PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Over-reduction of stroma and the intersystem chain upon illumination of photoinhibitory light. Tsuyoshi Endo^*,1, Toshiharu Shikanai², Fumihiko Sato¹, ¹ Graduate School of Biostudies, Kyoto University, Kyoto, JAPAN² Graduate School of Bioresource and Bioenvironment, Kyusyu University, Fukuoka, JAPAN

ABSTRACT- Over-reduction of the intersystem electron transport and stromal redox carriers, may be considered as a direct cause of photoinhibition of PSII and generation of reactive species of oxygen in PSI. However, the redox level of these electron carriers upon illumination with photoinhibitiory light has not been intensively studied. In this study we use simple non-destructive methods to estimate redox level of the stroma and the intersystem chain, that is, P700 oxidation level under far-red light (FR) and photochemical quenching of chlorophyll fluorescence under low light. Oxidation level of P700 induced by far-red light in briefly dark-adapted leaves in some sun plants decreased during daytime and recovered at night. The maximum level of P700 oxidation, represented by the peak level of P700 oxidation induced by a light flash, was also depressed during daytime. It is proposed that stromal over-reduction induced by sunlight was the direct cause of this depression since it was reproduced by controlled illumination with saturating white light to dark-adapted leaves. Simultaneous measurements of P700 redox and chlorophyll fluorescence showed that the depression of FR-induced P700 oxidation was associated with dark reduction of the plastoquinone, which may be associated with cyclic electron transport activity. A light-stressed transformant of tobacco, in which chloroplastic NAD(P)H dehydrogenase (NDH) was insertinally destroyed, showed stronger depression of FR-induced oxidation of P700 and higher rate of non-photochemical reduction of the plastoqinone pool than wild type control, confirming association of both phenomena. Here, we want to emphasize physiological role of chloroplastic NAD(P)H dehydrogenase in the relaxation of high-light induced over-reduction.

KEY WORDS: P700, photo-oxidative stress, chlorophyll fluorescence, over-reduction