Consistent with classic theories in behavioural ecology, plants make root foraging decisions as a function of the spatial distribution of both resources and competitors, although how these factors typically interact is not clear. Plants usually produce more roots in higher nutrient soil, and responses to neighbours range from avoiding neighbours to producing more roots when a neighbour is present. It has been theorized that some plants grown with neighbouring plants produce roots according to an ideal free distribution, with foraging effort corresponding to both nutrient levels and number of individuals. We test this by addressing two main questions: is foraging effort in soil proportional to local patch value when plants are presented with patches of varying quality, and do proportions change in the presence of competitors? We grew Helianthus annuus (common sunflower) plants in soil with high- and low-quality nutrient patches. We compared foraging effort (root length per unit soil) in patches of differing quality for plants grown individually or with a neighbouring plant. Plants were grown in root observational chambers for two weeks and foraging effort by each plant was characterized every 48 hours.
Results/Conclusions
Individually grown plants increased foraging effort in high-quality patches relative to low-quality patches. More total roots were produced by pairs of plants than by plants grown singly. However, the proportion of foraging effort expended in high- and low- quality patches did not appear to differ in plants grown together, consistent with an ideal free distribution. This was due to behavioural adjustments in root foraging at the level of the individual plant. Plants decreased foraging effort near neighbours, and increased foraging effort in soil opposite neighbours, when compared to plants grown alone. Responses to both neighbours and nutrients (decreased foraging effort near neighbours and increased foraging effort in high quality patches) could be detected after only 6 days of growth, soon after plant roots had reached those areas of the soil. The degree to which relative foraging effort was adjusted in these areas also changed over time, demonstrating the dynamic nature of root foraging behaviour. Plants grow in complex environments where frequent plant-plant root interactions and resource heterogeneity are the norm. Understanding how plants respond to and integrate these cues is essential to understanding plants, and behavioural approaches can shed new light on these questions.