Decrease In Photosynthesis Research Articles

Effects of elevated temperature on photosynthesis in desert plant Alhagi sparsifolia S

Most plants growing in temperate desert zone exhibit brief temperature-induced inhibition of photosynthesis at midday in the summer. Heat stress has been suggested to restrain the photosynthesis of desert plants like Alhagi sparsifolia S. It is therefore possible that high midday temperatures damage photosynthetic tissues, leading to the observed inhibition of photosynthesis. In this study, we investigated the mechanisms underlying heat-induced inhibition of photosynthesis in A. sparsifolia, a dominant species found at the transition zone between oasis and sandy desert on the southern fringe of the Taklamakan desert. The chlorophyll (Chl) a fluorescence induction kinetics and CO2 response curves were used to analyze the thermodynamic characters of both photosystem II (PSII) and Rubisco after leaves were exposed to heat stress. When the leaves were heated to temperatures below 43°C, the initial fluorescence of the dark-adapted state (Fo), and the maximum photochemical efficiency of PSII (Fv/Fm), the number of active reaction centers per cross section (RCs) and the leaf vitality index (PI) increased or declined moderately. These responses were reversed, however, upon cooling. Moreover, the energy allocation in PSII remained stable. The gradual appearance of a K point in the fluorescence curve at 48°C indicated that higher temperatures strongly impaired PSII and caused irreversible damage. As the leaf temperature increased, the activity of Rubisco first increased to a maximum at 34°C and then decreased as the temperature rose higher. Under high-temperature stress, cell began to accumulate oxidative species, including ammoniacal nitrogen, hydrogen peroxide (H2O2), and superoxide (O2 .-), suggesting that disruption of photosynthesis may result from oxidative damage to photosynthetic proteins and thylakoid membranes. Under heat stress, the biosynthesis of nonenzyme radical scavenging carotenoids (Cars) increased. We suggest that although elevated temperature affects the heat-sensitive components comprising of PSII and Rubisco, under moderately high temperature the decrease in photosynthesis is mostly due to inactivation of dark reactions.

Independent and Combined Effects of High Temperature and Drought Stress During Grain Filling on Plant Yield and Chloroplast EF-Tu Expression in Spring Wheat

High temperature and drought stress are among the two most important environmental factors influencing crop growth, development and yield processes. These two stresses commonly occur in combination. Objectives of this research were to investigate the independent and combined effects of high temperature and drought stress during grain filling on physiological, vegetative and yield traits and expression of a chloroplast protein synthesis elongation factor (EF-Tu) of wheat (Triticum aestivum L.). Two spring wheat cultivars (Pavon-76 and Seri-82) were grown at control temperatures (CT; day/night, 24/14 � C; 16/ 8 h photo/dark period) from sowing to heading. Thereafter, one half of the plants were exposed to high temperature stress (HT; 31/18 � C in Exp. 1 and 34/ 22 � C in Exp. 2), drought stress (withholding water), or a combination of both HT and drought stress. There were significant influences of HT and/or drought stress on physiological, growth and yield traits. There was no cultivar or cultivar by temperature or cultivar by drought interaction effects on most traits. The decreases in leaf photosynthesis were greater at HT compared with drought alone throughout the stress period, and the combination of HT and drought had the lowest leaf photosynthetic rates. Overall, HT or drought had similar effects (about 48‐56 % decrease) on spikelet fertility, grain numbers and grain yield. High temperature decreased grain numbers (by 56 % averaged across both experiments) and individual grain weight (by 25 %), while, respective decreases due to drought were 48 % and 35 %. This suggests that the grain numbers were more sensitive to HT and grain weights to drought for the range of temperatures tested in this research. The interaction between HT and drought stress was significant for total dry weights, harvest index and spikelet fertility, particularly when HT stress was severe (34/22 � C). The combined effects of HT and drought were greater than additive effects of HT or drought alone for leaf chlorophyll content, grain numbers and harvest index. High temperature stress and the combination of HT and drought stress but not drought stress alone resulted in the overexpression of EF-Tu in both spring wheat cultivars.

Responses to water stress of gas exchange and metabolites in Eucalyptus and Acacia spp.

