Chapter 4 The primary reactions of photosystems I and II of algae and higher plants

Does a Large Ear Type Wheat Variety Benefit More From Elevated CO2 Than That From Small Multiple Ear-Type in the Quantum Efficiency of PSII Photochemistry?

Recently, several reports have suggested that the growth and grain yield of wheat are significantly influenced by high atmospheric carbon dioxide concentration (CO₂) because of it photosynthesis enhancing effects. Moreover, it has been proposed that plants with large carbon sink size will benefit more from CO₂ enrichment than those with small carbon sink size. However, this hypothesis is yet to be test in winter wheat plant. Therefore, the aim of this study was to examine the effect of elevated CO₂ (eCO₂) conditions on the quantum efficiency of photosystem II (PSII) photochemistry in large ear-type (cv. Shanhan 8675; greater ear C sink strength) and small multiple ear-type (cv. Early premium; greater vegetative C source strength) winter wheat varieties. The experiment was conducted in a free air CO₂ enrichment (FACE) facility, and three de-excitation pathways of the primary reaction of PSII of flag leaf at the anthesis stage were evaluated under two CO₂ concentrations (ambient [CO₂], ∼415 μmol⋅mol^-1, elevated [CO₂], ∼550 μmol⋅mol^-1) using a non-destructive technique of modulated chlorophyll fluorescence. Additionally, the grain yield of the two varieties was determined at maturity. Although elevated CO₂ increased the quantum efficiency of PSII photochemistry (Φ_PSII) of Shanhan 8675 (SH8675) flag leaves at the anthesis stage, the grain number per ear and 1,000-kernel weight were not significantly affected. In contrast, the Φ_PSII of early premium (ZYM) flag leaves was significantly lower than that of SH8675 flag leaves at the anthesis stage, which was caused by an increase in the regulatory non-photochemical energy dissipation quantum (Φ_NPQ) of PSII, suggesting that light energy absorbed by PSII in ZYM flag leaf was largely dissipated as thermal energy. The findings of our study showed that although SH8675 flag leaves exhibited higher C sink strength and quantum efficiency of PSII photochemistry at the anthesis stage, these factors alone do not ensure improved grain yield under eCO₂ conditions.

Biochemical properties and ultrastructure of mesophyll and bundle sheath thylakoids from maize (Zea mays) chloroplasts

A characteristic feature of C4 plants is the differentiation of the photosynthetic leaf tissues into two distinct cell types: mesophyll (M) and bundle sheath (BS) cells. We have investigated several biochemical parameters, including pigment composition, polypeptide patterns, fluorescence at 77K, the activity of photosystems and ultrastructure of mesophyll and bundle sheath chloroplasts of maize (Zea mays L.) plants. It is shown that the BS chloroplasts have ~2-fold higher chlorophyll a/b ratio than M chloroplasts, 6.15 and 3.12 respectively. The PSI apoprotein (68 kDa) was more abundant in BS than in M thylakoids. Polypeptides belonging to PSII core antenna, are in similar amounts in both types of membranes, but the 45kDa band is more intensive in M thylakoids. Polypeptides in the region of 28-24 kDa of the light-harvesting complex of PSII (LHCII) are also present in both types of chloroplasts, though their amounts are reduced in BS thylakoids. The chlorophyll fluorescence emission spectra in M cells showed the presence of three bands at 686, 695 and 735 nm characteristics of LHCII, PSII core and PSI complexes, respectively. However, in the fluorescence spectrum of agranal plastids, there are almost traces of the band at 695 nm, which belongs to the PSII core complex. The research results revealed that the photochemical activity of PSII in BS chloroplasts is ~5 times less than in the chloroplasts of M cells. The highest PSI activity was found in maize BS chloroplasts.

