Can plants integrate information on above-ground competition in their directional responses below ground?

Background and Aims: Light competition can induce varying above-ground responses in plants. However, very little is known regarding the effect of above-ground light competition cues on plant responses below ground. Here we asked whether light competition cues that indicate the occurrence and direction of neighbours above ground might affect directional root placemat. Methods: In a common-garden experiment, we examined the integrated responses of the annual procumbent plant Portulaca oleracea to light competition cues and soil nutrient distribution. Soil nutrients were distributed either uniformly or in patches, and light competition was simulated using a transparent green filter, which was spatially located either in the same or opposite direction of the soil nutrient patch. Key Results: As predicted, root proliferation of P. oleracea increased in the direction of the enriched soil patches but was homogenously distributed under the uniform nutrient distribution. Interestingly, root distribution was also affected by the light competition cue and increased in its direction regardless of the location of the soil patches. Conclusions: Our results provide initial support to the idea that below-ground plant responses to competition might also be regulated by above-ground neighbour cues, highlighting the need to further investigate the combined effects of both above- and below-ground competition cues on root behaviour.