Plant-plant interactions play a crucial role in shaping the growth environment for crops, impacting their productivity and resilience to stress. Interactions between plants have been incorporated into breeding programmes by selecting new target traits that will advance plants' abilities to produce in high densities. The study of plant-plant interactions belowground promises new pathways and traits for crop improvement. This study focuses on the developmental and physiological responses of sorghum (Sorghum bicolor L.) genotypes to neighbouring sorghum plants. In this study, we used two growing methods: (i) a focal plant surrounded by neighbouring plants in the same pot but without shading, and (ii) a focal plant grown either alone or surrounded by neighbours, irrigated with nutrient solution that was passed through pots (leachates) with or without plants. Our results show that the presence of neighbours in the same pot led to reduced dry weight, plant height, and leaf area of the focal plant. In addition, the presence of neighbours reduced stomatal conductance and photosystem II quantum yield. While the response direction was similar across tested genotypes, the magnitude varied. The results were repeated when neighbouring plants were not grown in the same pot, but a nutrient solution was passed through the root systems of other plants into a separate pot containing another plant. Furthermore, we saw a reduction in assimilation rate and stomatal conductance when plants were exposed to either the physical presence of neighbours or leachate. We did not find differences in root architecture in either treatment. These results show that plants change their growth in response to neighbours and that the signal is carried through the liquid phase of the soil. Our findings provide insights into sorghum plants' responses to belowground signalling from neighbouring plants and lay the foundation for future studies enabling increased crop performance under high-density planting conditions.