Graduate Thesis Or Dissertation
 

The effects of ultraviolet irradiation on the photosynthetic apparatus

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  • The effects of ultraviolet (UV) irradiation on photosynthesis and various partial reactions involving the individual photoreactions of photosynthesis were examined. These reactions included photo-reduction in adapted green algae, Hill reaction, cyclic photophosphorylation, the 520 nm absorbance change and variable-yield fluorescence. The reactions were measured in spinach chloroplasts and isolated chloroplasts or whole cells of green and blue-green algae. Experiments showing that all system II requiring reactions, as well as photoreduction and cyclic photophosphorylation (which require only system I) were equally inhibited suggested that the site of the UV inhibition was at some point in the electron transport chain near system II. Ascorbate-DCIP mediated reduction of NADP was not affected. A peak in the action spectrum for UV inhibition at 250 nm and an apparent decrease in the plastoquinone (PQ) content of chloroplasts after irradiation suggested that PQ was the locus of the inhibition. However, experiments in which pure, unirradiated PQ was added to irradiated chloroplasts did not restore photochemical activity. In addition, examination of whole cell systems, using various algae, revealed that when photosynthetic activity had been totally abolished, only about 40% of the endogenous PQ had been destroyed. Experiments comparing the effects of petroleum ether extraction of the PQ from lyophilized chloroplasts versus UV irradiation of chloroplasts revealed striking differences. From this type of data it was concluded that destruction of PQ was not the major cause of the UV inhibition. The effects of UV irradiation on the 520 nm absorbance change were also examined. It was found that the system II (DCMU sensitive) portion of the absorbance change was inhibited at a rate parallel to the inhibition of photosynthesis. The system I (DCMU insensitive) portion was also inhibited, but required two to three times the amount of irradiation. Experiments to compare the rate of inhibition of the system I portion of the 520 nm absorbance change with PQ destruction did not reveal any apparent positive correlation. Periods of irradiation sufficient to abolish totally the system I portion of the absorbance change had no effect on the ability of chloroplasts isolated from this material to reduce NADP with the ascorbate-DCIP couple. Extraction of the PQ from chloroplasts abolished both portions of the 520 nm absorbance change, indicating that PQ is required for the change but is not the site of the UV inhibition. Carotenoid, lipid and protein content and complement in irradiated and unirradiated chloroplasts and cells were also compared. In no case was any major alteration noted. Measurements were made of the effects of UV irradiation on the variable-yield fluorescence of isolated chloroplasts and whole cells. It was expected that as photosynthetic activity decreased the variable-yield fluorescence would increase until the maximum attainable yield was reached when photosynthetic activity had been totally abolished. Surprisingly, it was found that the variable-yield fluorescence decreased after UV irradiation and, more important, the maximum attainable yield also decreased. This was interpreted as indicating that the fluorescence emitter, the system II trapping center, was destroyed. Considering all of the results discussed above, it seems likely that UV irradiation disrupts a physical entity consisting of the system II trapping center, the initial electron acceptor from system II, and at least one component beyond the site of DCMU inhibition. This interpretation further suggests that system II consists of a physical unit containing several components of the electron transport chain.
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file details MantaiKennethE1968_Redacted.pdf 2017-08-07 Pubblico Scaricare