How LEDs Impact Fruit Quality Development

• LEDs have become popular in horticulture grows due to their narrow spectrum and low thermal effect. As a result, they are being increasingly used for fruit quality development..
• LEDs impact on fruit quality proves to be more effective compared to the application of conventional white lights.
• Red, blue, and green LEDs are the crucial wavelengths for fruit quality development. Light intensity is vital for photosynthetic rate and timing to decide the fruit quality effect.
• LEDs increase fruit dry matter, soluble sugar, color, firmness, ripening, flavor, nutrition value, and safety.

Vertical and indoor farming is expected to grow by a CAGR of 24.13% from 2023-2027 and will be worth US$15.43 billion. Similarly, greenhouse cultivation worth US$26.69 billion in 2022 will grow at a CAGR of 10.6% up to 2027 to reach US$49.26 billion. These industries rely heavily on Light-emitting diodes as horticulture grow lights. Find out how this trend is affecting fruit quality.

What Makes LEDs Suitable for Horticulture?

Light-emitting diodes (LEDs) are preferred over traditional monochromatic lights since they have a narrow spectrum, minimal thermal effects, longevity, and low energy use. Conventional lights have a broad light spectrum and produce excessive heat that negatively impacts plant growth since they can contain ultraviolet (UV) or infrared (IR) radiation. LED light’s advantages are as follows:

• Narrow spectrum: LEDs emit light in a narrow band with varying intensity. Using color bands suitable for specific plant functions makes it possible to manipulate and optimize crop production, protection, and quality control. Horticulturists and scientists mix light colors in various intensities to customize LEDs for different species and cultivars for multiple purposes.
• Low thermal properties: Since the LEDs produce less heat, they can be integrated within closed or within-canopy systems of plants without harming them.
• Reduce energy use: LEDs can achieve significant energy consumption reduction. LED grow lights use 40% less energy than conventional light. Even in colder regions, where 9–49% power is necessary for heating greenhouses using LEDs, it still results in a 10–25% reduction in energy use (see Figure 1). Moreover, the lights come in a convenient casing and are easily integrated with smart electronic systems to reduce energy use.

Figure 1: Comparison of energy use by conventional and LED lights for horticulture, Katzin et al. 2021. (Image credits: https://doi.org/10.1016/j.apenergy.2020.116019)

Important Light Characteristics

Two characteristics are crucial while planning grow light recipes- quality and quantity. These factors will interact and also determine their relative importance.

• Light quality is the wavelength or color of light. The entire light spectrum is not helpful for plants. The bands of interest for plant physiology lie between 300 nm and 1,000 µm. The useful spectrum includes photosynthetically active radiation (PAR), IR, and UV. The most significant segments are blue (400−500 nm) and red (600−700 nm). Only blue or red would not be enough, a mixture is needed. Other wavelengths of interest are the far red and green (500–600 nm).
• Quantity is the intensity of light that is usually measured as total photosynthetic photon flux density (PPFD).

Light quality is more important when light intensity is less (1 to 2 μmol·m–2·s–1) than high. However, plant responses to light spectrum and intensities differ depending on species, growth stage, and growing conditions.

How the two light attributes can influence fruit sensory, nutritional, and physiochemical parameters is discussed below.

Fruit Physiochemical Quality Parameters

The internal physiochemical parameters determining fruit quality are dry matter content, soluble sugar content, and titrable acidity. All of them can be improved with LEDs.

LEDs Increase Dry Matter Accumulation

Figure 2: “Dry matter content of pepper fruit samples from ‘Eurix’ (panel A) and ‘Gina’ (panel B), respectively, grown under lighting treatments with different percentages of green light,” Lanous et al. 2022. (Image credits: https://doi.org/10.1016/j.scienta.2022.111350)

Artificial light from LEDs can supplement natural light in greenhouses or be the sole source in indoor vertical farms. In both cases, LEDs increase dry matter by increasing overall plant photosynthesis.

Light intensity will increase the photosynthetic rate, regardless of wavelengths. Light quality responses vary. Red, blue, and green light are essential for photosynthesis. There are plant photoreceptors for red and blue, the chlorophyll a and b molecules; however, a specific photoreceptor for green has not yet been discovered. Red light resulted in the highest carbon dioxide (CO2) assimilation, followed by green and blue light.