Studies of water stress commonly examine either gas exchange or leaf metabolites, and many fail to quantify the concentration of CO₂ in the chloroplasts (C(c)). We redress these limitations by quantifying C(c) from discrimination against ¹³CO₂ and using gas chromatography-mass spectrometry (GC-MS) for leaf metabolite profiling. Five Eucalyptus and two Acacia species from semi-arid to mesic habitats were subjected to a 2 month water stress treatment (Ψ(pre-dawn) = -1.7 to -2.3 MPa). Carbohydrates dominated the leaf metabolite profiles of species from dry areas, whereas organic acids dominated the metabolite profiles of species from wet areas. Water stress caused large decreases in photosynthesis and C(c), increases in 17-33 metabolites and decreases in 0-9 metabolites. In most species, fructose, glucose and sucrose made major contributions to osmotic adjustment. In Acacia, significant osmotic adjustment was also caused by increases in pinitol, pipecolic acid and trans-4-hydroxypipecolic acid. There were also increases in low-abundance metabolites (e.g. proline and erythritol), and metabolites that are indicative of stress-induced changes in metabolism [e.g. γ-aminobutyric acid (GABA) shunt, photorespiration, phenylpropanoid pathway]. The response of gas exchange to water stress and rewatering is rather consistent among species originating from mesic to semi-arid habitats, and the general response of metabolites to water stress is rather similar, although the specific metabolites involved may vary.

Sphagnum growth and ecophysiology during mire succession

Sphagnum mosses are widespread in areas where mires exist and constitute a globally important carbon sink. Their ecophysiology is known to be related to the water level, but very little is currently known about the successional trend in Sphagnum. We hypothesized that moss species follow the known vascular plant growth strategy along the successional gradient (i.e., decrease in production and maximal photosynthesis while succession proceeds). To address this hypothesis, we studied links between the growth and related ecophysiological processes of Sphagnum mosses from a time-since-initiation chronosequence of five wetlands. We quantified the rates of increase in biomass and length of different Sphagnum species in relation to their CO(2) assimilation rates, their photosynthetic light reaction efficiencies, and their physiological states, as measured by the chlorophyll fluorescence method. In agreement with our hypothesis, increase in biomass and CO(2) exchange rate of Sphagnum mosses decreased along the successional gradient, following the tactics of more intensely studied vascular plants. Mosses at the young and old ends of the chronosequence showed indications of downregulation, measured as a low ratio between variable and maximum fluorescence (F(v)/F(m)). Our study divided the species into three groups; pioneer species, hollow species, and ombrotrophic hummock formers. The pioneer species S. fimbriatum is a ruderal plant that occurred at the first sites along the chronosequence, which were characterized by low stress but high disturbance. Hollow species are competitive plants that occurred at sites with low stress and low disturbance (i.e., in the wet depressions in the middle and at the old end of the chronosequence). Ombrotrophic hummock species are stress-tolerant plants that occurred at sites with high stress and low disturbance (i.e., at the old end of the chronosequence). The three groups along the mire successional gradient appeared to be somewhat analogous to the three primary strategies suggested by Grime.

INTRACELLULAR METAL COMPARTMENTALIZATION IN THE GREEN ALGAL MODEL SYSTEM MICRASTERIAS DENTICULATA (STREPTOPHYTA) MEASURED BY TRANSMISSION ELECTRON MICROSCOPY-COUPLED ELECTRON ENERGY LOSS SPECTROSCOPY1.

Entry of metals in form of aerosols into areas of high air humidity such as peat bogs represents a serious danger for inhabiting organisms such as the unicellular desmid Micrasterias denticulata Bréb. ex Ralfs (Desmidiaceae, Zynematophyceae, Streptophyta). To understand cellular detoxification and tolerance mechanisms, detailed intracellular localization of metal pollutants is required. This study localizes the metals aluminum (Al), zinc (Zn), copper (Cu), and cadmium (Cd) in the green algal model system Micrasterias after experimental exposure to sulfate solutions by highly sensitive TEM-coupled electron energy loss spectroscopy (EELS). Concentrations of the metals shown to induce inhibiting effects on cell development and cytomorphogenesis were chosen for these experiments. Long-term exposure to these metal concentrations led to a pronounced impact on cell physiology expressed by a general decrease in apparent photosynthesis. After long-term treatment, Zn, Al, and Cu were detected in the cell walls by EELS. Zn was additionally found in vacuoles and mucilage vesicles, and Cu in starch grains and also in mucilage vesicles. Elevated amounts of oxygen in areas where Zn, Al, and Cu were localized suggest sequestration of these metals as oxides. The study demonstrated that Micrasterias can cope differently with metal pollutants. In low doses and during a limited time period, the cells were able to compartmentalize Cu the best, followed by Zn and Al. Cu and Zn were taken up into intracellular compartments, whereas Al was only bound to the cell wall. Cd was not compartmentalized at all, which explains its strongest impact on growth, cell division rate, and photosynthesis in Micrasterias.

