APPEARANCE AND DEVELOPMENT OF PHOTOSYNTHETIC ACTIVITIES IN WHEAT ETIOPLASTS GREENED UNDER CONTINUOUS OR INTERMITTENT LIGHT—EVIDENCE FOR WATER-SIDE PHOTOSYSTEM II DEFICIENCY AFTER GREENING UNDER INTERMITTENT LIGHT*

Reconstruction of the water-oxidizing complex in manganese-depleted Photosystem II using synthetic manganese complexes

The efficiency of a trinuclear and two binuclear manganese complexes in reconstituting electron transport and O(2) evolution activity in Mn-depleted Photosystem II preparations is analyzed. The trinuclear Mn-complex is more efficient than two binuclear Mn-complexes in restoring oxygen evolution, but it is less effective as an electron donor than binuclear Mn-complexes. It is inferred from our results that recovery of electron transport and O(2) evolution with polynuclear Mn-complexes is affected with different factors. Moreover, the trinuclear Mn-complex is extremely sensitive to the addition of CaCl(2). It is suggested that there is an interaction between Ca(2+) and carboxyl within the trinuclear Mn-complex during photoactivation and this interaction benefits the ligation of Mn atom to the apo-WOC and form an active WOC. Binuclear Mn(III)Mn(III) complex shows slightly higher efficiency than binuclear Mn(III)Mn(IV) complex in restoration of O(2) evolution activity. The efficiency of three Mn-complexes in the reconstitution of WOC is in an order: trinuclear Mn(3)(III)>binuclear Mn(III)Mn(III)>binuclear Mn(III)Mn(IV).

Metallo-radical hypothesis for photoassembly of (Mn)4-cluster of photosynthetic oxygen evolving complex

A new hypothetical mechanism is proposed for photoassembly of the (Mn)4-cluster of the photosynthetic oxygen evolving complex (OEC). In this process, a neutral radical of Y(Z) tyrosine plays a role in oxidizing Mn2+ associated with an apo-OEC, and also in abstracting a proton from a water molecule bound to the Mn2+ ion, together with D1-His190. This is in a similar fashion to the metallo-radical mechanism proposed for photosynthetic water oxidation by the (Mn)4-cluster. The model insists that a common mechanism participates in the photoassembly of the (Mn)4-cluster and the photosynthetic water oxidation.

Quenching of a room temperature fluorescence band at 693 nm during photoactivation of the water-splitting system of Photosystem II in flashed barley leaves

The modifications of the room temperature fluorescence spectrum during the photoactivation of the water-splitting system by continuous illumination were investigated in flashed barley leaves. A blue shift of the chlorophyll fluorescence band was detected during the first 2 min of illumination. During this shift, a decrease of the fluorescence intensity around 693 nm could be demonstrated in difference spectra and in second derivative spectra. This decrease is interpreted as a quenching of PS II fluorescence during the photoactivation. A relative fluorescence increase around 672 nm also occurred during the same period and is thought to reflect rapid light-induced chlorophyll formation. The flashed leaves contained small amounts of photoactive photochlorophyllide which could be removed by a short flash of intense white light given before continuous illumination. The fact that such flash had only weak effect on the 693 nm fluorescence decrease, whereas it strongly reduced the amplitude of the 672 nm fluorescence increase, favours the above interpretations.

A possible role of redox-active histidine in the photoligation of manganese into a photosynthetic O2-evolving enzyme

Our previous experiments with a histidine modifier suggest that in Mn-depleted photosystem (PS) II a histidine residue is photooxidized and charge recombination between the oxidized histidine and QA-emits the thermoluminescence AT-band [Ono, T., & Inoue, Y. (1991) FEBS Lett. 278, 183-186]. By use of the AT-band as an index for histidine oxidation, and EPR signals IIf and IIs as indexes for tyrosine oxidation, we studied the role of this putative redox-active histidine in the photoactivation of the O2-evolving enzyme in NH2OH-treated PSII. The following results have been obtained. (i) Strong-light photoinhibition of NH2OH-treated PSII quickly impaired both capabilities of photoactivation and AT-band emission with almost the same half-inhibition time of 1-2 s, while signal IIf was well retained and signal IIs was not affected at all after complete loss of photoactivation capability. (ii) The capability of exogenous Mn2+ photooxidation was relatively sensitive to strong-light photoinhibition, but DPC was relatively sensitive to strong-light photoinhibition, but DPC photooxidation was highly resistant. (iii) Weak-light photoinhibition simultaneously impaired the capabilities of photoactivation, AT-band emission, and signal IIf with the same half-inhibition time of 1 min, leaving signal IIs unaffected. (iv) It was inferred that the putative redox-active histidine is essential for the photooxidation of coordinated Mn2+, the probable initial step of photoactivation, and its photodamage results in the loss of the capabilities of photoactivation and AT-band emission. Based on these, a scheme of electron transfer on the donor side of PSII involving histidine oxidation via Z+ is proposed.

