Extracellular Matrix in Early Stages of Direct Somatic Embryogenesis in Leaves of Drosera spathulata

The growth and water relations of Paulownia fortunei in photoautotrophic cultures (nutrient medium lacking sucrose and growth regulator) with CO₂ enrichment (PWAH) or without CO₂ enrichment (PWAL) were compared with those in photomixotrophic shoot (PWC; 30 g dm⁻³ sucrose and 0.3 mg dm⁻³ N⁶-benzyladenine) and root cultures (PWR; 0.3 mg dm⁻³ indole-3-butyric acid). The photoautotrophic and photomixotrophic cultures were incubated under photosynthetic photon flux 125 and 60 μmol m⁻² s⁻¹, respectively. 100 % sprouting and significantly higher number of shoots (1.6) were obtained with PWAH as compared to PWAL and PWC. PWAH and PWAL stimulated spontaneous rooting from the cut end of axillary shoots. In PWAH, 84 % of shoots rooted with an average of 5.9 roots per shoot and 4.0 cm of root length in 21 d. Rooting of photomixotrophic shoot cultures were stimulated by an auxin treatment. In this case, 98.3 % of shoots were rooted with an average of 4.6 roots per shoot and 1.9 cm length. A microscopic observation on leaf abaxial surface prints from photomixotrophic shoot and root cultures showed widely open (6 – 8 μm) spherical stomata (12 – 14 μm) and from photoautotrophic cultures elliptical stomata (10 – 12 μm) with narrow openings (3 – 4 μm). Leaves from photomixo-trophic cultures had higher stomatal index as compared to photoautotrophic cultures. The rate of moisture loss from detached leaves was not varying significantly in different cultures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of activated charcoal and 6-benzyladenine on in vitro nitrogen uptake by Lagerstroemia indica

A sterile hydroponic culture system suitable for studying nitrogen (N) uptake ofLagerstroemia indica L.in vitro was developed. Four different treatments were assayed: with and without activated charcoal (AC and NAC, respectively), with and without 50 μM of 6-benzyladenine (+BA and −BA, respectively). Medium pH, electrical conductivity (EC), NO₃ ⁻ and NH₄ ⁺ concentrations were measured weekly. At the end of the culture, propagules were sampled and SPAD indices, and shoot and root fresh weights were determined. Explants grown in media with activated charcoal were able to take up both NO₃ ⁻ and NH₄ ⁺, although NH₄ ⁺ uptake was lower. Subsequently the pH of the media was maintained between 5.5–6.0. In treatments with no addition of activated charcoal, NH₄ ⁺ uptake was preferential and the pH dropped to 3.1. Explants in these conditions were unable to raise the pH by taking up NO₃ ⁻, especially when root morphogenesis was inhibited by addition of BA. Supply of this PGR produced root growth inhibition, which was almost complete in the treatment without activated charcoal. This component significantly reduced the inhibitory effect of 50 μM BA on root growth. This revised version was published online in June 2006 with corrections to the Cover Date.     

Contribution of Lolium perenne rhizodeposition to carbon turnover of pasture soil

Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by ¹⁴CO₂ pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total ¹⁴CO₂ efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after ¹⁴C pulse labelling decreased during plant development from 14 to 6.5% of the total ¹⁴C input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the ¹⁴C input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total ¹⁴CO₂ efflux from the soil was approximately 41%. Close correlation was found between cumulative ¹⁴CO₂ efflux from the soil and the time when maximum ¹⁴CO₂ efflux occurred (r=0.97). The average total of CO₂ Defflux from the soil with Lolium perenne was approximately 21 μg C-CO₂ d⁻¹ g⁻¹. It increased slightly during plant development. The contribution of plant roots to total CO₂ efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total ¹⁴C content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6–10 g C m⁻² at the beginning of the growth period and 50–65 g C m⁻² towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Carbon balance and allocation of assimilated CO2 in Scots pine, Norway spruce, and Silver birch seedlings determined with gas exchange measurements and 14C pulse labelling

