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

P700⁺ dark-reduction in leaves of Cucumis sativus L. photoinhibited at chilling temperature. Sridharan Govindachary^*,1, Caroline Bigras¹, Alain Gauthier¹, Johanne Harnois¹, Robert Carpentier¹, ¹ Groupe de Recherche en Énergie et Information Biomoléculaires, Trois-Rivières, Québec, Canada

ABSTRACT- P700⁺ dark-reduction was measured in leaves of Cucumis sativus L. subjected to photoinhibition of photosystem I (PSI) by moderate white light (300 mol m^-2 s^-1) at 4^oC. During the course of photoinhibition, the far-red light induced photooxidation of P700 progressively declined. Following 5-h of light treatment, the oxidation of P700 was completely abolished. The major constraint with these estimates of P700 oxidation is the considerable amount of photooxidized P700 being rapidly reduced by the electrons generated through PSII linear electron transport, PSI cyclic electron transfer and other electron recyclization pathways. Thus we sought to use a variety inhibitors that specifically intercept these electron donation pathways to obtain true rates of P700 oxidation in such leaves. When control, untreated leaves were infiltrated with DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) prior to measurements of A₈₃₀, the steady state level of P700 oxidation was reached faster. A drastic increase in the oxidation of P700 to the extent of 45% was found only if this inhibitor was used together with methyl viologen (MV). This effect is attributable to a robust inhibition of the rate limiting routes of electron donation to P700⁺. This approach was extended to evaluate the oxidation-reduction kinetics of P700 in photoinhibited leaves. We also compared the action of stigmatellin, which binds to the Q_o site of the Cyt b₆/f complex and prevents the electron flow to PSI centers. Analysis of the kinetics of P700⁺ dark-reduction during the course of chilling-induced photoinhibition will be discussed in detail.

KEY WORDS: Photoinhibition, Chilling stress, Photosystem I, P700