Total Electron Transport Research Articles

Effect of high salinity and of priming of non-germinated seeds by UV-C light on photosynthesis of lettuce plants grown in a controlled soilless system.

High salinity results in a decrease in plant photosynthesis and crop productivity. The aim of the present study was to evaluate the effect of UV-C priming treatments of lettuce seeds on photosynthesis of plants grown at high salinity. Non-primed and primed seeds were grown in an hydroponic system, with a standard nutrient solution, either supplemented with 100 mM NaCl (high salinity), or not (control). Considering that leaf and root K+ concentrations remained constant and that chlorophyll fluorescence parameters and root growth were not affected negatively in the high salinity treatment, we conclude that the latter was at the origin of a moderate stress only. A substantial decrease in leaf net photosynthetic assimilation (Anet) was however observed as a consequence of stomatal and non-stomatal limitations in the high salinity treatment. This decrease in Anet translated into a decrease in growth parameters; it may be attributed partially to the high salinity-associated increase in leaf concentration in abscisic acid and decrease in stomatal conductance. Priming by UV-C light resulted in an increase in total photosynthetic electron transport rate and Anet in the leaves of plants grown at high salinity. The increase of the latter translated into a moderate increase in growth parameters. It is hypothesized that the positive effect of UV-C priming on Anet and growth of the aerial part of lettuce plants grown at high salinity, is mainly due to its stimulating effect on leaf concentration in salicylic acid. Even though leaf cytokinins' concentration was higher in plants from primed seeds, maintenance of the cytokinins-to-abscisic acid ratio also supports the idea that UV-C priming resulted in protection of plants exposed to high salinity.

Maternal and Intrauterine Influences on Feto-Placental Growth Are Accompanied by Sexually Dimorphic Changes in Placental Mitochondrial Respiration, and Metabolic Signalling Pathways.

Adverse maternal environments such as small size, malnutrition, and metabolic conditions are known to influence fetal growth outcomes. Similarly, fetal growth and metabolic alterations may alter the intrauterine environment and affect all fetuses in multiple gestation/litter-bearing species. The placenta is the site of convergence between signals derived from the mother and the developing fetus/es. Its functions are fuelled by energy generated by mitochondrial oxidative phosphorylation (OXPHOS). The aim of this study was to delineate the role of an altered maternal and/or fetal/intrauterine environment in feto-placental growth and placental mitochondrial energetic capacity. To address this, in mice, we used disruptions of the gene encoding phosphoinositol 3-kinase (PI3K) p110α, a growth and metabolic regulator to perturb the maternal and/or fetal/intrauterine environment and study the impact on wildtype conceptuses. We found that feto-placental growth was modified by a perturbed maternal and intrauterine environment, and effects were most evident for wildtype males compared to females. However, placental mitochondrial complex I+II OXPHOS and total electron transport system (ETS) capacity were similarly reduced for both fetal sexes, yet reserve capacity was additionally decreased in males in response to the maternal and intrauterine perturbations. These were also sex-dependent differences in the placental abundance of mitochondrial-related proteins (e.g., citrate synthase and ETS complexes), and activity of growth/metabolic signalling pathways (AKT and MAPK) with maternal and intrauterine alterations. Our findings thus identify that the mother and the intrauterine environment provided by littermates modulate feto-placental growth, placental bioenergetics, and metabolic signalling in a manner dependent on fetal sex. This may have relevance for understanding the pathways leading to reduced fetal growth, particularly in the context of suboptimal maternal environments and multiple gestation/litter-bearing species.

Photosynthetic electron transport rate (ETR) in the littoral herb Launaea sarmentosa known as mole crab in Thailand.

