Effect of Timing and Duration of Blue Light Exposure On the Anthocyanin Content of Red Leaf Lettuce Cv. Outredgeous Grown Under Light Emitting Diodes

Future space missions beyond Low Earth orbit will expose crew members to higher doses of cosmic radiation than currently received onboard Space Shuttle or the International Space Station.  A diet rich in bioactive phytochemicals is a potential radiation countermeasure to both reduce the effective radiation dose and induce in situ repair of radiation damage during these long-duration space missions.  It has been shown that light quality can be use to increase the anthocyanins content, and thus, anti-oxidant value, of the salad crops. The use of solid state light emitting diodes (LEDs) to provide plant lighting systems is being evaluated for use during long duration space missions. A series of experiments were conducted to determine the effect of timing and duration of narrow band blue light (440 nm) on the concentration of anthocyanins in Lactuca sativa cv. Outredgeous (a red, loose-leaf type).  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.   An 18 h light/ 6 hr dark photoperiod was maintained under light emitting diodes (LEDs) at 300 μmol m^-2 s^-1.  Two control  treatments,  red LEDs only (640 nm) and   270 μmol m^-2 s^-1 red (640 nm) / 30 μmol m^-2 s^-1 blue (440 nm) were applied.   All treatments had red light throughout development [(0-21 days after planting (DAP)].  Timing experiments consisted of  blue for 0-14 DAP, 14-21 DAP, 7-14 DAP or 7-21 DAP.  Lettuce was harvested at 21 DAP and dry mass, leaf area, plant height, anthocyanin content and oxygen radical absorbance capacity (ORAC) value determined.  Results clearly showed that blue light was required for the induction of anthocyanin production in Outredgeous.  Continuous exposure (0-21 DAP) to blue light had little additional effect on ORAC, although total anthocyanin content was more than double.  Removing blue light from days 14-21 DAP resulted in significant reduction of both anthocyanin and ORAC values.  In marked contrast, applying blue light for the final 7 days of growth (14-21 DAP) increased anthocyanin levels over 4 fold and resulted in ~24% increase in ORAC values.   These data indicate that anthocyanin production is under blue light photoregulation in lettuce and that bioprotective and nutraceutical value can be enhanced by selective management of light spectra during development.