The effects of a neighbour and its identity on roots' plastic growth

Plant responses to the presence of neighbours and social interactions between them have the potential to alter fundamental aspects of plants' evolution, persistence, and coexistence. The present study employs a novel approach to investigate the three-dimensional movement of root tips in response to a neighbouring plant of the same or different species. We collected data from maize and pea plants in three experimental conditions: (i) individual condition, in which plants grew without neighbours; (ii) social growing condition with a conspecific neighbour, in which plants grew in the presence of another plant of the same species; and (iii) social growing with a heterospecific neighbour, in which plants grew in the presence of another plant of a different species. The results indicate that roots display a more pronounced 'exploratory' behaviour when growing under social conditions. For both maize and pea plants, a higher incidence of aggregative behaviour (primary root moving towards the neighbour) was observed when plants grew near a conspecific when compared with an heterospecific neighbour. According to our analyses, roots showing aggregative behaviour seem to detect the neighbouring root with a good level of geometrical precision as shown by the observed directional movement. We contend that this study provides for the first time quantitative information on the modulation of kinematic and oscillatory features of root movements, which are vital for a deeper understanding of plants' below-ground interactions.