Linear relationships between shoot magnesium and calcium concentrations among angiosperm species are associated with cell wall chemistry

Background Linear relationships are commonly observed between shoot magnesium ([Mg]shoot) and shoot calcium ([Ca]shoot) concentrations among angiosperm species growing in the same environment. Scope and Conclusions This article argues that, in plants that do not exhibit ‘luxury’ accumulation of Mg or Ca, (1) distinct stoichiometric relationships between [Mg]shoot and [Ca]shoot are exhibited by at least three groups of angiosperm species, namely commelinid monocots, eudicots excluding Caryophyllales, and Caryophyllales species; (2) these relationships are determined by cell wall chemistry and the Mg/Ca mass quotients in their cell walls; (3) differences between species in [Mg]shoot and [Ca]shoot within each group are associated with differences in the cation exchange capacity (CEC) of the cell walls of different species; and (4) Caryophyllales constitutively accumulate more Mg in their vacuoles than other angiosperm species when grown without a supra-sufficient Mg supply.