Early belowground interactions in lupin–wheat intercropping assessed by a simple root phenotyping approach

Legume-cereal intercropping is a common strategy to enhance agricultural sustainability. Many plant-plant interactions occur in the underground but remain poorly understood due to methodological challenges. In the current study, we developed an indoor rhizobox-based simple and rapid root phenotyping approach to study early interactions between roots of white lupin seedlings cultivated as a sole crop or in intercropping with bread wheat. A rhizobox-based root phenotyping approach assessed the belowground structure of white lupin in the two conditions during early growth stages. The method focused on spatial root metrics and a rapid graphic indicator of root overlap between neighbour plants, termed Root Merge (RM) index. Root traits that could promote facilitative processes were also recorded. Lupin modified its root architecture spatially in response to intercropping rather than simply altering total root production. The RM index clearly separated the two lupin cropping systems. Lupin in pure stand showed an avoidance behaviour between neighbours (RM = 0.17) indicating spatial compartmentalization, whereas in intercropping large lupin–wheat root intermingling was evident (RM = 0.69), suggesting mechanisms for nutrient-availability facilitation. Lupin aboveground and belowground growth was not inhibited by the great spatial soil exploration of wheat in intercropping. Lupin nodulation significantly increased in intercropping compared to pure stand, highlighting facilitative interaction through enhanced biological nitrogen fixation. The root visual parameters proved to be useful indicators of phenotypic plasticity in response to intercropping. The method could be easily extended to other legume-cereal combinations or to intercropping involving other plant types.