Changes in Photosystem II Photochemistry in Senescing Maple Leaves

Biochemical and functional changes in photosystem II (PSII) were investigated during natural senescence of maple (Acer platanoides L.) leaves. Three different types of senescing leaves were used: early senescent green (G), yellow green (YG) and yellow (Y). The main goal of this study was to investigate the dynamics of degradation of major photosynthetic proteins (D1, LHCII and Rubisco LSU) and how this would reflect on PSII photochemistry. It was revealed that D1 and LSU degrade faster than LHCII. Such differential degradation dynamics influenced PSII photochemistry. A decrease in the maximum (F-v/F-m) and effective (Delta F/F'(m)) quantum yields of PSII as well as a decrease in its capability for O-2 evolution were observed. Analysis of chlorophyll alpha fluorescence transient measurement (OJIP test) in senescent leaves showed striking decrease in performance index (PIABs), a measure of overall photosynthetic performance. Further, increased absorption, trapping and dissipation of excitation energy per reaction centre were found in senescent leaves. In addition, it was shown that a marked decrease in electron transport beyond the primary electron acceptor (Q(A)) in Y leaves was due to reduced electron transfer ability from QA(-) to Q(B). It can be concluded that molecular and functional modifications of a certain fraction of reaction centres from active to dissipative might be considered an important regulatory mechanism in chloroplast electron-transport chain redox poise control during natural senescence of maple leaves.