Recovery of Photosynthetic Efficiency at Night: Utilizing Quantum Yield of Photosystem II and Maximum Chlorophyll Fluorescence

In indoor growing systems or other applications where supplemental lighting is used, it is important to understand what light levels are ideal for plants without causing damage to the photosynthetic apparatus. Night-time quantum yields of photosystem II and maximum chlorophyll fluorescence data can be used as benchmarks for plant recovery after exposure to light and as an indication of photoinhibition and photodamage. Green Towers lettuce plants were grown inside a growth chamber at photosynthetic photon flux of 280 µmol∙m^-2∙s^-1. Using a chlorophyll fluorescence-based biofeedback control system that adjusts LED lighting to maintain desired electron transport rates (ETRs) through photosystem II, lettuce plants were maintained at constant ETRs for 14 hrs with 10 hrs of dark. The ETRs ranged between 50 and 120 µmol∙m^-2∙s^-1, in steps of 10 µmol∙m^-2∙s^-1, for a total of 8 ETRs. Photosynthetic photon flux ranged from 125 to 300 µmol∙m^-2∙s^-1 for plants with ETRs of 50 to 90 µmol∙m^-2∙s^-1, and from 340 to 520 µmol∙m^-2∙s^-1 for plants with ETRs of 100 to 120 µmol∙m^-2∙s^-1. The night time yields of plants exposed to ETRs of 90 and below returned to values comparable to those during the previous night, while plants maintained at ETRs above 90 did not fully recovered during the subsequent dark period. This lack of recovery indicates damage to photosystem II caused by relatively high light levels. Maximum fluorescence in the dark of plants maintained at ETRs of 90 µmol∙m^-2∙s^-1 and below demonstrated asymptotic increases, corresponding to a down regulation of NPQ and repair of photosystem II. These asymptotic increases typically plateaued after 2 hrs of darkness. The maximum fluorescence in plants maintained at ETRs above 90 µmol∙m^-2∙s^-1 also gradually increased after the lights were turned off, indicating a downregulation of NPQ. However, in these plants maximum fluorescence did not plateau after 2 hrs and kept increasing throughout the dark period. We postulate that this gradual, continuous increase in maximum fluorescence is indicative of repair of photosystem II that was damaged by the high light levels (> 300 µmol∙m^-2∙s^-1) during the preceding day.