Mg is essential for plant growth and is present in higher plants in amounts on the order of 80 μmol g dry weight. The amounts of Mg vary in different parts of the plant and are dependent upon nutritional status. In times of plenty, excess Mg may be stored in vascular cells (Stelzer et al., 1990; and in times of starvation Mg is redistributed, in many plants, from older to newer leaves.
Mg is taken up into plants via the roots. Interactions with other cations in the rhizosphere can have a significant effect on the uptake of the ion.(Kurvits and Kirkby, 1980; The structure of root cell walls is highly permeable to water and ions, and hence ion uptake into root cells can occur anywhere from the root hairs to cells located almost in the centre of the root (limited only by the Casparian strip). Plant cell walls and membranes carry a great number of negative charges, and the interactions of cations with these charges is key to the uptake of cations by root cells allowing a local concentrating effect. Mg binds relatively weakly to these charges, and can be displaced by other cations, impeding uptake and causing deficiency in the plant.
Within individual plant cells, the Mg requirements are largely the same as for all cellular life; Mg is used to stabilise membranes, is vital to the utilisation of ATP, is extensively involved in the nucleic acid biochemistry, and is a cofactor for many enzymes (including the ribosome). Also, Mg is the coordinating ion in the chlorophyll molecule. It is the intracellular compartmentalisation of Mg in plant cells that leads to additional complexity. Four compartments within the plant cell have reported interactions with Mg. Initially, Mg will enter the cell into the cytoplasm (by an as yet unidentified system), but free Mg concentrations in this compartment are tightly regulated at relatively low levels (≈2 mmol/L) and so any excess Mg is either quickly exported or stored in the second intracellular compartment, the vacuole. The requirement for Mg in mitochondria has been demonstrated in yeast and it seems highly likely that the same will apply in plants. The chloroplasts also require significant amounts of internal Mg, and low concentrations of cytoplasmic Mg. In addition, it seems likely that the other subcellular organelles (e.g., Golgi, endoplasmic reticulum, etc.) also require Mg.