Comparison of biomass and antioxidant phenolic compound of kale under supplemental UV-A and deep-blue LED lights

This study aimed to evaluate the effects of ultraviolet-A (UV-A) and deep-blue light-emitting diodes (LEDs) on the growth and antioxidant phenolic content of kale (Brassica oleracea var. acephala) and to explore potential boundary effects between UV-A and visible light. Five-week-old kale seedlings cultivated in a vertical farm module were exposed to supplemental UV-A (peak wavelength at 365, 385, and 395 nm) and deep-blue (peak wavelength at 405, 415, 430, and 440 nm) light at an intensity of 30 W m⁻² for 7 days.

The supplemental light treatments enhanced the biomass, showing the most significant improvement at 415 nm. During the light period, photosynthetic rate peaked at 415 nm, and carbon dioxide exchange was observed during the background-light-off period, with higher rates under deep-blue light. The maximum quantum efficiency of photosystem II (Fv/Fm) remained above 0.8 under deep-blue treatments but decreased under UV-A treatments. Hydrogen peroxide levels increased significantly across treatments from days 2 to 7. Except for the 365 nm treatment, phenylalanine ammonia-lyase enzyme activity also increased. Total phenolic content and antioxidant capacity significantly increased in the 395, 405, 415, 430, and 440 nm treatments, with values in the 415 nm treatment being 2-fold higher compared to the control.

These findings suggest that deep-blue light, particularly at 415 nm, may enhance biomass and secondary metabolite accumulation in kale. The results also indicate a potential functional transition region between UV-A and deep-blue light around 400 nm.