Control of Flowering Using LED Night Interruption and Day-Extension Techniques

When it comes to growing photoperiodic plants, controlling the timing of flowering is crucial for commercial growers. Plants respond to the length of night (dark period) in each 24-hour cycle, and by manipulating this, growers can encourage or delay flowering. This practice is particularly important for ornamental plants and specialty crops. But how can you adjust this timing in a way that’s efficient, cost-effective, and sustainable? LED lighting is stepping up to the challenge, offering growers a more energy-efficient and customizable option compared to older lighting methods. Let’s explore how night-interruption (NI) and day-extension (DE) LED lighting techniques work and how they can help you achieve the perfect flowering schedule.

Understanding Flowering and Photoperiod

First off, flowering in many plants is regulated by the length of the night. Long-day plants (like many ornamentals) need shorter nights to bloom, while short-day plants flower when the night is longer. This process is known as photoperiodic signaling and it’s triggered by low light intensity.

There are two key ways growers manipulate this:

1. Day Extension (DE): This involves extending the daylight after sunset.
2. Night Interruption (NI): A technique that breaks the long night by adding a pulse of light during the dark period.

Both methods fool the plant into experiencing shorter nights, which either promotes or delays flowering, depending on the species.

Old-School Lamps vs. LEDs

Before LED lighting became a game-changer, conventional lamps like incandescent (INC), halide, and compact fluorescent (CFL) bulbs were commonly used. These older technologies had some major drawbacks:

• They were energy inefficient.
• They had a short lifespan.
• They emitted light at wavelengths that weren’t always effective for flowering.

LED Lighting, on the other hand, has several advantages:

• Longer lifespan.
• Greater energy efficiency.
• Ability to emit specific wavelengths that directly impact plant growth and flowering.

Now let’s dive into how these LED systems are transforming the way we control flowering.

How LEDs Regulate Flowering: The Science

When it comes to photoperiodic control, specific wavelengths of light (think red, far-red, and blue) are key to signaling flowering. Plants have photoreceptors like phytochromes and cryptochromes that absorb these wavelengths and trigger responses in the plant, including flowering.

Long-Day Plants

For long-day plants, flowering is typically accelerated by a combination of red (R) and far-red (FR) light. Red light is absorbed by phytochromes, but when far-red is added, the process speeds up, making it highly effective for promoting flowering. Interestingly, red light alone can also be enough for some plants, though this might result in more compact plants—ideal for growers who want to limit stem elongation.

Tip:

• Want compact plants? Try using red + white LEDs. These are effective for flowering but reduce unwanted stem growth.

Short-Day Plants

In contrast, short-day plants need long, uninterrupted nights to flower. Here’s where red light comes in handy again: using a brief pulse of red light during the night can interrupt the darkness and prevent flowering.

Tip:

• Want to delay flowering in short-day plants? Use red LEDs during the night to extend the day length artificially.

Night Interruption vs. Day Extension: Which is Better?

Both NI and DE lighting techniques have their advantages, and which one you use depends on your crop and setup.

• Night Interruption (NI): A brief light pulse during the night can effectively promote or delay flowering in many crops. Usually, 4 hours of NI is enough to influence flowering in long-day plants.
• Day Extension (DE): This method involves extending the natural daylight to about 16 hours. DE is particularly useful for plants that need longer days to bloom.

Interestingly, studies have shown that a 4-hour NI gives a slightly stronger signal to long-day plants than a 5.5-hour DE, although both techniques can be effective.

Tip:

• Choosing between NI and DE? Try experimenting with both to see what works best for your plants. NI might give stronger results in some species, but DE could be more practical depending on your lighting setup.

Why LEDs Are the Future of Flower Control

LED technology isn’t just about saving energy; it also gives you unprecedented control over light quality and timing. You can tailor the spectrum to meet the exact needs of your crops. With LEDs, you’re not just getting light—you’re getting the right light.

Plus, with LED systems lasting longer and getting more affordable, they are quickly replacing older lamp technologies in greenhouses and indoor farms. LEDs with customizable spectra are perfect for research and production in controlled environments, especially when it comes to commercial-scale growing.

Key Takeaways for Growers:

• Use NI and DE techniques to manage flowering times effectively.
• Choose LEDs over traditional lamps for greater energy efficiency and more precise control over light quality.
• Red and far-red LEDs are ideal for promoting flowering in long-day plants, while red light alone can delay flowering in short-day plants.
• LEDs help control not just flowering but also plant morphology, making them a versatile tool for all growers.

