Nodulation,nitrogen fixation,leaf area,and sugars content inLablab purpureus as affected by sulfur nutrition

Summary In order to explore interrelations between S nutrition, soluble sugars, leaf area, nodulation and N₂ fixation, greenhouse experiments were done with several levels of S added to perlite-sand cultures or to a moderately S-deficient soil. Sulfur had indirect effects on nodulation and N₂ fixation, possibly by improving sugars supply and N metabolism.In perlite-sand culture, leaf area increased with concentrations of supplied S up to 50 and 200 M for symbiotic and N-treated plants respectively, then decreased at higher concentrations. Plant yield and total sugars content (mg per plant) for the N-treated plants behaved similar to leaf area in response to added S but in the symbiotic plants maximum values were obtained at 100 M S. In soil, Mo had no effect on growth but interacted significantly with S in affecting total sugars content. High levels of S depressed sugars content at low Mo but raised it at high Mo.Sulfur increased the N content of soil-grown plants. It increased the N content of plants grown in perlite-sand culture except at very high levels of S. There was little effect on concentration of N in the shoots. Nitrogen content correlated significantly with leaf area and sugar content, and highly significantly with S concentration in the shoots.     

Nodulation and growth ofLablab purpureus (Dolichos lablab) in relation to rhizobium strain,liming and phosphorus

Summary Greenhouse experiments were done with two purposes: (1) to identify strains of rhizobia effective and acid-tolerant in symbiosis withLablab purpureus, and (2) to determine whether soil acidity or the symbiotic condition increased the phosphate requirement for growth.Five rhizobial strains were tested in one neutral soil, two acid soils, and the two acid soils limed to pH 6.6. In the neutral and limed soils, three of the strains were effective (CB1024, CB756, TAL169), but only two strains (CB756, TAL169) remained effective in acid soil.Strain CB756 and plus-N treatments were further compared in a factorial trial involving combinations of five levels of P with lime, no lime and CaCl₂ treatments, applied to an acid soil. Some of the treatments were also applied to plants inoculated with CB1024. Between the N-fertilized and CB756 treatments there was no clear difference in growth response to applied P, and the critical internal concentration of P for 95% of maximal growth was the same (0.22% shoot dry weight). Increasing P beyond levels needed for maximal growth increased nodulation and N concentration in plants inoculated with CB756. It lowered N concentration in N-fertilized plants. There was evidence suggesting that the P requirement of symbiotic plants increased if the soil was acid, or if CB756 were replaced by CB1024 as microsymbiont; but the critical statistical interactions were not significant.     

Reducing carbon: phosphorus ratio can enhance microbial phytin mineralization and lessen competition with maize for phosphorus

We tested the hypothesis that reducing the carbon (C):Phosphorus (P) ratio in rhizosphere soil would reduce bacterial competition with the plant for P from phytin, which would then increase phytin use efficiency for the plant. A three-factor pot experiment was carried out to study the effect of inoculation with a phytin-mineralizing bacterium, Pseudomonas alcaligenes (PA), on maize P uptake from phytin. Two levels of organic P, two levels of inorganic P, and three different PA inoculation treatments were used. When maize plants were grown in low available P soil with phytin, PA transformed soil P into microbial biomass P, which caused competition for available P with plant and inhibited plant uptake. When 5 mg P kg^?1 as KH₂PO₄ was added, inoculation with PA increased soil acid phosphatase activity which enhanced the mineralization rate of phytin. PA mobilized more P than it immobilized in microbial pool and enhanced plant P uptake. We conclude that the decreased C:P ratio by adding small amount of inorganic P in the rhizosphere could drive phytin mineralization by the bacteria and improve plant P nutrition.     