Photosynthetic acclimation in relation to nitrogen allocation in cucumber leaves in response to changes in irradiance

Leaves deep in canopies can suddenly be exposed to increased irradiances following e.g. gap formation in forests or pruning in crops. Studies on the acclimation of photosynthesis to increased irradiance have mainly focused on the changes in photosynthetic capacity (A(max)), although actual irradiance often remains below saturating level. We investigated the effect of changes in irradiance on the photosynthesis irradiance response and on nitrogen allocation in fully grown leaves of Cucumis sativus. Leaves that fully developed under low (50 µmol m⁻² s⁻¹) or moderate (200 µmol m⁻² s⁻¹) irradiance were subsequently exposed to, respectively, moderate (LM-leaves) or low (ML-leaves) irradiance or kept at constant irradiance level (LL- and MM-leaves). Acclimation of photosynthesis occurred within 7 days with final A(max) highest in MM-leaves, lowest in LL-leaves and intermediate in ML- and LM-leaves, whereas full acclimation of thylakoid processes underlying photosystem II (PSII) efficiency and non-photochemical quenching occurred in ML- and LM-leaves. Dark respiration correlated with irradiance level, but not with A(max). Light-limited quantum efficiency was similar in all leaves. The increase in photosynthesis at moderate irradiance in LM-leaves was primarily driven by nitrogen import, and nitrogen remained allocated in a similar ratio to Rubisco and bioenergetics, while allocation to light harvesting relatively decreased. A contrary response of nitrogen was associated with the decrease in photosynthesis in ML-leaves. Net assimilation of LM-leaves under moderate irradiance remained lower than in MM-leaves, revealing the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to.

Burn severity influences postfire CO2exchange in arctic tundra

Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars are composed of a mosaic of patches that differ in burn severity, which may influence postfire carbon budgets through damage to vegetation and carbon stocks. We deployed three eddy covariance towers along a burn severity gradient (i.e., severely burned, moderately burned, and unburned tundra) to monitor postfire net ecosystem exchange of CO2 (NEE) within the large 2007 Anaktuvuk River fire scar in Alaska, USA, during the summer of 2008. Remote sensing data from the MODerate resolution Imaging Spectroradiometer (MODIS) was used to assess the spatial representativeness of the tower sites and parameterize a NEE model that was used to scale tower measurements to the landscape. The tower sites had similar vegetation and reflectance properties prior to the Anaktuvuk River fire and represented the range of surface conditions observed within the fire scar during the 2008 summer. Burn severity influenced a variety of surface properties, including residual organic matter, plant mortality, and vegetation recovery, which in turn determined postfire NEE. Carbon sequestration decreased with increased burn severity and was largely controlled by decreases in canopy photosynthesis. The MODIS two-band enhanced vegetation index (EVI2) monitored the seasonal course of surface greenness and explained 86% of the variability in NEE across the burn severity gradient. We demonstrate that understanding the relationship between burn severity, surface reflectance, and NEE is critical for estimating the overall postfire carbon balance of the Anaktuvuk River fire scar.

