In the life cycle of plants, most developmental processes are dependent on light. Significant biological processes such as germination, shade avoidance, circadian rhythms, and flowering are all affected by light. Recent advancements in the use of LED lighting in plant and vegetable production systems has researchers looking for insights into the effects of these artificial lights on the growth and yield of crops.

Scientists from the Department of Horticultural Science at Chungbuk National University, South Korea have studied the advantages and challenges of LEDs in lettuce cultivation. They looked at different wavelengths of light.  

The increased use of LEDs in environmentally controlled closed-type plant production systems allows crop production throughout the year, regardless of external weather conditions. Indoor production in urban agriculture operations, including the world's largest, Chicago's FarmedHere, is often dependent on these kinds of conditions.

According to authors Ki-Ho Son and Myung-Min Oh, LEDs have some advantages over traditional lighting sources in plant cultivation. "LEDs have the advantages of high light-conversion efficiency with low radiant heat output, semipermanence, and small mass," they noted. "In addition, LEDs are available in a variety of narrow wavebands; hence, it is possible to optimize light quality to improve both crop yield and quality."

They explained that blue and red LEDs are usually used for plant growth because chlorophyll a and b efficiently absorb wavelengths in the blue and red ranges.

As part of recent research, the scientists subjected red and green leaf lettuce to six LED lighting treatments. The scientists examined growth characteristics including the fresh and dry weights of shoots and roots, shoot-to-root ratio, total leaf area, and chlorophyll content. They also examined the chemical components of the lettuce to provide information on its pigments, antioxidant capacity, healthy response to stressors and other areas of interest.

"The growth characteristic results for the two lettuce cultivars grown under various ratios of blue to red LEDs confirmed that red LED serves as a major light source that improves lettuce growth rate," the authors said.

The experiments confirmed that both blue and red LEDs have a positive effect on the accumulation of antioxidant phenolic compounds and lettuce growth, respectively. Red light alone was effective at stimulating growth of lettuce plants, but this lighting alone produced abnormally shaped leaves and reduced antioxidant levels, whereas the mixture of the two had optimal effect on both yield and crop quality in "closed-type plant production systems."