Carbon dioxide is released from the soil to the atmosphere in heterotrophic respiration when the dead organic matter is used for substrates for soil micro-organisms and soil animals. Respiration of roots and mycorrhiza is another major source of carbon dioxide in soil CO₂ efflux. The partitioning of these two fluxes is essential for understanding the carbon balance of forest ecosystems and for modelling the carbon cycle within these ecosystems. In this study, we determined the carbon balance of three common tree species in boreal forest zone, Scots pine, Norway spruce, and Silver birch with gas exchange measurements conducted in laboratory in controlled temperature and light conditions. We also studied the allocation pattern of assimilated carbon with ¹⁴C pulse labelling experiment. The photosynthetic light responses of the tree species were substantially different. The maximum photosynthetic capacity (P _max) was 2.21 μg CO₂ s⁻¹ g⁻¹ in Scots pine, 1.22 μg CO₂ s⁻¹ g⁻¹ in Norway spruce and 3.01 μg CO₂ s⁻¹ g⁻¹ in Silver birch seedlings. According to the pulse labelling experiments, 43–75% of the assimilated carbon remained in the aboveground parts of the seedlings. The amount of carbon allocated to root and rhizosphere respiration was about 9–26%, and the amount of carbon allocated to root and ectomycorrhizal biomass about 13–21% of the total assimilated CO₂. The ¹⁴CO₂ pulse reached the root system within few hours after the labelling and most of the pulse had passed the root system after 48 h. The transport rate of carbon from shoot to roots was fastest in Silver birch seedlings.     

Characterization of natural variation for zinc, iron and manganese accumulation and zinc exposure response in Brassica rapa L.

Brassica rapa L. is an important vegetable crop in eastern Asia. The objective of this study was to investigate the genetic variation in leaf Zn, Fe and Mn accumulation, Zn toxicity tolerance and Zn efficiency in B. rapa. In total 188 accessions were screened for their Zn-related characteristics in hydroponic culture. In experiment 1, mineral assays on 111 accessions grown under sufficient Zn supply (2 μM ZnSO₄) revealed a variation range of 23.2–155.9 μg g⁻¹ dry weight (d. wt.) for Zn, 60.3–350.1 μg g⁻¹ d. wt. for Fe and 20.9–53.3 μg g⁻¹ d. wt. for the Mn concentration in shoot. The investigation of tolerance to excessive Zn (800 μM ZnSO₄) on 158 accessions, by using visual toxicity symptom parameters (TSPs), identified different levels of tolerance in B. rapa. In experiment 2, a selected sub-set of accessions from experiment 1 was characterized in more detail for their mineral accumulation and tolerance to excessive Zn supply (100 μM and 300 μM ZnSO₄). In this experiment Zn tolerance (ZT) determined by relative root or shoot dry biomass varied about 2-fold. The same six accessions were also examined for Zn efficiency, determined as relative growth under 0 μM ZnSO₄ compared to 2 μM ZnSO₄. Zn efficiency varied 1.8-fold based on shoot dry biomass and 2.6-fold variation based on root dry biomass. Zn accumulation was strongly correlated with Mn and Fe accumulation both under sufficient and deficient Zn supply. In conclusion, there is substantial variation for Zn accumulation, Zn toxicity tolerance and Zn efficiency in Brassica rapa L., which would allow selective breeding for these traits.     

Microplankton respiratory activity and CO2 production rates in the Otranto Strait (Mediterranean Sea)

Respiratory activity and metabolic CO₂production of the microplankton in the Otranto Strait (Mediterranean Sea) were determined by monitoring the Electron Transport System activity. Ten stations were repeatedly investigated during two oceanographic surveys in February–March and August 1994. Respiratory activity and CO₂ production, estimated from the surface to the bottom, were higher in the euphotic layers (0-200 m) during summer (mean values: Winter = 0.024 μg C h⁻¹ dm⁻³; Summer = 0.042 μg C h⁻¹ dm⁻³); in the aphotic zone (deeper than 200 m), the rates were similar throughout different seasons (0.013 and 0.014 μg C h⁻¹ dm⁻³, respectively). A comparison with data collected by other authors from the euphotic layers of the Mediterranean Sea was made. Respiratory activities decreased from Western to Eastern Mediterranean Basins. The values of CO₂ production, integrated between 200 and 1000 m in the Otranto Strait (mean value 237.7 mg C m⁻² d⁻¹), were compared with other data collected from the Mediterranean Sea as well as from the Pacific, Atlantic and Indian Oceans. The comparison showed the Otranto Strait to be a site of organic matter oxidation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of Elevated Atmospheric CO2 Concentrations on CH4 and N2O Emission from Rice Soil: An Experiment in Controlled-environment Chambers

