The correct answer is magnesium deficiency. This condition is often characterized by a specific yellowing of leaves, known as interveinal chlorosis, where the areas between the veins turn yellow while the veins themselves remain green. This is particularly evident in older leaves, as magnesium is a mobile nutrient within the plant, meaning it can be relocated from older to younger leaves as needed. When there is insufficient magnesium, the plant cannot produce enough chlorophyll, which leads to this distinctive yellowing pattern.
Magnesium plays a crucial role in photosynthesis, being a central component of chlorophyll, the pigment responsible for absorbing sunlight. Without adequate magnesium, plants struggle to convert sunlight into energy, impacting their growth and overall health. Observing this yellowing symptom is a clear indicator that the plant is lacking magnesium and requires attention to rectify the nutrient imbalance.