Photosynthesis Research

The functional site of ChlZ, an auxiliary electron donor to P680+, was determined by pulsed ELDOR applied to a radical pair of Y D • and Chlz+ in oriented PS II membranes from spinach. The radical-radical distance was determined to be 29.5 Å and its direction was 50° from the membrane normal, indicating that a chlorophyll on the D2 protein is responsible for the EPR Chlz+ signal. Spin polarized ESEEM (Electronin Spin Echo Envelop Modulation) of a 3Chl and Q A − radical pair induced by a laser flash was observed in reaction center D1D2Cytb559 complex, in which QA was functionally reconstituted with DBMIB and reduced chemically. Q A − ESEEM showed a characteristic oscillating time profile due to dipolar coupling with 3Chl. By fitting with the dipolar interaction parameters, the distance between 3Chl and Q A − was determined to be 25.9 Å, indicating that the accessory chlorophyll on the D1 protein is responsible for the 3Chl signal.

Exposure of algae or higher plants to bright light can result in a photoinhibitory reduction in the number of functional PS II reaction centers (n) and a consequential decrease in the maximum quantum yield of photosynthesis. However, we found that light-saturated photosynthetic rates (Pmax) in natural phytoplankton assemblages sampled from the south Pacific ocean were not reduced despite photoinhibitory decreases in n of up to 52%. This striking insensitivity of Pmax to photoinhibition resulted from reciprocal increases in electron turnover ( $${1 \mathord{\left/ {\vphantom {1 {\tau _{PSII} }}} \right. \kern-\nulldelimiterspace} {\tau _{PSII} }}$$ )through the remaining functional PS II centers. Similar insensitivity of Pmax was also observed in low light adapted cultures of Thalassiosira weissflogii (a marine diatom), but not in high light adapted cells where Pmax decreased in proportion to n. This differential sensitivity to decreases in n occurred because $${1 \mathord{\left/ {\vphantom {1 {\tau _{PSII} }}} \right. \kern-\nulldelimiterspace} {\tau _{PSII} }}$$ was close to the maximum achievable rate in the high light adapted cells, whereas $${1 \mathord{\left/ {\vphantom {1 {\tau _{PSII} }}} \right. \kern-\nulldelimiterspace} {\tau _{PSII} }}$$ was initially low in the low light adapted cells and could thus increase in response to decreases in n. Our results indicate that decreases in plant productivity are not necessarily commensurate with photoinhibition, but rather will only occur if decreases in n are sufficient to maximize $${1 \mathord{\left/ {\vphantom {1 {\tau _{PSII} }}} \right. \kern-\nulldelimiterspace} {\tau _{PSII} }}$$ or incident irradiance becomes subsaturating.

In this study we show that the diadinoxanthin cycle in the diatom Phaeodactylum tricornutum is stimulated by mild UV-B radiation. High steady state concentrations of diatoxanthin established during a period of strong actinic illumination with white light (300 μmol photons m-2 s-1 PAR) are further increased if weak UV-B (3 μmol photons m-2 s-1) is additionally applied. Short term increases in the diatoxanthin concentration caused by UV-B strongly correlate with a stoichiometric decrease in diadinoxanthin. The UV-B dependent increase in diatoxanthin is correlated with a concommitant enhancement of non-photochemical quenching of chlorophyll fluorescence and a decrease in the quantum efficiency of oxygen evolution. This indicates that UV-B induced diatoxanthin functions in thermal energy dissipation. Possible scenarios for a stimulation of the diadinoxanthin cycle by UV-B are discussed.

Using DTT and iodoacetamide as a novel irreversible method to inhibit endogenous violaxanthin de-epoxidase, we found that violaxanthin could be converted into zeaxanthin from both sides of the thylakoid membrane provided that purified violaxanthin de-epoxidase was added. The maximum conversion was the same from both sides of the membrane. Temperature was found to have a strong influence both on the rate and degree of maximal violaxanthin to zeaxanthin conversion. Thus only 50% conversion of violaxanthin was detected at 4 °C, whereas at 25 °C and 37 °C the degree of conversion was 70% and 80%, respectively. These results were obtained with isolated thylakoids from non-cold acclimated leafs. Pigment analysis of sub-thylakoid membrane domains showed that violaxanthin was evenly distributed between stroma lamellae and grana partitions. This was in contrast to chlorophyll a and β-carotene which were enriched in stroma lamellae fractions while chlorophyll b, lutein and neoxanthin were enriched in the grana membranes. In combination with added violaxanthin de-epoxidase we found almost the same degree of conversion of violaxanthin to zeaxanthin (73–78%) for different domains of the thylakoid membrane. We conclude that violaxanthin de-epoxidase converts violaxanthin in the lipid matrix and not at the proteins, that violaxanthin does not prefer one particular membrane region or one particular chlorophyll protein complex, and that the xanthophyll cycle pigments are oriented in a vertical manner in order to be accessible from both sides of the membrane when located in the lipid matrix.

