Etiolated pea explants were greened in the presence of the photosynthetic inhibitor CMU. Chlorophyll synthesis, except for the initial photoconversion of protochlorophyll(ide) to chlorophyll, was totally inhibited. In addition, the development of chloroplast fine structure subsequent to the prolamellar body dispersal was also inhibited, and the plastids remained functionally inactive. The addition of exogenous sucrose reversed the inhibition of both chlorophyll synthesis and fine structural development. Morphologically normal chloroplasts were formed which contained chlorophyll levels above those of the water control. Measurement of CO2 uptake demonstrated that leaves treated in this manner remained photosyn-thetically incompetent. This experimental procedure was utilised in order to compare the relationship between functional and structural development of the chloroplast in normal and functionally incompetent leaves, and to assess the exact contribution of photosynthesis towards this development. The results showed that the pigment composition of the CMU/ sucrose treated leaves developed normally, but PS II activity, as measured by ferricyanide reduction remained inactive. Manganese incorporation into these chloroplast membranes suggested that the oxygen evolving centres of PS II were developing in the absence of electron flow, and this was confirmed by detection of PS II activity through silicomolybdate reduction. It is concluded that, provided a substrate such as sucrose is present, the components associated with a functional PS II will be synthesised in the absence of photosynthetic electron flow. Similar conclusions were drawn in relation to the development of PS I, as measured by ascorbate photooxidation. However, measurement of PS I activity as NADP+ reduction demonstrated a 50% decrease in rates compared with those of the control. The synthesis of ferredoxin-NADP+ reductase is suggested to require an intact photosynthetic electron transport system, which cannot be replaced by an exogenous substrate. In vitro PMS-mediated cyclic photophosphorylation was detected in chloroplasts isolated from CMU/sucrose treated explants. In addition, evidence from light-stimulated chlorophyll synthesis in these explants supported the existence of in vivo cyclic phosphorylation. The role of this process in chloroplast development is discussed. A correlation of the onset and development of various portions of chloroplast electron transport with changes in fine structure, confirmed the theory that the organisation of the photosynthetic membrane is a predetermined multistep assembly process. The instigation of PS II was preceded by PS I which appeared to be formed from etioplast components. The synthesis of components associated with PS II was shown to correspond with grana formation, and the relationship between these two events is discussed. Two examples of PS I activation when PS II was impaired, have been outlined and discussed in relation to the concept of excitation energy spillover.
|Date of Award||1976|