Impact of Cultivar and Environment on Size Characteristics of Wheat Proteins Using Asymmetrical Flow Field‐Flow Fractionation and Multi‐Angle Laser Light Scattering

The use of multi‐angle laser light scattering (MALLS) in conjunction with asymmetrical flow field‐flow fractionation (A‐FFFF) was investigated for the determination of the molecular weight distribution (MWD) of wheat proteins. The wheat flour proteins were dissolved by sonication in 0.1M sodium phosphate (pH 6.9) containing 2% SDS. The results presented make it evident that efficient separation and size characterization of monomeric (M < 10⁵ g/mol) and polymeric protein (10⁵ ≤ M < 10⁸ g/mol) wheat proteins can be achieved with A‐FFFF/MALLS/UV in a single run. Therefore, this method appears to be able to detect significant modifications of MWD of wheat protein, whatever the factor inducing these alterations (i.e., genetic or environmental) and whatever the nature of these alterations (i.e., monomeric‐to‐polymeric ratio or MWD of polymeric protein). In the present study, we have indeed demonstrated that the MWD of wheat proteins can be altered from one cultivar to another in three main ways: by changing the relative amounts of monomeric and polymeric proteins, by changing the MWD of polymeric protein, and then by changing both the monomeric‐to‐polymeric ratio and the MWD of polymeric protein.     

Effects of mycorrhizal inoculation of upland rice on uptake kinetics of arsenate and arsenite

We assessed the effect of mycorrhizal inoculation on short‐term uptake kinetics of arsenate and arsenite by excised roots of upland rice (Oryza sativa L. cv. Zhonghan 221). A concentration of 0.01–0.05 mM arsenic (As) differentially affected the influx rates of both arsenate and arsenite into rice roots non‐inoculated or inoculated with Glomus mosseae and G. versiforme. While V_max for arsenate uptake by non‐mycorrhizal roots was 1.02 µmol g^?1 fresh weight h^?1, it was reduced by a factor of 2.4 for mycorrhizal roots (about 0.42 µmol g^?1 fresh weight h^?1) in the high‐affinity uptake system. However, at high concentrations of 0.5–2.5 mM As only G. versiforme was able to reduce As influx. The results show that mycorrhizal effects on As uptake of upland rice are both concentration and species‐specific.     

GROWTH AND POTASSIUM TRANSPORT IN COMMON AND DURUM WHEAT AS AFFECTED BY ALUMINUM AND NITRITE STRESS

Aluminum (Al) toxicity has been identified as one of the most important factors limiting plant growth in acid soil. Besides Al, nitrite (NO₂ ^?) may also be a significant stress factor in an acid environment. The objective of this study was to examine the effects of Al and NO₂ ^? stress on the growth and potassium (K⁺) uptake of roots and their transport toward the shoots of an Al-resistant common wheat (Triticum aestivum L. cv. Jubilejnaja 50) and an Al-sensitive durum wheat (T. durum Desf. cv. GK Betadur) grown in 0.5 mM CaSO₄ solution at pH 4.1 or 6.5. Root elongation of durum wheat was inhibited with 30% at 10 μM AlCl₃ treatment, while this low Al-concentration did not show a significant effect on root growth of common wheat. In all cases shoot growth was not influenced under low-salt conditions by 10 μ M AlCl₃, but exposure to 100 μM KNO₂ (alone or in combination with Al) had a definite stimulatory effect on growth. Aluminum was found to stimulate the K⁺(⁸⁶Rb) influx in short-term (6 h) experiments, but to inhibit it in long-term (3 days) experiments. This treatment was thought to damage the plasma membrane. When 10 μM 2,4-dinitrophenol was present in the uptake solution the Al-stimulated K⁺ uptake stopped even in short-term experiments. In the case of nitrite and nitrite + Al treatment combinations, however, a striking inhibition was observed in the K⁺(⁸⁶Rb) influx and the K⁺ concentration of the roots and shoots of both species.     

Oxygen enrichment with magnesium peroxide for minimizing hypoxic stress of flooded corn

Flooding/waterlogging is a major factor responsible for hypoxic stress in agriculture. The aim of this study was to develop an effective oxygen buffer with magnesium peroxide (MgO₂) to generate hydrogen peroxide (H₂O₂) and release bioavailable oxygen. MgO₂ provided a relatively stable level (approx. 300 µM) of bioavailable oxygen. The oxygen‐buffer system is adjustable and controllable by adding Mg²⁺ or EDTA to the aqueous system. Regular H₂O₂ was also able to provide bioavailable oxygen but the system was poorly buffered with respect to oxygen release. The accessibility of plants to bioavailable oxygen was indicated by the activity of alcohol dehydrogenase (ADHase, EC 1.1.1.1), an anaerobically induced enzyme of flooded plants. The application of MgO₂ to flooded soil reduced ADHase activity in corn‐root tips by 91.3%. This application of MgO₂ presents a novel pathway to significantly (P < 5%) minimize adverse impacts of hypoxia on flooded corn seedlings. This finding may have broad implications for addressing hypoxicity problems in crop science and technology.     