Launaea sarmentosa (Willd.) Sch. Bip ex Kunze (Asteracaeae) is a littoral sand dune herb found in the Indian Ocean region, used as a folk medicine and as a savory vegetable in Thailand. It is in the transition stage from a kitchen & cottage industry to a commercial proposition. Rapid light curves to measure the photosynthetic electron transport rate (ETR) were conducted on the plants over the course of daylight from 6:00 to 18:00 using a PAM fluorometer on plants grown under 50% (nominal) green horticultural and 20% black (nominal) shade cloth and in the open. Plants grown in the open were sun plants. Eopt (µmol photon m-2s-1) decreased slightly under shade cloth (open air: 890 > green shade cloth, 778 > green shade cloth, 713). Launaea shows limited shade adaptation. ETR decreased under shade cloth on both a surface area and Chl a basis (µmol e- g Chl a-1s-1) [open air: 388 > (green shade cloth, 209 = black shade cloth, 263)]. Maximum non-photochemical quenching (NPQmax) was significantly decreased under black shade cloth [(open air: 1.02 = green shade cloth, 0.969) > black shade cloth, 0.694]. ETR showed midday inhibition (9:00-15:00) when irradiance exceeded the Eopt of Launaea. Daily total photosynthetic electron transport was (mmol e- m-2 d-1): open air, 1890 ± 157; green shade cloth, 1620 ± 203, black shade cloth, 1217 ± 143. Green shade cloth has no effect on total daily photosynthetic electron transport of Launaea but offers some protection from desiccation and excessive evapotranspiration. Waterlogging decreases ETR by about 40%. Launaea can be grown watered with brackish (½ seawater) water unlike many other vegetables. Launeae is a physiologically undemanding cottage industry/market garden crop suitable for sandy coastline fishing communities.

Longer Photoperiods with the Same Daily Light Integral Increase Daily Electron Transport through Photosystem II in Lettuce.

Controlled environment crop production recommendations often use the daily light integral (DLI) to quantify the light requirements of specific crops. Sole-source electric lighting, used in plant factories, and supplemental electric lighting, used in greenhouses, may be required to attain a specific DLI. Electric lighting is wasteful if not provided in a way that promotes efficient photochemistry. The quantum yield of photosystem II (ΦPSII), the fraction of absorbed light used for photochemistry, decreases with increasing photosynthetic photon flux density (PPFD). Thus, we hypothesized that the daily photochemical integral (DPI), the total electron transport through photosystem II (PSII) integrated over 24 h, would increase if the same DLI was provided at a lower PPFD over a longer photoperiod. To test this, ΦPSII and the electron transport rate (ETR) of lettuce (Lactuca sativa ‘Green Towers’) were measured in a growth chamber at DLIs of 15 and 20 mol m−2 d−1 over photoperiods ranging from 7 to 22 h. This resulted in PPFDs of 189 to 794 μmol m−2 s−1. The ΦPSII decreased from 0.67 to 0.28 and ETR increased from 55 to 99 μmol m−2 s−1 as PPFD increased from 189 to 794 μmol m−2 s−1. The DPI increased linearly as the photoperiod increased, but the magnitude of this response depended on DLI. With a 7-h photoperiod, the DPI was ≈2.7 mol m−2 d−1, regardless of DLI. However, with a 22-h photoperiod, the DPI was 4.54 mol m−2 d−1 with a DLI of 15 mol m−2 d−1 and 5.78 mol m−2 d−1 with a DLI of 20 mol m−2 d−1. Our hypothesis that DPI can be increased by providing the same DLI over longer photoperiods was confirmed.

Accessions of Brazilian ginseng (Pfaffia glomerata) with contrasting anthocyanin content behave differently in growth, antioxidative defense, and 20-hydroxyecdysone levels under UV-B radiation.

