Summary 1We develop and test a game-theoretic model for considering the effects of intra-and interplant competition on root proliferation and reproductive yield. 2 We predict that if space and resources per individual are held constant, plants should produce more roots per individual and less reproductive yield per individual as the number of plants sharing the combined space increases. 3 We tested the predictions using soybean plants ( Glycine max ) cultivated in the glasshouse either as owners or as two individuals sharing twice the space and nutrients. 4 Sharing individuals produced 85% more root mass than owners. Owners, however, produced 30% more reproductive yield per plant (dry mass of seeds), as a result of significantly more seed pods (8.70 vs. 7.66), more seeds per pod (1.87 vs. 1.72) and larger seeds (0.205 vs. 0.195 g seed -1 ), than did sharing individuals. 5 Total plant biomass did not differ between owners and sharing individuals, but owners had significantly higher shoot to root ratios, produced significantly more seeds per unit root mass, and allocated a significantly higher percentage of total biomass production to seeds. 6 Possession of an evolutionarily stable strategy (ESS) of root competition suggests that different roots and parts of a plant assess and respond to opportunities in a manner that maximizes the good of the whole plant. Thus, plants may be more sophisticated and share more in common with animals in their non-cognitive behaviours than previously thought. A plant operating as a co-ordinated whole should, all else being equal, first proliferate roots in unoccupied soil, then in soil occupied by a conspecific competitor, and lastly in soil already occupied by its own roots.

Summary 1Competition usually involves the allocation of limiting resources to non-reproductive functions. Natural selection is expected to favour mechanisms that increase competition with non-self neighbours and limit wasteful competition with self. 2 We used Pisum sativum plants that had two roots and 'double plants' with two shoots and two roots that could be either longitudinally separated into two genetically identical but physiologically distinct individuals or left intact. 3 Root development was significantly greater in split-root plants whose neighbours belonged to different plants. Furthermore, root development was relatively greater in the presence of roots of a different plant, regardless of its identity. This discrimination had a vectorial component whereby plants developed more and longer lateral roots towards neighbouring roots of different plants than towards other roots of the same plant.4 The results thus demonstrate a mechanism of avoiding self-competition that is based on physiological co-ordination among different organs of the same plant rather than on allochemical recognition that depends on genetic differences. 5 The ability to discriminate between self and non-self could be expected to increase resource use efficiency and ecological performance in plants. It could also be expected that tight physiological co-ordination will be selected for between organs of the same plant that have greater probability of being engaged in direct competitive interactions with each other.

Here, we tested the predictions of a 'tragedy of the commons' model of below-ground plant competition in annual plants that experience spatial heterogeneity in their competitive environment. Under interplant competition, the model predicts that a plant should over-proliferate roots relative to what would maximize the collective yield of the plants. We predict that a plant will tailor its root proliferation to local patch conditions, restraining root production when alone and over-proliferating in the presence of other plants. A series of experiments were conducted using pairs of pea (Pisum sativum) plants occupying two or three pots in which the presence or absence of interplant root competition was varied while nutrient availability per plant was held constant. In two-pot experiments, competing plants produced more root mass and less pod mass per individual than plants grown in isolation. In three-pot experiments, peas modulated this response to conditions at the scale of individual pots. Root proliferation in the shared pot was higher compared with the exclusively occupied pot. Plants appear to display sophisticated nutrient foraging with outcomes that permit insights into interplant competition.

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Circumventing physical obstacles is critical for a plant's survival and performance. Although the ability of roots to circumvent obstacles has been known for over 100 years, the phenomena and its mechanisms have received relatively little attention. In this study it is demonstrated that roots of Pisum sativum are able to detect and avoid growth towards inanimate obstacles and the hypothesis that this behaviour is based on the sensitivity of roots to their own allelopathic exudates that accumulate in the vicinity of physical obstacles is tested. The development of lateral roots of Pisum sativum towards an obstacle (a piece of nylon string, similar in dimensions to a plant root) was followed. Lateral roots were similar in number, but significantly shorter in the direction of the nylon string. In addition, up to half of the lateral roots that developed towards the nylon string withered, whereas no withering was observed in the absence of the nylon string. These avoidance growth patterns were suppressed in the presence of potassium permanganate or activated carbon, indicating a role of allelopathic exudates in promoting obstacle avoidance. The demonstrated obstacle avoidance by self inhibition could increase plant performance by limiting resource allocation to less promising parts of the root system.