The recovery of photosynthesis in tomato subsequent to chilling exposure

The overall success of a plant in coping with low temperature sensitivity of photosynthesis is dependent not only on the maximum extent of inhibition suffered for a given time of low temperature exposure but also on the persistence of the inhibition after normal growth temperatures are restored. Thus the capacity of recovery and the speed with which a plant can recover from the effects of chilling exposure are important parameters in determining how devastating the chilling event will be on season-long growth and yields. We have studied the recovery of CO₂-saturated photosynthesis from the injury caused by exposing intact tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) or detached tomato leaves to a temperature of 1°C in the dark for varying periods of time. We found that net photosynthesis was fully recovered within 12 h after returning the plants to 25°C in the dark, even after chilling exposures as long as 45 h. This was true for intact plants as well as for detached leaves that were supplied with water. When chilling took place in the light (4°C, 1000 E · m^-2 · s^-1, PAR) inhibition of photosynthesis was more severe and appeared more quickly and the recovery was slower and incomplete. A 12 h chilling exposure in the light resulted in injury to net photosynthesis that was not fully recovered even after 50 h. Chilling damage to photosynthesis developing in the light was distinguished from chilling in the dark by the decreased photosynthetic quantum yield. Not only did high intensity illumination enhance chilling damage of photosynthesis but bright light subsequent to the chilling exposure also delayed the recovery of photosynthesis. At none of the three ambient CO₂ concentrations investigated (300, 1500 and 5000 1.1^-1) did the recovery of photosynthesis depend on stomatal conductance.     

Inhibition of photosynthesis by chilling in moderate light: a comparison of plants sensitive and insensitive to chilling

Photosynthetic activity, in leaf slices and isolated thylakoids, was examined at 25° C after preincubation of the slices at either 25° C or 4° C at a moderate photon flux density (PFD) of 450 mol·m^–2·s^–1, or at 4° C in the dark. The plants used wereSpinacia oleracea L.,Cucumis sativus L. andNerium oleander L. which was acclimated to growth at 20° C or 45° C. The plants were grown at a PFD of 550 mol·m^–2·s^–1. Photosynthesis, measured as CO₂-dependent O₂ evolution, was not inhibited in leaf slices from any plant after preincubation at 25° C at a moderate PFD or at 4° C in the dark. However, exposure to 4° C at a moderate PFD induced an inhibition of CO₂-dependent O₂ evolution within 1 h inC. sativus, a chilling-sensitive plant, and in 45° C-grownN. oleander. The inhibition in these plants after 5 h reached 80% and 40%, respectively, and was independent of the CO₂ concentration but was reduced at O₂ concentrations of less than 3%. Methyl-viologen-dependent O₂ exchange in leaf slices from these plants was not inhibited. There was no photoxidation of chlorophyll, in isolated thylakoids, or any inhibition of electron transport at photosystem (PS)II, PSI or through both photosystems which would account for the inhibition of photosynthesis. The conditions which inhibit photosynthesis in chilling-sensitive plants do not cause inhibition inS. oleracea, a chilling-insensitive plant, or in 20° C-grownN. oleander. The CO₂-dependent photosynthesis, measured at 5° C, was reduced to about 3% of that recorded at 25° C in chilling-sensitive plants but only to about 30% in the chilling-insensitive plants. Methyl-viologen-dependent O₂ exchange, measured at 5° C, was greater than 25% of the activity at 25° C in all the plants. The results indicate that the mechanism of the chilling-induced inhibition of photosynthesis does not involve damage to PSII. That inhibition of photosynthesis is observed only in the chilling-sensitive plants indicates it is related, in some way, to the disproportionate decrease in photosynthetic activity in these plants at chilling temperatures.Abbreviations Chl chlorophyll - DPIPH reduced form of 2,6-dichlorophenol-indophenol - DMQ 2,5-dimethyl-p-benzoquinone - MV methyl viologen - 20°-oleander Nerium oleander grown at 20° C - 45°-oleander N. oleander grown at 45° C - PFD photon flux density (photon fluence rate) - PSI and PSII photosystem I and II, respectively     