Response of Cotton and Wheat Cultivars to Soil-applied Boron in a Boron-deficient,Noncalcareous Typic Ustochrept

Field experiments were conducted to study the response of cotton genotypes (G. arboreum Bt cv. RCH 650 BGII; non-Bt cv. F 2228; G. herbaceum cv. FDK 124) and wheat and triticale genotypes (T. aestivum cv. PBW 622; T. durum cv. PDW 314; triticale cv. TL 2908) to direct and residual B application (0, 0.5, 1.0, and 2.0 kg B ha^?1 as borax) using a Typic Ustrochrept, neutral, noncalcareous, loamy sand and B-deficient soil. A significant response of 218 and 231 kg ha^?1 in seed cotton yield was recorded with an application of 1.0 kg B ha^?1 to cotton and 2.0 kg B ha^?1 to wheat. A significant response of 152 kg ha^?1 grain yield of wheat was observed with the application of 0.5 kg B ha^?1 to wheat, while no residual effect of B was observed when B was applied to cotton. On the basis of agronomic and B uptake efficiency, genotypes of cotton (RCH 650 BG II > FDK 124 > F 2228) and wheat (PDW 314> TL 2908> PBW 621) responded differentially to B application, thus indicating that yield of Bt cotton and durum wheat will be reduced more than the other cultivars under B deficiency.     

Diagnosis of zinc and boron availability in emerging vegetable‐based crop rotations in Nepal

Background : Nepal's traditional rice–wheat rotation systems are subject to continuing changes. Changing consumer demand currently drives a replacement of wheat by high‐value vegetables during the dry season, while emerging water shortages lead to a substitution of rice by maize in the wet season. Hence, associated changes in soil aeration status and shifting conditions of soil nutrient supply to match crop nutrient demand are expected to increase the requirements for the principle limiting micro‐nutrients such as boron (B) and zinc (Zn). Aim: Our aim was to investigate the changes in B and Zn availability as well as crop yields and nutrient uptake after system shifts from rice to maize and from wheat to vegetables. Method : We analyzed the B and Zn availability in rice‐ and maize‐based systems as well as crop yields and the nutrient uptake by wheat, cauliflower, and tomato during the dry season in Nepal. Plants were grown at two field sites (midhills vs. lowland) and under greenhouse conditions using soils from the field sites. Results : A change from irrigated rice to maize reduced soil C and N contents with resulting decreases in dry season crop yields. Low soil Zn after rice cultivation led to shortage in Zn uptake by vegetables in both greenhouse and field experiments. The shift from wheat to vegetables increased the demand for B and to a lesser extent for Zn, and consequently vegetables showed visual symptoms of B deficiency. Boron concentrations in dry biomass were below the critical limits with < 10 mg B kg^?1 in wheat, < 21 mg B kg^?1 in cauliflower, and < 23 mg B kg^?1 in tomato. Conclusions: Soils in larger parts of Nepal are low in available B and that the ongoing system shifts increase in the demand for B and Zn in the currently emerging and more diversified production systems.     

CO2浓度升高、氮和水分对春小麦养分吸收和土壤养分的效应

研究了 2种CO2浓度水平 ,2种土壤水分处理和 5种N肥施用水平对春小麦 (TriticumaestivumL cv DingxiNo. 8654)养分吸收和土壤速效养分的影响。结果表明 ,高CO2浓度 (700 molmol-1)明显降低春小麦对氮(N)的吸收 ,低N时降低更为明显 ,但对磷 (P)、钾 (K)吸收的影响不明显。小麦对N、P、K吸收 ,干旱处理明显比湿润处理低。CO2浓度增高对土壤速效N的影响与土壤水分状况有关。湿润处理 ,CO2浓度增加的处理速效N量比当前CO2浓度的处理低 ;而干旱处理 ,施N 50、100、150mgkg-1时 ,速效N则较高。高CO2 浓度对土壤速效P、K量的影响不明显 ,而低N和水分不足 ,土壤速效P、K量较高     

太湖地区稻麦轮作条件下施用包膜尿素的氮素循环和损失

A field experiment was conducted to investigate the fate of ^15N-labeled urea and its residual effect under the winter wheat （Triticum aestivum L.） and summer maize （Zea mays L.） rotation system on the North China Plain. Compared to a conventional application rate of 360 kg N ha^-1 （N360）, a reduced rate of 120 kg N ha^-1 （N120） led to a significant increase （P 〈 0.05） in wheat yield and no significant differences were found for maize. However, in the 0-100 cm soil profile at harvest, compared with N360, N120 led to significant decreases （P 〈 0.05） of percent residual N and percent unaccounted-for N, which possibly reflected losses from the managed system. Of the residual fertilizer N in the soil profile, 25.6%-44.7% and 20.7%-38.2% for N120 and N360, respectively, were in the organic N pool, whereas 0.3%-3.0% and 11.2%-24.4%, correspondingly, were in the nitrate pool, indicating a higher potential for leaching loss associated with application at the conventional rate. Recovery of residual N in the soil profile by succeeding crops was less than 7.5% of the applied N. For N120, total soil N balance was negative; however, there was still considerable mineral N （NH4^＋-N and NO3^--N） in the soil profile after harvest. Therefore, N120 could be considered ngronomically acceptable in the short run, but for long-term sustainability, the N rate should be recommended based on a soil mineral N test and a plant tissue nitrate test to maintain the soil fertility.     