The effects of elevated concentrations of atmospheric CO₂ on CH₄ and N₂O emissions from rice soil were investigated in controlled-environment chambers using rice plants growing in pots. Elevated CO₂ significantly increased CH₄ emission by 58% compared with ambient CO₂. The CH₄ emitted by plant-mediated transport and ebullition–diffusion accounted for 86.7 and 13.3% of total emissions during the flooding period under ambient level, respectively; and for 88.1 and 11.9% of total emissions during the flooding period under elevated CO₂ level, respectively. No CH₄ was emitted from plant-free pots, suggesting that the main source of emitted CH₄ was root exudates or autolysis products. Most N₂O was emitted during the first 3 weeks after flooding and rice transplanting, probably through denitrification of NO₃⁻ contained in the experimental soil, and was not affected by the CO₂ concentration. Pre-harvest drainage suppressed CH₄ emission but did not cause much N₂O emission (< 10 μg N m⁻² h⁻¹) from the rice-plant pots at both CO₂ concentrations.     

Mechanism of Activation of Caspase-9 and Caspase-3 during Hypoxia in the Cerebral Cortex of Newborn Piglets: The Role of Nuclear Ca<Superscript>2+</Superscript>-Influx

In previous studies, we have shown that cerebral hypoxia results in increased activity of caspase-9, the initiator caspase, and caspase-3, the executioner of programmed cell death. We have also shown that cerebral hypoxia results in high affinity Ca²⁺–ATPase-dependent increase in nuclear Ca²⁺-influx in the cerebral cortex of newborn piglets. The present study tests the hypothesis that inhibiting nuclear Ca²⁺-influx by pretreatment with clonidine, an inhibitor of high affinity Ca²⁺–ATPase, will prevent the hypoxia-induced increase in caspase-9 and caspase-3 activity in the cerebral cortex of newborn piglets. Thirteen newborn piglets were divided into three groups, normoxic (Nx, n = 4), hypoxic (Hx, n = 4), and hypoxic treated with clonidine (100 mg/kg) (Hx–Cl, n = 5). Anesthetized, ventilated animals were exposed to an FiO₂ of 0.21 (Nx) or 0.07 (Hx) for 60 min. Cerebral tissue hypoxia was documented biochemically by determining levels of ATP and phosphocreatine (PCr). Caspase-9 and -3 activity were determined spectrofluoro-metrically using specific fluorogenic synthetic substrates. ATP (μmoles/g brain) was 4.6 ± 0.3 in Nx, 1.7±0.4 in Hx (P < 0.05 vs. Nx), and 1.5 ± 0.2 in Hx–Cl (P < 0.05 vs. Nx). PCr (μmoles/g brain) was 3.6 ± 0.4 in Nx, 1.1 ± 0.3 in Hx (P < 0.05 vs. Nx), and 1.0 ± 0.2 in Hx–Cl (P < 0.05 vs. Nx). Caspase-9 activity (nmoles/mg protein/h) was 0.548 ± 0.0642 in Nx and increased to 0.808 ± 0.080 (P < 0.05 vs. Nx and Hx–Cl) in the Hx and 0.562 ± 0.050 in the Hx–Cl group (p = NS vs. Nx). Caspase-3 activity (nmoles/mg protein/h) was 22.0 ± 1.3 in Nx and 32 ± 6.3 in Hx (P < 0.05 vs. Nx) and 18.8 ± 3.2 in the Hx–Cl group (P < 0.05 vs. Hx). The data demonstrate that clonidine administration prior to hypoxia prevents the hypoxia-induced increase in the activity of caspase-9 and caspase-3. We conclude that the high afinity Ca²⁺–ATPase-dependent increased nuclear Ca²⁺ during hypoxia results in increased caspase-9 and caspase-3 activity.     