Ultraviolet-B (UV-B) radiation is an elicitor of secondary metabolites in plant tissue culture, but the effects on 20-hydroxyecdysone (20E) are still unclear. The 20E may show biotechnological, pharmacological, medical, and agrochemical applicability. Here, we use Pfaffia glomerata, a medically important species, to understand the impacts of UV-B radiation on their physiological performance, the expression of key genes involved in the 20E biosynthesis, and the 20E content. Two accessions (A22 and A43) of plants 20days old grown in vitro were exposed to 0 (control), 2 (6.84kJm-2), and 4 (13.84kJm-2)h UV-B radiation for 20 consecutive days. Our data showed that UV-B reduced glucose concentration in A22 and A43 under 4h of exposure (29 and 30%, respectively), while sucrose concentration increased (32 and 57%, respectively). UV-B also differentially impacted the accessions (A22 and A43), where the A22 under 4h of UV-B had reduced total dry weight (8%) and electron transport rate (31%); in contrast, A43 did not change. Also, only A22 had increased POD activity under 4h of UV-B (66%), as well as increased gene expression of the 20E pathway and the 20E content under 2 and 4h of UV-B in leaves (28 and 21%, respectively) and roots (16 and 13%, respectively). This differential performance to UV-B can be explained by the contrasting anthocyanin contents. Notably, A43 displayed 56% more anthocyanin to the former, a possible defense against UV-B. In conclusion, UV-B radiation is a potential elicitor for increasing 20E content in P. glomerata grown in vitro.

Accumulation of ammonium and reactive oxygen mediated drought-induced rice growth inhibition by disturbed nitrogen metabolism and photosynthesis

Nitrogen (N) deficiency and drought are two key limiting factors for rice production worldwide, but the relationship of drought stress with N homeostasis in rice is rarely advanced. The aim of this study was to dissect the physiological effects of drought stress on rice growth that coupled unbalanced N metabolism. Water-deficient stress (WD) limited stomatal aperture function and activity of Rubisco carboxylase to photosynthesis. The rate of total electron transport (Jt) and the electron to carboxylation (Jc) were considerably decreased, whereas the proportion of e− flow to photorespiration was stimulated by WD, especially at 1600 μmol m−2 s−1 PPFD. Concurrently, the expressions of glycolate oxidase genes (GOX1, GOX5) and glycine decarboxylase complex (GDCH, GDCP and GDCT) were significantly induced in leaves of WD treatment, which led to the accumulation of reactive oxygen species in leaves. With the photosynthetic change, nitrate uptake and reduction were suppressed. Moreover, the enhanced photorespiration generated excess NH3 accumulation in leaves and stimulated the expressions of GS1;1, GS1;2 and GS2, which were tightly coupled with the expressions of PEPC1 and PEPC2 under WD stress. Our results suggest that the inhibited nitrate reduction associated with diminished electron transport rate, and the photorespiration-associated accumulation of hydrogen peroxide and NH3 were critical in the drought-induced rice growth inhibition.

Plastid terminal oxidase acts as an alternative electron sink in a marine cyanobacterium Arthrospira sp.

Abstract In the marine cyanobacterium Arthrospira sp. under high light, the electron transport activity of photosystem (PS) II was much higher than the activities of PSI and the whole chain, indicating the existence of an alternative electron sink in PSII. Under high light, the addition of n-propyl gallate (PG), an inhibitor of plastid terminal oxidase (PTOX), decreased photosynthetic electron transport significantly as compared with that under low light. A significant residual level of photosynthetic electron transport remained in the presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) under high light. The extent of DBMIB insensitive electron transport was close to that of PG sensitive electron transport, suggesting that the PTOX acted as an alternative electron sink, accounting for 27% of total PSII electron transport in Arthrospira sp. cells under high light.

Evidence for a Role for NAD(P)H Dehydrogenase in Concentration of CO2 in the Bundle Sheath Cell of Zea mays.

Prior studies with Nicotiana and Arabidopsis described failed assembly of the chloroplastic NDH [NAD(P)H dehydrogenase] supercomplex by serial mutation of several subunit genes. We examined the properties of Zea mays leaves containing Mu and Ds insertions into nuclear gene exons encoding the critical o- and n-subunits of NDH, respectively. In vivo reduction of plastoquinone in the dark was sharply diminished in maize homozygous mutant compared to normal leaves but not to the extreme degree observed for the corresponding lesions in Arabidopsis. The net carbon assimilation rate (A) at high irradiance and saturating CO2 levels was reduced by one-half due to NDH mutation in maize although no genotypic effect was evident at very low CO2 levels. Simultaneous assessment of chlorophyll fluorescence and A in maize at low (2% by volume) and high (21%) O2 levels indicated the presence of a small, yet detectable, O2-dependent component of total linear photosynthetic electron transport in 21% O2 This O2-dependent component decreased with increasing CO2 level indicative of photorespiration. Photorespiration was generally elevated in maize mutant compared to normal leaves. Quantification of the proportion of total electron transport supporting photorespiration enabled estimation of the bundle sheath cell CO2 concentration (Cb) using a simple kinetic model of ribulose bisphosphate carboxylase/oxygenase function. The A versus Cb relationships overlapped for normal and mutant lines consistent with occurrence of strictly CO2-limited photosynthesis in the mutant bundle sheath cell. The results are discussed in terms of a previously reported CO2 concentration model [Laisk A, Edwards GE (2000) Photosynth Res 66: 199-224].

