Yellowing leaves is a very generic plant symptom. There can be many causes including diseases, environmental, insect, or nutrient deficiencies. The following are common causes of leaf yellowing on vegetable plants:
Environmental/cultural or caused by nonliving factors
Normal Aging of Older Leaves - older, lowers leaves of plants can turn yellow as they age. It is called leaf senescence. The rest of the plant will look healthy and be productive.