Sugar Content in Floral and Extrafloral Exudates of Orchids: Pollination, Myrmecology and Chemotaxonomy Implication

Abstract

SUMMARY Sugars present in the floral and extrafloral exudates from a number of orchid species have been analysed. All contain fructose, glucose and sucrose. Raffinose is the next most common sugar and stachyose occurs less frequently. Cellobiose, gentiobiose, lactose, maltose, melibiose, melezitose and a few large oligosaccharides may also be present. The distribution of these sugars in orchids may have some chemotaxonomic implications. There appears to be no correlation between the sugar content of exudates and orchid pollinators. This would seem to suggest that scent, form and colour are the major attractants to pollinators in orchids. Floral and extrafloral exudates in orchids may also function as attractants for ants which probably feed on them and repel grazers.

Nectar chemistry

“…The fact that this trisaccharide (also occurring in sugar beet) was not identified in the nectar of several Eucalyptus species (Nicolson, 1994) may merely reflect the conditions used in HPLC analyses (B.-E. van Wyk, personal communication). In orchid nectars analysed by thin-layer chromatography, raffinose was a common constituent (Jeffrey et al, 1970). High mannose levels in the nectar of lime trees (Tilia) during drought conditions may be due to unusual phloem sap composition, and are toxic to honeybees (Crane, 1977), owing to low activity in these insects of the enzyme mannosephosphate isomerase (Sols et al, 1960).…”

Post‐pollination Phenomena in Orchid Flowers

“…Elaborate mechanisms such as deception, mimicry, explosive anthers, food and/or scent production are employed to attract pollinators and insure pollination (Dodson, 1967;Dodson, et al, 1969;van der Pijl and Dodson, 1966). Moreover, in several instances exudates are produced both before and after pollination or fruit-set and act as attractants for ants which serve to protect the plant, flowers, or developing fruits from grazers (Dodson, 1967;Jeffrey, Arditti and Koopowitz, 1970;van der Pijl and Dodson, 1966). All these require considerable expenditures of energy and, indeed, respiration rate of orchid inflorescences are highest when the flowers are fully open and usually actively attracting pollinators (Rosenstock, 1956).…”

Nectar, nectaries, flower visitors, and breeding system in five terrestrial Orchidaceae from central Argentina

“…Most orchids are self-compatible, but the pollination mechanisms favour outcrossing (van der Pijl andDodson 1966, Dressier 1990) The interesting perspective of the Orchidaceae from the floral biology point of view contrasts with the comparatively small number of investigations carried out in such a big family. Although nectar is considered one of the most important rewards, sugar composition has only been identified in some 110species (Daumann 1941, FreyWyssling and H&userman 1960, Percival 1961, Payne 1965, Baskin and Bliss 1969, Jeffrey et al 1970 and, as far as we know, sugar proportions have been investigated in four species (Pais and Chaves das Neves 1980, Gottsberger et al 1984, Pais et al 1986, Freeman et al 1991. On the other hand, it is assumed the existence of floral nectaries, but little information is available on the structure and location of them (cf.…”