Optimization of UV-A/PAR ratio for phenolic accumulation in kale using 3D radiation interception modeling

In vertical farms using light-emitting diode radiation, supplemental ultraviolet (UV)-A radiation at specific wavelengths and energy levels has been shown to enhance photosynthesis and biomass accumulation, indicating its potential as an additional light source. However, plant responses to supplemental UV-A vary depending on the intensity of photosynthetically active radiation (PAR). In this study, three-dimensional (3D) radiation interception analysis based on ray-tracing simulation was conducted to examine how the UV-A/PAR ratio affects phenolic content and antioxidant capacity of kale and to determine appropriate UV-A levels, under different photosynthetic photon flux densities (PPFD), selected based on previous studies. Kale transplants were grown for 3 weeks under 150 or 300 µmol m− 2 s− 1 PPFD, and supplemented with UV-A radiation (385 nm) at 15, 30, or 45 W m− 2 for 6 days before harvest. Radiation interception was analyzed using a 3D plant model and ray-tracing simulation. Kale growth was substantially greater under 300 than under 150 µmol m− 2 s− 1 PPFD. Growth responses to supplemental UV-A radiation varied significantly with PPFD level. UV-A radiation increased shoot fresh weight and leaf area under 150 µmol m− 2 s− 1 PPFD, but not under 300 µmol m− 2 s− 1 PPFD at 6 days after the onset of treatment. Supplemental UV-A radiation also enhanced total phenolic content and antioxidant capacity, with the magnitude of these effects dependent on PPFD level. Under 150 µmol m− 2 s− 1 PPFD, the 15 W m− 2 UV-A treatment enhanced phenolic content and antioxidant capacity, whereas no increase was observed under 300 µmol m− 2 s− 1 PPFD. Radiation interception analysis indicated that differences in absorbed radiation between the two PPFD conditions were primarily attributable to plant size. The correlation between UV-A radiation intensity and phenolic accumulation was stronger when based on intercepted radiation than on measured intensity. Notably, higher phenolic content and antioxidant capacity were observed at UV-A/PAR ratio ranging from 0.1 to 0.2. These findings suggest that supplemental UV-A radiation should be optimized considering PPFD level and radiation interception to enhance phenolic content and antioxidant capacity in kale.