We describe a technique to measure the fluorescence decay profiles of intact leaves during adaptation to high light and subsequent relaxation to dark conditions. We show how to ensure that photosystem II reaction centers are closed and compare data for wild type Arabidopsis thaliana with conventional pulse-amplitude modulated (PAM) fluorescence measurements. Unlike PAM measurements, the lifetime measurements are not sensitive to photobleaching or chloroplast shielding, and the form of the fluorescence decay provides additional information to test quantitative models of excitation dynamics in intact leaves.

5-O-β-d-galactopyranosyl-7-methoxy-3′,4′-dihydroxy-4-phenylcoumarin isolated from Exostema caribaeum (Rubiaceae) has been found to act as an energy-transfer inhibitor in spinach chloroplasts. ATP synthesis and phosphorylating (coupled) electron flow were inhibited by 89 and 72%, respectively, at a concentration of 400 μM. H+-uptake, basal and uncoupled electron transport were not affected by the coumarin. The light-activated Mg+2-ATPase activity from bound membrane thylakoid chloroplasts was slightly inhibited by the coumarin. Also, the heat-activated Ca+2-ATPase activity of the isolated coupling factor protein was insensitive to this compound. In chloroplasts partially stripped of coupling factor 1 by an EDTA treatment, the coumarin showed a restoration of the proton uptake process. These results suggest that the 4-phenylcoumarin under investigation inhibited phosphorylation in chloroplasts by specifically blocking the transport of protons through a membrane-bound component or a carrier channel (CFO) located in a hydrophobic region at or near the functional binding site for the coupling factor 1.

Phosphoenolpyruvate carboxylase from leaves of the C4 plant Setaria verticillata (L.) Beauv. is activated by light; day levels of activity are reached after 30 minutes of illumination. Photoactivation is prevented by inhibitors of photosynthetic electron flow or of photophosphorylation and by D,L-glyceraldehyde, which inhibits the reductive pentose phosphate pathway. Although the extractable activity in the dark is not affected by temperature the photoactivation is prevented when both illumination and extraction are done under low temperature (5 C). High temperature (30 C) during either illumination or extraction is needed for activation. Once the enzyme is photoactivated at 30 C, a transfer of the leaves to 5 C does not abolish the extra activity. The results suggest that both unimpaired electron flow and photophosphorylation are prerequisites for the activation of phosphoenolpyruvate carboxylase. Low temperature apparently suppresses either the transport to the cytoplasm of a photosynthetic intermediate or the activating reaction itself. The inclusion of phosphoenolpyruvate in the extraction medium increases the night activity. On the basis of the available information, it is suggested that phosphoenolpyruvate could be the activator in vivo. In that case, the activation of phosphoenolpyruvate carboxylase would depend on internal CO2 level and prior photoactivation of both pyruvate, orthophosphate, dikinase and NADP malate dehydrogenase.

Cupric ion (Cu++) inhibits the rate of photosystem II electron transport and the intensity of the variable part of chl a fluorescence in isolated chloroplast thylakoids. The inhibition is markedly dependent on the nature of the buffer used in the assay medium. In MES and HEPES buffers, complete inhibition of photosystem II occurs at 30 μM of Cu++, while in Tricine no inhibition occurred even at 200 μM Cu++. In other buffers used (TES, Phosphate, Tris), the efficacy of Cu++ inhibition is intermediate. The calculated binding constants are found to correspond to the observed I50 values for the six buffers used. It is concluded that the previous reports on copper inhibition, where buffers have been used indiscriminately should be reconsidered. Certain reagents such as hydroxylamine, ascorbate and diphenyl carbazide, which react with Cu++, should be avoided.

Ngoài những hoàn cảnh đã được ghi nhận rõ ràng liên quan đến ribulose-1,5-bisphosphate carboxylase/oxygenase, các phép tính chỉ ra rằng còn có một số enzym khác trong chu trình khử cacbon quang hợp (ví dụ: glyceraldehyde-3-phosphate dehydrogenase, aldolase, phosphoribulokinase, transketolase) mà nồng độ trong stroma có thể dễ dàng đạt hoặc vượt qua nồng độ của một hoặc nhiều cơ chất của chúng. Những hoàn cảnh như vậy có tác động sâu rộng từ việc thay đổi hành vi động học của enzym và sự gắn kết của các chuyển hóa cho đến việc tạo ra các vi môi trường trong stroma có thể hỗ trợ việc kênh hóa hoặc ảnh hưởng đến sự phân bố của các chuyển hóa. Việc xem xét nồng độ tương đối của tất cả các enzym trong chu trình khử cacbon quang hợp sẽ được coi là thiết yếu cho việc hiểu biết đầy đủ về hoạt động và điều tiết của nó.