Supporting Online Material for Plants Integrate Information About Nutrients and Neighbors

Animals regularly integrate information about the location of resources and the presence of competitors, altering their foraging behavior accordingly. We studied the annual plant Abutilon theophrasti to determine whether a plant can demonstrate a similarly complex response to two conditions: presence of a competitor and heterogeneous resource distributions. Individually grown plants fully explored the pot using a broad and uniform rooting distribution regardless of soil resource distributions. Plants with competitors and uniform soil nutrient distributions exhibited pronounced reductions in rooting breadth and spatial soil segregation among the competing individuals. In contrast, plants with competitors and heterogeneous soil nutrient distributions reduced their root growth only modestly, indicating that plants synthesize information about both neighbor and resource distributions in determining their root behavior. Materials and Methods Study Species Abutilon theophrasti is an annual weed native to Asia and introduced into agricultural areas throughout the western hemisphere (S1). Prior studies indicate this species responds to soil heterogeneity through altered root growth (S2-4). Soil heterogeneity can cause increased plant biomass in individually grown plants (S4) and alter rooting breadth (S5), but has little impact on population characteristics (S2, S3). Whether this species demonstrates additive or situationspecific behavioral responses to variation in both resource distribution and the presence of competitors is unknown. Experimental Arenas In total there were six treatment combinations, representing all factorial combinations of soil heterogeneity (uniform, patch-center, patch-edge) and plant density treatments (alone vs. with competitor). We constructed plywood pots (mesocosms) that were 13.5cm long, 12.5 cm wide, and 25 cm deep. To record root growth over time, we placed a single clear acrylic tube horizontally through each mesocosm, 10 cm below the soil surface. One replicate of all six treatment combinations was placed onto a single tube, with a total of 10 replicate tubes used in the experiment. We were primarily interested in the interaction of roots between the focal and competitor plant. Therefore we limited our image analysis to only those locations under the focal plant, and in the direction towards the competitor plant. Thus, within each mesocosm we recorded 8 discrete, non-overlapping 13.5 x 18 mm image frames every two weeks for the duration of the experiment (from 2.7cm-13.5cm; Fig 1). Images were taken with a BTC 2 Minirhizotron camera system (Bartz Technology Corporation, Santa Barbara, CA, USA) at 15x magnification. During imaging, the camera was oriented upwards towards the soil surface, such that any roots which grew down would contact the tube and be visible to the camera. All mesocosms were filled with a ‘background soil’ consisting of a low nutrient 3:1 mixture of sand and topsoil. Nutrient levels in all mesocosms were augmented by the addition of a small volume (25 cm) of a 1:1 mixture of sterilized steer manure and the background soil. The three soil treatments varied in the spatial distribution of these nutrients with either: (1) all