Prolonged chilling under moderate light: effect on photosynthetic activity measured with the photoacoustic method

Abstract Exposure of tomato plants to a mild chilling temperature and relatively low ambient photon flux density for an extended period (10°C and 400 μmol photons m^?2 s^?1 d and 5°C night for 6 d) resulted in a significant decrease in the variable chlorophyll fluorescence, the quantum yield of oxygen evolution and the amount of total absorbed energy stored in photochemical intermediates, but not in the chlorophyll concentration or in the activity of ribulose biphosphate carboxylase. These results indicate that photochemical processes involving PSII were affected, and might reflect photoinhibitory effects on the photosynthetic apparatus. Chilling treatment had relatively small influence on the maximal extent of the Emerson effect. This observation, together with the sharp decrease found in the quantum yield of oxygen evolution, could be reconciled with the above results only if some dependency between the two photosystems was assumed. On the basis of this interpretation, it was concluded that the strong Emerson effect after chilling still reflects the typical imbalance between PSI and PSII centres, even though populations of such unaffected pairs are smaller than in the untreated plants. The relatively new photoacoustic technique employed in this study is shown to be useful both as a diagnostic tool and as a means of investigating changes in photochemical activity in the study of environmental stress effects on photosynthesis. The results support the view that photoinhibition can play an important role in limiting photosynthetic activity, and therefore productivity, in chilling-sensitive plants such as the tomato under the natural conditions that prevail during the winter in mediterranean climates.     

Separately and simultaneously induced dark chilling and drought stress effects on photosynthesis, proline accumulation and antioxidant metabolism in soybean

The effects of separately or simultaneously induced dark chilling and drought stress were evaluated in two Glycine max (L.) Merrill cultivars. For the separately induced dark chilling treatment (C), plants were incubated at 8 °C during 9 consecutive dark periods. During the days, plants were kept at normal growth temperatures. For the separately induced drought treatment (D), plants were maintained at normal growth temperatures without irrigation. For the simultaneously induced dark chilling and drought stress treatment (CD), plants were dark chilled without irrigation. All treatments caused similar decreases in pre-dawn leaf water potential, but resulted in distinct physiological and biochemical effects on photosynthesis. In Maple Arrow, where C had the smallest effect on photosynthesis, prolonged CD caused less inhibition of photosynthesis compared to D. Compared to Fiskeby V, the photosynthetic apparatus of Maple Arrow appears to possess superior dark chilling tolerance, a property which probably also conveyed enhanced protection against CD. Proline accumulation was prevented by CD at the ψ_PD where D already resulted in considerable accumulation. The superior capacity for proline accumulation in Maple Arrow would seem to be an important factor in its stress tolerance. Antioxidant activity evoked by CD and D was higher than for C alone. In Fiskeby V, the small increase in ascorbate peroxidase (EC 1.11.1.7) activity, which was in most cases not accompanied by increased gluthatione reductase (EC 1.6.4.2) activity, could impact negatively on its stress tolerance. These results demonstrate large genotypic differences in response to chilling and drought stress, even between soybean cultivars regarded as chilling tolerant.     

Interaction between light and chilling temperature on the inhibition of photosynthesis in chilling-sensitive plants*

Abstract. Fully expanded leaves of 25°C grown Phaseolus vulgaris and six other species were exposed for 3 h to chilling temperatures at photon flux densities equivalent to full sunlight. In four of the species this treatment resulted in substantial inhibition of the subsequent quantum yield of CO₂ uptake, indicating reduction of the photochemical efficiency of photosynthesis. The extent of inhibition was dependent on the photon flux density during chilling and no inhibition occurred when chilling occurred at a low photon flux density. No inhibition occurred at temperatures above 11.5°C, even in the presence of the equivalent of full sunlight. This interaction between chilling and light to cause inhibition of photosynthesis was promoted by the presence of oxygen at normal air partial pressures and was unaffected by the CO₂ partial pressure present when chilling occurred in air. When chilling occurred at low O₂ partial pressures, CO₂ was effective in reducing the degree of inhibition. Apparently, when leaves of chilling-sensitive plants are exposed to chilling temperatures in air of normal composition then light is instrumental in inducing rapid damage to the photochemical efficiency of photosynthesis.     