Imbalance between oxygen photoreduction and antioxidant capacities in Symbiodinium cells exposed to combined heat and high light stress

During the last decades, coral reefs have been affected by several large-scale bleaching events, and such phenomena are expected to increase in frequency and severity in the future, thus compromising their survival. High sea surface temperature accompanied by high levels of solar irradiance has been found to be responsible for the induction of oxidative stress ultimately ending with the disruption of the symbiosis between cnidarians and Symbiodinium. For two decades, many studies have pointed to the water–water cycle (WWC) as being one of the primary mediators of this phenomenon, but the impacts of environmental stress on the O2 reduction by PSI and the associated reactive oxygen species (ROS)-detoxifying enzymes remain to be determined. In this study, we analyzed the impacts of acute thermal and light stress on the WWC in the model Symbiodinium strain A1. We observed that the high light treatment at 26 °C resulted in the up-regulation of superoxide dismutase, ascorbate peroxidase, and glutathione reductase activities and an increased production of ROS with no significant change in O2-dependent electron transport. Under high light and at 33 °C, O2-dependent electron transport was significantly increased relative to total electron transport. This increase was concomitant with a twofold increase in ROS generation compared with the treatment at 26 °C, while enzymes involved in the WWC were largely inactivated. These data show for the first time that combined heat and light stress inactivate antioxidant capacities of the WWC and suggests that its photoprotective functions are overwhelmed under these conditions. This study also indicates that cnidarians may be more prone to bleach if they harbor Symbiodinium cells having a highly active Mehler-type electron transport, unless they are able to quickly up-regulate their antioxidant capacities.

Optimization of Quantum Dot Infrared Photodetectors Based on Noise Model

As an important property of the quantum dot infrared photodetector, the noise has attracted extensive attention. In this paper, the model for the noise of the QDIP is built. This model takes the total electron transport and the dependence of the drift velocity of electrons on the electric field into account. The corresponding calculated results not only show a good agreement with the published results, but also illustrate the dependence of the noise on the structure which can provide us with a method used to optimize the structure of the detector devices.

Performance characteristics of quantum dot infrared photodetectors under illumination condition

Quantum dot infrared photodetector (QDIP) has shown some good performances, for example, a high responsivity, a large detectivity, due to its quantum-dot nanostructure, and already attracted more and more attention in recent years. In this paper, a physical model used to evaluate the performance of the QDIPs under illumination condition is derived. This model supposes that the total electron transport and the self-consistent potential distribution determine together the performance of the QDIP under illumination, and the dependences of the drift velocity and the photoconductive gain on the applied electric field are also considered and included in this model. The corresponding results are calculated out and show a good agreement with the published results, which illustrates the validity and the correctness of the model.

Effect of nitrogen application and elevated CO<sub>2</sub> on photosynthetic gas exchange and electron transport in wheat leaves

