Mixing varieties mitigates early root competition in wheat under water and nutrient limitation.

Competition between plants can lead to a Tragedy of the Commons (TOCs) where over-investments into resource harvesting organs reduce collective performance. Mixing different crop varieties could resolve such TOCs and thus maintain high productivity while increasing within field diversity because varieties might have different resource requirements (positive niche complementarity effect) or because the most competitive varieties benefit from being mixed with weaker competitors (positive selection effect). To date, most studies on varietal mixtures have focused on the aboveground compartment while belowground competition remains poorly understood due to the challenges associated with accessing root traits. In the present study, we grew durum wheat (Triticum turgidum ssp. durum) varietal mixtures in a high-throughput root phenotyping platform to characterize early-stage belowground competition between varieties. We grew 36 varieties in pure stands and in 54 binary mixtures under both unlimited resource conditions (R+) and water and nutrient limitation (R-). Seedlings in mixed stands produced less biomass than predicted from their pure stands in R-, which mainly resulted from a negative complementarity effect. Around 50% of this biomass reduction was explained by the average projected area of the root systems of the two varieties in pure stands, and instead of a negative interaction between varieties, this negative complementarity effect reflected a relaxation of competition in mixed stands: varieties with high root projected area were strong competitors that over-invested in belowground organs in pure stands. They benefited from having, on average, a weaker competitor than themselves in mixed stands and thus disengaged from the arms race for biomass accumulation. Our results suggest that root area is a promising breeding targets to reduce intra-specific competition and a key trait to consider for mixture assembly.