Release and aggregation of the light-harvesting complex in intact leaves subjected to strong CO₂ deficit

Abstract

Tobacco plants were subjected to long-term CO₂ deficit. The stress caused photoinhibition 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 (F₆₉₉) in 77 K fluorescence emission spectra. LHC II aggregates are considered to quench fluorescence 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 total 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 F₆₉₉ revealed that mainly the bands at 680 and 699 nm, representing emission of LHC II aggregates, were responsible for the increase of the fluorescence yield. This shows an interruption of the excitation energy transfer between LHC II and both photosystems and, thus, a physical disconnection of LHC II from photosystems. PS II and PS I emissions were not quenched 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 decrease only after the large depression of PS II photochemistry, when LHC II aggregation was considerably speeded up and the fluorescence yields of PS I and II turned to decline.