The relationship between the redox state of Q A and photosynthesis in leaves at various carbon-dioxide,oxygen and light regimes

The response of chlorophyll fluorescence elicited by a low-fluence-rate modulated measuring beam to actinic light and to superimposed 1-s pulses from a high-fluence-rate light source was used to measure the redox state of the primary acceptor Q _A of photosystem II in leaves which were photosynthesizing under steady-state conditions. The leaves were exposed to various O₂ and CO₂ concentrations and to different energy fluence rates of actinic light to assess the relationship between rates of photosynthesis and the redox state of Q _A. Both at low and high fluence rates, the redox state of Q _A was little altered when the CO₂ concentration was reduced from saturation to about 600 l·l^-1 although photosynthesis was decreased particularly at high fluence rates. Upon further reduction in CO₂ content the amount of reduced Q _A increased appreciably even at low fluence rates where light limited CO₂ reduction. Both in the presence and in the absence of CO₂, a more reduced Q _A was observed when the O₂ concentration was below 2%. Q _A was almost fully reduced when leaves were exposed to high fluence rates under nitrogen. Even at low fluence rates, Q _A was more reduced in shade leaves of Asarum europaeum and Fagus sylvatica than in leaves of Helianthus annuus and Fagus sylvatica grown under high light. Also, in shade leaves the redox state of Q _A changed more during a transition from air containing 350 l·l^-1 CO₂ to CO₂-free air than in sun leaves. The results are discussed with respect to the energy status and the CO₂-fixation rate of the leaves.Abbreviations and symbols L 1,2 first and second actinic light beam - Q _A primary acceptor of photosystem II - q _Q Q-quenching     

Acclimation of photosynthesis to increasing atmospheric CO2: The gas exchange perspective

The nature of photosynthetic acclimation to elevated CO₂ is evaluated from the results of over 40 studies focusing on the effect of long-term CO₂ enrichment on the short-term response of photosynthesis to intercellular CO₂ (the A/C_i response). The effect of CO₂ enrichment on the A/C_i response was dependent on growth conditions, with plants grown in small pots (< 5 L) or low nutrients usually exhibiting a reduction of A at a given C_i, while plants grown without nutrient deficiency in large pots or in the field tended to exhibit either little reduction or an enhancement of A at a given C_i following a doubling or tripling of atmospheric CO₂ during growth. Using theoretical interpretations of A/C_i curves to assess acclimation, it was found that when pot size or nutrient deficiency was not a factor, changes in the shape of A/C_i curves which are indicative of a reallocation of resources within the photosynthetic apparatus typically were not observed. Long-term CO₂ enrichment usually had little effect or increased the value of A at all C_i. However, a minority of species grown at elevated CO₂ exhibited gas exchange responses indicative of a reduced amount of Rubisco and an enhanced capacity to metabolize photosynthetic products. This type of response was considered beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity. The ratio of intercellular to ambient CO₂ (the C_i/C_a ratio) was used to evaluate stomatal acclimation. Except under water and humidity stress, C_i/C_a exhibited no consistent change in a variety of C₃ species, indicating no stomatal acclimation. Under drought or humidity stress, C_i/C_a declined in high-CO₂ grown plants, indicating stomata will become more conservative during stress episodes in future high CO₂ environments.Abbreviations A net CO₂ assimilation rate - C_i (C_a) intercellular (ambient) partial pressure of CO₂ - operational C_i intercellular partial pressure of CO₂ at a given ambient partial pressure of CO₂ - g_s stomatal conductance - normal CO₂ current atmospheric mole fraction of CO₂ (330 to 355 mol mol^–1) - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

An evaluation of light and CO2 limitation of leaf photosynthesis by CO2 gas-exchange analysis

Numerical values which define the relative limitation of photosynthesis by light and CO₂ were computed from the slopes of light-and CO₂-response curves of photosynthesis. This method offers an easy approach for the characterization of photosynthesis of leaves.     

