Supplementary lighting can be beneficial or even necessary for the production of high quality plants. Many greenhouse crops benefit from supplemental light in the winter months when there is relatively little sunlight. For production of horticultural crops in plant factories, supplemental light is critical, since crops are grown inside buildings, where they may not receive any sunlight. Unfortunately, supplemental lighting is expensive, because of the large amounts of energy required for electric lights.
Why use LEDs?
The obvious, but also incorrect answer is: because LEDs (light emitting diodes) are much more energy efficient than other lamps. In reality, LEDs are about equally efficient as the best high intensity discharge (HID) lamps, which are by far the most common bulbs used for photosynthetic lighting. Since LEDs are substantially more expensive than HID lights, there seems little reason to use LEDs. However, LEDs have some important advantages over HID lights:
New. This is not yet a mature technology and LEDs are constantly getting more efficient and cheaper. That will continue to make LEDs more and more competitive.
Color. LEDs are available in many colors and we can take advantage of this in horticulture by using LED colors that elicit specific plant responses.
Cool. Although LEDs and HID lights produce about the same amount of heat, the way they do this is very different. With HID lights, the bulbs get very hot, which means that the bulbs need to be placed a safe distance from the plants, or they will burn the plants. With LEDs, it is not the LED itself that gets hot. Instead the electronics board that the LEDs are mounted on gets hot. To keep LEDs from overheating, powerful LEDs need to be mounted on a heat sink, and the heat can then easily be dissipated with a fan. Because the LEDs themselves do not get hot, they can be placed close to, or even inside, the canopy. And the more closely the lights are to the crop, the more efficiently the crop can be lit. This gives LEDs are major advantage over other kinds of lights.
Control. For our purposes, this is the biggest advantage of LEDs. They can be controlled in ways other lights cannot. The typical way most light are dimmed is simply by restricting the current going to those lights. That is one option with LEDs, but not the most common way. To understand how we can dim LEDs, two terms are important:
- Frequency: LEDs can easily be turned on and off thousands of times per second. Because this happens so fast, our eyes cannot detect that the LEDs go on and off
- Duty cycle: We can control how long the LEDs are on and off during one on/off cycle A duty cycle of 0 means the LEDs are always off, a duty cycle of 1 mean they’re always on. We can set the duty cycle to any value we want and that is the most common way LEDs are dimmed: the greater the duty cycle to brighter the lights.
The ability to control both duty cycle and frequency gives us precise control over LED lighting. The challenge is how to use that control to supply plants with the amount of light they can efficiently use. Since that is different for different species, and possibly even for different cultivars of the same species, it is not practical to experimentally determine optimal lighting levels for all horticultural species. So why not ask the plants and let them decide?
Photosynthetic light use efficiency
By measuring the efficiency which with plants use light, we can make automated, informed decisions about how much light to supply. Chlorophyll fluorescence measurements provide a great tool do this, because it is a relatively simple method to measure exactly what we need to know: how efficiently do plants use the absorbed light to drive the light reactions of photosynthesis?
When plants absorb a photon, that photon excites an electron in a chlorophyll or other pigment molecule. That excitation energy can be transferred from one chlorophyll molecule to another, until it finally reaches a photosystem I or photosystem II reaction center. At the point, the excitation energy is transferred to a special chlorophyll dimer. This chlorophyll dimer in photosystem II can use the excitation energy to take an electron away from a water molecule. This electron than moves through the electron transport chain in the thylakoid membrane of chloroplasts. Halfway through this pathway, photosystem I uses the energy from another absorbed photon to give this electron an additional boost. An in-depth review of electron transport is not needed here, but ultimately, the electron transport chain is used to make two valuable compounds: NADPH and ATP. ATP is the basic form of chemical energy in all life, while NADPH is a reducing agent: it drives many chemical reactions in plants. For more detail on this see: Photosystem II or Light-dependent reactions. Photosystem II is the rate-limiting step in this process, so we are especially interested in what happens there.
Unfortunately, plants cannot use all of the light that is absorbed by the leaves. And plants need to dissipate that excess light energy. There are two competing ways in which plants do this: non-photochemical quenching (which converts the light energy into heat) and chlorophyll fluorescence. Yes, chlorophyll actually fluoresces. We normally can’t see this, because the intensity of fluorescence is very low compared to ambient light. But measuring chlorophyll fluorescence is fairly straightforward, using a technique called pulse-amplitude modulated fluorescence measurements. And it turns out the chlorophyll fluorescence measurements can give us a tremendous amount of information about what happens inside chloroplasts, and specifically with regard to photosystem II. Using chlorophyll fluorescence measurements we can quantify the following physiological parameters:
Maximum (or dark-adapted) quantum yield of photosystem 11, often referred to as FvFm: The maximum efficiency at which photosystem can use absorbed photons to drive electron transport. This value is typically 0.8 to 0.84 in healthy plants and lower values indicate damage to photosystem II. Fv/Fm is measured after plants have fully acclimated to dark conditions.
The actual quantum yield of photosystem II (φPSII): the fraction of absorbed light used by photosystem II to drive electron transport. This is measured when plants are exposed to light and this value is lower than Fv/Fm.
Electron transport rate (J): If we know φPSII, the photosynthetic photon flux, and what fraction of absorbed light is actually absorbed by a leaf, we can calculate the electron transport rate, or the number of electrons moving through photosystem II. This allows for determination of the rate of the light reactions of photosynthesis.
Non-photochemical quenching (NPQ): Estimates the rate constant for the reactions involved in dissipating absorbed light energy into heat. Higher values indicate that more light energy is converted into heat, and typically mean that φPSII is reduced.
The fraction of open reaction centers of photosystem II qL). When plants are exposed to light, some photosystem II reaction centers will be open (able to accept the energy from an absorbed photon), while other are closed (and unable to accept the energy from a photon). This provides an indication of the overall ability of the photosystem II reaction centers to use light energy to drive electron transport.
Biofeedback control of supplemental lighting
By measuring chlorophyll fluorescence and calculating the above parameters, we can design a biofeedback system to control LED lights. Although the principle is simple (measure photosynthetic efficiency and adjust the light intensity accordingly) developing optimal control methods is not. Increasing the light level will reduce φPSII, but increase the electron transport rate. In other words, at high light the rate of electron transport is higher, potentially resulting in more ATP and NADPH production and more photosynthesis, but the plant will use the light less efficient. Achieving maximum efficiency is easy: grow plants at very low light levels and the light is used most efficiently. But at the same time, the electron transport rate will be very low and there will be little photosynthesis. An optimal control algorithm needs to result in relatively high φPSII and electron transport rate. We hope to achieve this by keeping NPQ low and qL high.
Here is a link to the program we use to run our Biofeedback system:
CR1000 Datalogger program for biofeedback control of LEDs EEB