TheParasponia parviflora — Rhizobium symbiosis. Isotopic nitrogen fixation,hydrogen evolution and nitrogen-fixation efficiency,and oxygen relations

Summary Isotopic¹⁵N₂ experiments confirmed nitrogen fixation inParasponia parviflora. The conversion ratio C₂H₄/N₂ was 6.7 under the experimental conditions employed. Measurements of the δ¹⁵N in leaves of Parasponia and Trema showed on the basis of these determinations thatParasponia parviflora possesses N₂-fixing capacity and can be distinguished in this respect from the non-nitrogen-fixingTrema cannabina tested by the same method. Therefore, δ¹⁵N can be used to monitor N₂ fixation in natural ecosystems. Hydrogen evolution and the relative efficiency of N₂ fixation in this relation have been determined. DetachedParasponia parviflora root nodules grown in soil and tested in an argon/oxygen atmosphere produced appr. 4 μmol H₂.h⁻¹.g⁻¹ fresh weight root nodules. The relative efficiency of hydrogen utilization as measured in argon, air, and in the presence of C₂H₂ 10% (v/v) was for both equations used for to express this efficiency 0.96 and 0.97, respectively. This indicates that Parasponia like the root nodules of some actinorhizal symbioses (Alnus, Myrica, Elaeagnus) and some tropical legumes (Vigna sinensis) has evolved mechanisms of minimizing net hydrogen production in air, thus increasing the efficiency of electron transfer to nitrogen. The oxygen relation of nitrogen fixation (C₂H₂) inParasponia parviflora root nodules was determined. The nitrogenase activity of Parasponia root nodules increased at increasing oxygen concentrations up till c. 40% O₂. At oxygen levels above 40% O₂, the nitrogenase activity of the root nodules was nil or very erratic suggesting that at these oxygen levels the nitrogenase is not longer protected against the harmful effect of oxygen. In this respect Parasponia root nodules differ from actinorhizal root nodules in other nonlegumes, where optimal nitrogenase activity was observed in the range of 12–25% oxygen. Respiration experiments with Parasponia root nodules showed that in the range 10, 20, and 40% oxygen, the respiration rate (CO₂ evolution) increased concomitantly with an increase of the acetylene reduction rate. The CO₂/C₂H₄ values obtained varied between 8.1 and 19.2, being therefore 2–3 times higher than similar estimations in some actinorhizal and legume root nodules. The respiratory quotient (RQ) of detachedParasponia parviflora root nodules was in air initially approximately 2.0, but this value dropped to about 1.0 in a 3-hours period.     

The kinetics of NH4 + and NO3 − uptake by Douglas fir from single N-solutions and from solutions containing both NH4 + and NO3 −

The kinetics of NH₄ ⁺ and NO₃ ⁻ uptake in young Douglas fir trees (Pseudotsuga menziesii [Mirb.] Franco) were studied in solutions, containing either one or both N species. Using solutions containing a single N species, the V_max of NH₄ ⁺ uptake was higher than that of NO₃ ⁻ uptake. The K_m of NH₄ ⁺ uptake and K_m of NO₃ ⁻ uptake differed not significantly. When both NH₄ ⁺ and NO₃ ⁻ were present, the V_max for NH₄ ⁺ uptake became slightly higher, and the K_m for NH₄ ⁺ uptake remained in the same order. Under these conditions the NO₃ ⁻ uptake was almost totally inhibited over the whole range of concentrations used (10–1000 μM total N). This inhibition by NH₄ ⁺ occurred during the first two hours after addition. ei]{gnA C}{fnBorstlap}     

Response of the plant root to aluminum stress: Analysis of the inhibition of the root elongation and changes in membrane function

Pea root elongation was strongly inhibited in the presence of a low concentration of Al (5 μM). In Al-treated root, the epidermis was markedly injured and characterized by an irregular layer of cells of the root surface. Approximately 30% of total absorbed Al accumulated in the root tip and Al therein was found to cause the inhibition of whole root elongation. Increasing concentrations of Ca²⁺ effectively ameliorated the inhibition of root elongation by Al and 1 mM of CaCl₂ completely repressed the inhibition of root elongation by 50 μM Al. The ameliorating effect of Ca²⁺ was due to the reduction of Al uptake. H⁺-ATPase and H⁺-PPase activity as well as ATP and PPidependent H⁺ transport activity of vacuolar membrane vesicles prepared from barley roots increased to a similar extent by the treatment with 50 μM AlCl₃. The rate of increase of the amount of H⁺-ATPase and H⁺-PPase was proportional to that of protein content measured by immunoblot analysis with antibodies against the catalytic subunit of the vacuolar H⁺-ATPase and H⁺-PPase of mung bean. The increase of both activities was discussed in relation to the physiological tolerance mechanism of barley root against Al stress.     

