Guarana (Paullinia cupana) rewards seed dispersers without intoxicating them by caffeine
1995
Baumann, T.W. | Schulthess, B.H. | Hanni, K.
The fruit of the Amazonian guarana liana (Paullinia cupana) looks like a human eye, and undoubtedly shows the 'bird dispersal syndrome'. The seeds were reported to be ingested by large birds such as toucans and guans. We determined the purine alkaloid content of the various fruit and seed parts. The two aspects of defence and dispersal are reflected in the differential seed alkaloid distribution: the seed kernel (embryo with bulky cotyledons) and the seed coat (testa) accumulate much caffeine, i.e. 4.28 and 1.64%, respectively, whereas the 'white of the eye', the aril, is virtually alkaloid-free, but contains glucose, fructose and sucrose up to almost 70% of aril dry weight. Furthermore, the aril is strongly hygroscopic and it is suggested that it extends germination power by preventing seed desiccation. Experiments simulating pH and temperature conditions in the avian stomachs showed rapid desintegration of the aril and no caffeine release by the intact seed at pH 4.5 (crop) during the first 30 min of 'digestion'. Only a tiny fraction (between 0.025 and 0.07%) of total seed caffeine left the intact seed after 60 min at pH 4.5 or during the incubation at pH 2.3 (gizzard), indicating the presence of a very powerful diffusion barrier in the seed coat which at least theoretically should prevent intoxication of the dispersing bird even after an assumed foraging bout of 50 seeds. The cracked seed, however, releases a considerable fraction of its caffeine, considered harmful to destructive birds, if a few seeds were processed in this way. Absence of caffeine in the aril could well be the result of a 'secondary' degradation during maturation, analogous to hypoglycin A in the aril of the closely related sapindaceous Blighia sapida.
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