Nitrogen (N) availability is a critical factor affecting photosynthetic acclimation of C3 plants under elevated atmospheric CO2 concentration ([CO2]e). However, current understanding of N effects on photosynthetic electron transport rate and partitioning, as well as its impact on photosynthesis under [CO2]e, is inadequate. Using controlled environment open-top chambers, wheat (Triticum aestivum L.) was grown at two N levels (0 and 200 mg(N) kg−1 soil) and two atmospheric CO2 concentrations of 400 ([CO2]a) and 760 μmol mol−1([CO2]e) during 2009 and 2010. Under [CO2]e high N availability increased stomatal conductance and transpiration rate, reduced limitations on the activity of triose phosphate isomerase, a Calvin cycle enzyme, and increased the rate of net photosynthesis (PN). Considering photosynthetic electron transport rate and partitioning aspects, we suggest that greater N availability increased PN under [CO2]e due to four following reasons: (1) higher N availability enhanced foliar N and chlorophyll concentrations, and the actual photochemical efficiency of photosystem (PS) II reaction centers under irradiance increased, (2) increase of total electron transport rate and proportion of open PSII reaction centers, (3) enhancement of the electron transport rate of the photochemical and carboxylation processes, and (4) reduced limitations of the Calvin cycle enzymes on the photosynthetic electron transport rate. Consequently, sufficient N improved light energy utilization in wheat flag leaves under [CO2]e, thus benefiting to photosynthetic assimilation.

Comparison of a global model to semi-kinetic fluid simulations for atmospheric pressure radio-frequency plasmas

A global model of a homogeneous plasma bulk oscillating between electron-free radio-frequency (rf) sheaths is developed. Particle and power balance, including ohmic heating loss for bulk-electrons and ions in the sheaths, yield the bulk electron temperature and density. Explicit time dependence of the reduced bulk electric field and correspondingly of the total ionization rate and electron transport coefficients is accounted for. Results for a gas mixture of 0.5 vol% oxygen in helium at atmospheric pressure within a 1 mm discharge gap as a function of the 13.56 MHz rf power density are presented and compared with a semi-kinetic 1D-fluid simulation. The quality of agreement is critically analyzed and correlated with the individual global model assumptions.

Nutrient supply has greater influence than sink strength on photosynthetic adaptation to CO2 elevation in white birch seedlings

To study the effects of source–sink ratio and nutrient supply on photosynthetic acclimation to CO2 elevation, we subjected white birch seedlings to two levels of nutrient supply (high vs. low) and CO2 concentrations (ambient vs. doubled [CO2]) for two months and then shaded the lower canopy on half of the seedlings to reduce source/sink ratio for an additional month. The CO2 elevation significantly increased Pn and IWUE at both nutrient levels but the increase was greater in the high than low nutrient treatment. The CO2 elevation resulted in a down-regulation of Vcmax in the low nutrient treatment but up-regulation of Jmax, TPU, (F′m−F)/F′m and Jc in the high nutrient after 3 months of treatment. Both the CO2 elevation and high nutrient supply increased the partition of total electron transport to carboxylation at the expense of oxidation. The seedlings responded to the shading of the lower canopy by reducing biomass allocation to roots rather than making physiological adjustments to unshaded leaves in the upper canopy. Our results suggest that the direction of photosynthetic acclimation to CO2 elevation in white birch was nutrient-dependent and an increase in sink strength could reduce the feedback inhibition of photosynthesis.

Mitochondria express enhanced quality as well as quantity in association with aerobic fitness across recreationally active individuals up to elite athletes

Changes in skeletal muscle respiratory capacity parallel that of aerobic fitness. It is unknown whether mitochondrial content, alone, can fully account for these differences in skeletal muscle respiratory capacity. The aim of the present study was to examine quantitative and qualitative mitochondrial characteristics across four different groups (n = 6 each), separated by cardiorespiratory fitness. High-resolution respirometry was performed on muscle samples to compare respiratory capacity and efficiency in active, well-trained, highly trained, and elite individuals. Maximal exercise capacity (ml O(2)·min(-1)·kg(-1)) differed across all groups, with mean ± SD values of 51 ± 4, 64 ± 5, 71 ± 2, and 77 ± 3, respectively. Mitochondrial content assessed by citrate synthase activity was higher in elite trained compared with active and well-trained (29 ± 7 vs. 16 ± 4 and 19 ± 4 nmol·min(-1)·mg wet wt(-1), respectively). When normalizing respiration to mitochondrial content, the respiratory capacities during maximal fatty acid oxidation (P = 0.003), maximal state 3 respiration (P = 0.021), and total electron transport system capacity (P = 0.008) improved with respect to maximal exercise capacity. The coupling efficiency of β-oxidation, however, expressed no difference across groups. These data demonstrate the quantitative and qualitative differences that exist in skeletal muscle mitochondrial respiratory capacity and efficiency across individuals that differ in aerobic capacity. Mitochondrial-specific respiration capacities during β-oxidation, maximal oxidative phosphorylation, and electron transport system capacity all correspondingly improve with aerobic capacity, independent of mitochondrial content in human skeletal muscle.

