Nitrate, nitrite, protein, amino acid contents, and photosynthetic and growth characteristics of tatsoi cultivated under various photon flux densities and spectral light compositions

Abstract This study evaluated the effects of light intensity and the spectrum produced by light-emitting diodes (LEDs) on the growth and nitrate assimilation of tatsoi (Brassica rapa var. rosularis), grown in a controlled environment. A system of high-powered solid-state lighting modules with blue 445, red 640 and 660 and far-red 731 nm LEDs were used for illumination. Two experiments were performed seeking to explore the light parameter effects on tatsoi growth, nitrate, nitrite, total protein, and free amino acid contents, as well as soluble sugar contents and chlorophyll indexes: (1) investigation of the effects of a photosynthetically active photon flux density (PPFD) of 100, 200, 300, 400 and 500 μmol m–2 s–1 and (2) evaluation of the effects of different combinations of red (R660 and R660 + R640), red supplemented with blue (R660 + B445 and R660 + R640+B445) and far red (R660 + R640+B445+FR731) LEDs. A deficient light intensity resulted in nitrate and nitrite accumulation in tatsoi leaves; under the exposure of 100 μmol m–2 s–1 PPFD, tatsoi contained 3.4 times higher NO3 and 34 times higher NO2 contents, compared to that under 200 μmol m–2 s–1. The photosynthetic photon flux density of ∼300 μmol m2 s−1 allowed the obtaining of an optimal tatsoi production with high biomass and minimal nitrate and nitrite contents in the leaves. Long term exposure of sole red light was stressful for tatsoi growth; however the addition of blue light to the red 660- and 640- nm LEDs resulted in the highest total protein and essential amino acid contents, leaf area and biomass, and significantly reduced nitrate but increased nitrite content.