Foliar Accumulation of Melatonin Applied to the Roots of Maize (Zea mays) Seedlings

Plants absorb melatonin from the environments as well as they synthesize the regulatory molecule. We applied melatonin to the roots of maize (Zea mays) seedlings and examined its accumulation in the leaves. Melatonin accumulation in the leaves was proportional to the exogenously applied concentrations up to 5 mM, without saturation. Time-course analysis of the accumulated melatonin content did not show an adaptable (or desensitizable) uptake system over a 24-h period. Melatonin accumulation in the leaves was reduced significantly by the plant hormones abscisic acid (ABA) and salicylic acid (SA), which commonly cause stomatal closure. The application of ABA and benzo-18-crown-6 (18-CR, a stomata-closing agent) induced stomatal closure and simultaneously decreased melatonin content in the leaves. When plants were shielded from airflow in the growth chamber, melatonin accumulation in the leaves decreased, indicating the influence of reduced transpiration. We conclude that melatonin applied exogenously to the root system is absorbed, mobilized upward according to the transpirational flow, and finally accumulated in the leaves.