Dark-interval relaxation kinetics (DIRK) of absorbance changes as a quantitative probe of steady-state electron transfer

We introduce a new, non-invasive technique to measure linear electron transfer in intact leaves under steady-state illumination. Dark-interval relaxation kinetic or ‘DIRK’ analysis is based on measurements of the initial rates of relaxation of steady-state absorbance signals upon a rapid light-dark transition. We show that estimates of electron flux by DIRK analysis of absorbance signals, reflecting redox changes in the photosynthetic electron transfer chain, can yield quantitative information about photosynthetic flux when the light-dependent partitioning of electrons among redox components of the electron transfer chain are considered. This concept is modeled in computer simulations and then demonstrated in vivo with tobacco plants under non-photorespiratory conditions resulting in linear relationships between DIRK analysis and gross carbon assimilation (A_G). Estimation based on DIRK analysis of the number of electrons transferred through the photosynthetic apparatus for each CO₂ fixed was within 20% of the theoretical value. Possible errors and future improvements are discussed. We conclude that the DIRK method represents a useful tool to address issues such as plant stress and photosynthetic regulation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar light effects on growth,net photosynthesis,and leaf morphology of in vitro kiwifruit (Actinidia deliciosa) CV hayward

Summary Proliferating axillary shoots of kiwifruit (Actinidia deliciosa A. Chev., C. F. Liang and A. R. Ferguson), var.deliciosa, cv. ‘Hayward’ were grown under solar (SL), white (WL), and blue (BL) light regimens to determine the accumulation of fresh and dry weight, proliferation rate, shoot growth (length), and the net leaf photosynthetic capacity at the CO₂ concentration ranges of 200 to 350, 400 to 600, and 1200 to 1500 ppm. An histologic study determined the effects of light source on leaf stomatal density and tissue morphology. Dry and fresh matter accumulation was greatest, but callus development most limited under the SL regimen. Shoot proliferation was highest under WL and length under BL. Net photosynthetic capacity was highest for leaves grown under SL and lowest for those under BL; the leaves exposed to the latter regimen were also thinner and exhibited a less compact arrangement of palisade cells than those under WL and SL. Leaf stomata density was highest under the BL source.     

Inhibition of photosynthesis by osmotic stress in pea (Pisum sativum) mesophyll protoplasts is intensified by chilling or photoinhibitory light; intriguing responses of respiration

The effects of reduced osmotic potential on photosynthesis and respiration were studied in mesophyll protoplasts of pea (Pisum sativum). Osmotic stress was induced by increasing the sorbitol concentration in the medium from 0·4 kmol m⁻³ (-1·3 MPa) to 1·0 kmol m⁻³ (-3·1 MPa). Protoplasts lost up to 35% of the maximum capacity of photo-synthetic carbon assimilation (but not PS II mediated activity) soon after exposure to 1·0 kmol m⁻³ sorbitol. The response of protoplast respiration to osmotic stress was intriguing. Respiration was stimulated if stress was induced at 25°C, but was inhibited when protoplasts were subjected to osmotic stress at 0°C. Photosynthesis was also much more sensitive to osmotic stress at 0°C than at 25°C. The inhibitory effects of osmotic stress on photosynthesis as well as respiration were amplified by not only chilling but also photoinhibitory light. The photosynthetic or respiratory activities of protoplasts recovered remarkably when they were transferred from hyperosmotic (1·0 kmol m⁻³ sorbitol) back to iso-osmotic medium (0·4 kmol m⁻³ sorbitol), demonstrating the reversibility of osmotic-stress-induced changes in protoplasts. Respiration was more resistant to osmotic stress and was quicker to recover than photosynthesis. We suggest that the experimental system of protoplasts can be useful in studying the effects of osmotic stress on plant tissues.     

