What condition does a lack of magnesium cause in plants?

A lack of magnesium in plants leads to chlorosis and yellow leaves, which is a key symptom of magnesium deficiency. Magnesium is an essential element needed for the production of chlorophyll, the pigment responsible for the green color in plants and vital for photosynthesis. When magnesium is deficient, chlorophyll production decreases, resulting in the leaves turning yellow due to insufficient green pigmentation. This yellowing often starts in the older leaves and can severely impact the plant's overall health by reducing its ability to photosynthesize effectively, which in turn can limit growth and development.

The other options do not accurately reflect the primary deficiency symptom associated with magnesium shortage. Root rot is related to overwatering and soil conditions rather than a nutrient deficiency. Excessive flowering and poor fruit development can stem from various factors, but they are not directly caused by magnesium deficiency. Thus, chlorosis and yellow leaves remain the clear indication of insufficient magnesium in plants.