Is Magnesium (Mg) mobile in the plant?

Magnesium (Mg) is indeed considered a mobile nutrient within plants. This mobility allows magnesium to move from older leaves to newer growth during the plant's development. When magnesium is in short supply, it tends to be preferentially redistributed from mature tissues to developing tissues. This feature is particularly significant because it means that deficiency symptoms, such as interveinal chlorosis, will often appear first on older leaves. Understanding the mobility of magnesium is crucial for effective nutrient management, as it influences how and when to apply fertilizers to prevent deficiencies in crops.

The other options suggest varying degrees of immobility or specific conditions under which magnesium may behave differently, which does not align with the established understanding of magnesium's behavior in plants.