Abstract
This study aimed to evaluate the effect of dynamic red and blue light parameters on the physiological responses and key metabolites in lettuce and also the subsequent impact of varying light spectra on nutritive value. We explored the metabolic changes in carotenes, xanthophylls, soluble sugars, organic acids, and antioxidants; the response of photosynthetic indices [photosynthetic (Pr) and transpiration (Tr) rates]; and the intracellular to ambient CO2 concentration ratios (Ci/Ca) in lettuce (Lactuca sativa L. “Lobjoits Green Cos”). They were cultivated under constant (con) or parabolic (dyn) blue (B, 452 nm) and/or red (R, 662 nm) light-emitting diode (LED) photosynthetic photon flux densities (PPFDs) at 12, 16, and 20 h photoperiods, maintaining consistent daily light integrals (DLIs) for each light component in all treatments, at 2.3 and 9.2 mol m–2 per day for blue and red light, respectively. The obtained results and principal component analysis (PCA) confirmed a significant impact of the light spectrum, photoperiod, and parabolic profiles of PPFD on the physiological response of lettuce. The 16 h photoperiod resulted in significantly higher content of xanthophylls (neoxanthin, violaxanthin, lutein, and zeaxanthin) in lettuce leaves under both constant and parabolic blue light treatments (BconRdyn 16 h and BdynRdyn 16 h, respectively). Lower PPFD levels under a 20 h photoperiod (BdynRdyn 20 h) as well as higher PPFD levels under a 12 h photoperiod (BdynRdyn 12 h) had a pronounced impact on leaf gas exchange indices (Pr, Tr, Ci/Ca), xanthophylls, soluble sugar contents, and antioxidant properties of lettuce leaves. The parabolic PPFD lighting profile over a 16 h photoperiod (BdynRdyn 16 h) led to a significant decrease in Ci/Ca, which resulted in decreased Pr and Tr, compared with constant blue or red light treatments with the same photoperiod (BconRdyn and BdynRcon 16 h). Additionally, constant blue lighting produced higher α + β-carotene and anthocyanin (ARI) content and increased carotenoid to chlorophyll ratio (CRI) but decreased biomass accumulation and antioxidant activity.
Highlights
The effects of light on plants are complex, as it controls photosynthesis, plant growth, and developmental processes
A parabolic photosynthetic photon flux densities (PPFDs) profile of 16 h BdynRdyn 16 h resulted in a significant accumulation of Violax (Figure 2A) and hexoses (Fru and Glu) (Figure 2D), along with a significant decrease in pigment reflectance indexes (CRI and anthocyanin reflectance index (ARI)) (Figure 2C), Ci/Ca, Pr, and Tr (Figure 2E)
The total amounts of xanthophylls were similar to those accumulated under lower PPFD levels under a 20 h photoperiod (BdynRdyn 20 h) or under the constant red (BdynRcon 16 h) PPFD treatment
Summary
The effects of light on plants are complex, as it controls photosynthesis, plant growth, and developmental processes. Various studies have demonstrated that controlled amounts or specific spectra of light affect photosynthetic characteristics (Dong et al, 2014; Samuolieneet al., 2020), metabolism (Samuolieneet al., 2013; Lee et al, 2016), and antioxidant properties (Samuolieneet al., 2012; Lekkham et al, 2016). These experiments are typically performed in controlled environment chambers with constant irradiance during the day and abrupt transitions between light and dark at dawn and dusk. The light in controlled environment chambers differs from natural sunlight in several ways, including qualitative spectral and quantitative intensity changes over the course of the day (Annunziata et al, 2017)
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