Release and aggregation of the light-harvesting complex in intact leaves subjected to strong CO (2) deficit

Tobacco plants were subjected to long-term CO2 deficit. The stress caused p hotoinhibition of Photosystem (PS) II photochemistry and the aggregation of the light-harvesting complex of PS II (LHC II). The aggregation was shown by the appearance of the characteristic band at 698-700 nm (F-699) in 77 K fluorescence emission spectra. LHC II aggregates are considered to quench f luorescence and, therefore, the fluorescence yield was determined to verify their quenching capability. PS II photochemistry, measured as F-V/F-M, was largely depressed during first 4 days of the stress. Unexpectedly, the tot al fluorescence yield increased in this period. Fitting of emission spectra by Gaussian components approximating emission bands of LHC II, PS II core, PS I and F-699 revealed that mainly the bands at 680 and 699 nm, represent ing emission of LHC II aggregates, were responsible for the increase of the fluorescence yield. This shows an interruption of the excitation energy tr ansfer between LHC II and both photosystems and, thus, a physical disconnec tion of LHC II from photosystems. PS II and PS I emissions were not quenche d in this period. Therefore, it was concluded that these LHC II aggregates were accumulated out of PS II antenna, and, thus they cannot be involved in dumping of excess excitation. The total fluorescence yield turned to decre ase only after the large depression of PS II photochemistry, when LHC II ag gregation was considerably speeded up and the fluorescence yields of PS I a nd II turned to decline.