Testing the Effects of Light Quality on Red Lettuce

Health-conscious consumers increasingly demand nutritious, high-quality food, causing stakeholders to closely monitor the bio composition of food products. Phytochemicals with antioxidant properties are some of the most sought-after nutraceuticals. 

Phytochemical levels can be, in large part, determined by light quality, which is necessary for crop productivity. Therefore, a study wanted to find what effect light-filtering by shades has on lettuce. 

Appearances Matter

Lettuce (Lactuca sativa) contains two main types of antioxidants- phenolic acids and flavonoids (anthocyanin and quercetin). These are also the chemicals that make the lettuce red, adding to consumer appeal.

Both classes of phytochemicals also protect the plants from intense solar radiation, while phenolic acids make the leaves bitter and astringent and defend the plant against pest attacks.

Growers use high tunnel (HT) systems to protect lettuce crops from extreme environments to extend growing time, as they are more cost-effective than greenhouses. However, the material used for the HT systems alters the microclimate and changes crop productivity.

Environmental factors can also affect the accumulation of phenolic compounds. For example, the UV light band of solar radiation and cooler temperatures enhance anthocyanin synthesis and make lettuce redder. Since HT materials change the quality of light they transmit, they will also affect phenolic compounds accumulation.

Gude, Talavera, Sasse, Rivard, and Pliakoni, a team of horticulturists and food scientists from the Kansas State University, tested different HT coverings to find what influence the material had.

Testing the HT coverings 

The scientists used four HTs, which were 39.6 m long × 3.7 m wide × 2.1 m high. Each tunnel was a replicate and had six plots randomly allocated with different coverings. Open fields adjacent to each HT tunnel acted as control plots. The six coverings used were:

1. Standard polyethylene film (Standard) with 92% PAR (photosynthetically active radiation) transmission that blocked ultraviolet light (wavelengths lower than 350 nm). 
2. Movable standard polyethylene film covering (Movable), which was removed two weeks before harvest.
3. Diffuse polyethylene (Diffuse), which eliminated infrared radiation and blocked UV radiation. 
4. Clear polyethylene (Clear) without any UV inhibitor.
5. UV-A/B block polyethylene (Block), which blocked UV rays below 400 nm.
6. A 55% shade cloth with standard poly underneath to reduce temperatures. 

The scientists experimented with the lettuce variety ‘New Red Fire.’ They germinated the seeds, then transplanted seedlings after four weeks and grew them under the same agricultural practices. 

Sensory Profiles

As a first step, the team of scientists developed sensory profiles, defined not only by color, but also by the flavor, texture, and mouthfeel characteristics of red lettuce.

Sample collections were carried out on six separate days. 

• Two samples of full-grown lettuce were harvested minus the root system from each plot on the same day. 
• The lettuce from the first two collections was used for orientation and lexicon development. 
• The other four harvests of three lettuce plants were used to evaluate the samples based on the newly developed lexicon of sensory attributes.

A trained panel of five (four women and one man), who had previous experience with sensory testing of vegetables were selected to evaluate the samples. The panelists had at least 2000 hours of sensory testing experience and 120 hours of training.  

Four to five samples of leaves from each of the three lettuces were used for evaluation. Each panelist evaluated one leaf for flavor, texture, and mouthfeel.

The scientists developed a lexicon, consisting of 20 attributes for appearance, flavor, texture, and mouthfeel for red lettuce.

Microclimate

Next, the scientists measured the effect of coverings on microclimate or soil temperature. They then evaluated the microclimate impact on leaf color and anthocyanin accumulation. 

To do this, they measured UV radiation, PAR, and soil and canopy temperatures inside the HT tunnels. Sensors connected to a data logger were used for making temperature measurements for 12 days. 

PAR measurements (μmol/m2/s) were made three times during the experiment, each time between 10:00 and 13:00 on cloudless days using the CI-340 Handheld Photosynthesis System. The CI-340 is a handheld open gas exchange system, manufactured by CID Bio-Science. Three photosynthesis measurements were made for one leaf from each plot. This was always a healthy leaf from the lowermost whorl from lettuce in the center of the plot. 

