Nitrogen supply and neighbour identity shape responses to belowground interactions in a cultivated rice genotype and its wild relative

BACKGROUND AND AIMS: The outcomes of mesh-mediated belowground interactions depend on resource availability and neighbour identity, but how these factors jointly affect plant performance across planting contexts remains unclear. We tested how nitrogen (N) supply and neighbour identity affected responses to belowground interactions in a cultivated rice genotype and its wild relative grown in monocultures or mixtures. We examined whether interaction outcomes differed between intra- and interspecific neighbour treatments, whether low N increased asymmetry between the two rice species in biomass allocation and N acquisition, and whether these whole-plant responses corresponded with leaf physiological traits. METHODS: Plants were grown under low- and high-N supply in monocultures and mixtures, with adjacent root compartments separated either by a permeable mesh barrier or an impermeable plastic barrier. We used the relative interaction index (RII) to quantify mesh-plastic differences in biomass production and allocation, root biomass components, N accumulation, and leaf physiological traits; positive RII estimates indicate higher trait values under the mesh treatment, negative estimates indicate lower values, and estimates near zero indicate little difference between barrier treatments. KEY RESULTS: Responses to mesh-mediated belowground interactions depended on neighbour identity and N availability. RII estimates were close to zero or positive in some monocultures but predominantly negative in mixtures. Within mixtures, the O. sativa genotype 'Huanghuazhan' better maintained whole-plant performance, whereas the O. nivara accession showed larger trait reductions. This divergence was most apparent under low N. Leaf physiological responses did not consistently parallel the cumulative changes in whole-plant traits. CONCLUSIONS: These results indicate that N supply and neighbour identity modulate divergent responses to mesh-mediated belowground interactions between a cultivated rice genotype and its wild relative. Because this study compared one cultivated rice genotype with one wild relative, broader genotype sampling will be needed to test whether these patterns reflect domestication-associated divergence.