The chilling sensitivity of root ammonium influx in a cultivated and wild tomato

A chilling episode of a few hours damaged root ammonium absorption in a cultivated tomato ( Lycopersicon esculentum cv. T-5), but not in a wild congener from high altitudes ( Lycopersicon hirsutum LA1778). In the cultivar, ammonium influx was strongly temperature dependent and showed the residual effects of chilling, whereas ammonium efflux was nearly temperature invariant and showed no persistent effects. A 2 h exposure to 5 °C significantly depressed subsequent ammonium absorption at 20 °C, and about 12 h at 20 °C was required for recovery. For both the cultivated and wild species, rerooted cuttings were slightly less sensitive to chilling than seedlings. The relative inhibition (mean ± SE) of ammonium absorption before and after chilling was 58·4 ± 2·5% for the cultivated species and 29·0 ± 9·1% for the wild species. The F₁ hybrid between the species showed a relative inhibition of 52·4 ± 3·6%, suggesting that chilling sensitivity may be dominant. In a backcross of the hybrid to L. esculentum , the phenotypic distribution of the relative inhibition of ammonium absorption indicated that this trait is segregating.     

High CO2 partial pressure effects on dark and light CO2 fixation and metabolism in Vicia faba leaves

Stomatal opening on Vicia faba can be induced by high CO₂ partial pressures (10.2%) in dark as well as in light. Stomatal aperture was measured in both cases with a hydrogen porometer. The distribution of ¹⁴C among early products of photosynthesis was studied. Comparisons are made with carboxylations occurring when stomata were open in the dark with CO₂-free air and in light with 0.034% CO₂. Results showed that in high CO₂ partial pressure in light, less radioactivity was incorporated in Calvin cycle intermediates and more in sucrose. carboxylations and photorespiration seemed to be inhibited. In the dark in both CO₂ conditions, ¹⁴C incorporation was found in malate and aspartate but also in serine and glycerate in high CO₂ conditions. In light these changes in metabolic pathways may be related with the deleterious effects recorded on leaves after long-term expositions to high partial pressure of CO₂.Abbreviations DHAP dihydroxyacetone phosphate - PEP phosphonenolpyruvate - PEPCK phosphonenolpyruvatecarboxykinase - PGA 3-phosphoglyceric acid - RUBPc ribulose 1,5-bisphosphate carboxylase     

Opposite effects of exogenous sucrose on growth, photosynthesis and carbon metabolism of in vitro plantlets of tomato (L. esculentum Mill.) grown under two levels of irradiances and CO2 concentration

The long-term effects of exogenous sucrose (3 percnt;) on growth, photosynthesis and carbon metabolism ofin vitro tomato plantlets were investigated under two sets of growth conditions that respectively favor source- or sink-limitations of photosynthesis: 1) low photosynthetic photon flux (PPF) (50 μmol m⁻² · s⁻¹) and low CO₂ concentration (400 μmol mol⁻¹) and 2) high PPF (500 μmol m⁻² · s⁻¹ and high CO₂ concentration (4000 μmol mol⁻¹). The supply of sucrose under source-limitation conditions increased the growth, the maximal photosynthetic rate, the chl content, the maximal quantum yield of Photosystem II estimated by the Fv/Fm chl fluorescence ratio as well as the soluble sugars (hexoses, sucrose) and starch contents in roots, young and mature leaves when compared to those of photo-autotrophic plantlets. Also, sucrose feeding under these conditions strongly increased the activity of sucrose synthase (SS) (EC 2.4.1.13) in roots and young leaves whereas the activities of sucrose phosphate synthase (SPS) (EC 2.4.1.14), acid invertase (INV) (EC 3.2.1.26) and ADP-glucose pyrophosphorylase (ADPGppase) (EC 2.7.7.27) were highly stimulated in roots and mature leaves. Contrary to these observations, the supply of sucrose to plantlets developed under high PPF and CO₂ concentration decreased growth and led to a somewhat lower maximal photosynthetic rate relative to photo-autotrophic plantlets. These negative responses to exogenous sucrose were accompanied by stronger accumulations of hexose and starch, larger stimulation of INV in mature leaves developed under conditions of sink limitation than those from source limitation conditions. Moreover, under high PPF and high CO₂ concentration, exogenous sucrose led to a marked repression of the SPS activity and caused much lower stimulations of ADPGppase in mature leaves than those observed at low PPF and low CO₂ concentration. We therefore conclude that under our experimental conditions, the interactive effects of exogenous sucrose and environmental conditions on growth and photosynthesis could be rationalized by the source-sink equilibrium of thein vitro tomato plantlets.     