Influence of phosphorus and zinc fertilizers on the uptake of P and Zn by corn plants grown in highly calcareous soils

Summary The influence of heavy applications of P (100, 200 and 400 ppm P) and Zn (12.5 and 25 ppm) fertilizers on their extractabilities, availabilities and uptake by corn grown in highly calcareous soil was investigated.A significant increase was found in the levels of (NH₄)₂CO₃-EDTA-extractable Zn either by Zn-applications alone or together with P. The amounts of NaHCO₃-extractable P were also increased with P additions and the influence of Zn applications was not clear.Phosphorus application generally increased the plant dry weight. In the soils treated with P and Zn fertilizers, that increase was mostly related to P rather to Zn.In the soils not treated with Zn, P additions increased Zn uptake by the plants. On the other side, in the soils treated with Zn, P additions decreased Zn uptake.Phosphorus concentration in the whole plant and/or in the different plant parts was increased by P application without being significantly affected by Zn addition. The plants showed greater response to 12.5 ppm Zn application than to 25 ppm.Plants grown for 4 weeks contained lower amounts of Zn relative to those grown for 8 weeks. The influence of plant age on P content was not as clear as occurred with Zn.     

Interrelationship between phosphorus toxicity and sugar metabolism in Verticordia plumosa L

Phosphorus, an essential plant nutrient, may become toxic when accumulated by plants to high concentrations. Certain plant species such as Verticordia plumosa L. suffer from P toxicity at solution concentrations far lower than most other plant species. In this study, exposure of V. plumosa plants to a solution containing as low as 3 mg l^–1 P resulted in significant growth inhibition and typical symptoms of P toxicity. In a wide range of P levels studied, micronutrient concentrations in V. plumosa leaves were within the range considered adequate for optimal growth. Notably, tomato plants with high hexokinase activity due to overexpression of Arabidopsis hexokinase (AtHXK1) exhibited senescence symptoms similar to those of P toxic V. plumosa. The resemblance in senescence symptoms between P-toxic tomato plants and those with high hexokinase activity suggested that increased sugar metabolism could play a role in P toxicity in plants. To test this hypothesis, we determined the amount of hexose phosphate, the product of hexokinase, in V. plumosa leaves grown at various P levels in the nutrient solution. Positive correlations were found between concentration in the medium, P concentration in the plant, hexose phosphate concentration in leaves and P toxicity symptoms. Foliar Zn application suppressed P toxicity symptoms and reduced the level of hexose phosphate in leaves. Furthermore, Zn also inhibited hexokinase activity in vitro. Based on these results we suggest that P toxicity involves sugar metabolism via increased activity of hexokinase that accelerates senescence     

Responsiveness of Carob (Ceratonia siliqua L.) Plants to Arbuscular Mycorrhizal Symbiosis Under Different Phosphate Fertilization Levels

This experiment was carried out in pots in a greenhouse to evaluate the effects of arbuscular mycorrhizal fungi (Funneliformis mosseae, Rhizophagus intraradices and Rhizophagus fasciculatus) on carob plant performance under different levels of phosphate fertilization. Non-mycorrhizal (NMyc) and mycorrhizal (Myc) carob plants were subjected to three levels of phosphate fertilization, L1 (0 mg P kg⁻¹ soil), L2 (25 mg P kg⁻¹ soil) and L3 (100 mg P kg⁻¹ soil). Results showed that under L1 and L2 P-fertilization levels, arbuscular mycorrhizal symbiosis significantly improved growth and biomass production of carob plants. Moreover, mineral nutrient (P, K, Na and Ca) acquisition, photosynthetic activity (F_v/F_m), stomatal conductance, total chlorophyll content, and soluble sugar accumulation were also strongly improved in Myc plants in comparison with NMyc ones. Under L1 P-fertilization level, Myc plants showed strongly increased acid phosphatase activity in roots and in the rhizospheric soil than NMyc plants. Furthermore, Myc plants maintained high membrane integrity (over 80%) and low hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents, associated with increased activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (G-POD), and catalase (CAT) compared to NMyc plants. However, high phosphorus input (L3) negatively affected root colonization and mycorrhizal plant performance. Thus, carob plants associated with Funneliformis mosseae performed best under phosphorus deficiency and were the least sensitive to the variations of phosphorus input levels.

