The Red Lettuce Effect

Researchers have recently reported that adding far-red light to the lighting regimen of a greenhouse-grown lettuce crop (‘Rex’) enhanced its color and antioxidant capacity. The far-red wavelengths were added to the LED array at different photon flux densities, or photosynthetic photons per square meter (PPFD), and were found to be most beneficial for this cultivar under a PPFD of 180 umol m-2 s-1. The researchers also examined the effects of these light wavelengths on baby lettuce grown hydroponically in an indoor vertical farm and found that the light spectrum was important for foliar growth, as well as color, leaf size, and dry matter content.

The phenolic compounds that give red leaf lettuce its characteristic color are anthocyanins, which have antimicrobial, anti-fungal, and antioxidant properties. Accumulation of anthocyanins depends on several factors, including light quality, temperature, and genetic control. A study that compared the impact of various climatic variables on plant pigmentation and phenolic compounds in red lettuce concluded that cooler environmental conditions enhanced anthocyanin synthesis, and this effect was stronger in February than May.

In the field, a change in environmental conditions that enhances anthocyanin accumulation can be achieved by using high tunnel (HT) systems to protect crops from extreme weather. However, the HT covering material alters the spectral quality of light, and this can affect the production of phenolic compounds. In addition, HTs can lower the soil temperature and this affects anthocyanin synthesis.

To determine the influence of HT coverings on the color and anthocyanin content of red leaf lettuce, scientists conducted a multi-factorial experiment with six contrasting high tunnel coverings. The randomized complete block design included two replications of each covering type, and the data were statistically analyzed to reveal significant differences among attributes. Color intensity and initial crispness accounted for 73% of the variance observed between the different coverings. Lettuce grown under diffuse, standard, and clear coverings grouped together with the highest color intensity scores, while the shade group had the lowest. The other three covering types (movable, block, and standard) fell into the second cluster.

Sensory evaluations were conducted on the lettuce produced under each HT covering, and anthocyanin concentration was determined by spectrophotometer. The flavor and mouthfeel attributes water-like, initial crispness, and sweetness registered the highest intensity levels. Astringency, parsley, woody, earthy, and sweet overall flavors exhibited moderate intensity scores.

The results from this experiment provide an understanding of the interaction between PPFD and the anthocyanin content of red leaf, but not green or green/red lettuce. This information can be used to develop a better understanding of the relationship between phenolic content and growing conditions, as well as to produce improved lettuce cultivars with desirable characteristics. Moreover, it is hoped that the results can be applied to other fruit and vegetable crops as well. The results also indicate that the addition of far-red LEDs is an effective and affordable method for increasing red lettuce color under controlled conditions. This can be useful for growers looking to maximize their profitability by improving crop yield and quality.