Plants absorb more red and blue light and green only in small amounts. Red and blue wavelengths are absorbed in the top 20% of the leaf tissue; green goes deeper, distributes evenly, and is absorbed more in tissues deeper than 250 µm.

Greenlight also penetrates deeper into the canopy and increases photosynthesis in areas with less sunlight. For example, Lanoue et al. (2022) found adding green to red and blue light for pepper increased light intensity availability by 43-158% in the lower canopy.

When chlorophyll a and b absorb red and blue wavelengths in both cases, the light goes deeper into the canopy, and tissue is rich in green light.

Any additional CO2 fixation increases the whole plant photosynthesis to benefit fruit quality. CO2 fixation is higher at low intensity of light. Since fruits are sinks for photosynthates and their derivatives, fruit weight increases depending on species and cultivars. For example, in the pepper experiment, fruit weight increased by 2-15%, and dry matter, the sum of all solids minus water content in fruits, increased linearly with the percentage of green light used, see Figure 2.

Higher dry matter content increases the taste and nutrition quality of fruits. Dry matter is increasingly recognized as one of the most vital attributes defining fruit quality, so this influence by LEDs over conventional monochromatic light is significant.

Soluble Sugars Content

LEDs indirectly increase soluble sugar content (SSC) by increasing dry matter content. In climacteric fruits, starch, part of dry matter, is converted to SSC during ripening. A higher dry matter content leads to more SSC.

There is also a direct influence of LEDs on SSC.

Besides its use in CO2 assimilation, solar radiation can influence other light-dependent plant physiological processes. Light influences plant absorption of water and nutrients, plant metabolic processes, and metabolite production.

These influences occur through photoreceptors, red/far-red light-sensing phytochrome, and blue light-sensing cryptochrome found in leaves and fruits. Red and blue LEDs promote protein accumulation in glucose metabolic pathways compared to white light. As a result, fruits can increase glucose, fructose, and sucrose, the crucial sugars making up soluble solid contents. Also, combined with ambient light, blue, red, green, red:blue, or red:far-red LEDs increase soluble sugar accumulation.

Varying light recipes using different ratios of colored LEDs can produce varying amounts of SSC. Therefore, it is possible to manipulate LED lights towards a targeted SSC.

Titratable Acidity

Organic acids are another vital group of compounds for fruit quality as they influence taste and flavor. As fruits ripen, the acidity content decreases, and the fruit grows less sour. However, a fruit without any acidity is also not tasty.

LEDs increase the production of organic acids. Blue, red, green, red: blue, or red:far-red LEDs, in combination with ambient light, increase organic acid accumulation in various crops. For example, high-intensity blue LEDs in tomatoes stimulate antioxidants and improve ascorbic acid content. While red LEDs increase phenolic acid.

Appropriate LED lighting, therefore, can enhance the physicochemical quality of fruits.

Nutritional Quality Is Boosted

Nutritional quality has increasingly also become essential in marketing fresh produce. Different LEDs wavelengths have great potential in increasing various bioactive compounds and antioxidants that make fruits nutritious.

The quality of light is vital to the accumulation of primary and secondary metabolites in plants.

• Primary metabolites: Combining blue and red LEDs increases the accumulation of primary metabolites plants need for their growth and development, like carbohydrates, amino acids, proteins, organic acids, nucleic acids, sterols, and esters. Accumulation of primary metabolites could occur by LED preventing translocation of photosynthates.
• Secondary metabolites: These compounds are not evenly spread in all plants and can be species-specific. They aren’t necessary for growth and development but are produced to perform particular plant functions, like plant defense, pigmentation, fragrance, etc. These can be vitamins, ascorbic acid, phytic acid, carotenoids, flavonoids, and polyphenols, which increase with single spectral blue or red LEDs compared with white light. Red LEDs are better than blue for anthocyanin accumulation, while blue, red, green, red: blue, or red:far-red LEDs enhance the accumulation of organic acids, vitamin-C, phenolic compounds, α-tocopherol, and nitrates, in different crops, see Figure 3. Secondary metabolite accumulation could result from plants trying to protect themselves from ionizing radiation from light, especially UV light.
• Antioxidants: Single or mixed spectral LEDs also trigger antioxidant accumulation in plants. Blue and red increase antioxidants in peas, tomatoes, etc. Green, yellow, red:white LEDs also increase antioxidant properties.