     

Effects of soil acidification on the chemical extractability of Fe,Mn, Zn and Cu and the growth and micronutrient uptake of highbush blueberry plants

Summary The effects of soil acidification and micronutrient addition on levels of extractable Fe, Mn, Zn and Cu in a soil, and on the growth and micronutrient uptake of young highbush blueberry plants (Vaccinium corymbosum L. cv. Blueray) was investigated in a greenhouse study.Levels of 0.05M CaCl₂-extractable Fe, Mn, Zn and Cu increased as the pH was lowered from 7.0 to 3.8. However, the solubility (CaCl₂-extractability) of Fe and Cu was considerably less pH-dependent than that of Mn and Zn. With the exception of HCl-and DTPA-extractable Mn, micronutrients extractable with 0.1M HCl, 0.005M DTPA and 0.04M EDTA were unaffected or raised only slightly as the pH was lowered from 6.0 to 3.8. Quantities of Mn and Zn extractable with CaCl₂ were similar in magnitude to those extractable with HCl, DTPA and EDTA whilst, in contrast, the latter reagents extracted considerably more Cu and Fe than did CaCl₂. A fractionation of soil Zn and Cu revealed that soil acidification resulted in an increase in the CaCl₂- and pyrophosphate-extractable fractions and a smaller decrease in the oxalate-extractable fraction.Plant dry matter production increased consistently when the soil pH was lowered from 7.0 to 4.6 but there was a slight decline in dry matter as the pH was lowered to 3.8. Micronutrient additions had no influence on plant biomass although plant uptake was increased. As the pH was lowered, concentrations of plant Fe first decreased and then increased whilst those of Mn, and to a lesser extent Zn and Cu, increased markedly.     

Growth effects of the vesicular-arbuscular mycorrhizal fungusGlomus constrictum on maize plants in pot trials

Maize plants were inoculated withGlomus constrictum in soil of low phosphorus content amended with five rates of P in the form of Ca₃(PO₄)₂. Maize dry matter yield was increased by addition of P up to 30 and/or 60 mg P/kg soil, above that it began to decrease to reach at 100 mg P/kg a value similar to that of the control. At all P levels used, the shoot and root (total plant) dry mass of inoculated plants was significantly increased compared with the non-inoculated controls and this increment ranged in some cases between 50 and 70%. Development of vesicular-arbuscular mycorrhizal fungus (VAM) monitored in terms of P contents in dry matter of maize revealed that the P content of plants not inoculated withG. constrictum was not influenced by P addition to soil. On the other hand, P content of maize plants inoculated with VAM was dramatically increased by increasing P levels of soil and was maximum at 30 mg P; above that it began to decline. Mycorrhizal root infection (expressed as percentage of root length infected) increased by increasing the P concentrations above the soil basal level up to 80 mg P where the infected root length was 72% of the total root length after 28 d of planting. The increase in VAM spore formation in soil was similar to that of root infection except that the highest spore number was sieved from soil at 60 mg P/kg soil.     

Influence of the arbuscular mycorrhizal fungus Glomus mosseae on uptake of arsenate by the As hyperaccumulator fern Pteris vittata L.

We report for the first time some effects of colonization by an arbuscular mycorrhizal (AM) fungus (Glomus mosseae) on the biomass and arsenate uptake of an As hyperaccumulator, Pteris vittata. Two arsenic levels (0 and 300 mg As kg^–1) were applied to an already contaminated soil in pots with two compartments for plant and hyphal growth in a glasshouse experiment. Arsenic application had little or no effect on mycorrhizal colonization, which was about 50% of root length. Mycorrhizal colonization increased frond dry matter yield, lowered the root/frond weight ratio, and decreased frond As concentration by 33–38%. Nevertheless, transfer of As to fronds showed a 43% increase with mycorrhizal colonization at the higher soil As level. Frond As concentrations reached about 1.6 g kg^–1 (dry matter basis) in non-mycorrhizal plants in the As-amended soil. Mycorrhizal colonization elevated root P concentration at both soil As levels and mycorrhizal plants had higher P/As ratios in both fronds and roots than did non-mycorrhizal controls.     