Summary (for Canva Infographics):

• What is Photoperiodic Lighting? Using light to manipulate the night length and control flowering.
• Why LEDs? More energy-efficient, customizable wavelengths, longer-lasting.
• Two Key Techniques:
  • Day Extension (DE): Extends daylight after sunset to promote flowering.
  • Night Interruption (NI): Interrupts the night with brief light pulses to influence flowering.
• Best for Long-Day Plants: Red + far-red LEDs accelerate flowering.
• Best for Short-Day Plants: Red light during the night delays flowering.
• LED Benefits:
  • More efficient than incandescent or fluorescent lamps.
  • Reduces unwanted growth (compact plants).
  • Customizable light spectrums to suit specific crops.

This passage discusses how different types of light, particularly specific wavelengths, affect the flowering of short-day and long-day plants. It focuses on the regulatory role of red (R), far-red (FR), blue (B), and green (G) radiation in inhibiting or promoting flowering based on the plant type and intensity of the light used. Some key points include:

1. Photoreversibility in Short-Day Plants: Short-day plants respond to red radiation by inhibiting flowering, but this inhibition can be reversed by subsequent exposure to far-red radiation. This is a function of how light affects the phytochrome photoreceptor system, particularly the conversion between its active (PFR) and inactive (PR) forms.
2. Intensity Dependency: The intensity of blue light (B radiation) plays a critical role in how plants perceive light signals for flowering. For example, in duckweed, B radiation needed to be significantly higher than red or green radiation to achieve a similar inhibitory effect on flowering. High-intensity B radiation also influenced short-day plants by mimicking long-day signals.
3. Effectiveness of Green Radiation: Green light (G radiation) has varying effects on different plant species. Some studies showed that it could inhibit flowering similarly to red light in certain short-day plants. However, its effectiveness depends on factors like intensity and duration, which differ among species. The combination of green and red radiation has been suggested to be particularly effective for inhibiting flowering in some short-day plants.
4. LED vs. Conventional Lamps: LED technology offers advantages in energy efficiency and longevity compared to traditional lamps (incandescent, fluorescent, etc.). LEDs can be customized to emit specific wavelengths, making them particularly effective for regulating plant flowering with less energy waste. Trials comparing LED and conventional lamps showed that LEDs are at least as effective in controlling flowering, especially in combination with red and far-red radiation.
5. Economic and Practical Considerations: Over time, the cost of LED lighting systems is expected to decrease, making them a more economical choice for commercial growers. Their ability to regulate flowering efficiently through tailored light spectra makes them a promising tool for the future of horticultural lighting.

Overall, the passage highlights the complexity of light interactions in plant photoperiodism and the potential of advanced LED systems to improve control over flowering in commercial horticulture.

This document discusses the control of plant morphology through the manipulation of light quality and daily light integral (DLI) using LEDs (light-emitting diodes) in greenhouses. Key highlights include:

1. Daily Light Integral (DLI)

• DLI represents the cumulative amount of photosynthetically active photons received by plants over 24 hours.
• Increasing DLI generally enhances plant growth, biomass accumulation, and overall health.
• Studies show that increasing DLI significantly improves growth attributes such as stem, leaf, and root biomass in plants like geranium, impatiens, and petunia.

2. Light Quality

• Red Light: Essential for photosynthesis, driving biomass increase. However, relying solely on red light can cause undesirable elongation in plants.
• Blue Light: Plays a critical role in inhibiting excessive stem elongation. Combining red and blue light results in more compact and sturdy plants, promoting better quality.
• Far-Red Light: Impacts flowering and growth by affecting phytochrome photoreceptors. A lack of far-red light can delay flowering in some species.

3. LED Advantages

• LEDs allow for specific wavelengths to be selected, enabling precise control of plant morphology and growth.
• They offer energy efficiency by targeting only the necessary wavelengths for growth, compared to traditional High-Pressure Sodium (HPS) lamps.
• LED arrays with a mix of red and blue light are shown to produce more compact plants, improve chlorophyll content, and achieve better overall quality.

4. Supplemental Lighting

• In northern climates with lower natural light during winter months, supplemental lighting becomes necessary for optimal plant growth.
• LEDs, with their wavelength specificity, provide more consistent growth results for crops like ornamental seedlings compared to traditional HPS lamps, especially in terms of stem length suppression and biomass production.

5. Impact on Commercial Greenhouse Production

• LEDs are seen as a future technology with potential to outperform traditional lighting in energy efficiency, lifespan, and plant-specific growth responses.
• Their use in ornamental plant production has shown that LED lighting (with specific red-to-blue ratios) can enhance plant morphology and health, making them suitable for large-scale greenhouse operations during low-light seasons.

This research highlights how manipulating light quality and quantity can optimize plant production in controlled environments, positioning LEDs as a promising tool for future agricultural practices.

