title: Study of tobacco transformants to assess the role of chloroplastic NAD(P)H dehydrogenase in photoprotection of photosystems I and II.
abstract: Nicotiana tabacum L. wild-type plants and transformants (DeltandhCKJ), deficient in functional NAD(P)H dehydrogenase (NDH), were subjected to high light at 20 degrees C and 4 degrees C for 2 h to examine a possible role of NDH-mediated cyclic electron flow in protecting photosystems I and II from photoinhibition. Photochemical activity of photosystem I (PSI) was assessed by means of P700 absorbance changes at 810 nm. In addition, potential photosystem II (PSII) efficiency was determined by measuring the 'dark-adapted' ratio of variable to maximum chlorophyll fluorescence, F(v)/ F(m). Both photosystems were more susceptible to photoinhibition at 4 degrees C than at 20 degrees C. However, the degree of photoinhibition was not less in the wild type than in the NDH-deficient plants. To evaluate the efficiency of P700 oxidation in far-red light, a saturation constant, K(s), was determined, representing the far-red irradiance at which half of the maximum P700 absorbance change was reached. In photoinhibited leaves, a decrease in the efficiency of P700 oxidation (increase in K(s)) was observed. The increase in K(s) was more pronounced at 4 degrees C than at 20 degrees C, but not significantly different between wild-type and DeltandhCKJ plants. Re-reduction kinetics of oxidised P700 in the dark were accelerated to a similar extent in photoinhibited samples of both genotypes and at the two temperatures tested. The data indicate that NDH-mediated cyclic electron flow does not protect PSI against short-term light stress. It is proposed that the observed increase in K(s) represents a protective mechanism that is based on accelerated charge recombination in PSI and facilitates thermal dissipation of excessive light energy.
authors.affiliation: Boyce Thompson Institute for Plant Research at Cornell University, Tower Road, Ithaca, NY 14853, USA.
authors.forename: Carina
authors.initials: C
authors.lastname: Barth
authors.forename: G Heinrich
authors.initials: GH
authors.lastname: Krause
mesh_terms: D000222:Adaptation, Physiological / Q000528:radiation effects; D002734:Chlorophyll / Q000378:metabolism / Q000528:radiation effects; D002736:Chloroplasts / Q000201:enzymology* / Q000235:genetics; D004579:Electron Transport / Q000235:genetics / Q000502:physiology / Q000528:radiation effects; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes; D009252:NADPH Dehydrogenase / Q000235:genetics* / Q000378:metabolism; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism* / Q000528:radiation effects; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D030821:Plants, Genetically Modified; D013696:Temperature; D014026:Nicotiana / Q000201:enzymology* / Q000235:genetics
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; C001785:chlorophyll P 700; D009252:NADPH Dehydrogenase