Characterization by FTIR spectroscopy of the photoreduction of the primary quinone acceptor QA in photosystem II

Molecular changes associated with the photoreduction of the primary quinone acceptor Qa of photosystem II have been characterized by Fourier transform infrared spectroscopy. This reaction was light-induced at room temperature on photosystem II membranes in the presence of hydroxylamine and diuron. A positive signal at 1478 cm-1 is assigned to the C---O stretching mode of the semiquinone anion, and can be correlated to the negative C=O mode(s) of the neutral QA at 1645 cm-1 and/or 16 cm-1. Analogies with bacterial reaction center are found in the amide I absorption range at 1672 cm-1, 1653 cm-1 and 1630 cm-1. The stabilization of QA- does not result from a large protein conformation change, but involves perturbations of several amino acid vibrations. At 1658 cm-1, a negative feature sensitive to 1H-2H exchange is tentatively assigned to a NH2 histidine mode, while tryptophan D2252 could contribute to the signal at 1560/1550 cm-1.

Electron spin echo envelope modulation spectroscopy in photosystem I

The applications of electron spin echo envelope modulation (ESEEM) spectroscopy to study paramagnetic centers in photosystem I (PSI) are reviewed with special attention to the novel spectroscopic techniques applied and the structural information obtained. We briefly summarize the physical principles and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. In PSI, ESEEM spectroscopy has been used to the study of the cation radical form of the primary electron donor chlorophyll species, P(700)(+), and the phyllosemiquinone anion radical, A(1)(-), that acts as a low-potential electron carrier. For P(700)(+), ESEEM has contributed to a debate concerning whether the cation is localized on a one or two chlorophyll molecules. This debate is treated in detail and relevant data from other methods, particularly electron nuclear double resonance (ENDOR), are also discussed. It is concluded that the ESEEM and ENDOR data can be explained in terms of five distinct nitrogen couplings, four from the tetrapyrrole ring and a fifth from an axial ligand. Thus the ENDOR and ESEEM data can be fully accounted for based on the spin density being localized on a single chlorophyll molecule. This does not eliminate the possibility that some of the unpaired spin is shared with the other chlorophyll of P(700)(+); so far, however, no unambiguous evidence has been obtained from these electron paramagnetic resonance methods. The ESEEM of the phyllosemiquinone radical A(1)(-) provided the first evidence for a tryptophan molecule pi-stacked over the semiquinone and for a weaker interaction from an additional nitrogen nucleus. Recent site-directed mutagenesis studies verified the presence of the tryptophan close to A(1), while the recent crystal structure showed that the tryptophan was indeed pi-stacked and that a weak potential H-bond from an amide backbone to one of the (semi)quinone carbonyls is probably the origin of the to the second nitrogen coupling seen in the ESEEM. ESEEM has already played an important role in the structural characterization on PSI and since it specifically probes the radical forms of the chromophores and their protein environment, the information obtained is complimentary to the crystallography. ESEEM then will continue to provide structural information that is often unavailable using other methods.

Spin-lattice relaxation of the pheophytin, Pheo-, radical of photosystem II

The spin-lattice relaxation times (T1) of the pheophytin anion radical, Pheo-, of the PSII reaction center, were measured between 5 and 80 K by electron spin-echo spectroscopy. The Pheo- was studied in Mn-depleted PSII reaction centers in which the primary quinone, QA, was doubly reduced. The selective conversion of the non-heme Fe2+ into its low-spin (S = O) state, in CN-treated PSII, allowed the measurement of the intrinsic T1 of the Pheo- radical. The temperature dependence of the intrinsic (T1)-1 was found to be approximately T1.3 +/- 0.1. In Mn-depleted PSII membranes the high-spin (S = 2) non-heme iron, enhances the spin-lattice relaxation of Pheo-. By analyzing the data with a dipolar model, the dipolar interaction (k1d) between the Pheo and the Fe2+ (S = 2) is estimated over the temperature range 5-80 K. Comparison with the dipolar coupling between the iron and the tyrosine, YD+, shows that the Pheo is much closer to the iron than the YD+ in the PSII reaction center. By scaling the reported Fe(2+)-YD+ distance by the ratio [k1dPheo-]/[k1dYD+], we estimate the Fe(2+)-Pheo- distance in PSII to be 20 +/- 4.2 A. This distance is close to the Fe(2+)-BPheo- distance in the bacterial reaction center, and this result provides further evidence that the acceptor sides of the reaction centers in PSII and bacteria are homologous.