Symbiotic N2-fixation by the cover crop Pueraria phaseoloides as influenced by litter mineralization

The perennial legume Pueraria phaseoloides is widely used as a cover crop in rubber and oil palm plantations. However, very little knowledge exists on the effect of litter mineralization from P. phaseoloides on its symbiotic N₂-fixation. The contribution from symbiotic N₂-fixation (Ndfa) and litter N (Ndfl) to total plant N in P. phaseoloides was determined in a pot experiment using a ¹⁵N cross-labeling technique. For determination of N₂-fixation the non-fixing plant Axonopus compressus was used as a reference. The experiment was carried out in a growth chamber during 9 weeks with a sandy soil and 4 rates of ground litter (C/N=16,2.8% N). P. phaseoloides plants supplied with the highest amount of litter produced 26% more dry matter and fixed 23% more N than plants grown in soil with no litter application, but the percentage of Ndfa decreased slightly, but significantly, from 87 to 84%. The litter N uptake was directly proportional to the rate of application and constituted 10% of total plant N at the highest application rate. Additionally, a positive correlation was found between litter N uptake and the amount of fixed N₂. The total recovery of litter N in plants averaged 26% at harvest (shoot + root) and was not affected by the quantity added. A parallel incubation experiment also showed that, as an average of all litter levels, 26% of the litter N was present in the inorganic N pool. The amounts of fertilizer and soil N taken up by plants decreased with litter application, probably due to microbial immobilization and denitrification. It is concluded that, within the litter levels studied, litter mineralization will result in a higher amount of N₂-fixed by P. phaseoloides.     

不同水肥因子与AM真菌对黄芩生长和营养成分的交互效应

利用盆栽接种试验,探讨不同水肥条件下AM真菌双网无梗囊霉Acaulospora bireticulata对黄芩生长、养分含量和次生代谢产物的影响,为黄芩水肥合理施用提供理论依据。结果表明,不同水肥条件下,AM真菌能与黄芩根系形成良好共生关系,接种AM真菌能显著提高黄芩根系菌根侵染率和生物量,水分和施肥处理对菌根侵染率和黄芩生长具有显著交互作用。不同水肥条件下,接种AM真菌提高了植株保护酶活性和叶片渗透调节物质含量,降低了脯氨酸和丙二醛含量;显著增加了黄芩苷和N、P、K、Ca、Mg、Fe和Zn含量,降低了Mn和Cu含量。N和P含量随施肥量增加而提高,其余矿质元素在施肥量N 0.383 g、P 0.564 g、K 0.251 g时含量最高,说明AM真菌能够促进宿主植物根系对水分和矿质元素的吸收和利用,提高水分和肥料利用率,具有明显的节水节肥作用,其中50%相对含水量,施肥量N 0.383 g、P 0.564 g、K 0.251 g时,接种AM真菌的促生效应最佳。     

Soil-sodicity-induced zinc deficiency in maize

Summary Maize (Zea mays L. cv. Ganga-2) plants were grown in pot culture on a loamy alluvial soil of Lucknow district (India) alkalinized to graded levels of ESP (Exchangeable Sodium Percentage) ranging from 15.5 to 55.3. Before sowing maize seeds the soil was fertilised with NPK, Fe, Mn and Cu. At and above ESP 34 Zn-deficiency symptoms first appeared at 30 days. The symptoms gradually became pronounced with increase in age and at 60 days they were found even at ESP 15.5. The severity of symptoms was related to increase in sodicity. Alkalinization of soils depressed available soil Zn and tissue Zn and increased tissue ratios of Na/Zn and P/Zn. It also decreased the total plant content of Zn, Fe, Mn, Cu and even Na. Increase in soil sodicity increased both tissue concentration and total content of P in plants upto ESP 34 beyond which it decreased it. Among different extractants, 0.1N HCl, DTPA pH 7.3 and EDTA-(NH₄)₂ CO₃ pH 8.6, for measuring available soil Zn the latter showed best correlations with soil ESP (−), tissue P (−), P/Zn ratio (−), dry matter yield (+) and tissue Zn (+). Tissue Zn was related to yield (+), tissue Na (−) and soil ESP (−). Mild, moderate, severe and very severe Zn deficiency in maize was induced by soil ESP levels, 18, 25, 33 and 45, respectively.     

