Abstract
Evolutionary game theory posits that plants competing with neighbours preferentially allocate resources towards resource-foraging structures, which reduces the marginal gain per investment unit and the ability of plants to produce seeds. This phenomenon is known as the tragedy of the commons (TOC), and undesirable in grain crops. Specifically, changes in light quality due to neighbour plants often induce costly responses like petiole or stem elongation, increasing individual competitiveness at the expense of stand-level yield. Similarly, it has been postulated that belowground competition can induce increased root production in the vicinity of neighbours, which can also reduce seed yield.
There has been a long-standing debate about the interpretation of resource allocation patterns observed in plant-plant competition experiments in crops. Experimental setups using pot systems and artificial dividers to manipulate competitive interactions have been questioned because of the difficulty of simultaneously controlling pot volume, nutrient amount, and nutrient concentration. Moreover, observations of root growth under competition have been inconsistent, ranging from increased root production to spatial root system segregation. Here, using soybean as a model system, we conducted a competition experiment to disentangle above- and belowground drivers of biomass allocation patterns. Specifically, we used above- and belowground physical dividers and varied inter-plant distances (between 3 and 24 cm) to avoid a confounding of divider treatments with plant-available soil volume. Plants closer together had higher root-to-shoot ratios, indicating that an increased fraction of assimilated was diverted to root growth, but this only happened when they competed both above- and belowground (no dividers). Aboveground competition triggered shade avoidance responses, resulting in slender plant habit and reduced total root and shoot biomass. Overall, our results provide evidence for a preferential allocation to root biomass in soybean plants when exposed simultaneously to above and belowground competitions. As effects of belowground competition became only evident when plants were allowed to also compete aboveground, we interpret the competition cues to be processed hierarchically, with aboveground signals having precedence over belowground signals. More generally, our findings show that varying the distance between competing plants allows to avoid the confounding of divider treatments with plant-available soil volume. A reduced total biomass production of plants competing above- and belowground may be interpreted as a consequence of lowered marginal gains arising from allocation patterns that optimize individual but not group performance.
Competing Interest Statement
The authors have declared no competing interest.