The effect of potassium upon the uptake and distribution of magnesium in plants

Applications of potassium fertilisers may reduce the amount of magnesium in plants, at times to a degree which produces symptoms of magnesium deficiency. As well as its effects on the availability of soil magnesium, potassium affects the amount of magnesium in the plant in two ways: by competition at the site of uptake, thus resulting in a lower total plant magnesium, or by reducing transport of magnesium between plant organs. There is disagreement on the relative importance of these factors. The kinetics of short term uptake of potassium and magnesium by detached barley roots were studied by methods based on the Michaelis-Menten theory of enzyme-substrate relationships. These experiments showed competitive inhibition of magnesium by potassium but not of potassium by magnesium. This may be explained by proposing that there are many sites of potassium uptake and that some of them are free of magnesium competition. Competitive inhibition of a divalent ion by a monovalent ion has not been observed previously, earlier reports being confined to competition between ions of like valency. An apparent stimulation of potassium uptake by magnesium was found to be due to the influence of a common anion (chloride).