Starch grain characterization of Prosopis chilensis (Mol.) Stuntz and P. flexuosa DC, and the analysis of their archaeological remains in Andean South America

The fruit (pods) of Prosopis (Fabaceae) are frequently recovered from pre-Hispanic Argentinian archaeological sites, suggesting that this genus was of importance in ancient economies in this region. Yet it is only recently that archaeobotanists have begun to carry out systematic research into this genus. Therefore many questions remain to be addressed concerning the food value of Prosopis fruit, and its potential contribution. This paper examines starch from the pods of two species, Prosopis flexuosa and Prosopis chilensis, for the purposes of describing and classifying their morphological features and biometrical parameters. Pods of both species were gathered from two extremes (northern and southern) of the Hualfín Valley, Catamarca, Argentina. Starch abundance is estimated and compared with that of Zea mays, an economically important plant with high concentrations of starch. This paper reveals that Prosopis pods contain very low amounts of starch compared to starch-rich edible species. Despite this, the recovery of Prosopis starch grains can be successfully used to assess archaeological tool uses. Prosopis starch grains were found to be highly variable in shape. Grain size ranged between 10 and 20 μm. Granule irregularity and the high birefringence under polarized light are two of the most important diagnostic features. No significant statistical differences were found in the structure (morphology, size and hilum) of the starch of the same species from both localities. Finally, the implications for the role of this plant in past societies are discussed.