Structural basis for electron and methyl-group transfer in a methyltransferase system operating in the reductive acetyl-CoA pathway

Several anaerobic acetogenic, methanogenic, hydrogenogenic, and sulfate-reducing microorganisms are able to use the reductive acetyl-CoA (Wood-Ljungdahl) pathway to convert CO₂ into biomass. The reductive acetyl-CoA pathway consists of two branches connected by the Co/Fe-containing corrinoid iron-sulfur protein (CoFeSP), which transfers a methyl group from a methyltransferase (MeTr)/methyltetrahydrofolate (CH₃-H₄ folate) complex to the reduced Ni-Ni-[4Fe-4S] cluster (cluster A) of acetyl-CoA synthase. We investigated the CoFeSP and MeTr couple of the hydrogenogenic bacterium Carboxydothermus hydrogenoformans and show that the two proteins are able to catalyze the methyl-group transfer reaction from CH₃-H₄ folate to the Co(I) center of CoFeSP. We determined the crystal structures of both proteins. The structure of CoFeSP includes the previously unresolved N-terminal domain of the large subunit of CoFeSP, revealing a unique four-helix-bundle-like architecture in which a [4Fe-4S] cluster is shielded by hydrophobic amino acids. It further reveals that the corrinoid and the [4Fe-4S] cluster binding domains are mobile, which is mandatory for the postulated electron transfer between them. Furthermore, we solved the crystal structures of apo-MeTr, CH₃-H₄-folate-bound MeTr, and H₄-folate-bound MeTr, revealing a substrate-induced closure of the CH₃-H₄ folate binding cavity of MeTr. We observed three different conformations of Asn200 depending on the substrate bound in the active site, demonstrating its conformational modulation by hydrogen-bonding interactions with the substrate. The observed flexibility could be essential to stabilize the transition state during methyl-group transfer. The conformational space and role of Asn200 are likely conserved in homologous cobalamin-dependent MeTrs such as methionine synthase.     

Measurements of mesophyll conductance,photosynthetic electron transport and alternative electron sinks of field grown wheat leaves

Photosynthetic electron transport drives the carbon reduction cycle, the carbon oxidation cycle, and any alternative electron sinks such as nitrogen reduction. A chlorophyll fluorescence— based method allows estimation of the total electron transport rate while a gas-exchange-based method can provide estimates of the electron transport needed for the carbon reduction cycle and, if the CO₂ partial pressure inside the chloroplast is accurately known, for the carbon oxidation cycle. The gas-exchange method cannot provide estimates of alternative electron sinks. Photosynthetic electron transport in flag leaves of wheat was estimated by the fluorescence method and gasexchange method to determine the possible magnitude of alternative electron sinks. Under non-photorespiratory conditions the two measures of electron transport were the same, ruling out substantial alternative electron sinks. Under photorespiratory conditions the fluorescence-based electron transport rate could be accounted for by the carbon reduction and carbon oxidation cycle only if we assumed the CO₂ partial pressure inside the chloroplasts to be lower than that in the intercellular spaces of the leaves. To further test for the presence of alternative electron sinks, carbon metabolism was inhibited by feeding glyceraldehyde. As carbon metabolism was inhibited, the electron transport was inhibited to the same degree. A small residual rate of electron transport was measured when carbon metabolism was completely inhibited which we take to be the maximum capacity of alternative electron sinks. Since the alternative sinks were small enough to ignore, the comparison of fluorescence and gas-exchange based methods for measuring the rate of electron transport could be used to estimate the mesophyll conductance to CO₂ diffusion. The mesophyll conductance estimated this way fell as wheat flag leaves senesced. The age-related decline in photosynthesis may be attributed in part to the reduction of mesophyll conductance to CO₂ diffusion and in part to the estimated decline of ribulose 1,5-bisphosphate carboxylase amount.     