Less Chlorophyll Does not Necessarily Restrain Light Capture Ability and Photosynthesis in a Chlorophyll‐Deficient Rice Mutant

AbstractA chlorophyll‐deficient rice mutant, yellow‐green leaf 1 (ygl1), with a photosynthetic rate similar to that of wild type (Oryza sativa L., cv. Zhenhui 249 China) is presented in this study. Forty percent of the chlorophyll in the mutant captured 70 % of the light in photosystem II (PSII) compared with wild type. The 30 % decreased light in the mutant was compensated for by a relatively higher quantum yield of PSII, which conferred a total electron transport rate (JT) equal to that in wild type. More photons were absorbed by wild type and exhausted through thermal dissipation. Gene expression analysis of the thylakoid membrane showed that the chlorophyll deficiency in the mutant did not impair the electron transport chain. To determine why the photosynthetic rate was similar between the chlorophyll‐deficient mutant and wild type, the leaf nitrogen (N) and Rubisco contents, stomatal and mesophyll CO2 conductance, and chloroplast development were investigated. Our results indicate that the chlorophyll deficiency in the mutant had no negative impact on the chloroplast development in terms of size and grana stacking; moreover, the chloroplastic CO2 concentration and Rubisco content were comparable to those in wild type. We conclude that the light‐capturing ability of normal rice plants is not fully utilized and that absorbed light in the chlorophyll‐deficient mutant is more efficient in participating photosynthesis.

Performance investigations of quantum dot infrared photodetectors

Quantum dot infrared photodetectors (QDIPs) have already attracted more and more attention in recent years due to a high photoconductive gain, a low dark current and an increased operating temperature. In the paper, a device model for the QDIP is proposed. It is assumed that the total electron transport and the self-consistent potential distribution under the dark conditions determine the dark current calculation of QDIP devices in this model. The model can be used for calculating the dark current, the photocurrent and the detectivity of QDIP devices, and these calculated results show a good agreement with the published results, which illustrate the validity of the device model.

Low soil temperature inhibits the effect of high nutrient supply on photosynthetic response to elevated carbon dioxide concentration in white birch seedlings

To investigate the interactive effects of soil temperature (T(soil)) and nutrient availability on the response of photosynthesis to elevated atmospheric carbon dioxide concentration ([CO(2)]), white birch (Betula papyrifera Marsh.) seedlings were exposed to ambient (360 micromol mol(-1)) or elevated (720 micromol mol(-1)) [CO(2)], three T(soil) (5, 15 and 25 degrees C initially, increased to 7, 17 and 27 degrees C, respectively, 1 month later) and three nutrient regimes (4/1.8/3.3, 80/35/66 and 160/70/132 mg l(-1) N/P/K) for 3 months in environment-controlled greenhouses. Elevated [CO(2)] increased net photosynthetic rate (A(n)), instantaneous water-use efficiency (IWUE), internal to ambient carbon dioxide concentration ratio (C(i)/C(a)), triose phosphate utilization (TPU) and photosynthetic linear electron transport to carboxylation (J(c)), and it decreased actual photochemical efficiency of photosystem II (DeltaF/F(m)'), the fraction of total linear electron transport partitioned to oxygenation (J(o)/J(T)) and leaf N concentration. The low T(soil) suppressed A(n), transpiration rate (E), TPU, DeltaF/F(m)' and J(c), but it increased J(o)/J(T). The low nutrient treatment reduced A(n), IWUE, maximum carboxylation rate of Rubisco, light-saturated electron transport rate, TPU, DeltaF/F(m)', J(c) and leaf N concentration, but increased C(i)/C(a). There were two-factor interactions for C(i)/C(a), TPU and leaf N concentration, and a significant effect of CO(2) x T(soil) x nutrient regime on A(n), IWUE and J(c). The stimulations of A(n) and IWUE by elevated [CO(2)] were limited to seedlings grown under the intermediate and high nutrient regimes at the intermediate and high T(soil). For J(c), the [CO(2)] effect was significant only at intermediate T(soil) + high nutrient availability. No significant [CO(2)] effects were observed under the low T(soil) at any nutrient level. Our results support this study's hypothesis that low T(soil) would reduce the positive effect of high nutrient supply on the response of A(n) to elevated [CO(2)].

