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In modern controlled environment agriculture, precise regulation of lighting conditions has become an important means to improve crop quality and reduce the accumulation of harmful substances.
LED lighting, with its tunable spectrum and energy efficiency, has shown great potential in the field of plant growth illumination. This article focuses on exploring how LED light quality affects plants, particularly the nitrate content in vegetable crops.
The accumulation of nitrate in vegetables involves complex physiological processes, including the absorption and reduction assimilation of nitrate nitrogen, a process regulated by lighting conditions.
Plants reduce nitrate nitrogen to ammonia through the action of key enzymes such as nitrate reductase, which further participates in amino acid synthesis and transformation. These amino acids serve as substrates for protein synthesis, and based on this foundation, proteins undergo modification, classification, transportation, and storage, jointly constituting the basis of plant life activities.
Excessive accumulation not only affects the nutritional value of food but may also pose potential threats to human health. Scientific research has revealed that light quality at different wavelengths has significant regulatory effects on plant carbon-nitrogen metabolism processes, with red and blue light being particularly critical.
Next, we will explore how LED light sources of different proportions and wavelengths effectively reduce nitrate content in vegetables by adjusting nitrate reductase activity, influencing nitrogen absorption and assimilation in plants, as well as the accumulation of related carbohydrates and antioxidants.
The influence of light quality on plant carbon-nitrogen metabolism manifests at multiple levels, wherein different wavelengths of light have significant regulatory effects on photosynthesis, nitrogen absorption, transformation, and utilization in plants.
Red light (wavelength approximately in the range of 600-700 nanometers) enhances plants’ photosynthetic rate.
It is efficiently absorbed by chlorophyll and converted into chemical energy, thereby promoting the assimilation of CO₂ during carbon fixation and increasing the accumulation of carbohydrates in plant tissues.
Plants grown in a red-light environment typically have higher carbohydrate content, which is beneficial for plant growth and biomass accumulation.
Blue light (wavelength approximately in the range of 400-500 nanometers) has a more pronounced effect on plant nitrogen metabolism.
It can directly or indirectly influence the activity of key enzymes such as nitrate reductase, thereby promoting the reduction of nitrate to ammonia, and increasing the availability of ammonia sources for plants.
Blue light also stimulates nitrogen absorption and assimilation in plants, enhancing plant nitrogen metabolism, and thereby affecting the synthesis of amino acids and proteins.
The combination of red and blue light can more effectively regulate the carbon-nitrogen balance in plants.
Red light primarily promotes the accumulation of carbohydrates, while blue light plays a role in nitrogen metabolism.
When both lights act together, they can regulate metabolic pathways within plants, ensuring a more rational allocation and utilization of carbon and nitrogen resources, thereby enhancing plant growth efficiency and product quality.
Blue light can indirectly promote nitrogen assimilation and transport by influencing the intensity of plant respiration, such as enhancing mitochondrial dark respiration and adjusting the enzyme activity in glycolysis and the tricarboxylic acid cycle, thereby indirectly promoting the activity of nitrogen metabolism-related enzymes, thus affecting nitrogen assimilation and transport.
In an experiment conducted by Qi Liandong et al. in 2007, using colored fluorescent lamps to provide red, blue, and yellow light sources, the effects of different light qualities on spinach yield and nitrate accumulation were studied.
The study indicated that, compared to white and yellow light, although biomass was not high under red light treatment, it favored the formation and accumulation of dry matter and carbohydrates. Additionally, it could reduce nitrate content.
In a study conducted by Urbonaviciute et al. in 2007, using fluorescent lamps as a control, the effects of different LED light compositions on lettuce growth and nitrate content were investigated. The compositions tested included 92% LED red light (640nm) + 8% near-ultraviolet light, 86% LED red light + 14% LED blue light, and 90% LED red light + 10% green light.
The treatment with 86% LED red light + 14% LED blue light showed significantly higher sugar content compared to the other two combinations and the control group. However, the sugar content in the other two combinations was significantly lower than that in the control group.