Quantification of N2-fixation and survival of inoculated diazotrophs associated with roots of Kallar grass

White clover plants were grown for 97 days under two temperature regimes (20/15°C and 8/5°C day/night temperatures) and were supplied with either small amounts (a total of 80 mg N pot^–1) of ammonium (NH ₄ ⁺ ) or nitrate (NO ₃ ^– ) nitrogen, or received no mineral N and relied on N₂ fixation. Greatest growth and total leaf area of clover plants occurred in N₂ fixing and NO ₃ ^– -fed plants grown at 20/15°C and poorest growth occurred in NH ₄ ⁺ -fed plants grown at 8/5°C. Nodule mass per plant was greater at 8/5°C due to increased nodule numbers rather than increased dry weight per nodule. This compensated to some extent for the reduced N₂-fixing activity per unit dry weight of nodule tissue found at the low growth temperature up to 116 d after sowing, but thereafter both activity per nodule dry weight and activity per plant were greater at the low temperature. Highest nitrate reductase activity (NRA) per g fresh weight and total activity per leaf, petiole or root occurred in NO ₃ ^– -fed plants at 8/5°C. Low growth temperature resulted in a greater partitioning of total plant NRA to the roots of NO ₃ ^– -fed plants. The results are considered in relation to the use of N fertiliser in the spring under field conditions.     

Effect of light and atmospheric carbon dioxide concentration on nitrogen fixation by herbage legumes

Summary Lucerne, red clover and white clover were grown at two atmospheric concentrations of CO₂ (300 and 1000 μl l⁻¹) and the effects on N₂ fixation, nodule mass/number and root/shoot dry matter production determined. Pea plants were similarly evaluated as a comparison with grain legumes. CO₂ enrichment increased N₂ fixation activity in all cases but activity/unit nodule mass was significantly increased only in the pea. The enhancement of N₂ fixation in herbage legumes by CO₂ enrichment reflected an increase in nodule mass which in turn was attributed to increased nodule number, and results show that under the experimental conditions obtaining here photosynthate supply did not limit nodule N₂ fixation in these plants though it was limiting in the case of peas. White clover growing in a 6 and 14 hour photoperiod was studied for response of the N₂ fixing system to light. Long photoperiod (14 hour) plants assayed at constant temperature (20°C) did not show a significant response to light at the end of the dark period either in terms of fixation per plant or per unit nodule mass, in contrast with short photoperiod (6 hour) plants which showed significant responses. Short photoperiod plants compensated for reduced photosynthates by maintaining only half the root nodule mass and fixation activity of 14 hour photoperiod plants though plants in both systems supported similar rates of N₂ fixation per unit mass of nodule during the photoperiod. Comparison of N₂ fixation activities in whole and decapitated plant systems indicates the importance of shoot reserves for sustaining nitrogenase activity in white clover during short-term interruption of photosynthesis. These results support the conclusion of the CO₂ enrichment studies, that herbage legumes have the potential for supplying their nodule photosynthate requirements for sustaining optimum rates of N₂ fixation and excess carbon supply is used solely to promote further nodulation. Nodules of short photoperiod white clover plants were less efficient in N₂ fixation in that they evolved more H₂ relative to N₂ (C₂H₂) reduced than did long photoperiod plants.     

Optimizing environmental factors for large-scale multiplication of chrysanthemum (<Emphasis Type="Italic">Chrysanthemum grandiflorum</Emphasis>) in balloon-type bioreactor culture

Summary We have developed optimum culture conditions for the large-scale propagation of chrysanthemum in balloon-type bioreactors to achieve vigorous growth and quality. The effects of NH ₄ ⁺ /NO ₃ ⁻ ratio, air volume, air temperature, photosynthetic photo flux, and an inoculation density on the growth and quality of plantlets were investigated. The best production conditions were an NH ₄ ⁺ :NO ₃ ⁻ ratio of 20∶40 mM, air exchange of 0.1 vvm min⁻¹, air temperature 25°C, photosynthetic photo flux (PPF) at 100 μmol m⁻² s⁻¹, and an inoculation density of 40 nodes Chrysanthemum grandiflorum. Under each of these conditions, the maximum growth rate reached 279.0, 260,0, 20.0, 23.3, and 94.5 (g-fresh weight per plantlet d⁻¹), respectively, at 12 wk of culture. These results specify the key environmental factors that can be regulated to improve the quality and quantity of flowers and increase yield in large-scale bioreactor cultures of chrysanthemum.     

