By Dr. Marc van Iersel and Claudia Elkins, University of Georgia
Perennials are often grown in winter to get them ready for Spring sales. And that means that supplemental lighting is often required to produce high-quality plants in a timely manner. The electricity costs associated with supplemental lighting can be high. So, it’s important that supplemental light is provided in the most efficient way possible. For a long time, lighting recommendations have been based on the daily light integral (DLI.) DLI is the total amount of light received by a crop over a day. DLI is calculated from photosynthetic photon flux density (PPFD). By integrating these instantaneous measurements of the intensity of photosynthetic light, the DLI can be calculated. But basing lighting decisions solely on DLI may not be optimal.
In our research, funded by American Floral Endowment, we take a systematic approach towards finding the best supplemental lighting strategies. Our goal is to help growers produce high-quality crops, while assuring that lighting costs are not excessive. This starts with understanding the basic physiology of how efficiently different species use light. Then, developing lighting strategies based on those physiological responses.
Light use efficiency of perennials
Plants need light for photosynthesis, but as plants receive more light, they use that light less efficiently. Understanding how efficiently different species use light is important. Supplemental light should only be provided when plants can use that light efficiently. Measuring a plant’s light use efficiency is surprisingly easy and takes advantage of a little-known property of all plants. This is known as chlorophyll fluorescence.
When plants are exposed to light, much of that light is absorbed by chlorophyll and associated pigments in the leaves. Much like a solar panel, that light is used to create tiny electrical currents inside leaves and the energy from that current drives the reactions of photosynthesis. And, it indirectly provides the energy for all life on earth.
Not All Absorbed Light is Used to Create a Current…
However, not all the absorbed light is used to create a current. Some of the energy is converted into heat, while a small fraction of that light energy is converted into fluorescence. All leaves exposed to light give off a small amount of red light. This fluorescence is not enough to see with the naked eye, but can easily be measured. And by measuring fluorescence, we can determine exactly how much of the light absorbed by a leaf is converted into current and used for photosynthesis.
We use chlorophyll fluorescence measurements to quantify how the light use efficiency of different perennials is affected by the PPFD. Two things are clear. First, the light use efficiency of all species decreases at higher PPFDs. Second, there are important differences among species (Figure 1). And those differences have important implications for supplemental lighting. At very low PPFDs, all species use light with similar efficiency: between 70 and 80% of the light is used for photosynthesis.
However, how rapidly the light use efficiency decreases with increasing PPFD depends on the species. The light use efficiency of Heucherella, a plant that thrives in shade, decreases rapidly with increasing PPFD. On the other hand, Rudbeckia, a plant that does well in full sun or partial shade, is much more capable of using higher PPFDs efficiently. And perhaps not surprisingly, the response of Hosta, which does well in partial or full shade, falls in between that of Heucherella and Rudbeckia.
Figure 1. Light use efficiency of three perennial species in response to increasing PPFD. Note that the light use efficiency of all species decreases at higher PPFDs, but this decrease is more pronounced in shade-obligate Heucherella than in sun-loving Rudbeckia.
Using light use efficiency to develop better lighting strategies
So how can this basic information be used to help growers manage their lighting? There are three important lessons that can be drawn from this physiological information:
1) All species will use supplemental light more efficiently when that supplemental light is provided during periods with little sunlight. The common threshold-control long used for HPS lights is based on one principle. The lights are turned on when sunlight levels drop below a specific threshold and turned off above that threshold.
2) Appropriate thresholds are species-specific: providing Rudbeckia with supplemental light when sunlight provides a PPFD of 500 µmol/m2/s, allows those plants to use that supplemental light with an efficiency of over 60%; however, Heucherella would be able to use that same light with an efficiency of only about 30%. Because of such differences among species, it is important to provide a crop like Heucherella with supplemental light only when there is little sunlight; otherwise, the plants will not be able to use the supplemental light efficiently. For Rudbeckia, as well as sun-loving crops like lantana and rose, it is less important to provide supplemental light only when there is little sunlight. Those crops can still use supplemental light with reasonable efficiency at higher PPFDs.
3) These findings suggest that not all DLIs are equal. Because light is used more efficiently when the PPFD is low, our findings suggest that the overall light use efficiency can be increased by providing light at lower PPFDs and longer photoperiods, while providing the same DLI. In other words, spreading the light out over more hours each day should improve the overall light use efficiency and thus increase growth.
Not all DLIs are created equal
To test whether spreading the light out more hours each day increases growth, we grew Rudbeckia seedlings at a DLI of 12 mol/m2/day, with that light provided over photoperiods of 12, 15, 18, or 21 hours per day. To ensure that all plants received the same amount of light, we developed a new control approach for dimmable LED lights. Our system measures the PPFD at the crop level and calculates how much light is needed to reach the DLI by the end of the photoperiod. The controller then sends a signal to the LED lights so they provide just enough supplemental light to ensure that the plants receive a DLI of 12 mol/m2/day by the end of the photoperiod.
This control approach to supplemental lighting has two advantages: 1) the DLI can be precisely controlled, regardless of weather conditions and 2) the supplemental light is provided preferentially when there is little sunlight (and thus when plants can use the supplemental light most efficiently). Using this control approach to supplemental lighting, the PPFD received by the plants decreased from 275 to 160 µmol/m2/s as the photoperiod increased from 12 to 21 hours.
As We Hypothesized…
As we hypothesized, the Rudbeckia seedlings grew substantially faster with longer photoperiods and lower PPFDs; with a 21-hour photoperiod, plants were about 30% larger than those grown under a 12-hour photoperiod, even though all plants received the same total amount of light (Figure 2). And the longer photoperiod had no negative effects on plant quality, as determined from root fraction (an important measure for seedlings, since good root growth is critical) and compactness. Based on our findings, using a longer photoperiod can decrease the crop cycle by at least one week. Another advantage of using longer photoperiods is the instantaneous amount of supplemental light that needs to be provided is lower. That means that fewer light fixtures are needed to provide the supplemental light. Thus, lowering the cost of installing a lighting system.
Keep in mind that not all species will respond the same to longer photoperiods. The flowering of many crops is photoperiod-dependent. And while flowering is desired when the crop is finished, premature flowering can slow down growth.
Figure 2. Longer photoperiods result in better growth of Rudbeckia seedlings. The control plants on the left did not receive supplemental light (and an average DLI of about 5 mol/m2/d. The other plants all received a DLI of 12 mol/m2/d, but that light was spread out over photoperiods ranging from 12 to 21 hours.
What does this mean to the floriculture industry?
More efficient lighting strategies can improve the production of perennials while lowering electricity costs. Our approach is most easily implemented with dimmable LED light fixtures, but similar approaches can be implemented using HPS lights. Several lighting companies now offer lighting control systems that take advantage of the ability to dim LED fixtures in response to changing levels of sunlight. This can provide a more consistent light environment in your greenhouse. It can also make crop production more predictable while assuring that no excess light is provided.
What is next?
Industry support of the American Floral Endowment has helped to make this research possible. AFE’s financial support for our work helped us get a $5,000,000 grant from USDA’s Specialty Crops Research Initiative. This project, titled Lighting Approaches to Maximize Profits, brings together scientists and engineers from around the country. A diverse team working on lighting issues in the controlled environment agriculture industry helps integrate horticultural production, economics, and engineering. This will result in holistic approaches to optimize supplemental lighting strategies. To learn more about project LAMP, please visit www.hortlamp.org.
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