Bicarbonate effects in leaf discs from spinach

Abstract

In this paper, we show the unique role of bicarbonate ion in stimulating the electron transfer of photosystem II (PS II) in formate-treated leaf discs from spinach. This is referred to as the "bicarbonate effect" and is independent of the role of CO2 in CO2 fixation. It is shown to have two sites of action: (1) the first, described here for the first time, stimulates the electron flow between the hydroxylamine donation site ("Z" or "D") and QA, the first plastoquinone electron acceptor and (2) the other accelerates the electron flow beyond QA, perhaps at the QA QB complex, where QB is the second plastoquinone electron acceptor. The first site of inhibition by formate-treatment is detected by the decrease of the rate of oxygen evolution and the simultaneous quenching of the variable chlorophyll a (Chl a) fluorescence of leaf discs infiltrated with 100 mM formate for about 10 s followed by storage for 10 min in dark. This is referred to as short-term formate treatment. Addition of bicarbonate reverses this short-term formate effect and restores fully both Chl a fluorescence and oxygen evolution rate. Reversible quenching of variable Chl a fluorescence of heated and short-term formate treated leaf discs, in the presence of hydroxylamine as an artificial electron donor to PS II, is also observed. This suggests that the first site of action of the anion effect is indeed between the site of donation of hydroxylamine to PS II (i.e. "Z" or "D") and QA. The second site of the effect, where bicarbonate depletion has its most dramatic effect, as well known in thylakoids, is shown by an increase of Chl a fluorescence of leaf discs infiltrated with 100 mM formate for about 10 min followed by storage for 10 min in dark. This is referred to as the long-term formate treatment. Addition of bicarbonate fully restores the variable Chl a fluorescence of these leaf discs. Chl a fluorescence transient of DCMU-infiltrated (10 min) leaf discs is similar to that of long-term formate-treated one suggesting that the absence of bicarbonate, like the presence of DCMU, inhibits the electron flow beyond QA.

Bicarbonate requirement for the donor side of photosystem II

“…In the early 1970s the high-affinity pool had been ascribed to the donor side of PS-II [2,5,6] and a model including BC as a mediator for photosynthetic water oxidation had been suggested [5,6] which however was in contradiction with the results of isotopic experiments [7]. Wydrzynski and Govindjee [3] observed an accelerated rise of Chl fluorescence in BC-depleted chloroplasts which demonstrated that reoxidation of the first plastoquinone acceptor, Qa, by the plastoquinone pool has been impaired by the removal of BC which was supported by a number of data ( [8][9][10][11] and references therein). The non-heme Fe between QA and QB has been shown to play an essential role in BC binding [8].…”

“…E-mail: Klimov@issp.serpukhov.su Abbreviations: BC, bicarbonate; DCMU, 3-(3,4-dichlorphenyl)-l, 1-dimethylurea; DCPIE 2,6-dichlorophenol-indophenol; DPC, diphenylcarbazide; EDTA, ethylenediaminetetraacetic acid. effect) confirmed that BC evidently has two sites of action: the first accelerates the electron flow beyond Qa, and the other stimulates it between the secondary electron donor, Yz, and QA (the site between the primary electron acceptor, pheophytin, and QA was suggested for the latter case [10]). In this paper we present evidence for a BC requirement on the donor side of PS-II and for involvement of BC in reconstitution of the Mn cluster in Tris-washed PS-II membranes.…”

“…1) while NaHCO3 reverses this effect. This effect was investigated in detail earlier [3,10,11] and it was ascribed to binding of BC to the non-heme iron acting between QA and QB [8]. The quite opposite effect with low (5-20/JM) concentrations of formate on AF (Fig.…”

“…However, there are data showing that BC-depletion affects both the electron acceptor and the donor side of PS-II [9]. Govindjee and co-workers [10][11] (using formate for accentuating the BC *Corresponding author, Fax: (7) (095) 923 3602. E-mail: Klimov@issp.serpukhov.su Abbreviations: BC, bicarbonate; DCMU, 3-(3,4-dichlorphenyl)-l, 1-dimethylurea; DCPIE 2,6-dichlorophenol-indophenol; DPC, diphenylcarbazide; EDTA, ethylenediaminetetraacetic acid.…”

Modeling of the D1/D2 proteins and cofactors of the photosystem II reaction center: Implications for herbicide and bicarbonate binding

“…The depletion of bicarbonate by its analogue formate results in a significant inhibition of the electron transfer on the acceptor side, i.e., from QA to the plastoquinone pool (see references in Blubaugh & Govindjee, 1988a;Govindjee & van Rensen, 1993). Studies have also suggested that bicarbonate may affect the donor side function of PSII (El-Shintinawy & Govindjee, 1990;Jursinic & Dennenberg, 1990;Stemler & Jursinic, 1993;Klimov et al, 1995;Wincencjusz et al, 1996).…”