<p>Variety mixtures represent a promising option to sustainably increase the productivity of grain crops, but the underlying processes driving potential yield benefits remain poorly understood. Notably, the role of variety-specific phenotypic changes in mixtures — defined here as plasticity — and their effects on plant-plant interactions have scarcely been evaluated. Here, we examined the trait responses of eight Swiss wheat varieties when grown in mixtures, and how these plastic changes contributed to overyielding, that was further partitioned into complementarity and selection effects for the first time in wheat mixtures under real conditions. For this, we conducted an outdoor field experiment over three years and at three sites, where wheat varieties were grown in 2-way mixtures and in pure stands. We used a visual criterion (awns) to differentiate individuals of the different varieties in mixtures. We found significant plastic changes in response to mixing for several traits in seven varieties. Furthermore, mixture-induced plasticity in ear density was the main driver of overyielding, itself largely dominated by complementarity effects. An additional experiment allowed us to positively link plasticity in ear density to the speed of tillering onset under shading. This study improves our understanding of the plastic processes fostering overyielding in variety mixtures and provides a key criterion — tillering onset under shade — as a potential breeding target for cultivars for mixtures.</p>