Color Assessment

Red lettuce’s color was measured by a colorimeter on two sides of the midrib. The leaf contents of three lettuce were lyophilized and the anthocyanin content was estimated by a spectrophotometer. 

Coverings Influence Different Sensory Parameters

The six different coverings produced lettuce that differed in eight attributes- 

• Color intensity 
• Parsley
• Water-like 
• Woodiness 
• Toothetch 
• Sweetness 
• Astringency
• Initial crispness 

Among them, color intensity showed the greatest difference. Lettuce in open fields had the most intense color, followed by clear and movable coverings. Clear coverings let in UV radiation, and the movable coverings exposed lettuce to sunlight two weeks before harvest, allowing for 85% and 100% of PAR exposure, respectively.

Besides light, the lower soil temperature in the open fields also makes lettuce redder.

Colorimetric and spectrophotometric measurements showed a similar trend. Open field and movable coverings with 100% PAR and 100% UV radiation, and the clear coverings, had a lower L*, hue, and chroma values, indicating darker color, and higher a* values indicating more redness. 

As the anthocyanin levels increased the hue values decreased. The panelists gave higher color intensity scores to lettuce that had more anthocyanin, see Figure 1. 

Warmer soil temperatures produced similar initial crispness under the movable, standard, block, and diffuse coverings. Lettuce in these plots had more initial crispness than those since they mature faster. Lower soil temperatures in the open field slow maturation, so the lettuce had softer and less initial crispness than those grown in under the four coverings.

Of the flavors defined in the lexicon, green-overall, green peapod, green-grassy, and green-viney, had low to moderate intensity scores. Flavors, spinach, parsley, woody, earthy, sweet-overall, sour, bitter, salty, and umami (savoriness) registered low intensity with minor differences between the coverings. Lettuce and bitter flavors were moderate. Only overall sweetness showed a difference due to the coverings. Red lettuce under diffuse coverings was sweeter than standard, shade, and open field lettuce.

Among the mouthfeel attributes, “water-like” was most significant and higher for standard covering compared to the others. Lettuce under all coverings had similarly low values for mouthfeel attributes of toothetch and metallic. Though astringent values were also low, open fields had significantly higher values than the block and clear coverings.

The three attributes that differed significantly, color, water-like, and initial crispness, were responsible for the two clusters seen in Figure 2. Lettuce grown under open, clear, and movable coverings had the most color intensity. The second group of diffuse, standard, and block coverings had more initial crispness and waterlike attributes. Shade had low color and anthocyanin content because they had only 40% PAR and 7% UV radiation. 

The changes in spectral quality and UV radiation coverings influenced color intensity the most, which accounted for 73% of the difference seen in lettuce. Texture and mouthfeel parameters were the two other parameters that differed due to the type of covering and accounted for 13% of the difference seen. 

Since these three parameters, color, texture, and flavor, are important for consumers, the choice of coverings used for HT does matter. 

More Anthocyanins in Plants are Good 

Among the parameters, color matters the most, as it influences the appearance and also the concentrations of the healthy antioxidant anthocyanins. Therefore, clear and movable coverings would be most useful for HT tunnels and greenhouses in the case of the cultivar ‘New Red Fire.’ However, the responses of lettuce to coverings will be cultivar-specific, so HT cover recommendations could differ for other varieties of lettuce.

Food crop studies like this, using advanced technology, such as IRGA photosynthesis devices, can pave the way to a better understanding of our food supply. By measuring plant parameters effectively, we can achieve better quality and nutritional outcomes for all people.

—

Vijayalaxmi Kinhal
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture

Read the peer-reviewed publication for more details on the research: 

Gude, K., Talavera, M., Sasse, A.M., Rivard, C.L., & Pliakoni, E. (2021). Effect of Light Characteristics on the Sensory Properties of Red Lettuce (Lactuca sativa). Foods, 10, 2660.

https://doi.org/10.3390/foods10112660