Grain weight changes over time in ancient cereal crops: Potential roles of climate and genetic improvement

Inferences of grain weight (GW) for cereals cultivated in the past could be useful to track early agronomic improvements. Though strongly genetically determined, GW also depends on environmental conditions, such as water availability. In the archaeological context, we can estimate GW and plant water status from grain dimensions and carbon isotope discrimination (Δ13C), respectively. In this study we attempted to determine the role of environmental changes on the evolution of GW of naked wheat (Triticum aestivum/durum) and hulled barley (Hordeum vulgare) in the Ebro Depression (NE Spain) during the last four millennia. To that end, we examined charred grains from nine archaeological sites, from the Bronze Age (ca. 1950 BCE) to the end of the Middle Ages (XVth century). Estimated GW in archaeological grains (averaging ca. 19.5 mg for both species) was significantly lower than in present rainfed cereals in the area (ca. 35 mg). In contrast, for both species mean Δ13C values in archaeological grains were similar to or even greater than in modern material (16.3‰ and 15.5‰, respectively). Accordingly, we conclude that the most recent increase in GW cannot be attributed to increased water availability, but probably to empirical breeding efforts which would have started not before the beginning of the CE.