Dynamics of electron transfer within and between PS II reaction center complexes indicated by the light-saturation curve of in vivo variable chlorophyll fluorescence emission

The dynamics of light-induced closure of the PS II reaction centers was studied in intact, dark-adapted leaves by measuring the light-irradiance (I) dependence of the relative variable chlorophyll fluorescence V which is the ratio between the amplitude of the variable fluorescence induced by a pulse of actinic light and the maximal variable fluorescence amplitude obtained with an intense, supersaturating light pulse. It is shown that the light-saturation curve of V is a hyperbola of order n. The experimental values of n ranged from around 0.75 to around 2, depending on the plant material and the environmental conditions. A simple theoretical analysis confirmed this hyperbolic relationship between V and I and suggested that n could represent the apparent number of photons necessary to close one reaction center. Thus, experimental conditions leading to n values higher than 1 could indicate that, from a macroscopic viewpoint, more than one photon is necessary to close one PS II center, possibly due to changes in the relative concentrations of the different redox states of the PS II reaction center complexes at the quasi-steady state induced by the actinic light. On the other hand, the existence of environmental conditions resulting in n noticeably lower than 1 suggests the possibility of an electron flow between PS II reaction center complexes.

A guide to electron paramagnetic resonance spectroscopy of Photosystem II membranes

This guide is intended to aid in the detection and identification of paramagnetic species in Photosystem II membranes, by electron paramagnetic resonance spectroscopy. The spectral features and occurrence of each of the electron paramagnetic resonance signals from Photosystem II are discussed, in relation to the nature of the moiety giving rise to the signal and the role of that species in photosynthetic electron transport. Examples of most of the signals discussed are shown. The electron paramagnetic resonance signals produced by the cytochrome b6f and Photosystem I complexes, as well as the signals from other common contaminants, are also reviewed. Furthermore, references to seminal experiments on bacterial reaction centers are included. By reviewing both the spectroscopic and biochemical bases for the electron paramagnetic resonance signals of the cofactors that mediate photosynthetic electron transport, this paper provides an introduction to the use and interpretation of electron paramagnetic resonance spectroscopy in the study of Photosystem II.

Contribution of vitamin K1 to the electron spin polarization in spinach photosystem I

The electron spin polarized (ESP) electron paramagnetic resonance (EPR) signal observed in spinach photosystem I (PSI) particles was examined in preparations depleted of vitamin K1 by solvent extraction and following biological reconstitution by the quinone. The ESP EPR signal was not detected in the solvent-extracted PSI sample but was restored upon reconstitution with either protonated or deuterated vitamin K1 under conditions that also restored electron transfer to the terminal PSI acceptors. Reconstitution using deuterated vitamin K1 resulted in a line narrowing of the ESP EPR signal, supporting the conclusion that the ESP EPR signals in the reconstituted samples arise from a radical pair consisting of the oxidized PSI primary donor, P700+, and reduced vitamin K1.

Nanosecond flash studies of the absorption spectrum of the Photosystem I primary acceptor Ao

Photosystem I particles devoid of the secondary electron acceptor A1 were studied by nanosecond flash absorption. The primary radical pair (P-700(+), A0 (-)) decays with a half-time of 35 ns. The difference spectrum was measured (400-870 nm). After subtraction of the P-700(+)/P-700 difference spectrum, the A0 (-)/A0 was obtained. It includes bleachings centered at 690 and 430 nm, and broad positive bands in the near infra-red and the blue-green. This spectrum is consistent with A0 being chlorophyll a absorbing at 690 nm.