Laidback a citrus leaf with the center rib turning yellow, and then also spreading out a little from the center is quite common. Also it is common that this type of symptom is happening on the older lower leaves and not the younger leaves. This symptom would not indicate an iron deficiency, because the element iron in citrus is NOT mobile in plant tissues. Therefore, iron deficiency symptoms are expressed only in new leaves, and not in the older bottom leaves of a young tree. Magnesium, on the other hand, is very mobile in the plant; therefore, the symptoms develop first on older leaves. However, the symptom you describe is certainly not a magnesium deficiency. I believe, your tree is only discarding the old leaves that are no longer useful. When a citrus leaf no longer supplies more energy to the tree than it uses, it is dropped. I don't believe you have much to worry about. Its nice to hear from you again. Merry Christmas. - Millet