Photosynthetic response and recovery of Antarctic marine benthic microalgae exposed to elevated irradiances and temperatures

Exposure to high temperatures affects the photosynthetic processes in marine benthic microalgae by limiting the transport of electrons, thus reducing the ability of the cell to use light. This causes damage to the Photosystem II (PSII) and may lead to photoinhibition. However, the PSII of benthic microalgal communities from Brown Bay, eastern Antarctica, were relatively unaffected by significant changes in temperature. Benthic microalgae exposed to temperatures up to 8°C and an irradiance of 450 μmol photons m−2 s−1 did not experience any photosynthetic damage or irreversible photoinhibition. The effective quantum yield (∆F/Fm′) at 8°C (0.433 ± 0.042) was higher by comparison to cell incubated at −0.1°C (0.373 ± 0.015) with similar irradiances. Temperatures down to −5°C at a similar irradiance showed a decrease in photosynthesis with decreasing temperature, but no severe photoinhibition as the cells were able to dissipate excess energy via non-photochemical quenching and recover from damage. These responses are consistent with those recorded in past studies on Antarctic benthic microalgae and suggest that short-term temperature change (from −5 to 8°C) will not do irreversible damage to the PSII and will not affect the photosynthesis of the benthic microalgae.

Effect of panicle removal on photosynthetic acclimation under elevated CO<sub>2</sub> in rice

To examine the role of sink size on photosynthetic acclimation under elevated atmospheric CO2 concentrations ([CO2]), we tested the effects of panicle-removal (PR) treatment on photosynthesis in rice (Oryza sativa L.). Rice was grown at two [CO2] levels (ambient and ambient + 200 μmol mol-1) throughout the growing season, and at full-heading stage, at half the plants, a sink-limitation treatment was imposed by the removal of the panicles. The PR treatment alleviated the reduction of green leaf area, the contents of chlorophyll (Chl) and Rubisco after the full-heading stage, suggesting delay of senescence. Nonetheless, elevated [CO2] decreased photosynthesis (measured at current [CO2]) of plants exposed to the PR treatment. No significant [CO2] × PR interaction on photosynthesis was observed. The decrease of photosynthesis by elevated [CO2] of plants was associated with decreased leaf Rubisco content and N content. Leaf glucose content was increased by the PR treatment and also by elevated [CO2]. In conclusion, a sink-limitation in rice improved N status in the leaves, but this did not prevent the photosynthetic down-regulation under elevated [CO2].

Salicylic acid alleviates decreases in photosynthesis under salt stress by enhancing nitrogen and sulfur assimilation and antioxidant metabolism differentially in two mungbean cultivars

Salicylic acid (SA) is known to affect photosynthesis under normal conditions and induces tolerance in plants to biotic and abiotic stresses through influencing physiological processes. In this study, physiological processes were compared in salt-tolerant (Pusa Vishal) and salt-sensitive (T44) cultivars of mungbean and examined how much these processes were induced by SA treatment to alleviate decrease in photosynthesis under salt stress. Cultivar T44 accumulated higher leaf Na + and Cl − content and exhibited greater oxidative stress than Pusa Vishal. Activity of antioxidant enzymes, ascorbate peroxidase (APX) and glutathione reductase (GR) was greater in Pusa Vishal than T44. Contrarily, activity of superoxide dismutase (SOD) was greater in T44. The greater accumulation of leaf nitrogen and sulfur through higher activity of their assimilating enzymes, nitrate reductase (NR) and ATP-sulfurylase (ATPS) increased reduced glutathione (GSH) content more conspicuously in Pusa Vishal than T44. Application of 0.5 mM SA increased nitrogen and sulfur assimilation, GSH content and activity of APX and GR. This resulted in the increase in photosynthesis under non-saline condition and alleviated the decrease in photosynthesis under salt stress. It also helped in restricting Na + and Cl − content in leaf, and maintaining higher efficiency of PSII, photosynthetic N-use efficiency (NUE) and water relations in Pusa Vishal. However, application of 1.0 mM SA resulted in inhibitory effects. The effect of SA was more pronounced in Pusa Vishal than T44. These results indicate that SA application alleviates the salt-induced decrease in photosynthesis mainly through inducing the activity of NR and ATPS, and increasing antioxidant metabolism to a greater extent in Pusa Vishal than T44.

