July 2024 Controlled Environment Agriculture, Lighting, Research

Supplemental lighting strategies that let plants control the conditions By By Suyun Nam, Marc W. van Iersel and Rhuanito S. Ferrarezi

Light is a crucial factor affecting plant growth, especially during the winter or on cloudy days. To ensure optimal crop yield and quality, growers often utilize supplemental lighting strategies such as HPS or LED lights, but the high installation and electricity costs associated with these lights can reduce or eliminate profitability. 

It’s essential to strike a balance between the energy input for lighting and the resulting increase in productivity and profits. In response to this challenge, various “smart” lighting control strategies have been explored. While HPS remains popular, dimmable LEDs are increasing in utilization since they offer precise control with higher energy efficiency. 

One straightforward lighting strategy is to add a certain amount of supplemental light over the sunlight. Supplemental lighting is also used to extend the photoperiod, providing additional light at dawn and dusk. However, it is crucial to consider whether your crops are sensitive to day length, especially concerning flowering or fruiting. 

There is an advanced lighting control strategy, also called an adaptive lighting system, that can dynamically adjust supplemental light levels based on changing sunlight conditions (van Iersel and Gianino, 2017). When sunlight is ample, supplemental light levels are decreased or switched off, while they are increased under insufficient sunlight. 

It makes sense to provide more supplemental lighting under low sunlight conditions because plants use light less efficiently when they are exposed to high light levels, matching the light use efficiency. 

EFFICIENCY 

Light use efficiency refers to how efficiently plants use light energy to grow. For sure, we hope our crops use supplemental lighting as efficiently as possible so that we can save electrical costs, since light level is one of the biggest factors affecting light use efficiency. However, the light use efficiency varies also with other environmental factors such as temperature, humidity and carbon dioxide levels. 

For example, on a cold winter day, plants might use supplemental light less efficiently in the morning than in the afternoon, even under the same sunlight levels. This is because low temperatures in the morning might stress plants, making them unable to utilize light efficiently. 

In this case, we would like to provide more supplemental lighting in the afternoon rather than in the morning. Conversely, hot temperatures in the afternoon on a hot summer day will reduce the light use efficiency. 

Even though the discrepancies may vary depending on species and growth stage, it is still worthwhile to take into account the light use efficiency when controlling supplemental lighting.

Then how can we measure the light use efficiency? 

CHLOROPHYLL FLUORESCENCE 

For all plants, a small fraction of absorbed light is re-emitted from chlorophyll molecules, which is called chlorophyll fluorescence. 

Chlorophyll fluorescence can be a good option that represents photosynthetic light use efficiency in scientific settings. We can measure a plant’s stress status and photosynthetic capacity using the chlorophyll fluorescence parameters. The electron transport rate (ETR) and quantum yield of photosystem II (ΦPSII) represent photosynthesis speed and photosynthesis efficiency, respectively. Those physiological responses can be monitored in real-time by using a fluorometer. 

CHLOROPHYLL FLUORESCENCE-BASED BIOFEEDBACK SYSTEM 

A novel supplemental light control strategy has been developed utilizing chlorophyll fluorescence parameters. A chlorophyll fluorescence-based biofeedback system, also known as the biofeedback system, adjusts LED light levels based on a target value of either ETR or ΦPSII. 

For example, if you want to achieve a certain amount of photosynthesis speed of your crop, you can use this biofeedback system to maintain specific ETR (e.g., 80 μmol·m-2·s-1). Or, if you want to provide LED light only when your crops can utilize it efficiently, ΦPSII can be used as a target parameter. In other words, by using this innovative biofeedback system, plants can decide their light conditions depending on their physiological status instead of providing just a fixed amount of light level. Not only sunlight levels but also other environmental factors,such as temperature and humidity, are reflected when determining supplemental light levels. 

THE PROCESS 

Two trials were tested to evaluate the biofeedback system’s applicability. The biofeedback system was installed in a climate control chamber with dimmable LED lighting. 

During the first experiment, a wide range of ETR and ΦPSII target values were maintained for a single day using lettuce and cucumber plants. The chlorophyll fluorescence parameters were monitored every 15 minutes, and the LED light levels were adjusted whenever the current ETR and ΦPSII differed from the target values. 

The objectives of the second experiment were to test if the biofeedback system can be used throughout the growing cycle and to find out which level of target ETR is optimal for growing lettuce in terms of yield and energy efficiency. 

RESULTS 

From the first experiment, we could confirm that the biofeedback system precisely maintained the specific level of photosynthetic capability (either ETR or ΦPSII) constantly, and also adjusted the LED light levels according to the target values. 

Lettuce and cucumber plants showed slightly different levels of LED light because there is a crop-specific difference in photosynthetic efficiency. The second experiment showed that lettuce plants acclimated to the new light conditions and increased photosynthetic efficiency over time. Also, the high target ETR produced greater biomass with a higher growth rate but required high electrical use at the same time leading to low energy efficiency. 

In summary, the biofeedback system could successfully maintain specific levels of physiological response as we designed, and the system adjusted LED light levels considering environmental factors, crop-specific response and acclimation process. Optimal target ETR can be determined based on the grower’s objectives such as crop yield, electrical energy use and growth cycle.

WHAT’S NEXT? 

Ultimately, the biofeedback system is expected to perform a distinctive role in optimizing the energy efficiency of supplemental light in greenhouses. There are fluctuating environmental factors in greenhouse conditions that affect a crop’s photosynthetic performance in real-time, and we have an ongoing trial with lettuce in a glass-covered greenhouse at the University of Georgia CAES PGF. 

The biofeedback system will determine how much supplemental light levels should be provided depending on the specific sunlight level, temperature, humidity, crop type and growth stage — this can lead to dynamic and energy-efficient supplemental light control in commercial settings.

Reference: van Iersel, M. W., & Gianino, D. (2017). An adaptive control approach for light-emitting diode lights can reduce the energy costs of supplemental lighting in greenhouses. HortScience, 52(1), 72-77.