Chlorophyll Fluorescence Measurements Can Indicate Carbon Fixation Rates of Lettuce

Chlorophyll fluorescence is a fast, easy, and reliable method for gaining information about the light reactions of photosynthesis. This technique is highly versatile and invaluable for plant physiological studies because it is non-destructive and responsive to a variety of biotic and abiotic stressors. Recently, we developed a prototype biofeedback control system that uses chlorophyll fluorescence to automatically adjust the intensity of light-emitting diode (LED) grow lights to maintain a constant electron transport rate (ETR). However, chlorophyll fluorescence is not a direct indicator of the rate of carbon fixation. We conducted a series of studies to determine the effect of LED light intensity on net photosynthesis (P_n), and to elucidate the relationship between electron transport rate and P_n. Dark-adapted lettuce plants were placed in a growth chamber under warm-white LED lights. After dark-adapted fluorescence values were recorded, the plants were exposed to a constant light intensity for 20 hours. Chlorophyll fluorescence, leaf gas exchange, and light intensity were continuously monitored during this time, and for 4 hours of darkness immediately after the 20-hour light exposure period. This experiment was repeated over a range of light intensities (100 to 800 µmol·m^-2·s^-1). In all experiments, ETR and quantum yield of photosystem II initially reached a high level then decreased gradually. There was a strong positive correlation between P_n and ETR averaged over 20 hours (r = 0.87). When the lights were turned off, ETR decreased to near-zero, P_n became negative (because of respiration), and quantum yield rapidly increased to levels similar to dark-adapted values. These results suggest that chlorophyll fluorescence can be used as an indirect measure of carbon fixation. Future research will focus on how LED light intensity can be adjusted to optimize photosynthetic processes for the most efficient conversion of light into biomass, and thereby developing methods for reducing the overall energy cost of indoor crop production.