Figure 3: “Effect of LEDs on (A) bioactive compounds production and  (B) disease resistance against different pathogens,” Adapted from Hasan et al. 2017. (Image credits: https://doi.org/10.3390/molecules22091420)

The light recipe of single or mixed colors will vary depending on the species and the phytochemicals that have to be enhanced. Single spectral blue or red LEDs increase anticancer compounds like paclitaxel and baccatin in grapes, or blue LEDs can increase total ginsenosides from 2 to 74% in ginseng plants.

Besides light quality, the timing of light exposure can change the composition of metabolites produced by LEDs. As Figure 4 shows, exposure for three hours to a combination of red and blue LEDs in the morning and evening can produce different quality tomatoes. There is a difference in the compounds or their concentrations.

• Morning LEDs exposure increased the nutritional value of tomatoes by increasing organic acids, long-chain alkanes, amino acids, phenolic acids, carotenoids, and ammonia aromatics.
• Evening LEDs exposure increased flavor quality due to more sugars, amino acids, long-chain alkanes, carotenoids, flavonoids, ammonia and benzene aromatics, and fewer minerals.

There was no difference in organic acids and phenolic acids content.

Figure 4. “Effects of LED supplementation on fruit quality of tomato in the morning and evening. Red represents an increase in the substance content, orange also represents an increase in the substance content, but the increase is weaker than the red box, and blue represents a decrease in the substance content,” Wang et al., 2022. (Image credits: https://doi.org/10.3389/fnut.2022.833723)

Sensory Parameters Improve

LED grow lights enhance sensory qualities like fruit size, external and internal color, firmness, flavor, and ripening.

• Fruit Size: Since LEDs increase photosynthesis, the size of fruits is also increased by red, blue, and green colors by providing more biomass.
• Ripening: Red and blue LEDs increase ethylene production and respiration rate, boosting ripening. LEDs also increase melatonin levels, which help in the ripening process and carbohydrate accumulation.
• Firmness: The improved ripening red and blue LEDs make fruits softer and better suited for consumer taste.
• Flavor: The light and dark phases influence fruit flavor development as hormone composition can alter. Therefore, flavor consistency can be controlled by adjusting light recipes and timing of exposure after understanding the hormone compositions and concentrations at different times of the day. For example, in Figure 4, evening exposure to LEDs improves the flavor of tomatoes.

Growing fruits and vegetables in winter results in less flavor than outdoor summer crops. Using LEDs over the plant life cycle to increase sugars, organic acids, and the metabolites contributing to aroma can increase and maintain flavor standards. For example, shifting wavelengths to red and far red improves bio-compound concentrations that enhance flavor in tomatoes, strawberries, and blueberries.

• Color: Ripening processes will improve color. LEDs also directly improve internal and external color by increasing the accumulation of pigments. Red and blue LEDs increase lycopene in tomatoes; and anthocyanins and flavonoids in other fruits.

Plant Defense

LEDs improve plant defense against a wide range of plant pathogens. Red, blue, and green grow lights can induce disease resistance and protect fruits from infection and microbial spoilage to maintain fruit quality, see Figure 3.

Red lights are known to inhibit lesion development and increase the expression of defense genes; red color also stimulates the synthesis of stilbenic compounds, which are involved in plant defense responses. Other wavelengths can also help to activate defense-related genes.

Preventing diseases improves food safety and limits premature senescence of fruits.

The Right Light Recipe

LED light recipes customized for species and cultivars are crucial in growing crops in greenhouses, vertical farms, and urban indoor farms to extend growing possibilities spatially and temporally. LEDs impact on fruit quality and plant growth is more significant during winter than in summer.

Researchers and horticulturists developing the optimal light recipe will find precision tools like Near Infrared spectroscopy based Quality Meters produced by Felix Instruments Applied Food Science interesting. IN REAL-TIME, the F-750 Produce Quality Meter and F-751 series make nondestructive estimates of dry matter content, SSC, titrable acidity, and internal and external color, allowing people to track fruit quality development due to LEDs.

Source

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