Kinetics of photoinhibition in hydroxylamine-extracted photosystem II membranes: relevance to photoactivation and sites of electron donation

Kinetic analyses were made of the effects of weak-light photoinhibition on the capacity of NH2OH-extracted photosystem II membranes to photooxidize the exogenous electron donors Mn2+, diphenylcarbazide, and I- or to assemble functional water-oxidizing complexes during photoactivation. The loss of capacity for photooxidation of the donors showed two first-order components (half-times of 2-3 min and 1-4 h) with relative amplitudes dependent on the donor, suggesting two photodamageable sites of electron donation (sites 1 and 2, respectively), a conclusion confirmed by analyses of velocity curves of electron donation by each donor. All of the donors appear to be oxidized preferentially by site 1 both at saturating and at limiting light intensity; however, the contribution by site 2 was nearly comparable in saturating light. Loss of photoactivation also exhibited biphasic kinetics, with components having half-times of approximately 0.8 and 3.2 min. The major component (t1/2 = 3.2 min) corresponded to loss of site 1; essentially no photoactivation was observed after its loss. From these and other analyses, we conclude (1) the relative contributions of site 1 and site 2 to the photooxidation of various exogenous electron donors is determined largely by the rates of equilibration of the donors with the two sites, and (2) only site 1 contributes to photoactivation of the water-oxidizing complex. Site 1 is attributed to tyrosine Z of the reaction center's D1 polypeptide. The molecular identity of site 2 is unknown but may be tyrosine D of the D2 polypeptide.

Energy transfer in the photochemical apparatus of flashed bean leaves

Fluorescence and energy transfer properties of bean leaves greened by brief, repetitive xenon flashes were studied at -196 degrees C. The bleaching of P-700 has no influence on the yield of fluorescence at any wavelength of emission. The light-induced fluorescence yield changes which are observed in both the 690 and 730 nm emission bands in the low temperature fluorescence spectra are due to changes in the state of the Photosystem II reaction centers. The fluorescence yield changes in the 730 nm band are attributed to energy transfer from Photosystem II to Photosystem I. Such energy transfer was also confirmed by measurements of the rate of photooxidation of P-700 AT -196 degrees C in leaves in which the Photosystem II reaction centers were either all open or all closed. It is concluded that energy transfer from Photosystem II to Photosystem I occurs in the flashed bean leaves which lack the light-harvesting chlorophyll a/b protein.

Light-dependent development of thermoluminescence, delayed emission and fluorescence variation in dark-grown spruce leaves

Thermoluminescence profiles of spruce leaves grown under various light or dark conditions were measured after excitation at a low temperature (-70 to -20 degrees C) by 1-min illumination with red light, and the following results were obtained. Mature spruce leaves showed five thermoluminescence bands at -30, -5, +20, +40 (or +35) and +70 degrees C (denoted as Zv, A, B1, B2 and C bands, respectively), but dark-grown spruce leaves with a similar chlorophyll content showed only two bands, at -30 and +70 degrees C (the Zv and C bands) and were devoid of the three other bands (the A, B1, and B2 bands). On exposure of the dark-grown leaves to continuous red light, the A, B1and B2 bands were rapidly developed, and the development was accompanied by enhancement of delayed emission, fluorescence variation and the Hill activity (photoreduction of 2,6-dichlorophenolindophenol with water as electron donor). It was demonstrated that the dark-grown spruce leaves are devoid of the water-splitting system in Photosystem II, and that the latent water-splitting activity is rapidly photoactivated by exposure of the leaves to continuous red light. These results on the gymnosperm spruce leaves, in which greening proceeds in complete darkness, being independent of the development of the water-splitting system in light, were discussed in relation to previous observations on angiosperm leaves, in which both greening and the activity generation proceed in the light.

Multiple-flash development of thermoluminescence bands in dark-grown spruce leaves

Spruce leaves greened in darkness were devoid of three of the five thermoluminescence bands found for mature leaves. These bands were developed rapidly by exposure of the dark-grown leaves to continuous light. The development of these bands was studied by illumination with repetitive flashes at varied intervals. Flashes at intervals of 1 s were the most effective in inducing these bands. Those at shorter or longer intervals were less effective. It was deduced from these data that the development of these bands is a multiquantum process which involves at least two photo-events with a dark reaction between them.