The nitrate content in all three treatments was lower than the control by 15% to 20%. Red light plays a crucial role in stimulating nitrate reductase, while the combination of red and blue light enhances nitrogen absorption and assimilation in plants.
Through optimization of light quality, nitrate content can be reduced by more than 20%. However, there was no significant difference in nitrate content among the three combinations, indicating that red light may play the primary role in reducing nitrate levels.
Effects of Different Light Qualities on Lettuce Quality and Nutrient Uptake
Light Quality | AsA Content(mg/kg) | Nitrate Content(mg/kg) | Calcium(mg/g) | Magnesium(mg/g) | Potassium(mg/g) |
White Fluorescent Lamps | 100.25a | 3500a | 8.42b | 3.61a | 74.7a |
Red LED | 79.00b | 2350b | 8.37b | 3.69a | 75.77a |
Blue LED | 93.25b | 3710a | 9.88a | 3.48a | 72.48a |
Red + Blue | 103.25a | 2174b | 8.36b | 3.72a | 78.32a |
Comparing the experimental data, it is evident from the graph that the LED red light treatment significantly reduced the AsA content in the tested loose-leaf lettuce variety compared to the control. LED blue light and LED red-blue light did not affect the AsA content.
In comparison to the control, the LED red light treatment significantly decreased the nitrate content in the tested loose-leaf lettuce variety, while LED blue light did not affect nitrate content in lettuce.
The LED red light treatment also led to a reduction in calcium content in the leaves of the tested variety compared to the control, although the difference was not significant.
The calcium content in the leaves of loose-leaf lettuce reached its maximum under LED blue light treatment, significantly higher than the control, while the calcium content in the leaves of the tested variety under LED red-blue light treatment showed no difference compared to the control.
Different LED light qualities had no significant effect on the total magnesium and potassium content in the leaves.
Samuoliene et al. (2011) conducted a study on the effect of LED supplementary lighting on three lettuce varieties grown under high-pressure sodium lamps (16-h) in a greenhouse.
Three days before harvest, supplementary lighting with 638nm 300umol/m2·s LED red light for 16 hours significantly reduced nitrate content in red and light green lettuce by 56.2% and 20.0%, respectively, but increased nitrate content in pale green lettuce by 6 times.
LED supplementary lighting increased total phenolic content (52.7% and 14.5%) and free radical scavenging capacity (2.7% and 16.4%) in red and pale green lettuce, respectively, but decreased in green lettuce. After treatment, only the AsA content in red lettuce significantly increased (63.3%).
In summary, the research on the impact of LED light quality on plant nitrate content suggests a straightforward analogy: different colors of LED lights act like various ingredients in a customized nutritional meal plan for plants, each with unique effects on growth and nutrient composition.
Interestingly, the proper combination of red and blue light resembles a carefully crafted dish, which can more efficiently induce plants to reduce nitrate content.
However, when it comes to the task of reducing nitrate content, it seems that red light takes the lead. Moreover, different light qualities have distinct effects on other nutritional components in plants, such as vitamin C (Ascorbic Acid), calcium, magnesium, and potassium content.
This indicates that the choice of light quality is indeed a technical matter, requiring flexible adjustments based on the specific needs of the plant.
Based on the above experimental data, the influence of red light on lettuce’s antioxidant capacity reveals the impact of light quality on plant physiological metabolic processes.
However, the effect of red light supplementation varies depending on the variety, with each variety’s sensitivity to the light environment determined by the accumulation level of antioxidant substances in lettuce leaves.
As for the application of LED supplementary lighting, it is akin to providing plants with strength training. Especially with red light supplementation, it can effectively reduce nitrate content in certain lettuce varieties before harvest while enhancing antioxidant capacity.
However, this does not apply to all varieties, indicating that plants’ requirements for the light environment vary depending on their characteristics.
Therefore, overall, the use of LED light sources with different light qualities allows us not only to regulate the nitrate content of plants but also to improve the overall quality and nutritional content of plants by optimizing the light environment.
This undoubtedly provides modern agriculture with another new tool and a new approach to precision management.
From custom light planning, to tailored quotes, and everything in between, our team of horticulture experts are always ready to assist.
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