The text outlines various studies examining how different light qualities, particularly with LEDs and high-pressure sodium (HPS) lamps, influence plant growth and morphology in greenhouse environments. The key points can be summarized as follows:

1. Propagation Phase Impact: Supplemental light during propagation had little effect on subsequent flowering and plant morphology (e.g., stem length, flower bud number, etc.), likely due to the overwhelming presence of solar radiation in greenhouses. This suggests that background sunlight provides enough intensity to mask significant effects of varying light spectra.
2. Blue and Red Light in Seedling Growth: Studies on tomato and bedding plants, like petunias and geraniums, under various red/blue LED light ratios (e.g., 70:30, 85:15) in greenhouses showed that blue light had limited impact on seedling morphology due to the ample sunlight. However, sole-source LED lighting (without solar input) resulted in more compact plants with increased root mass, darker foliage, and improved space efficiency.
3. LEDs for Plant Growth Regulation: LED lighting can be used to control plant height and morphology by adjusting the red and blue light ratios, potentially eliminating the need for growth regulators. The lack of radiant heat from LEDs also allows close canopy placement, optimizing space usage.
4. Poinsettia Growth with Supplemental Blue Light: Studies showed poinsettia plants grown under LEDs emitting a higher blue light fraction (e.g., 80:20 red/blue) were more compact and had reduced leaf area and dry mass compared to plants grown under HPS lamps.
5. Far-Red (FR) Light Influence: Petunia seedlings grown under supplemental light with different far-red (FR) and blue light treatments demonstrated shorter shoots under red LEDs but promoted flowering and elongation with blue LEDs, particularly under low natural light conditions.
6. Pigmentation Enhancement: Geranium and purple fountain grass exhibited more vibrant foliage color under blue LED light at 100 μmol m² s⁻¹, with the lowest hue values indicating deeper red hues.

The section discusses the use of LED supplemental lighting for vegetable production in greenhouses, particularly during the winter months when natural sunlight is limited. Several key findings and insights from research are highlighted:

1. Marketability and Yield: Supplemental LED lighting has been studied primarily for its impact on yield and marketability in various vegetable crops, such as tomatoes, cucumbers, and lettuce. Specific wavelengths of light, including red and blue light, are used to influence crop characteristics like color and size to increase consumer appeal.
2. Coloration in Lettuce: Research demonstrated that LED light ratios of red and blue, such as 50:50 or 100:0, can enhance the red pigmentation of lettuce varieties like ‘Cherokee’ and ‘Ruby Sky.’ As little as 5-7 days of exposure to these light conditions before harvest can boost anthocyanin production, resulting in darker red foliage, which is desirable for aesthetic and nutritional quality.
3. Hypocotyl Elongation: Studies showed that using red-orange and blue LEDs can reduce hypocotyl elongation (excessive stem growth) in lettuce seedlings compared to HPS lamps. The blue light, comprising 20% of the LED spectrum, plays a significant role in this growth regulation, keeping seedlings compact and market-ready.
4. Internode Length in Tomato and Cucumber: Including blue light in intracanopy supplemental lighting (lighting positioned within the plant canopy) helps reduce the internode length (the distance between leaves along the stem) in tomato and cucumber plants. This compact growth, along with increased fruit coloration, results in better-quality produce.
5. Far-Red Light Influence: Manipulating the red/far-red light ratio also influences crop morphology by reducing elongation and enhancing fruit color, particularly in tomato and cucumber plants. The interaction between intracanopy lighting and natural light during production enables better control of plant growth, despite the ambient sunlight.

Conclusion:

While LED supplemental lighting in greenhouses may have minimal effects when natural sunlight is abundant, it becomes valuable for intracanopy lighting, end-of-production treatments, and sole-source lighting (without natural sunlight). The ability to control specific light wavelengths can lead to enhanced crop quality, compact growth, and increased pigmentation, making LED lighting a versatile tool for vegetable production. As LED technology becomes more affordable and efficient, its adoption in commercial agriculture is expected to grow.

The chapter on supplemental lighting (SL) for greenhouse-grown fruiting vegetables highlights the role of lighting technology in improving crop yield, particularly during periods of low solar radiation. This is especially important in higher latitudes like the northern U.S., Canada, and northern Europe where darker months reduce natural light availability. SL can enhance photosynthesis and crop yields by compensating for insufficient natural light, particularly for crops like tomatoes, sweet peppers, and cucumbers.