Long-term acclimation of mesophyll conductance, carbon isotope discrimination and growth in two contrasting Picea asperata populations exposed to drought and enhanced UV-B radiation for three years

Two Picea asperata Mast. populations originating from wet and dry climate regions of China were subjected to enhanced UV-B radiation, drought and their combination in a greenhouse for 3 years. Drought treatment caused a significant decrease in photosynthesis, mesophyll conductance ( g m), carbon isotope discrimination ( Δ) and growth characteristics when compared with well-watered treatment. In contrast to the great effects of drought stress, enhanced UV-B radiation showed some but not as substantial negative effects on these parameters. Little interaction effect between drought and UV-B radiation was detected, and the drought effect in combination with enhanced UV-B was not more pronounced than with no enhanced UV-B radiation. These results suggest that enhanced UV-B radiation does not aggravate the drought effect on P. asperata seedlings. The results also showed that the proportional role of the intercellular CO 2 ( C i) decreased, while the role of chloroplastic concentration ( C c) became increasingly important in explaining the high values of carbon isotope composition ( δ 13C), when the water stress progressed in time. In addition, multivariate causal models proposed that there is a direct causal relationship between specific leaf area (SLA) and g m, which is not mediated by leaf N. Besides, there are functional links between g m and photosynthetic capacity ( V cmax). On the other hand, the study showed that net assimilation rate (NAR) is the main driving force for changes in relative growth rate (RGR), especially in low-water environments, and the degree of acclimation of respiration in the light ( R L) is of central importance to the greater role played by NAR in determining variation in RGR.

NUTRIENT ACCUMULATION AND PHOTOSYNTHESIS IN GLYPHOSATE-RESISTANT SOYBEANS IS REDUCED UNDER GLYPHOSATE USE

Global production of glyphosate-resistant (GR) soybean [Glycine max (L.) Merr.] continues to increase annually; however, there are no particular specific fertilizer recommendations for the transgenic varieties used in this system largely because reports of glyphosate effects on mineral nutrition of GR soybeans are lacking. Several metabolites or degradation products of glyphosate have been identified or postulated to cause undesirable effects on GR soybeans. In this work we used increasing glyphosate rates in different application on cv. ‘BRS 242 GR’ in order to evaluate photosynthetic parameters, macro- and micronutrient uptake and accumulation and shoot and root dry biomass production. Increasing glyphosate rates revealed a significant decrease in photosynthesis, macro and micronutrients accumulation in leaf tissues and also decreases in nutrient uptake. The reduced biomass in GR soybeans represents additive effects from the decreased photosynthetic parameters as well as lower availability of nutrients in tissues of the glyphosate treated plants.

Photosynthetic down-regulation under elevated CO 2 exposure can be prevented by nitrogen supply in nodulated alfalfa

Increasing atmospheric CO 2 concentrations are expected to enhance plant photosynthesis and yield. Nevertheless, after long-term exposure, plants acclimate and show a reduction in photosynthetic activity (called down-regulation), which may cause a reduction in potential yield. Some authors suggest that down-regulation is related to nutrient availability, and more specifically, to an insufficient plant C sink strength caused by limited N supply. In this paper, we tested whether or not N availability prevents down-regulation of photosynthesis in nodulated alfalfa plants ( Medicago sativa L.). To do so, we examined the effect of the addition of different levels of NH 4NO 3 (0, 10, and 15 mM) to 30-day-old nodulated alfalfa plants exposed to ambient (approximately 400 μmol mol −1) or elevated CO 2 (700 μmol mol −1) during a period of 1 month in growth chambers. After 2 weeks of exposure to elevated CO 2, no significant differences were observed in plant growth or photosynthesis rates. After 4 weeks of treatment, exclusively N 2 fixing alfalfa plants (0 mM NH 4NO 3) showed significant decreases in photosynthesis and Vc max . Photosynthetic down-regulation of these plants was caused by the C/N imbalance as reflected by the carbohydrate and N data. On the other hand, plants supplied with 15 mM NH 4NO 3 grown under elevated CO 2 maintained high photosynthetic rates owing to their superior C/N adjustment. The intermediate N treatment, 10 mM NH 4NO 3, also showed photosynthetic down-regulation, but to a lesser degree than with 0 mM treatment. The present study clearly shows that external N supply can reduce or even avoid acclimation of photosynthesis to elevated CO 2 as a consequence of the increase in C sink strength associated with N availability.

