Solid-state illuminator used in greenhouses decreases nitrates, increases nutrient levels

Treatment results in decreased nitrates, increased antioxidants

Searching for ways to improve the nutritional quality of leafy green vegetables, Lithuanian researchers have found success with new technology that features high-density photosynthetic photon flux generated by a solid-state illuminator. The technology, which can be applied in greenhouses for preharvest treatment of leafy vegetables, was found to decrease concentrations of harmful nitrates while allowing some beneficial nutrient levels to increase. The research results were published in a recent issue of HortScience.

The researchers experimented with a solid-state illuminator to provide short-term preharvest light treatment of lettuce, marjoram, and green onions. The vegetable plants were grown to harvest time in a greenhouse under daylight with supplementary lighting provided by standard high-pressure sodium lamps. A subsequent 3-day treatment within a phytotron under light-emitting diodes resulted in the reduction of nitrate concentration by 44% to 65%.

According to Giedre Samuoliene, lead author of the report, the technology is different from the usual practice of using high-pressure sodium lamps; solid-state illuminators limit the amount of radiant heat, allowing a high intensity of photosynthesis. Additionally, the technique allows for short-term treatment of plants rather than for full-cycle growth.

In vegetable leaves exposed to light generated by the solid-state illuminator, nitrate concentration was reduced by two to three times in comparison with those kept under high-pressure sodium lamps. The highest nitrate reduction rate was observed in hydroponically grown lettuce; after a 3-day treatment under red LEDs, tests showed a 65% relative decrease of nitrate concentration. The relative decrease of nitrates was similar in all species tested. "The results of our study indicate that nitrate content in lettuce, marjoram, and green onions can be considerably reduced by several times using short-term preharvest treatment under purely red light with high PPFD", stated Samuoliene.

A significant outcome of the research is the finding that leafy vegetables can be produced under normal lighting conditions, while the health quality can be improved with a relatively short treatment using an advanced solid-state illuminator. The new technology may be expensive, but can prove economically viable in terms of production costs and the benefits of vegetables with added nutritional value. Since the treatment is conducted only over 10% of the overall growth cycle, the capital cost limitations for the application of solid-state lighting in horticulture are mitigated.

The researchers noted that the technology may be particularly practical for leafy vegetable production in northern countries where greenhouse plants are often grown under poor lighting conditions.