Sulfur and nitrogen nutrition status in flag leaf and shoot samples collected from wheat growing areas in Çukurova,Central Anatolia and GAP regions of Turkey

Sulfur (S) deficiency in soils and plants has been increased in the recent decade which is reducing crop yield and quality. Unfortunately, no extensive study has been conducted on S nutritional status of plants in Turkey. In this study, soil and plant samples were collected from Çukurova, Central Anatolia and GAP regions where wheat is extensively cultivated. Plant samples either as flag leaf or the whole shoot were collected depending on growth stage of wheat crop at sample collection. Similarly, surface (0–20 cm) and sub-surface (20–40 cm) soil samples were collected from plant sampling sites and a total 963 plant and 1947 soil samples were collected during the study. The S concentration in flag leaf samples varied between 0.18 and 0.67%, 0.11–0.59% and 0.17–0.82% for central Anatolia, Çukurova and GAP regions, respectively. According to S concentration in flag leaf samples, 99% of the plants in Çukurova region were found sufficient in S nutrition. However, 49% of the samples collected from central Anatolia and GAP regions were deficient in S. Critical N:S ratio indicating S nutrition status of plants was lower than the widely accepted critical value of 17. This low N:S ratio was a consequence of deficient N nutrition rather than S nutrition. Moreover, it was observed that plant available SO₄-S concentration of soils varied within and among sampled provinces with an average value of 20.6 and 31.6 mg kg⁻¹ for surface and sub-surface samples, respectively. The SO₄-S concentration increased with increasing soil depth. The results indicate a significantly positive correlation between S concentration in plant shoot and plant available SO₄-S concentration in soils. In conclusion, S-containing fertilizer use in central Anatolia and GAP regions must be considered as an important approach for the prevention of yield and quality losses. Furthermore, rapid and sensitive plant and soil analysis methods are needed, which must also consider the local and site-specific conditions.     

Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2

Although legumes showed a clearly superior yield response to elevated atmospheric pCO₂ compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N₂ fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne (Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO₂ response. Under elevated atmospheric pCO₂, effectively nodulating lucerne, thus capable of symbiotically fixing N₂, strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO₂ when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO₂, although not as strongly as effectively nodulating plants. The CO₂‐induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N₂ fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO₂. This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N₂ fixation per se is responsible for the considerably greater amount of above‐ground biomass and the higher N yield under elevated atmospheric pCO₂. This supports the assumption that symbiotic N₂ fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO₂‐rich world.     

Phosphorus sorption isotherm for evaluating phosphorus requirements of lettuce at five temperature regimes

A study was conducted to determine the influence of temperature on solubility and subsequent availability of P to plants. Phosphorus sorption curves were used to fertilize the soil with six P rates corresponding to initial sorption equilibrium solution P levels in the range of 0.03 to 0.72 ppm. Yield response to P measured for temperatures was in controlled environment chambers. Growth temperatures ranged 12.7°C to 29.4°C. Sorption of added P and desorption of sorbed P increased with increase in temperature. Yields of lettuce increased with the increased sorption equilibrium solution P levels at all the temperatures. Initial sorption-equilibrium solution P levels necessary for 95% yield had to be increased from 0.20 to 0.64 ppm when temperature was lowered from 29.4° C to 12.7°C. Desorption-equilibrium solution P levels for 95% yield were nearly constant at all temperatures. The log of initial sorption equilibrium solution P as a function of temperature for 95% yield was linear between 17.2°C and 29.4°C. Comparison of the data for 95% yield in solution cultures showed that critical nutrient solution P levels were very close to the desorption equilibrium P levels for 95% yield of plants grown in soil at similar temperatures. Relative yields of lettuce and desorption equilibrium solution P levels showed the same trend to changes in temperature. The study suggests that the major role of temperature on P uptake between 17.2°C and 29.4°C is due to desorption of P from the soil. Physiological influences of temperature on plant growth and consequent P requirement play a minor role.Contribution from the Department of Plant and Soil Sciences, University of Idaho, Moscow. Approved for publication by the Director of the Agricultural Experiment Station as Research Paper No.74740. Presented before Div. S-4, Western Regional Meeting, ASA, Irvine, Calif., June 19, 1974. Part of senior author's Ph.D. dissertation.     

Influence of sulphur on the yield and concentration of copper,manganese, iron and molybdenum in berseem (Trifolium alexandrinum) grown on two different soils

A screen-house experiment with 0, 25, 50 and 75 ppm S was conducted to study the effect of S on the yield and, Mn, Cu, Fe and Mo concentrations of berseem (Trifolium alexandrinum) at three stages of growth in a normal and reclaimed soil. Higher levels of S decreased the yield in both the soils. Yield in normal soil was much higher than in reclaimed soil. Application of S increased Cu, Mn, and Fe concentrations in all cuts in both soils. Molybdenum concentration was lowered following S application in both the soils in all the cuts. Molybdenum content increased with the stage of plant growth.     