Physiological aspects of tolerance in Atriplex halimus L. to NaCl and drought

Forty-day-old seedlings of Atriplexhalimus were treated either with NaCl (50, 300 and 550 mM) for the subsequent 30 days or with 15% PEG for the subsequent 10 days. As much as 50 mM of NaCl significantly increased shoot fresh and dry weight and height; nevertheless, 300 or 550 mM NaCl seemed to have no effect. On the other hand, these growth parameters were not affected by drought after 3 or 6 days, but were reduced after 10 days. The gas exchange parameters (photosynthetic rate, stomatal conductance and transpiration rate) were increased by 50 mM NaCl, but decreased by 300 and 550 mM. These parameters were decreased in response to drought only after 10 days of withholding water. In contrast to Na+, K+ was significantly decreased by NaCl but not by drought. The time course effect revealed that phosphoenol pyruvate carboxylase (PEPC) protein was doubled in response to NaCl after 1 and 5 h and continued thereafter, higher than control, while drought had no significant effect. Rubisco seemed unchanged by NaCl or drought. It could be concluded that the decrease in fresh weight might be attributed to the decrease in water content. Moreover, the decrease in photosynthesis could result from a decrease in stomatal conductance, a protective mechanism against water loss to improve water use efficiency. These findings indicate that Atriplexhalimus tolerates NaCl and drought through decreasing growth, reducing gas exchange parameters to improve water use efficiency, uptake Na+ and saving, if any, the photosynthetic enzyme particularly PEPC.

Effect of nutrient supply on photosynthesis and pigmentation to short-term stress (UV radiation) in Gracilaria conferta (Rhodophyta)

The effects of increased photosynthetic active radiation (PAR), UV radiation (UVR), and nutrient supply on photosynthetic activity, pigment content, C:N ratio and biomass yield were studied in tank cultivated Gracilaria conferta (Rhodophyta). Electron transport rate (ETR) and biliprotein content were higher under high nutrient supply (HNS), obtained from fishpond effluents, compared to low nutrient supply (LNS), in contrast to mycosporine-like amino acids (MAAs) dynamic. The high MAA content in LNS-algae could be explained by higher UVR penetration in the thallus and by the competition for the use of nutrients with other processes. Effective quantum yield decreased after short-term exposure to high irradiance whereas full recovery in shade was produced only under slightly heat shock. UVA radiation provoked an additional decrease in photosynthesis under high water temperature. UVB radiation reversed UVA’s negative effect mainly with HNS. Results support that nutrient-sufficiency help G. conferta to resist environmental changes as short-term temperature increase.

Glyphosate affects photosynthesis in first and second generation of glyphosate-resistant soybeans

The crop area planted to conventional soybeans has decreased annually while that planted to glyphosate-resistant (RR) soybean has drastically increased mainly due to the wide adoption of glyphosate in current weed management systems. With the extensive use of glyphosate, many farmers have noted visual plant injury in RR soybean varieties after glyphosate application. A new generation designated as “second generation—RR2” has been recently developed and these RR2 cultivars already are commercially available for farmers and promoted as higher yielding relative to the previous RR cultivars. However, little information is currently available about the performance of RR2 soybean beyond commercial and farmer testimonial data. Thus, an evaluation of different glyphosate rates applied in different growth stages of the first and second generation of RR soybeans, revealed a significant decrease in photosynthesis. In general, increased glyphosate rate and late applications (V6) pronounced decrease photosynthetic parameters and consequently decreased in leaf area and shoot biomass production. In contrast, low rate and early applications were less damage for the RR soybean plants, suggesting that with early applications (V2), plants probably have more time to recover from glyphosate or its metabolites effects regarding late applications.

Improvement of heat and drought photosynthetic tolerance in wheat by overaccumulation of glycinebetaine

