Leaf Lettuce, is one the most common choice for growing under grow lamps as it is a high-value vegetable. The effect of red, blue, green, white and yellow light on its morphology and growth varies at different stages of the plants’ life-cycle.
Light affects many important processes in plants. In leafy greens, the effect of light on germination, photosynthesis, shade avoidance, vegetative growth, and leaf shape and size is important. In addition to morphology, light can also influence the secondary chemicals produced in the plant. In the case of lettuce, the amount of antioxidant phenolic compounds is important (1, 2).
Using LED lighting has many advantages over other light sources, in closed and indoor plant production systems. Its low gradient heat output is important in terms of influencing plant processes. Since LEDs are available in narrow wave bands, it is possible to manipulate the environment to optimize plant characteristics. The ratios in which different colors are used have an important effect on crop yield and quality (1).
Far-red light should not be used during germination of lettuce. It has been found that red light is much better than far-red to ensure more and quicker germination in case of lettuce. Far-red light tends to inhibit seed germination (3, 4).
Vegetative Growth and Weight
Since lettuce is a leafy plant, its vegetative growth has been extensively researched. Thus we know that there are specific ratios of red and blue light that can be used for different stages of the lettuce’s lifecycle in order to maximize its growth. Light requirements can even differ through the day.
During the younger seedling stage, more blue light promotes growth. Prolonged use of only blue lights is, however not suitable (5). Using no blue light produced lettuces that were four times heavier than those produced with 59% blue light after 4 weeks of treatment (5).
It was found that increasing red to blue ratio, i.e., increasing red light produced better vegetative and root growth, and correspondingly increased the weight of the lettuce. Red light increases the height of plants. Lettuces grown under 100% red color were around three times (2.6-3.4) large than the lettuce grown under 100% blue light (1).
Increasing blue LED lights had no influence on leave shape. On the other hand, increasing red light produced longer and larger leaves (1).
A combination with equal blue and red light can increase both lettuce weight and leaf area, which could be the most desirable pick. Some people recommend adding a far-red component as well to increase the weight of lettuce (6).
Adding far-red to the combination, can further increase leaf area and therefore the fresh weight of lettuce (7). The same amount of far-red can have different effects when used in combination with different ratios of blue-red, and the variety of the lettuce.
More and more colors are gradually being used in different combinations. Green light has been found to reduce photosynthesis (6). However, it seems to produce no ill-effects. Applying green light has to be found to increase leaf area, plant growth and fresh weight of the whole plant (8).
Latest recommendations include adding some yellow and white light to optimize lettuce crops (9). There is a difference of opinion in terms of yellow light, where some say it can slow down the plant growth. However, used in combination with blue and red, lettuce growth rates increased two to three times than when only red and blue are used (10).
Moving away from the concept that using particular colors of the spectrum is the best way to maximize yield, people are giving more attention to white light. White light used with red and /or blue can also increase weight and the nutritional value of lettuce (11). Warm white LED lighting supplemented with blue can get the same amount of growth and quality compared to high-pressure sodium lighting (10).
Different colored lights can influence the color of lettuce. Far-red is detrimental to pigment accumulation, so red lettuce can turn completely green. Adding more blue and UV radiation, on the other hand, can turn plants completely red from green (7).
Blue light is conducive for the formation of chlorophyll that ensures better photosynthesis. Red light is necessary to break seed dormancy that can lead to germination (12). Moreover, a combination of mixed blue and red LEDs are more efficiently absorbed by the leaves and lead to better net photosynthesis. So a combination of blue and red LEDs are good to stimulate germination. It is important to note that the amount of red light should not be used in excess as it can negatively affect both the photosynthetic rate and chlorophyll formation (4).
Chlorophyll and Antioxidant Phenolic Production
Besides photosynthesis, light regulates other secondary compound formation in the plant. These can, in turn, affect plant growth and its quality.
Increasing blue LED can result in plants with greater concentrations of chlorophyll, antioxidant phenolics, and flavonoids. These are one of the most common compounds in plants and are important since they can prevent diseases such as cancer and atherosclerosis. Since people consume greens to improve their health, A high concentration of these beneficial compounds in leafy greens is desirable (1, 2). It is also possible to increase the amount of many vitamins by using a mixed red-blue spectrum (10).
The concentration of these compounds depends not just on the amount of light but also on the variety of lettuce. For example, the amounts of the photo chemicals measured in red lettuce are more at all of the ratios of blue and red LED lights, than in green lettuce (1).
Besides photosynthesis, plants also use light to regulate other secondary chemical pathways in the plants. So UV radiation is necessary to start protective mechanisms against stress in plants (7).
More is Better
Though growers are interested in increasing the weight and size of lettuce, using only red light, would not be the ideal solution. Red light does increase the weight of lettuce, but used alone it can produce abnormal leaf shape. Moreover, it decreases the amount of polyphenolics and antioxidant levels. So using a combination of colors is recommended to improve both the quality and yield in closed plant production systems. Pulsed LEDs had similar effects on lettuce growth as continuous LED lighting, so energy and money can be saved by using them instead of continuous LED (8).
- Son KH And MM Oh. 2013. Leaf Shape, Growth, and Antioxidant Phenolic Compounds of Two Lettuce Cultivars Grown under Various Combinations of Blue and Red Light-emitting Diodes. HortScience.
- Han T, et.al. 2017. Improving “color rendering” of LED lighting for the growth of lettuce. Nature.DOI: 10.1038/srep45944
- Son KH, Jeon YM and MM Oh. 2016. Application of supplementary white and pulsed light-emitting diodes to lettuce grown in a plant factory with artificial lighting.Horticulture, Environment, and Biotechnology 57: 6.https://DOI 10.1007/s13580-016-0068-y
- doi: http://dx.doi.org/10.1101/215095
- Hytönen T, et. al. 2017. Effects of LED light spectra on lettuce growth and nutritional composition. Lighting Research and Technology. https://doi.org/10.1177/1477153517701300
- Muneer S et al. 2014. Influence of Green, Red and Blue Light Emitting Diodes on Multiprotein Complex Proteins and Photosynthetic Activity under Different Light Intensities in Lettuce Leaves (Lactuca Sativa L.). International Journal of Molecular Sciences. 15: 4657–4670. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3975419/
- Contreras S, et. al. 2009. Red to Far-red Ratio During Seed Development Affects Lettuce Seed Germinability and Longevity. HortScience February 44:130-134
- Shimizu H et. al. 2011. Light Environment Optimization for Lettuce Growth in Plant Factory.IFAC Proceedings 44: 605-609, https://doi.org/10.3182/20110828-6-IT-1002.02683