Plant stress responses can be observed in plants that are under- or over-supplied with Mg. The first observable signs of Mg stress in plants for both starvation and toxicity is a depression of the rate of photosynthesis, it is presumed because of the strong relationships between Mg and chloroplasts/chlorophyll. In pine trees, even before the visible appearance of yellowing and necrotic spots, the photosynthetic efficiency of the needles drops markedly. In Mg deficiency, reported secondary effects include carbohydrate immobility, loss of RNA transcription and loss of protein synthesis. However, due to the mobility of Mg within the plant, the deficiency phenotype may be present only in the older parts of the plant. For example, in Pinus radiata starved of Mg, one of the earliest identifying signs is the chlorosis in the needles on the lower branches of the tree. This is because Mg has been recovered from these tissues and moved to growing (green) needles higher in the tree.
A Mg deficit can be caused by the lack of the ion in the media (soil), but more commonly comes from inhibition of its uptake. Mg binds quite weakly to the negatively charged groups in the root cell walls, so that excesses of other cations such as K, NH, Ca, and Mn can all impede uptake.(Kurvits and Kirkby, 1980; In acid soils Al is a particularly strong inhibitor of Mg uptake. The inhibition by Al and Mn is more severe than can be explained by simple displacement, hence it is possible that these ions bind to the Mg uptake system directly. In bacteria and yeast, such binding by Mn has already been observed. Stress responses in the plant develop as cellular processes halt due to a lack of Mg (e.g. maintenance of ΔpH across the plasma and vacuole membranes). In Mg-starved plants under low light conditions, the percentage of Mg bound to chlorophyll has been recorded at 50%. Presumably, this imbalance has detrimental effects on other cellular processes.
Mg toxicity stress is more difficult to develop. When Mg is plentiful, in general the plants take up the ion and store it (Stelzer et al., 1990). However, if this is followed by drought then ionic concentrations within the cell can increase dramatically. High cytoplasmic Mg concentrations block a K channel in the inner envelope membrane of the chloroplast, in turn inhibiting the removal of H ions from the chloroplast stroma. This leads to an acidification of the stroma that inactivates key enzymes in carbon fixation, which all leads to the production of oxygen free radicals in the chloroplast that then cause oxidative damage.