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fulltext - PHYSIOL PLANT 57 422-427 Copenhagen 1983...

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PHYSIOL. PLANT. 57: 422-427. Copenhagen 1983 Kinetics of the reactivation of the Hill reaction in CO2-depleted chloroplasts by addition of Mcarbonate in the absence and in the presence of herbicides Jan F. H. Snel and Jack J. S. van Rensen Snel, J. F, H, and van Rensen, J. J, S, 1983. Kinetics of the reactivation of the Hill reaction in COj-depleted chloroplasts by addition of bicarbonate in the absence and in the presence of herbicides. - PhysioL Plant. 57: 422^27. In isolated broken chloroplasts photosynthetic electron transport requires the pre- sence of COj and/or bicarbonate. This bicarbonate effect on electron flow was meas- ured in a medium containing 100 mM sodium formate. In this medium a dark incu- bation time with bicarbonate is required for the reactivation of the Hill reaction. We have measured the kinetics of the reactivation of electron flow by varying the dark incubation of COj-depleted pea (Pisum sativum L., cv. Rondo) chloroplasts with bicarbonate. The half-time of this reactivation appears to be 25 s when 2 mM bicar- bonate is added. The dinitrophenol herbicide, i-dinoseb, is shown to be a competitive inhibitor of the bicarbonate dependent Hill reaction with an inhibitor constant (Kj) of 31 nM, In the presence of 100 nM i-dinoseb or 100 nM DCMU the half-time of the reactivation by 2 mM bicarbonate appears to increase to about 58 s. We provide an explanation for these phenomena by analyzing the bicarbonate-thylakoid interaction on the basis of a simple reaction scheme. "The binding of bicarbonate to the thyiakoids appears to be a second order reaction with pseudo-first order kinetics. According to our analysis, any inhibitor, which is competitive with respect to the bicarbonate stimulation of the Hill reaction, should increase the half-time of the reactivation of the Hill reaction. Additional key words Electron transport, Pisum sativum, thylakoids, /, F. H. Snel and J. J. S. van Rensen (reprint requests). Lab. of Plant Physiological Research, Agricultural Univ. Wageningen, Gen. Foulkesweg 72, 6703 BW Wageningen, The Netherlands. ^ in a COj-free medium at a higher pH with FeCy as an electron acceptor, it is almost completely lost. The Hill Carbon dioxide (or bicarbonate) is required for photo- reaction cao be partially restored by incubating the synthetic electron transport between Photosystem II chloroplasts with bicarbonate in the dark, according to and Photosystem 1 in isolated broken chloroplasts. The Stemler and Govindjee (1973), Stemler (1979) and major effect of bicarbonate on electron transport is lo- Vermaas and Van Rensen (1981b), Maximal restora- cated between the reoxidation of the first stable Photo- tion occurs at pH 6,5 (Vermaas and Van Rensen system II acceptor Q and the reduction of the PQ pool. 1981a). About 60-80% of the original activity remains For reviews on this matter, see Govindjee and Van lost, and this irreversible deactivation is probably lo- Rensen (1978) and Vermaas and Govindjee (1981), cated at or before the reoxidation of Q~ (Khanna et al.
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