were the hummingbird-visited species that showed a similar pattern in both cases, i.e., there is a convergence among plants offering nectar with a predominance of sucrose. To evaluate whether closely related species within a family have similar nectar composition and flower visitors, regardless of their region, sugar proportions were compared in several families. Hexose nectars were predominant in Asteraceae, Fabaceae, Solanaceae, and Verbenaceae, while sucrose nectars prevailed in Bromeliaceae and Onagraceae. Nectar composition seems to be a more conservative trait than flower morphology. This may be a reason to explain the absence of a convergence in sugar composition between plants growing in different biogeographical regions that share the same animal visitor guilds.