Citation
Meng, Q., J. Boldt, and E.S. Runkle. 2020. Blue radiation interacts with green radiation to influence growth and predominately controls quality attributes of lettuce. Journal of the American Society for Horticultural Sciences 145(2):75-87. https://doi.org/10.21273/JASHS04759-19
With recent shifts in modern agriculture to more urban environments, indoor farming has become increasingly popular. Such an environment allows growers to control everything from air flow to water. Arguably the most important aspect that can be controlled is lighting, specifically with the use of light-emitting diodes (LEDs) that allow for customizable wavelengths for varying stages in a plant’s life cycle. Research has shown that different wavelengths can produce different results. For instance, exposing certain varieties of lettuce to blue radiation (400-500 nm) has been tied to a significant reduction in biomass weight, as well as an increase in the production of secondary metabolites. Growers will often combine blue with red and far-red radiation (600-800 nm) to achieve desired results. Green radiation (500-600 nm), however, does not have much of a history of being used by growers, despite its ability to penetrate deep into the leaves. This has huge implications on its ability to drive photosynthesis and has recently been studied as a substitute for blue radiation. Previous research on this subject has shown an increase in biomass of several lettuce varieties, but the authors believe this could have been attributed to the shifting levels of blue radiation. To combat this, they designed a new experiment to keep levels of blue radiation constant and substitute red radiation for green.
In this experiment, researchers focused on ‘Rouxai’ red leaf lettuce and tested the effects of varying wavelengths using LED lighting. During the light quality treatment phase of the experiment, each treatment had a 20 hour photoperiod and a total photosynthetic photon flux density (PPFD) of 180 μmol m-2 s-1 . They exposed the lettuce to nine different treatments of lighting, including combinations of blue and red radiation, as well as introducing green radiation at a photon flux density (PFD) of 60 μmol m-2 s-1 in place of a reduction in red radiation. Researchers measured biomass accumulation including fresh and dry mass, different morphological features such as plant diameter and leaf number, and coloration of the foliage. They found that at 20 μmol m-2 s-1 of blue radiation, the presence of 60 μmol m-2 s-1 green radiation increased the fresh mass of lettuce, but had negative effects on the weight at any higher levels of blue radiation. Additionally, an increase in blue radiation with 60 μmol m-2 s-1 of green radiation decreased leaf diameter, width, and length. With an increase in blue radiation, lettuce is known to have an increase in red color of the foliage. Without green radiation, the foliage became saturated at 20 μmol m-2 s-1 of blue radiation. However, with 60 μmol m-2 s-1 of green radiation, the saturation point increased to 60 μmol m-2 s-1 of blue radiation. This has interesting implications for growers wishing to increase the coloration of their foliage.
This study provides valuable information regarding the role green radiation may have in a plant’s life cycle. With an increase in fresh mass at low levels of blue radiation, incorporating green radiation at the right stages could potentially increase yield for growers. Additionally, the presence of green light has been shown to vary depending on the species and age of the crop, which implies further research is needed on the subject. Having this research and knowledge that green radiation influences photosynthesis and other varying characteristics of lettuce growth is critical for growers looking to optimize their lighting in controlled environments.
Knowing that some growers only implement blue radiation toward the end of a crop’s grow cycle to produce more color in the foliage (but maintain biomass early on), I’m curious to see where the research might point to regarding green radiation. Based on this paper, it seems like it might be most beneficial during early stages of production when blue radiation is low, but I’m sure there’s a lot of room for research here.
I’m hoping that there are some follow up studies using green radiation that simulate what growers do by adding more blue in at the end of the cycle to really increase that nice red color in the lettuce.
Figure 2 in this paper is such a great visual for how each treatment affected the lettuce size and color. From that, you can really see that 20Blue adds some red color to the lettuce but slightly decreases the size, however, that size decrease can seemingly be combated by adding green into the light with a slight reduction in the lettuce’s red color. So it stands to reason that maybe a light ratio with green, red, and low blue can keep the size high while still adding some red color, but then maybe switching to more blue at the end of the cycle could increase the red color in the lettuce even more. It’s such a simple thing to be able to change but it has such dramatic effects which is so interesting!
It’s interesting that different wavelengths affect specific crops differently. When reading about this, I kept thinking of thermal imaging and its use in precision agriculture. Does anybody have an idea regarding if (or how) these two could possibly be connected?
The choice and inclusion of the greenhouse treatment among the 10 treatments at first appeared out of place. In this treatment, many variables like nutrients, substrate, air movement, humidity, etc. were changed or not controlled compared to the other 9 growth room treatments. This very likely confounded the effects of lighting and radiation on growth, morphology, and quality of greenhouse-grown plants. Despite its experimental design issue, the greenhouse treatment, which is the more common and practical way to produce lettuce in large scale settings, still provided valuable information. Based on the results on sensory attributes, lettuce grown in the greenhouse tasted more bitter and less sweet. Thus it was less likely to meet buyers’ expectations and to get bought compared to lettuce from some of the growth room treatments. This suggests further experiments may be done to apply different radiation treatments to greenhouse settings or other large scale production system and potentially improve greenhouse-grown produce quality.
I too was surprised by the inclusion of the greenhouse treatment with substrate based culture and different environmental conditions. I wonder if the results in terms of consumer preferences might have been different had the authors used a setup more common in leafy green greenhouses (like nutrient film technique or deep water culture). I also found it very interesting that the consumer preference rating for appearance and color of the lettuce was not significantly different for any of the treatments (including the greenhouse) but ‘willingness to buy’ and meets expectations ratings significantly lower for greenhouse grown lettuce compared the single source lighting treatments. This highlights the importance of considering the effect of environmental conditions such as lighting on more than just visual attributes.
In future studies looking at radiation in CEA, it would be interesting to see how varying the wavelengths of LED lighting may affect different varieties and crops grown in a controlled environment scenario. This paper showed how altering the blue and green radiations had adverse effects on the coloration, mass, and dimensions of the red leaf lettuce variety ‘Rouxai’, which sparked my curiosity and made me think, what would this mean in other lettuce varieties, or crops commonly grown in greenhouses such as tomatoes or cucumbers? How would it affect their coloration/growth/taste/yield? I’m curious to see if there are future studies in this area, and how they could benefit controlled environment settings in optimizing important factors such as lighting.
It would be interesting to see how breeding programs are affected by a growing use of LED lights for growing, especially with how variations in light wavelengths affect lettuce flavor. I believe it makes more sense to adjust light to produce favorable flavors and yields than to breed for more flavorful and higher-yielding crops under different LED lights, but I could see breeders and greenhouse growers meeting somewhere in the middle. I believe some varieties could be developed that produce well under energy-efficient lighting in greenhouse production systems.
Taste and willingness to buy were measured along with demographic data. The authors also asked participants for their frequencies of consuming lettuce, cruciferous vegetables, and coffee. Lettuce consumptions obviously makes sense as a metric for normalizing the willingness to buy statistic. Cruciferous vegetables and coffee are both bitter and would serve as reasonable metrics for normalizing perception of bitterness. However, I cannot find where or if the authors use theses consumption frequencies in their analysis. The description of the statistical analysis appears to leave room for the reader to guess which terms were used in the SAS statistical models.