Long-term chilling of young tomato plants under low light and subsequent recovery

The influence of unfavourable climatic conditions at the onset of the growth period on chilling-sensitive tomato (Lycopersicon esculentum Mill., cv. Abunda) was studied by exposing young plants to combinations of low temperature and low light (60–100 mol quanta · m^–2 · s^–1) for several weeks. When the temperature did not decrease below a critical point (8 ° C) no loss of developmental capacity of the plants was detected. However, while new leaves were readily formed upon return to normal growth conditions (22/18 °C, day/night, in a greenhouse), net accumulation of biomass showed a lag phase of approximately one week. This delay was accompanied by a strong, irreversible inhibition of photosynthesis in the fully expanded leaves which had been exposed to the chilling treatment. When plants were subjected to temperatures below 8 ° C, survival rates decreased after three weeks at 6 ° C and irreversible damage of apical meristematic tissue occurred. Drought-hardening prior to chilling ensured survival at 6 ° C and protected the plants against meristem loss.Abreviation Chl chlorophyll Thanks are due to G.P. Telkamp for technical assistance. This research is financially supported by the Netherlands Technology Foundation (STW, Utrecht, The Netherlands), and is coordinated by the Foundation for Biological Research (BION, 's-Gravenhage, The Netherlands).     

The pathway of electron transfer in the dimeric QH2: Cytochromec oxidoreductase

The experimental data currently available suggest that QH₂: cytochromec oxidoreductase functions according to a Q-cycle type of mechanism. The molecular weight of the enzyme in a natural or artificial phospholipid bilayer or in solution corresponds to that of a dimer. The pre-steady state kinetics of reduction of the prosthetic groups indicate that the enzyme is functionally dimeric. A double Q cycle is proposed, describing the pathway of electron transfer in the dimeric QH₂: cytochromec oxidoreductase. According to this scheme, the two monomeric halves of the enzyme act in a cooperative fashion to complete the catalytic cycle. It is proposed that high-potential cytochromeb-562 and low-potential cytochromeb-562 act cooperatively, viz. as a functional pair, in the antimycin-sensitive reduction of ubiquinone to ubiquinol.     

Combined effects of enhanced ultraviolet-B radiation and doubled CO2 concentration on growth, fruit quality and yield of tomato in winter plastic greenhouse

Five different doses of ultraviolet-B (UV-B) radiation were supplied to tomato (Lycopersicon esculeutum. Mill) with the doubled CO₂ concentration (700 μmol · mol⁻¹) in the winter plastic greenhouse. The influences on the seedling growth, fruit quality and yield of tomato were investigated. Results showed that the seedling growth, and the contents of UV absorbing compounds, soluble sugar, organic acid, vitamin C and lycopene of tomato fruits, and yield of tomato increased under doubled CO₂ concentration. Under the doubled CO₂ concentration the effects of lost doses of UV-B radiation could further promote the effects of doubled CO₂ concentration. However, there is no significant increase in yield of tomato. The best dose of UV-B radiation is about 1.163 kJ·m⁻². When the dose of UV-B radiation is more than it, the effects of UV-B will be reduced. __________ Translated from Journal of Wuhan Botanical Research, 2006, 24(1): 49–53 [译自: 武汉植物学研究]