Effect of Overexcitation of Photosystem <scp>II</scp> on Chlorophyll Fluorescence Quenching Parameters in <scp><i>Arabidopsis thaliana</i></scp> State Transition Mutants

Photosynthesis is driven by light absorbed by pigments situated in the antenna of the photosystems. Depending on the light quality, there is a difference in the absorption of Photosystems I and II. A balanced light absorption between both photosystems is required for optimal photosynthesis. State transitions are the acclimation response of the photosynthetic apparatus that allows adjusting the antenna size of the two photosystems. The dependency of chlorophyll fluorescence quenching parameters on the light quality was investigated in Arabidopsis wild type and state transition mutants. The parameter qL, indicating the fraction of open Photosystem II reaction centers, showed a large dependency on the quality of the actinic light in state transition mutants incapable of performing the transition to State 2. While the difference between wild type and these mutants was small in blue or red light, the difference was much more pronounced when cyan or green light was used instead. The phosphatase mutant pph, with the mobile part of the light harvesting Complex II locked at Photosystem I, the chlorophyll b-less mutant ch1, and phototropin mutants phot1 and phot2 affected in chloroplast movement did not show such low qL in cyan or green light. This shows that overexcitation of Photosystem II was responsible for the low qL in Arabidopsis state transition mutants stn7, nsi1, and nsi2.