Question

if older leaves turn yellow first, what is the most likely cause? virus infection sunburn iron deficiency nitrogen deficiency

Explanation:

Nitrogen is a mobile nutrient in plants. When there is a nitrogen deficiency, the plant can move nitrogen from older leaves to younger, more actively - growing leaves. As a result, the older leaves, which lose nitrogen, turn yellow first. Iron is a relatively immobile nutrient. Iron deficiency usually shows symptoms in younger leaves first. Sunburn typically causes bleaching or browning of the affected leaf areas rather than a uniform yellowing starting from older leaves. Virus infections can cause various symptoms like mottling, stunting, etc., but not specifically a pattern of older - leaf - first yellowing as a most common and characteristic sign compared to nitrogen deficiency.

Answer:

Nitrogen deficiency