| 17459329 |
Tripathy BC, Mohapatra A, Gupta I: Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX. Biochim Biophys Acta. 2007 Jun;1767(6):860-8. Epub 2007 Mar 19.
Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen ((1) O (2))-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F (v)/F (m) (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH (2) OH failed to restore the F (v)/F (m) ratio suggesting that the reduction of F (v)/F (m) reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q (A)(-) with S (2)) and B (due to recombination of Q (B)(-) with S (2)/S (3)) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F (695) band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by (1) O (2) produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings. |
81(1,1,1,1) |