Phosphorus application and elevated CO2 enhance drought tolerance in field pea grown in a phosphorus-deficient vertisol

Background and Aims Benefits to crop productivity arising from increasing CO₂ fertilization may be offset by detrimental effects of global climate change, such as an increasing frequency of drought. Phosphorus (P) nutrition plays an important role in crop responses to water stress, but how elevated CO₂ (eCO₂) and P nutrition interact, especially in legumes, is unclear. This study aimed to elucidate whether P supply improves plant drought tolerance under eCO₂.Methods A soil-column experiment was conducted in a free air CO₂ enrichment (SoilFACE) system. Field pea (Pisum sativum) was grown in a P-deficient vertisol, supplied with 15 mg P kg⁻¹ (deficient) or 60 mg P kg⁻¹ (adequate for crop growth) and exposed to ambient CO₂ (aCO₂; 380–400 ppm) or eCO₂ (550–580 ppm). Drought treatments commenced at flowering. Measurements were taken of soil and leaf water content, photosynthesis, stomatal conductance, total soluble sugars and inorganic P content (Pi).Key Results Water-use efficiency was greatest under eCO₂ when the plants were supplied with adequate P compared with other treatments irrespective of drought treatment. Elevated CO₂ decreased stomatal conductance and transpiration rate, and increased the concentration of soluble sugars and relative water contents in leaves. Adequate P supply increased concentrations of soluble sugars and Pi in drought-stressed plants. Adequate P supply but not eCO₂ increased root length distribution in deeper soil layers.Conclusions Phosphorus application and eCO₂ interactively enhanced periodic drought tolerance in field pea as a result of decreased stomatal conductance, deeper rooting and high Pi availability for carbon assimilation in leaves.     

Methyl jasmonate counteracts boron toxicity by preventing oxidative stress and regulating antioxidant enzyme activities and artemisinin biosynthesis in <Emphasis Type="Italic">Artemisia annua</Emphasis> L.

Boron is an essential plant micronutrient, but it is phytotoxic if present in excessive amounts in soil for certain plants such as Artemisia annua L. that contains artemisinin (an important antimalarial drug) in its areal parts. Artemisinin is a sesquiterpene lactone with an endoperoxide bridge. It is quite expensive compound because the only commercial source available is A. annua and the compound present in the plant is in very low concentration. Since A. annua is a major source of the antimalarial drug and B stress is a deadly threat to its cultivation, the present research was conducted to determine whether the exogenous application of methyl jasmonate (MeJA) could combat the ill effects of excessive B present in the soil. According to the results obtained, the B toxicity induced oxidative stress and reduced the stem height as well as fresh and dry masses of the plant remarkably. The excessive amounts of soil B also lowered the net photosynthetic rate, stomatal conductance, internal CO₂ concentration and total chlorophyll content in the leaves. In contrast, the foliar application of MeJA enhanced the growth and photosynthetic efficiency both in the stressed and non-stressed plants. The excessive B levels also increased the activities of antioxidant enzymes, such as catalase, peroxidase and superoxide dismutase. Endogenous H₂O₂ and O₂⁻ levels were also high in the stressed plants. However, the MeJA application to the stressed plants reduced the amount of lipid peroxidation and stimulated the synthesis of antioxidant enzymes, enhancing the content and yield of artemisinin as well. Thus, it was concluded that MeJA might be utilized in mitigating the B toxicity and improving the content and yield of artemisinin in A. annua plant.     

13C and 15N in microarthropods reveal little response of Douglas-fir ecosystems to climate change

Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO₂) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO₂ increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass (n= 4, r²= 0.96, P= 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8–1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25–26) and litter (36–48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO₂ increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments.     

Biosynthesis of plant phenolic compounds in elevated atmospheric CO2

Experiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO₂) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO₂, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO₂ on plant biomass and carbon and nitrogen content were species‐ and generation‐specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO₂ in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO₂ in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO₂ over the three generations. When the species were grown alone in pots, elevated CO₂ increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO₂ in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO₂ level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species‐specific and changed markedly between generations.