Acetylene reduction and indoleacetic acid production by Azospirillum isolates from Cactaceous plants

Azospirillum isolates were obtained from rhizosphere soil and roots of three cactaceae species growing under arid conditions. All Azospirillum isolates from rhizosphere and roots ofStenocereus pruinosus andStenocereus stellatus were identified asA. brasilense; isolates of surface-sterilized roots fromOpuntia ficus-indica were bothA. brasilense andA. lipoferum. Azospirilla per g of fresh root in the three species ranged from 70×10³ to 11×10³. The most active strains in terms of C₂H₂ reduction (25–49.6 nmol/h·ml) and indoleacetic acid (IAA) production (36.5–77 μg/ml) were those identified asA. brasilense and isolated from Stenocereus roots.A. lipoferum isolated from Opuntia roots produced low amounts of IAA (6.5–17.5 μg/ml) and low C₂H₂-reduction activity (17.8–21.2 nmol/h·ml).     

免耕稻田氮肥运筹对土壤NH3挥发及氮肥利用率的影响

通过大田试验,设置5种不同的施肥比例(基肥:分蘖肥:拔节肥:穗肥-2:2:3:3(R1)、3:2:2:3(R2)、4:2:2:2(R3)、4:3:1:2(R4)与0:0:0:0(CK)),研究氮肥运筹对稻田NH₃挥发和氮肥利用率的影响。结果表明,(1)相对于不施肥,施肥显著提高了稻田NH₃挥发量。氮肥施用后,NH₃挥发损失量占施氮量的6.2%-8.5%,其中,以分蘖期NH₃挥发损失量最大,齐穗期次之,苗期和拔节期最小。施肥处理间,处理R1稻田累积NH₃挥发量最小,显著低于其它施肥处理,比处理R2、R3和R4分别低9.1%(P<0.05)、10.9%(P<0.05)和17.7%(P<0.05)。(2)相关分析表明,田面水NH₄⁺、pH值和土壤NH₄⁺和pH值均与稻田土壤NH₃挥发通量呈显著或者极显著相关;(3)处理R1水稻氮肥利用率相对于处理R2、R3和R4增加了28.4%(P<0.05)、55.4%(P<0.05)和74.9%(P<0.05)。研究表明,氮肥后移能有效降低免耕稻田NH₃挥发,提高水稻的氮肥利用率。     

Grazing and Ecosystem Carbon Storage in the North American Great Plains

Isotopic signatures of ¹³C were used to quantify the relative contributions of C₃ and C₄ plants to whole-ecosystem C storage (soil+plant) in grazed and ungrazed sites at three distinct locations (short-, mid- and tallgrass communities) along an east–west environmental gradient in the North American Great Plains. Functional group composition of plant communities, the source and magnitude of carbon inputs, and total ecosystem carbon storage displayed inconsistent responses to long-term livestock grazing along this gradient. C₄ plants [primarily Bouteloua gracilis (H.B.K.) Lag ex Steud.] dominated the long-term grazed site in the shortgrass community, whereas the ungrazed site was co-dominated by C₃ and C₄ species; functional group composition did not differ between grazed and ungrazed sites in the mid- and tallgrass communities. Above-ground biomass was lower, but the relative proportion of fine root biomass was greater, in grazed compared to ungrazed sites at all three locations. The grazed site of the shortgrass community had 24% more whole-ecosystem carbon storage compared to the ungrazed site (4022 vs. 3236 g C m⁻²). In contrast, grazed sites at the mid- and tallgrass communities had slightly lower (8%) whole-ecosystem carbon storage compared to ungrazed sites (midgrass: 7970 vs. 8683 g C m⁻²; tallgrass: 8273 vs. 8997 g C m⁻²). Differential responses between the shortgrass and the mid- and tallgrass communities with respect to grazing and whole-ecosystem carbon storage are likely a result of: (1) maintenance of larger soil organic carbon (SOC) pools in the mid- and tallgrass communities (7476–8280 g C m⁻²) than the shortgrass community (2517–3307 g C m⁻²) that could potentially buffer ecosystem carbon fluxes, (2) lower root carbon/soil carbon ratios in the mid- and tallgrass communities (0.06–0.10) compared to the shortgrass community (0.20–0.27) suggesting that variation in root organic matter inputs would have relatively smaller effects on the size of the SOC pool, and (3) the absence of grazing-induced variation in the relative proportion of C₃ and C₄ functional groups in the mid- and tallgrass communities. We hypothesize that the magnitude and proportion of fine root mass within the upper soil profile is a principal driver mediating the effect of community composition on the biogeochemistry of these grassland ecosystems.     

