PHYSIOL. PLANT. 57: 422-427. Copenhagen 1983
and in the
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
L., cv. Rondo) chloroplasts with
bicarbonate. The half-time of this reactivation appears to be 25 s when 2
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
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,
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.
Usually broken chloroplasts are depleted of COj in a