Enrichment of Tissue Iron (Fe) Contents of Leaf Lettuce Grown Hydroponically

This experiment was conducted to enhance tissue concentrations of iron (Fe⁺⁺) in leaf lettuce (Lactuca stiva cv. Grand Rapids) plants grown hydroponically. The nutrient solution containing the standard macronutrient concentrations (meq/litter: 2 NH₄⁺, 10 NO₃^-, 4 Ca⁺⁺, 6 K⁺, 2 Mg²⁺, 2 SO₄^2-, 2 H₂PO₄^-) was modified to contain 0, 0.01, 0.1, 1, 2, 4 meg/liter Fe⁺⁺ using iron sulfate (FeSO₄). The concentration of SO₄^2- increased from 2 meq/liter to 2.01, 2.1, 3, 4, 6 meq/liter to accommodate the elevated levels of Fe⁺⁺ in the nutrient treatment solutions. All treatment solutions contained the standard micronutrient concentrations of Hoagland solution. Compared to the control treatment (0.01 meq/liter), the growth of lettuce was reduced when the nutrient solution contained 1 meq/liter or higher Fe⁺⁺ concentrations. The leaf tissue Fe⁺⁺ content was elevated linearly (R²=0.893) as nutrient solution Fe⁺⁺ levels increased, with the highest tissue Fe⁺⁺ content (1.53 g/kg, dry weight basis) being achieved when plants were grown with 4 meq/liter Fe⁺⁺ in the hydroponic solution for 35 days. However, the use of FeSO₄ to increase Fe⁺⁺ in the nutrient solution has a limitation due to suppression of plant growth on Grand Rapids lettuce, especially when nutrient Fe⁺⁺ concentrations exceeded 0.1 meq/liter. The possible use of chelated forms of iron in the nutrient solution may be tested as a means of reducing plant toxicity during hydroponic culture.