Spinach: Nitrate Analysis of an Advanced Life Support (ALS) Crop Cultured Under ALS Candidate Artificial Light Sources

Nitrate concentration in spinach and lettuce is known to be influenced by light quantity. The enzyme nitrate reductase is regulated by phytochrome in some species, and in the presence of light, electrons that reduce nitrite to ammonium come from photosynthetic electron transport. It was hypothesized that light quality as well as light quantity may be used to manipulate nitrate concentration in spinach. To test this, narrow-band wavelength light-emitting diode (LED) sources (670 nm and 735 nm peak emission) were utilized in combination with cool white fluorescent (CWF) lamps. Nitrate concentration was compared in spinach seedlings grown for four weeks under CWF, followed by one of three 5-day pre-harvest light treatments. The three different light quality regimes were 1) CWF, 2) CWF + RED (670 nm) LED, and 3) CWF + FR (735 nm LED). Ion chromatograph analysis of freeze-dried tissue showed a 10-fold increase in nitrate concentration in the CWF+ FR (0.12 mmoles/g dw) treatment over the CWF (0.01 mmoles/g dw) and CWF+Red (0.01 mmoles/g dw) treatments. The results of this study suggest that selection of ALS light sources should not only consider crop yield and energy efficiency, but should also take into account the influences of light quality on food safety and human nutrition.