Zinc sulphate improved microspore embryogenesis in barley

The effect of ZnSO₄ concentration on barley (Hordeum vulgare L.) microspore embryogenesis was investigated using cultivars of different androgenetic response. Concentrations from 0 (control) to 600 μM in the stress pre-treatment medium alone or in combination with 30 (control) to 600 μM in the embryo induction medium were assayed in anther culture. Incorporation of Zn²⁺ in the pre-treatment medium itself did not affect microspore embryogenesis. The optimum concentration in the stress pre-treatment and induction media was 180 μM for cultivars (cvs.) Igri and Reinette, and 90 μM for cv. Hop. A significant increase of 30 and 300% in cv. Igri and Reinette, respectively, were produced with 180 μM ZnSO₄ in both the number of embryos and green plants. In order to confirm the effect of Zn²⁺ on microspore embryogenesis this micronutrient was incorporated in the induction medium of isolated microspore cultures of cv. Igri. Concentrations of 90–300 μM ZnSO₄ resulted in an increase of 40–53% in the number of embryos and green plants. All these results indicate that the beneficial effect of Zn²⁺ is exerted mainly during the culture phase, increasing the number of embryos, leading to an increased number of green plants, but it had no effect on percentage of regeneration or green plants.     

Evidence against associative N2 fixation as a significant N source in long-term wheat plots

In monocropped cereal systems, annual N inputs from non-fertilizer sources may be more than 30 kg ha^-1. We examined the possibility that these inputs are due to biological N₂ fixation (BNF) associated with roots or decomposing residues. Wheat was grown under greenhouse conditions in pots (34 cm long by 10 cm diameter) containing soil from a plot cropped to spring wheat since 1911 without fertilization. The roots and soil were sealed from the atmosphere and exposed to a¹⁵N₂-enriched atmosphere for three to four weeks during vegetative, reproductive or post-reproductive stages. This technique permitted detection of as little as 1 μg fixed N plant^-1 in plant material and 40 μg fixed N plant^-1 in soil. No fixation of¹⁵N₂ occurred during either of the first two labelling periods. In the final labelling period, straw returned to the soil was significantly enriched in¹⁵N, especially in a pot with a higher soil moisture content. Total BNF in this pot was 13 μg N plant^-1, or about 30 g N ha^-1. In a separate experiment with soil from the same plot, we detected BNF only when soil was amended with glucose at a high soil moisture content. Measured associative BNF was insufficient to account for observed N gains under field conditions. Lethbridge Research Centre contribution no. 3879488. Lethbridge Research Centre contribution no. 3879488.     

Chlorophyll fluorescence imaging of photosynthetic activity in sun and shade leaves of trees

The differences in pigment levels, photosynthetic activity and the chlorophyll fluorescence decrease ratio R _Fd (as indicator of photosynthetic rates) of green sun and shade leaves of three broadleaf trees (Platanus acerifolia Willd., Populus alba L., Tilia cordata Mill.) were compared. Sun leaves were characterized by higher levels of total chlorophylls a + b and total carotenoids x + c as well as higher values for the weight ratio chlorophyll (Chl) a/b (sun leaves 3.23–3.45; shade leaves: 2.74–2.81), and lower values for the ratio chlorophylls to carotenoids (a + b)/(x + c) (with 4.44–4.70 in sun leaves and 5.04–5.72 in shade leaves). Sun leaves exhibited higher photosynthetic rates P _N on a leaf area basis (mean of 9.1–10.1 μmol CO₂ m⁻² s⁻¹) and Chl basis, which correlated well with the higher values of stomatal conductance G _s (range 105–180 mmol m⁻² s⁻¹), as compared to shade leaves (G _s range 25–77 mmol m⁻² s⁻¹; P _N: 3.2–3.7 μmol CO₂ m⁻² s⁻¹). The higher photosynthetic rates could also be detected via imaging the Chl fluorescence decrease ratio R _Fd, which possessed higher values in sun leaves (2.8–3.0) as compared to shade leaves (1.4–1.8). In addition, via R _Fd images it was shown that the photosynthetic activity of the leaves of all trees exhibits a large heterogeneity across the leaf area, and in general to a higher extent in sun leaves than in shade leaves.