Phytoliths as indicators of plant water availability: The case of millets cultivation in the Indus Valley civilization

Abstract

The interpretation of crop water management practices has been central to the archeological debate on agricultural strategies and is crucial where the type of water strategy can provide fundamental explanations for the adoption and use of specific crops. Traces of water administration are difficult to detect and are mostly indirect, in the form of water harvesting or distribution structures. Attempts have been made to infer plant water availability directly from archaeobotanical remains. Current evidence suggests that the ratio of sensitive to fixed phytolith morphotypes can be used as a proxy for water availability in C₃ crops, as well as in sorghum and maize. Nevertheless, the controversy on whether genetically and environmentally controlled mechanisms of biosilica deposition are directly connected to water availability in C₄ crops is open, and several species remain to be tested for their phytolith production in relation to water levels. This research aims at clarifying whether leaf phytolith assemblages and concentration, silica skeleton size and ratio of sensitive to fixed morphotypes can be related to different water regimes in Eleusine coracana Gaertn., Pennisetum glaucum (L.) R. Br., and Sorghum bicolor (L.) Moench. We cultivated 5 traditional landraces for each species in lysimeters, under different watering conditions and analyzed their phytolith content/production in leaves. Results show higher proportions of long cells, bulliforms and stomata produced in well watered conditions. The model built on the basis of phytolith composition has been then applied to interpret archeological phytolith assemblages recovered from a single phase at four different sites of the Indus Civilisation: Harappa, Kanmer, Shikarpur and Alamgirpur. The results show that most probably C4 crops grew under water stress conditions, providing new data on the interpretation of ancient agricultural management in the Indus Valley.