[Photosynthetic activity in the absence of CPl and CP2 pigmentary complexes (author's transl)]

Various photochemical activities were tested on chloroplasts of Zea mays that received 4 s of light every 4 h during the culture period. Photosystem I and Photosystem II were functioning, as well as the photosynthetic electron transport. These chloroplasts exhibited upon sodium dodecyl sulphate gel electrophoresis neither Complex 1 (Mr 70 000) generally associated with Photosystem I nor Complex 2 Mr 25 000) generally associated with Photosystem II. Chlorophyll is indeed attached to polypeptides of molecular weight 21 000 and 29 000. These results lead us to question the functional role of chloroplast protein-pigment complexes observed by sodium dodecyl sulphate gel electrophoresis.

Appearance and development of P700 oxidation and photosystem I activity in etio-chloroplasts prepared from greening barley leaves

The development of photosystem I activity of plastids isolated from greening barley (Hordeum distichum, L.) leaves was studied. The electron transport activity in photosystem I was measured as anthraquinone-mediated oxygen uptake and as light induced absorbance changes of the reaction centre molecule P700. P700 oxidation was observed after one hour of greening though an electron transport leading to oxygen uptake was observed after 30 minutes. Phenazine methosulphate had no effect on the oxidation of P700 until after four hours of greening. The ratio chlorophyll/P700 decreased from about 2300/l at one hour to 640/l at sixteen hours of greening. The light intensity dependence of the electron transport of photosystem I showed that the photosynthetic units gradually increased in size as the greening proceeded. The increase of the rate of the oxygen uptake, calculated on plastid basis, decreased after eight hours while the P700 content, calculated on plastid basis, increased continuously between three and sixteen hours. Chromatographic separations and fluorimetric analyses of the chlorophyll pigments showed that the reaction centre molecule could not be protochlorophyllide or chlorophyllide.

Characteristics of thermoluminescence bands of intact leaves and isolated chloroplasts in relation to the water-splitting activity in photosynthesis

Plant materials (intact leaves, chloroplasts or subchloroplast particles) pre-illuminated at a low temperature (e.g. -60 degrees C) were rapidly cooled to -196 degrees C and then the luminescence emitted from the sample on raising the temperature was measured as a function of temperature, by means of a sensitive photo-electron counting technique. Mature spinach leaves showed five luminescence bands at different temperatures which were denoted as ZV, A, B1, B2 and C bands. The A, B1, B2 and C bands appeared at constant temperatures, -10, +25, +40 and +55 degrees C, respectively, being independent of the illumination temperature, but the ZV band appeared at a variable temperature slightly higher than the illumination temperature. The B1 and B2 bands were absent in the thermoluminescence profiles of samples devoid of the oxygen-evolving activity, such as heat-treated spinach leaves, wheat leaves greened under intermittent illumination and photosystem-II particles prepared with Triton X-100. It was deduced that these luminescence bands arise from the energy stored by the electron flow in photosystem II to evolve oxygen, and other bands were ascribed to charge-separation in some other sites not related to the oxygen evolving system.

Multiple-flash activation of the water-photolysis system in wheat leaves as observed by delayed emission

The chloroplasts from wheat leaves greened under intermittent illuminations (1 ms in duration) at long intervals (5 min) are capable of photoreducing DCIP (2,6-dichlorophenolindophenol) with diphenylcarbazide as an electron donor but are incapable of photoreducing DCIP with water as the donor. On exposure of such intermittently illuminated leaves to flashes spaced at intervals of less than 10s, the delayed light emission from the leaves was greatly enhanced in parallel with the generation of Hill activity. The mechanism of this photoactivation was studied by following the changes of the delayed emission from intermittently illuminated leaves exposed to short-interval flashes programmed in various ways. Analysis of the kinetic data indicated that the photoactivation involves three consecutive photoreactions with a rate-limiting dark reaction between them; P-light leads to A0-light leads to A1-dark leads to A2-light leads to A3 in which P is a precursor convertible to A0, the first intermediate with a longer lifetime of t 1/2 approximately 100s and A3 is the final activated compound or state converted by short-interval flashes from A0 through A1 and A2, two other intermediates with shorter lifetimes of t 1/2 approximately 0.4s and 5s, respectively.