High nitrate concentrations in vegetables (particularly if the vegetables are subject to low-light conditions, such as in winter greenhouses) pose possible human health risks and are therefore subject to regulatory limits in Europe.
Researchers in Lithuania developed a way to reduce nitrates in leafy vegetables: short-term (three-day, round-the-clock) exposure immediately before harvest by narrow-bandwidth red light provided by light-emitting diodes (LEDs).
LED light before harvest resulted in significantly lower (by 44-65%) nitrate concentrations in lettuce, marjoram, and green onions relative to the concentrations in control vegetables. (under regular high-pressure greenhouse lighting) LED light also boosted carbohydrate concentrations. In some cases, vitamin C concentrations were raised or lowered by LED light.
If you’re growing leafy vegetables in hydroponic conditions during the winter months, it might pay you to add some extra red LED lights for the last three days before harvest.
Reference: Giedre· Samuolene·, et al., “Decrease in Nitrate Concentration in Leafy Vegetables Under a Solid-State Illuminator,” HortScience 44(7), December 2009, 1857-1860. (American Society for Horticultural Science, 113 S. West St., Suite 200, Alexandria, VA 22314-2851.)