Seedlings of tomato and tobacco were V‐grafted reciprocally and intraspecifically and cultured for 22 days in 2‐liter containers of nutrient solution with normal or no iron. The pH of solutions containing iron increased slightly, while the pH of solutions with no iron decreased. There was little decrease in the pH of tomato grafts on tomato, while the pH of other combinations decreased from 1.5 to 3.1 pH units. Riboflavin concentrations in nutrient solution were highest with tobacco on tobacco, and were about one‐third less with tomato on tobacco. The riboflavin values for tobacco on tomato and tomato on tomato were barely detectable like those for plants supplied with iron. Roots of iron‐deficient tobacco plants at harvest contained much more total riboflavin than roots of iron‐deficient tomato plants and roots of iron‐supplied plants contained less total riboflavin than iron‐deficient plants. Total riboflavin in roots of tobacco plants exceeded that in roots of tomato plants. The total riboflavin concentrations in leaves of iron‐deficient plants were elevated in plants on tobacco roots but not in plants on tomato roots. At harvest, after some regreening, tobacco leaves on tomato roots were more chlorotic than when on tobacco roots and tomato leaves on tomato plants were more chlorotic than when on tobacco roots. Results suggest that the roots are the site for the increased synthesis of the riboflavin in tobacco. Riboflavin may partially substitute for iron in roots and leaves.