Key lighting technologies discussed include:

1. High-Intensity Discharge (HID) Lamps:
   • Includes Metal Halide (MH) and High-Pressure Sodium (HPS) lamps.
   • HPS lamps, more commonly used, provide adequate intensity for photosynthetic activity and are relatively more efficient than MH lamps, converting electrical energy to photosynthetically active radiation (PAR).
   • HPS lamps also provide a thermal benefit in cold weather, reducing greenhouse heating costs.
   • Despite efficiency, their high life-cycle costs and environmental impact are drawbacks.
2. Light-Emitting Diodes (LEDs):
   • LEDs surpass HID lamps in terms of energy efficiency and longevity. As of 2015, LEDs were 50% efficient, surpassing HPS lamps’ 34%.
   • They emit narrow-spectrum light optimized for plant growth and are suited for both overhead and intracanopy lighting due to their cooler operating temperatures, allowing placement close to plants.
   • Overhead LEDs, however, require dense, high-output arrays, which can be costly and inefficient compared to other uses of LEDs.
3. Combination of Overhead and Intracanopy Lighting:
   • Tall crops, like high-wire tomatoes, can benefit from combining overhead and intracanopy lighting.
   • Intracanopy lighting targets lower, shaded foliage, which is otherwise unproductive under purely overhead lighting. LEDs are ideal for this due to their cool photon emission, avoiding the risk of overheating.
   • Studies have shown that intracanopy lighting, often combined with overhead SL, improves light distribution, increases yield, and enhances energy efficiency by reducing electricity consumption.
4. Intracanopy Lighting Alone:
   • In conditions with sufficient sunlight in the upper canopy, intracanopy SL can be used to boost growth in the lower, shaded parts of the plant.
   • This method is especially effective during certain stages of plant development, and research has demonstrated increased yield and fruit quality when intracanopy LED lighting is applied.
   • Intracanopy lighting systems using red and blue LEDs can save significant energy compared to HPS lamps while delivering comparable yield improvements.

The chapter emphasizes that proper SL management, including light intensity, photoperiod, and distribution, is crucial for maximizing crop productivity. Researchers found that SL intensities between 100 to 220 μmol m² s⁻¹ for 12–20 hours per day are optimal for various crops like tomatoes, sweet peppers, and cucumbers. Moreover, the daily light integral (DLI), the total amount of PAR received by the plants, is also a key metric. For tomatoes, a DLI of 25 mol m² d⁻¹ is suggested, and SL systems need to be adjusted to ensure this light requirement is met for consistent and enhanced yields.

Ultimately, the research points towards LEDs, particularly when used in combination with intracanopy systems, as a promising solution for energy-efficient, high-yield greenhouse crop production.

The use of Supplemental Lighting (SL), particularly LED technology, has shown promise in improving the quality and marketability of greenhouse-grown fruiting vegetables, such as tomatoes, cucumbers, and peppers. Beyond increasing yield, SL can enhance key aspects of fruit quality that are crucial for consumer acceptance, such as flavor, sugar content, color, and nutritional value (ascorbic acid, lycopene, and antioxidants like ORAC).

For example:

• LED intracanopy lighting has been proven to increase the sugar content and ascorbic acid (vitamin C) levels in tomatoes, especially in winter. Mini-cucumbers also exhibited improved fruit color, and green peppers ripened faster, allowing growers to take advantage of early-season market prices.

Factors to consider when using SL in greenhouses:

1. Solar radiation: The natural light available at a specific location, affected by the season and greenhouse structure.
2. Light distribution: Effective SL depends on uniform lighting, considering both overhead and intracanopy sources.
3. DLI (Daily Light Integral): Growers should focus on this metric instead of only light intensity or duration. DLI ensures stable production, especially during low-light seasons.
4. Crop type and lighting needs: Different crops have varying light requirements in terms of intensity and duration, making it crucial to customize lighting strategies.
5. Economy: Growers should design SL systems to balance light hours and intensity, making sure the benefits of SL outweigh the energy and installation costs.

Economic Aspects:

• The standard SL source, HPS (High-Pressure Sodium) lamps, has been widely used due to their established cost-effectiveness. However, LEDs offer advantages in energy efficiency by reducing electrical consumption and heat output, allowing closer positioning to plants. Despite their higher initial capital costs, the long lifespan and energy savings of LEDs make them a viable option in the long run, especially as prices continue to drop with advances in LED technology.
• Over time, LEDs have a lower environmental impact than HPS and may offer potential nutritional value by providing targeted light to fruit, unlike HPS fixtures. However, their economic viability depends on greenhouse layout, crop type, and electricity costs.

Overall, LEDs represent a growing trend in SL for fruiting vegetables, as they provide both quality enhancements and energy savings, with an expectation of increased profitability for growers through technological advancements.

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