Effect of Supplemental UV Light Exposure On the Anthocyanin Content of Two Red Lactuca Sativa Cv. Outredgeous and Firecracker Grown Under Light Emitting Diodes and Fluorescent Lamps

The use of naturally occurring antioxidants can mitigate the effects of radiation damage and enhance healing from oxidative damage which may occur on long-duration space missions.  Light quality can affect the content of bioprotective anthocyanins and change the antioxidant value of salad crops grown in space.  A series of experiments were conducted to determine the effect of both UVA and/or UVB light on the anthocyanin concentrations of two different cultivars of red leaf lettuce.  Lettuce was grown in controlled environment chambers (EGC M-36 Environmental Growth Chambers, Chagrin Falls, OH) at 23^oC, 65% RH and 1200 ppm CO₂.   A 16 h light/ 8 hr dark photoperiod was maintained under either light emitting diodes (LEDS) or triphosphor fluorescent lamps (TFP, Sylvania).  Treatments consisted of 300 μmol m^-2 s^-1 red LEDs only (640 nm) or 300 μmol m^-2 s TFP lamps only throughout development [(0-21 days after planting (DAP)] as controls.  UVA (365 nm peak, JKL (F8T5BLT) or UVB (312 nm peak, Phillips PL-S9W/01) supplemental light was added from 0-21.  Acrykuc barriers were used to block transmission of UVA and UVB as the “control” treatment in all experiments.  A UV-transmitting acrylic (ACrylite OP-4) was used in the UVA and UVB treatments.  Lettuce was harvested at 21 DAP and dry mass, leaf area, plant height, anthocyanin content and oxygen radical absorbance capacity (ORAC) value determined.

Results show a significant increase in anthocyanin per gram in both cultivars with the addition of UVB light to the red LEDs.  UVA was ineffective at inducing anthocyanin formation in cv. Outredgeous, but was inductive in cv. Firecracker.  Firecracker exhibited enhanced anthocyanin even from the supplemental transmission of UVA from TPF lamps through the OP-4 acrylic.  There was also a difference in the overall ORAC potential in response to UVA and UVB in both cultivars.  This suggests that there are genotypic differences in UV sensitivity to anthocyanin induction in lettuce which should be considered in any future development of salad crop production systems for long duration space missions.