Effects of Cu deficiency on photosynthetic electron transport

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Abstract
The role of Cu in photosynthetic electron transport was explored by using Cu deficiency in sugar beet [Beta vulgaris] as an experimental approach. Cu influenced electron transport at 2 sites in addition to plastocyanin. Under mild deficiency (0.84 nmol of Cu per cm2 of leaf area), electron transport between the 2 photosystems (PS) is inhibited but not electron transport within PS I or PS II measured separately. The chlorophyll/plastoquinone ratio was normal in Cu-deficient plants. However, the breakpoint in the Arrhenius plot of electron transport was shifted towards a higher temperature. Cu is necessary to maintain the appropriate membrane fluidity to ensure the mobility of plastoquinone molecules to transfer electrons between the 2 photosystems. Under severe deficiency (0.22 nmol of Cu per cm2 of leaf area) both PS II and PS I electron transports were inhibited and to the same extent. PS II electron transport activity could not be restored by adding artificial electron donors. Polypeptides with MW of 28,000 and 13,500 were missing in Cu-deficient chloroplast membranes. In PS II particles prepared from normal chloroplasts of spinach, 2 atoms of Cu per reaction center are present. Cu influences PS II electron transport either directly, by participation in electron transfer as a constituent of an electron carrier, or indirectly, via the polypeptide composition of the membrane in the PS II complex.