A Biofeedback System for Plant-driven Photosynthetic Lighting

Plant factories produce crops in closed growing environments with only artificial light. The electrical energy required for photosynthetic lighting is a significant expense, accounting for up to 60% of the overall of production costs. To reduce energy expenses, we have developed a biofeedback system that can automatically, and in real-time, adjust light levels based on the efficiency with which plants use the photosynthetic light provided by light emitting diodes (LEDs). Chlorophyll fluorescence measurements are used to determine the quantum yield of photosystem II (the fraction of absorbed light that is used to drive the light reactions of photosynthesis) and electron transport rate (ETR; the rate of the light reactions). The light output of the LEDs is adjusted by changing their duty cycle.  To maintain a specific ETR over time, the biofeedback system automatically adjusts the light intensity whenever the measured ETR is different from a specified rate. We tested this approach with pothos, lettuce, and sweet potato. The biofeedback system was able to maintain a wide range of ETRs (up to about 70 to 100 µmol·m^-2·s^-1, depending on the species), even though quantum yield decreased with increasing light levels. Electron transport rates could be controlled more precisely at lower rates. Attempts to control the light level based on quantum yield were not very successful: the system gradually decreased the light level more and more to maintain steady quantum yields. Although quantum yields were maintained close to specified values, the lights were dimmed so much that ETRs were very low, resulting in very low photosynthetic rates. Our trials show that chlorophyll fluorescence can be used as a tool for automating the biofeedback control of LED lights. An important next step will be to determine how this approach can be used to optimize the light environment in plant factories, balancing the need for high ETRs with efficient light use.