Graduate Research Fellow Washington State University Pullman, WA, United States
How plants use the carbon gained from photosynthesis is a central question in plant ecology. Carbon is used by plants to grow, synthesize defense compounds, and meet many other metabolic needs (referred to collectively as carbon sinks). Although the specific needs of a given plant depend on an array of variables, all plants face the same dilemma: how to allocate carbon between sinks. The prevailing hypothesis for how plants allocate carbon, termed ‘active allocation’, suggests that plants favor non-structural carbohydrate accumulation during periods of low carbon supply. In contrast, passive allocation suggests that plants allocate carbon along a fixed hierarchy. Under passive allocation, carbon is first allocated to growth and maintenance, then to defense, and finally to storage. Although we have several hypotheses for how plants allocate carbon, it is unclear what exactly the null hypothesis is. Clarifying this null hypothesis and the carbon allocation patterns produced by it will undoubtedly improve our capacity to interpret carbon allocation data in the future. Using the biochemical network of plant carbon metabolism, probability theory, and linear algebra, I provide the first mathematical treatment of passive, active, and neutral plant carbon allocation. This approach accurately reflects the hierarchy of passive allocation and high storage of active allocation. Notably however, this approach suggests that under passive allocation, allocation to storage occurs with similar probability as does allocation to growth. The argument that NSC accumulation is prioritized under active allocation is replicated exactly by neutral theory. Thus, neutral theory suggests that current observations of plant carbon allocation may be artefacts of the biochemical network of plant carbon metabolism coupled with limitations in how we measure allocation to growth, storage, and other carbon sinks. Finally, I use optimal transport theory to demonstrate the relative efficiency of neutral carbon allocation relative to passive and active allocation.
