House & Garden Nutrients MgO 8% Magnesium gives plants their fresh, healthy and green appearance. The nutrient is an important building block for chlorophyll and plays a key role in photosynthesis. It contributes to the strength of the tissue and the cell wall and is a building block for various enzymes. A lack of Magnesium will cause the leaf to turn yellow while the veins will stay green. The leaves will also fall earlier and have a reduced capacity for photosynthesis.