Résumé
In this study we investigate the relationship between phytolith formation and transpiration rate in Eleusine coracana (finger millet), Cenchrus americanus (syn. Pennisetum glaucum, pearl millet) and Sorghum bicolor (sorghum). The aim is to produce a prediction model to reconstruct water management for agriculture in archaeological contexts in drylands. Two kinds of phytolith proxy evidence have been tested in modern experimental growing seasons as indicators of water availability, the ratio of sensitive to fixed morphotypes and also a logistic regression predictive model built on the complete assemblage of all morphotypes of the three species. Our results show a relationship between total water transpired and phytolith formation, which can be best predicted by the application of statistical logistic regressions. This is because some morphotypes are positively correlated with water availability, others are negatively correlated, and the significance of specific morphotypes in response to water availability varies according to the species and the part of the plant where the phytolith is formed. Indeed, water stress prompts each plant to alter its phytolith production in a distinct manner. The outcomes of this investigation should be of interest to archaeobotanists seeking a way of detecting the past growing conditions of C4 crops, but also to physiologists and ecologists who are interested in the study of phytolith formation.