If the younger leaves of plants are showing chlorosis with dark green major veins, which nutrient deficiency is present?

The presence of chlorosis in younger leaves accompanied by dark green major veins is indicative of an iron deficiency. Iron is an essential micronutrient that plays a critical role in the process of photosynthesis, particularly in the formation of chlorophyll. When a plant lacks sufficient iron, it struggles to produce chlorophyll, resulting in chlorosis, especially in the new leaves that are still developing. The veins remain green while the surrounding leaf tissue turns yellow because iron is more mobile within the plant, affecting newer growth.

In contrast, other nutrient deficiencies present different symptoms. For instance, nitrogen deficiency typically leads to general yellowing of older leaves, as nitrogen is mobile in plants and can be relocated to newer growth when needed. Magnesium deficiency also results in chlorosis, but the yellowing often appears between the veins rather than leaving them dark green. Calcium deficiencies lead to symptoms primarily associated with shoot and root development, such as tip burn in lettuce or blossom end rot in tomatoes, rather than the specific chlorosis pattern seen with iron deficiency. Thus, the described symptoms clearly point to a deficiency of iron in the plant.