Within their natural habitat, crops are often subjected to drought and heat stress, which suppress crop growth and decrease crop production. Causing overaccumulation of glycinebetaine (GB) has been used to enhance the crop yield under stress. Here, we investigated the response of wheat (Triticum aestivum L.) photosynthesis to drought, heat stress and their combination with a transgenic wheat line (T6) overaccumulating GB and its wild-type (WT) Shi4185. Drought stress (DS) was imposed by controlling irrigation until the relative water content (RWC) of the flag leaves decreased to between 78 and 82%. Heat stress (HS) was applied by exposing wheat plants to 40°C for 4 h. A combination of drought and heat stress was applied by subjecting the drought-stressed plants to a heat stress as above. The results indicated that all stresses decreased photosynthesis, but the combination of drought and heat stress exacerbated the negative effects on photosynthesis more than exposure to drought or heat stress alone. Drought stress decreased the transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci), while heat stress increased all of these; the deprivation of water was greater under drought stress than heat stress, but heat stress decreased the antioxidant enzyme activity to a greater extent. Overaccumulated GB could alleviate the decrease of photosynthesis caused by all stresses tested. These suggest that GB induces an increase of osmotic adjustments for drought tolerance, while its improvement of the antioxidative defense system including antioxidative enzymes and antioxidants may be more important for heat tolerance.

Carbon Dioxide Enrichment Inhibits Nitrate Assimilation in Wheat and Arabidopsis

The concentration of carbon dioxide in Earth's atmosphere may double by the end of the 21st century. The response of higher plants to a carbon dioxide doubling often includes a decline in their nitrogen status, but the reasons for this decline have been uncertain. We used five independent methods with wheat and Arabidopsis to show that atmospheric carbon dioxide enrichment inhibited the assimilation of nitrate into organic nitrogen compounds. This inhibition may be largely responsible for carbon dioxide acclimation, the decrease in photosynthesis and growth of plants conducting C(3) carbon fixation after long exposures (days to years) to carbon dioxide enrichment. These results suggest that the relative availability of soil ammonium and nitrate to most plants will become increasingly important in determining their productivity as well as their quality as food.

Sub-cellular damage by copper in the cnidarian Zoanthus robustus

Sessile organisms may experience chronic exposure to copper that is released into the marine environment from antifoulants and stormwater runoff. We have identified the site of damage caused by copper to the symbiotic cnidarian, Zoanthus robustus (Anthozoa, Hexacorallia). External changes to the zoanthids were apparent when compared with controls. The normally flexible bodies contracted and became rigid. Histological examination of the zoanthid tissue revealed that copper had caused sub-cellular changes to proteins within the extracellular matrix (ECM) of the tubular body. Collagen in the ECM and the internal septa increased in thickness to five and seven times that of controls respectively. The epithelium, which stained for elastin, was also twice as thick and tough to cut, but exposure to copper did not change the total amount of desmosine which is found only in elastin. We conclude that copper stimulated collagen synthesis in the ECM and also caused cross-linking of existing proteins. However, there was no expulsion of the symbiotic algae ( Symbiodinium sp.) and no effect on algal pigments or respiration (44, 66 and 110 µg Cu L − 1). A decrease in net photosynthesis was observed only at the highest copper concentration (156 µg Cu L − 1). These results show that cnidarians may be more susceptible to damage by copper than their symbiotic algae.

Burn severity influences post-fire CO2exchange in arctic tundra

Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars are composed of a mosaic of patches that differ in burn severity, which may influence postfire carbon budgets through damage to vegetation and carbon stocks. We deployed three eddy covariance towers along a burn severity gradient (i.e., severely burned, moderately burned, and unburned tundra) to monitor postfire net ecosystem exchange of CO2 (NEE) within the large 2007 Anaktuvuk River fire scar in Alaska, USA, during the summer of 2008. Remote sensing data from the MODerate resolution Imaging Spectroradiometer (MODIS) was used to assess the spatial representativeness of the tower sites and parameterize a NEE model that was used to scale tower measurements to the landscape. The tower sites had similar vegetation and reflectance properties prior to the Anaktuvuk River fire and represented the range of surface conditions observed within the fire scar during the 2008 summer. Burn severity influenced a variety of surface properties, including residual organic matter, plant mortality, and vegetation recovery, which in turn determined postfire NEE. Carbon sequestration decreased with increased burn severity and was largely controlled by decreases in canopy photosynthesis. The MODIS two-band enhanced vegetation index (EVI2) monitored the seasonal course of surface greenness and explained 86% of the variability in NEE across the burn severity gradient. We demonstrate that understanding the relationship between burn severity, surface reflectance, and NEE is critical for estimating the overall postfire carbon balance of the Anaktuvuk River fire scar.