Abstract
Phenolic compounds in basil (Ocimum basilicum) plants grown under a controlled environment are reduced due to the absence of ultraviolet (UV) radiation and low photosynthetic photon flux density (PPFD). To characterize the optimal UV-B radiation dose and PPFD for enhancing the synthesis of phenolic compounds in basil plants without yield reduction, green and purple basil plants grown at two PPFDs, 160 and 224 μmol·m−2·s−1, were treated with five UV-B radiation doses including control, 1 h·d−1 for 2 days, 2 h·d−1 for 2 days, 1 h·d−1 for 5 days, and 2 h·d−1 for 5 days. Supplemental UV-B radiation suppressed plant growth and resulted in reduced plant yield, while high PPFD increased plant yield. Shoot fresh weight in green and purple basil plants was 12%–51% and 6%–44% lower, respectively, after UV-B treatments compared to control. Concentrations of anthocyanin, phenolics, and flavonoids in green basil leaves increased under all UV-B treatments by 9%–18%, 28%–126%, and 80%–169%, respectively, and the increase was greater under low PPFD compared to high PPFD. In purple basil plants, concentrations of phenolics and flavonoids increased after 2 h·d−1 UV-B treatments. Among all treatments, 1 h·d−1 for 2 days UV-B radiation under PPFD of 224 μmol·m−2·s−1 was the optimal condition for green basil production under a controlled environment.
Highlights
Decreasing arable land, rising urbanization, water scarcity, and climate change exert pressure on agricultural producers [1]
Photosynthetic photon flux density of 224 μmol·m−2 ·s−1 for basil plants was selected according to our previous study [21], and a low photosynthetic photon flux density (PPFD) of 160 μmol·m−2 ·s−1 was selected to test if Ultraviolet B (UV-B) radiation can compensate for the reduced accumulation of phenolic compounds in basil plants grown under low PPFD
Concentrations of phenolic compounds in basil plants grown under low PPFD with UV-B radiation was significantly higher compared to those of plants grown under high PPFD without UV-B radiation, suggesting that UV-B
Summary
Decreasing arable land, rising urbanization, water scarcity, and climate change exert pressure on agricultural producers [1]. Synthesis of anthocyanin and other polyphenols in another red leaf lettuce cultivar (‘Red Fire’, controlled environment, PPFD of 150 μmol·m−2 ·s−1 ) significantly increased after 3-days UV-B radiation at a much lower dose, 1.5 μmol·m−2 ·s−1 for 16 h·d−1 prior to harvest [4]. Considering energy saving, the photosynthetic photon flux density (PPFD) in controlled environment systems is much lower compared to sunlight intensity in open field, resulting in further reduction of secondary plant metabolites [21]. To identify the optimal combination of UV-B radiation dose and PPFD that enhance concentrations of phenolic compounds without significant yield reduction, further investigation is warranted to characterize the physiological, morphological, and biochemical responses in basil plants to supplemental UV-B radiation and different PPFDs under a controlled environment. Photosynthetic photon flux density of 224 μmol·m−2 ·s−1 for basil plants was selected according to our previous study [21], and a low PPFD of 160 μmol·m−2 ·s−1 was selected to test if UV-B radiation can compensate for the reduced accumulation of phenolic compounds in basil plants grown under low PPFD
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