Partitioning of photosynthetic electron flow between CO<sub>2</sub> assimilation and O<sub>2</sub> reduction in sunflower plants under water deficit

In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P N) and stomatal conductance (g s) of attached leaves decreased as leaf water potential (Ψw) declined from -0.3 to -2.9 MPa. Although g s decreased over the whole range of Ψw, nearly constant values in the intercellular CO2 concentrations (C i) were observed as Ψw decreased to -1.8 MPa, but C i increased as Ψw decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (qNP) increased progressively. A highly significant negative relationship between qNP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 µmol m-2 s-1, the allocation of electrons from photosystem (PS) 2 to O2 reduction was increased by 51 %, while the allocation to CO2 assimilation was diminished by 32 %, as Ψw declined from -0.3 to -2.9 MPa. A significant linear relationship between mean P N and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas qP declined only at very severe stress and the excess photon energy was dissipated by increasing qNP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of qNP against photoinhibition in sunflower.

Relationship between photosynthesis and leaf nitrogen concentration in ambient and elevated [CO2] in white birch seedlings

To study the effects of elevated CO2 concentration ([CO2]) on relationships between nitrogen (N) nutrition and foliar gas exchange parameters, white birch (Betula papyrifera Marsh.) seedlings were exposed to one of five N-supply regimes (10, 80, 150, 220, 290 mg N l(-1)) in either ambient [CO2] (360 micromol mol(-1)) or elevated [CO2] (720 micromol mol(-1)) in environment-controlled greenhouses. Foliar gas exchange and chlorophyll fluorescence were measured after 60 and 80 days of treatment. Photosynthesis showed a substantial down-regulation (up to 57%) in response to elevated [CO2] and the magnitude of the down-regulation generally decreased exponentially with increasing leaf N concentration. When measured at the growth [CO2], elevated [CO2] increased the overall rate of photosynthesis (P(n)) and instantaneous water-use efficiency (IWUE) by up to 69 and 236%, respectively, but decreased transpiration (E) and stomatal conductance (g(s)) in all N treatments. However, the degree of stimulation of photosynthesis by elevated [CO2] decreased as photosynthetic down-regulation increased from 60 days to 80 days of treatment. Elevated [CO2] significantly increased total photosynthetic electron transport in all N treatments at 60 days of treatment, but the effect was insignificant after 80 days of treatment. Both P(n) and IWUE generally increased with increasing leaf N concentration except at very high leaf N concentrations, where both P(n) and IWUE declined. The relationships of P(n) and IWUE with leaf N concentration were modeled with both a linear regression and a second-order polynomial function. Elevated [CO2] significantly and substantially increased the slope of the linear regression for IWUE, but had no significant effect on the slope for P(n). The optimal leaf N concentration for P(n) and IWUE derived from the polynomial function did not differ between the CO2 treatments when leaf N was expressed on a leaf area basis. However, the mass-based optimal leaf N concentration for P(n) was much lower in seedlings in elevated [CO2] than in ambient [CO2] (31.88 versus 37.00 mg g(-1)). Elevated [CO2] generally decreased mass-based leaf N concentration but had no significant effect on area-based leaf N concentration; however, maximum N concentration per unit leaf area was greater in elevated [CO2] than in ambient [CO2] (1.913 versus 1.547 g N m(-2)).