Horticulture is the branch of agriculture that deals with the science, art, technology, and business of plant cultivation. The role of light illumination for plant growth is an important factor that needs to be considered and improved. The Photosynthesis process in plants uses water, carbon dioxide, and incident light as the source of energy to produce glucose, an essential nutrient for the plant, and oxygen, as shown in Figure 1.
In the past, plant cultivators in greenhouse environments always used either natural sunlight, High Pressure Sodium (HPS), or Fluorescent lamps to illuminate crops. There were certain disadvantages to using these light sources: natural sunlight is obviously only available during the day, fluorescent lighting consumes energy and generates heat, which prevents it from being placed close to the plant, and it contains toxic materials such as mercury, making proper disposal costly.
The development of Light Emitting Diodes (LEDs) over the last few decades has introduced a new lighting source to horticulture, offering many superior advantages. First of all, the plant does need all wavelengths in the visible region (400-700 nm) in equal proportion. Photosynthetic Photon Flux (PPF) refers to the range of visible spectral radiation that plants use in photosynthesis. Plants use more red and blue light for photosynthesis than green, as shown in Figure 2.
The absorption spectrum of plants can be matched by using tunable LEDs, as shown in Figure 3 below. This illumination source is much more suitable than an HSP source, whose peak emissions differ significantly from the absorption spectrum of green plants.
This illumination source is much more suitable than an HSP source whose peak emissions widely differ from the absorption spectrum of green plants, as shown in Figure 4.
Some wavelengths of interest for LEDs as applicable to plant growth are:
- 200-280nm or UVC radiation is present in sunlight, but harmful to plants.
- 320-340nm may have a negligible effect on cryptochrome.
- 365nm is a “wavelength of interest”.
- 439nm, the blue absorption peak of chlorophyll A
- 450-460nm royal blue is absorbed by one of beta-carotene's peaks. It is a readily available LED wavelength, commonly used to excite the remote-phosphor in “white” LED lamps
- 469nm is the blue absorption peak of chlorophyll B
- 430-470nm range is important for the absorption of chlorophyll A and B. This is key for vegetative growth
- 480-485nm is the second absorption peak of beta-carotene
- 525nm, this is a phototropic activator that our researchers are still trying to find the chromophore of. It is apparent that plants are receiving direction and environmental signals from it, and that it affects internodal spacing. 525nm is also the wavelength of GaN or InGaN green LEDs commonly used in RGB and tunable applications.
- 590nm is key for carotenoid absorption. Carotenoids are both starch-storing, structural compounds and nutritional compounds. With thanks to Jeffery Bucove, who increased the harvest bulk of his plants by adding this wavelength.
- 590nm, in addition, is the phycoerythrin single absorption wavelength.
- 625nm is the phycocyanin single absorption peak.
- 642-645nm is the peak absorption point of chlorophyll B
- 660nm, often called the super-red LED wavelength, is vital for flowering
- 666-667nm is actually the peak red absorption point for chlorophyll A.
- 700nm light is to be avoided. It confuses the phytochrome recycling systems in green plants.
- 730nm, often referred to as Far Red, is essential for phytochrome recycling. It is needed for all kinds of morphogenic processes. A few minutes of 730nm light treatment after the full light cycle is over will revert the Pfr (activated) form of the phytochrome chromophore to the Pr (inactive) form. This resets the chemistry for another “lights-on” cycle and may help shorten the classic dark phase of the photoperiod. 735nm is the closest available standard LED wavelength to the above 730nm.
LEDs offer the horticulture industry a unique opportunity to use narrow-band illumination. Several LEDs at different wavelengths can be combined to provide an illumination source that follows the plant sensitivity curve. Aside from this, there are several other advantages of using LEDs in horticulture, which include:
- Geometry: Since radiation falling on a plant is inversely proportional to the square of the distance between the source of radiation and the plant, it is advantageous to bring the plant close to the light source. This is possible with LED sources because they are cooler, whereas with fluorescent lamps, the heat produced can burn the leaf at close distances.
- Efficiency: The Electrical efficiency of LEDs is much higher than that of Fluorescent lamps, which helps the crop grower save on electrical bills.
- Durability: By definition, the lifetime of an LED is defined as the duration at which the intensity drops to 70% of its original value, and this is about 50,000 hours, much higher than a typical lifetime for a Fluorescent lamp.
- Spectral quality: The Spectral quality of carefully chosen LED illumination sources can have dramatic effects on plant anatomy, morphology, and pathogen development.
- Small size: Allows a bigger space for installing the light source
The wavelength selection of illumination is observed in the figure. Several researchers have investigated the effects of different intensities and wavelengths on the growth of various crops. It is essential to understand that different crops may behave differently under different illumination levels, and a distinct “light recipe” may be needed for each crop. The PPF is measured in units of mmol/m2/s (One mole is 6.023 × 1023 photons), and crop growers have experimented with different levels of light intensity. Increased PPF has led to increased plant growth. Although red light is sufficient for plant growth, blue light is important for increased leaf thickness and the number of chloroplasts/cell. Rice plants grown a combination of blue and red LED’s showed a higher photosynthetic rates than those grown under red illumination alone (Reference 1). Although a combination of Red and Blue LED illumination improves crop growth, its purplish-grey appearance makes it difficult to observe the disease visually. The addition of green light, although not crucial for plant growth, allows the human eye to assess damage.
Another critical issue is the development of metrics to quantify PPF and crop light absorption. Crop growers need to calibrate their LED light sources and find the optimal light recipe for flux efficacy, appropriate wavelengths for different crops, and optimal illumination geometry. Allied Scientific Pro has recently introduced the SGAL App, which enables photometric measurements of LED light sources used in horticulture. The PPF measurement is done by pointing the device at the light source and pressing a button. The software also has features that allow recording data on a day-to-day basis and monitoring the plant's growth. These diaries will help the grower closely monitor the best course of action.