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Functional and molecular adjustment of PSII in bleached leaves of silver maple

Lower chlorophyll content and maximum quantum yield of PSII (Fv/F’m) in prematurely aged bleached leaves of silver maple (Acer saccharinum L.) were revealed during our current investigations on that topic. In this study we aimed to investigate physiological and molecular mechanisms of PSII adjustment during such impaired photosynthetic performance compared to healthy green leaves. We used in vivo chlorophyll fluorescence to analyze photochemical efficiency of PSII as well as immunodetection of several proteins of the photosynthetic apparatus (D1, cytochrome f, LSU, LHCII) in order to show changes in molecular organization. Although, total chlorophylls content in bleached leaves was lowered to a great extent (28 % of that in green leaves), density of reaction centers on chlorophyll basis (RC/ABS) in bleached leaves was 34 % lower than in green leaves. Overall photosynthetic activity expressed as photosynthetic performance index (Pi) was considerably lowered (0.32 and 2.48 in bleached and green leaves, respectively). This was due to lower electron transport per reaction center (ETo/RC was 1.23 and 1.54 in bleached and green leaves, respectively) and high capability for dissipation of excess light energy (DIo/RC was 1.58 and 0.55 in bleached and green leaves, respectively). Lower content of all investigated proteins (from 62 to 72 % of that in green leaves) was shown in bleached leaves. All obtained results indicate that modifications in molecular organization of photosynthetic electron chain components in bleached leaves led to functional adaptation of the photosynthetic apparatus, especially PSII, which enabled transfer and dissipation of excess light energy and efficient protection of the PSII from photodamage.

photosynthesis; chlorophyll fluorescence; photosystem II; Acer; premature ageing

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