Mild-Intensity UV-A Radiation Applied Over a Long Duration Can Improve the Growth and Phenolic Contents of Sweet Basil.
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UV-A radiation (320-400nm) is an abiotic stressor that may be used to enhance the production of beneficial secondary metabolites in crops such as leafy vegetables. However, tradeoffs between enhanced phytochemical contents and overall growth/yield reductions have been reported. The responses varied depending on the UV-A intensity, spectral peak, exposure time, species, and varieties. We quantified the changes in growth, morphology, photosynthesis, and phenolic contents of sweet basil grown under a base red/blue/green LED light with four supplemental UV-A intensity treatments (0, 10, 20, and 30Wm-2) in an indoor environment over 14days. The objective was to determine whether UV-A radiation could be utilized to improve both yield and quality of high-value sweet basil in a controlled production environment. Biomass harvested at 14days after treatment (DAT) was highest under mild-intensity UV-A treatment of 10Wm-2 and lowest under high-intensity UV-A treatment of 30Wm-2. The total leaf area and the number of leaves were significantly lower under the 30Wm-2 treatment than under the 10 and 20Wm-2 treatments at 14 DAT. The maximum quantum efficiency of photosystem II (PSII) for photochemistry (Fv/Fm ) showed a gradual decrease under the 20 and 30Wm-2 treatments from 3 to 14 DAT, whereas Fv/Fm remained relatively constant under the 0 and 10Wm-2 treatments over the entire 14days. The leaf net photosynthesis rate showed a significant decrease of 17.4% in the 30Wm-2 treatment compared to that in the 10Wm-2 treatment at 14 DAT. Phenolic contents (PAL enzyme activity, total phenolic concentration, and antioxidant capacity) were the highest under the 20Wm-2 treatment, followed by the 10, 30, and 0Wm-2 treatments. Overall, our results indicate that the biomass production and accumulation of beneficial phenolic compounds in sweet basil varied depending on the intensity and duration of UV-A application. Mild UV-A radiation (10-20Wm-2) can be a beneficial stressor to improve sweet basil yield and quality over relatively long-term cultivation.
