Photorespiration Process and Nitrogen Metabolism in Lettuce Plants (Lactuca sativa L.): Induced Changes in Response to Iodine Biofortification

Iodine is vital to human health, and iodine biofortification programs help improve human intake through plant consumption. There is no research on whether iodine biofortification influences basic plant physiological processes. Because nitrogen (N) uptake, utilization, and accumulation are determining factors in crop yield, the aim of this work was to establish the effect of the application of different doses (20, 40, and 80 μM) and forms of iodine (iodate [IO₃ ⁻] vs. Iodide [I⁻]) on N metabolism and photorespiration. For this study we analyzed shoot biomass and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AAT), glutamate dehydrogenase (GDH), glycolate oxidase (GO), glutamate:glyoxylate aminotransferase (GGAT), serine:glyoxylate aminotransferase (SGAT), hydroxypyruvate reductase (HR) and catalase (CAT), nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), organic and total N, amino acids, proteins, serine (ser), malate, and α-ketoglutaric acid in edible lettuce leaves. Application of I⁻ at doses of at least 40 μM reduced the foliar concentration of NO₃ ⁻ with no decrease in biomass production, which may improve the nutritional quality of lettuce plants. In contrast, the application of 80 μM of I⁻ is phytotoxic for lettuce plants, reducing the biomass, foliar concentration of organic N and NO₃ ⁻, and NR and GDH activities. HR activity is significantly inhibited with all doses of I⁻; the least inhibition was at 80 μM. This may involve a decrease in the incorporation of carbonated skeletons from photorespiration into the Calvin cycle, which may be partially associated with the biomass decrease. Finally, the application of IO₃ ⁻ increases biomass production, stimulates NO₃ ⁻ reduction and NH₄ ⁺ incorporation (GS/GOGAT), and optimizes the photorespiratory process. Hence, this appears to be the most appropriate form of iodine from an agronomic standpoint.