Nitrate control using LED lights

Light emitting diodes (LEDs) have become very efficient light sources for application in horticulture. Light sources based on LEDs are relatively easy to control in intensity and spectrum. Light-source optimization allows growth efficiency and quality of leafy greens to be influenced, especially the nutrient content. Leafy green vegetables contain relatively high concentrations of nitrate compared with other foods, and constitute a significant source of dietary nitrate to the consumer. Because of health concerns, nitrate content is regulated in Europe. Nitrate levels in vegetables grown in plant factories or greenhouses can be reduced significantly with an appropriate light strategy. In this paper, we present such a strategy, as well as results that show that, with optimized light recipes, the level of nitrate at harvest can be controlled, while the growth efficiency can be kept high. When grown in a closed environment without daylight, we have found that the daily light integral has a strong influence on the final nitrate level. For an optimum daily light integral, we have found that using a continuous pre-harvest light treatment for 2-5 days before harvesting can significantly reduce the nitrate level in lettuce. Light composed of red and blue or red and white can achieve a similar reduction of the nitrate level for various cultivars of baby-leaf lettuce. We present several results on lettuce, spinach and rocket. We also discuss how to achieve extremely low nitrate levels (<500 mg kg-1) and how to find the optimum light sum and spectrum during growth or pre-harvest treatment in combination with an adapted irrigation strategy. Finally, we applied our findings in a greenhouse trial during winter and compare the results with those obtained under plant factory growth conditions.