Study of tobacco transformants to assess the role of chloroplastic NAD(P)H dehydrogenase in photoprotection of photosystems I and II.

Nicotiana tabacum L. wild-type plants and transformants (ΔndhCKJ), deficient in functional NAD(P)H dehydrogenase (NDH), were subjected to high light at 20 °C and 4 °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, Fv/Fm. Both photosystems were more susceptible to photoinhibition at 4 °C than at 20 °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, Ks, 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 Ks) was observed. The increase in Ks was more pronounced at 4 °C than at 20 °C, but not significantly different between wild-type and ΔndhCKJ 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 Ks represents a protective mechanism that is based on accelerated charge recombination in PSI and facilitates thermal dissipation of excessive light energy.