Linear relationships between shoot magnesium concentration ([Mg]shoot) and shoot calcium concentration ([Ca]shoot) are commonly reported among angiosperm species growing together. White et al. present an original hypothesis to explain these relationships, proposing that the Mg/Ca selectivity of cell wall binding determines the gradient, whilst intracellular concentrations plus cell wall cation exchange capacity determines the absolute concentrations.
They observe different relationships between [Mg]shoot and [Ca]shoot for commelinid monocots, non-commelinid monocots, eudicots excluding Caryophyllales, and Caryophyllales species. They propose that these relationships are associated with the distinct cell wall chemistries of these groups and greater vacuolar Mg accumulation in Caryophyllales species.
In their paper, White and colleagues conclude: “The arguments and analysis presented lead to several hypotheses, i.e. that in plants that do not exhibit ‘luxury’ accumulation of Ca or Mg, (1) distinct linear relationships between [Mg]shoot and [Ca]shoot are exhibited by at least three groups of angiosperm species, namely commelinid monocots, eudicots excluding Caryophyllales, and Caryophylalles 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 [Ca]shoot and [Mg]shoot within groups are associated with their cell wall CEC; and (4) Caryophyllales constitutively accumulate more Mg in their vacuoles than other angiosperm species.”