The criteria for biomass partitioning of the current shoot: water transport versus mechanical support

In this study, we determine the theoretical criteria for biomass partitioning into the leaf and stem of the current shoot, using two quantitative models. The water transport model, based on the biochemical model of CO2 assimilation, predicts the relationship between the water transport capacity per biomass investment in the stem (stem mass specific conductivity) and the partitioning of biomass that maximizes shoot productivity. The mechanical support model, based on Eulerʹs buckling formula, predicts the relationship between the mechanical strength per biomass investment in the stem (the inverse relationship of stem mass density) and the partitioning of biomass to avoid mechanical failures such as lodging. These models predict the stem properties of mass specific conductivity and stem mass density that result in optimum partitioning just sufficient to provide adequate water transport and static mechanical support. In reality, the stem properties of plants differ from those predicted for optimum partitioning: the partitioning of biomass in the current shoot of both angiosperms and gymnosperms is mainly governed by the mechanical support criterion, although gymnosperms are probably more affected by the water transport criterion. This tendency is supported by actual measurements of biomass partitioning in plants.