Specific Leaf Area and Specific Leaf Weight in Small Grain Crops Wheat,Rye, Barley,and Oats Differ at Various Growth Stages and NPK Source

Leaf thickness plays an important role in leaf and plant functioning and is related to species’ strategies of resource acquisition and use. Leaf thickness in small grains crops was measured as specific leaf area (SLA) (leaf area in cm² produced g^?1 leaf dry weight plant^?1) and specific leaf weight (SLW) (leaf dry weight in mg produced cm^?2 leaf area plant^?1). The four small grains crops (cool season C₃ cereals) studied were wheat (Triticum aestivum L.), rye (Secale cereale L.), barley (Hordeum vulgare L.) and oats (Avena sativa L.). The null hypothesis tested was that SLA and SLW in small grain crops do not differ at different growth stages and nitrogen, phosphorus, and potassium (NPK) source. Eight NPK sources used in the experiment were: S₁ = 20-20-20, S₂ = 20-27-5, S₃ = 7-22-8, S₄ = 10-10-10-20S, S₅ = 11-15-11, S₆ = 31-11-11, S₇ = 24-8-16, and S₈ = 19-6-12, and each was applied at the rate of 300 mg pot^?1 at the time of sowing to each crop species. The experiment was performed in completely randomized design with three repeats at the Green House of Dryland Agriculture Institute, West Texas A&M University, Texas, during winter 2009-10. Based on the results, it was observed that both SLA and SLW differed with change in crop species, growth stage and NPK source. Among the crop species, SLA was greater in oats > barley > rye > wheat at different growth stages; while SLW of wheat and rye > barley and oats. The differences in the SLW between wheat vs. rye; and between oats vs. barley at different growth stages were not significant (P ≤ 0.05). Among the NPK sources, application of S₇ (24: 8: 16) had higher SLA at 60 and 90 DAE (days after emergence). Application of S₆ (31: 11: 11) had higher SLW at 60 DAE due to the production of the lowest leaf area plant^?1, but the increase in SLW at 90 DAE with S₅ and S₄ was due to the higher leaf dry weight plant^?1produced. Leaf dry weight and leaf area plant^?1 were considered the two most important leaf characteristics that influenced SLA and SLW. On the average, SLA increased with passage of time i.e. 0.70 < 520 < 600 cm² g^?1 at 30, 60 and 90 DAE, respectively; but SLW on the other hand, was highest 3.5 at 60 DAE > 2.2 at 90 DAE > 1.5 mg cm^?2 at 30 DAE. The increase in leaf area plant^?1 and decline in the leaf dry weight plant^?1 increased SLA; in contrast, increase in leaf dry weight plant^?1 and reduction in leaf area plant^?1 increased SLW indicated reciprocal relationship between SLA and SLW in the crop species.     

Effect of bulk density and soil water tension on denitrification in the rhizosphere of spring wheat (Triticum vulgare

Summary Pot experiments were carried out to study the influence of bulk density (D _b), soil water tension (pF) and presence of plants (spring wheat) on denitrification in a low-humus B_t-horizon of a udalf. Pots of only 5-cm depth were found to be most suitable for the experiments when using the acetylene inhibition method. Almost homogeneous soil compaction between 1.1 and 1.6g soil cm^–3 was achieved by a Proctor tamper. Water tensions were adjusted by means of ceramic plates on which negative pressure was applied. No denitrification was detected in unplanted pots. With planted pots and increasing bulk density denitrification increased more in pots with 14-day-old plants than in pots with 7-day-old plants. With 14-day-old plants N₂O emission pot^–1 increased steadily from 2 mol at D _b 1.1 to 8 mol at D _b 1.6, when soil moisture was adjusted to pF 1.5, although root growth was impaired by higher bulk density. From an experiment with different bulk densities and water tensions it could be deduced that the air-filled porosity ultimately determined the rate of denitrification. When low water tension was applied for a longer period, water tension had an overriding effect on total denitrification. Denitrification intensity, however, i.e. the amount of N₂O g^–1 root fresh weight, was highest when low water tension was accompanied by high bulk density. The results suggest that the increase in denitrification intensity at oxygen stress is partly due to higher root exudation.     

Silicon availability of the nursery bed soils and effects of silicon fertilizer applied on the growth and silicon uptake of rice Oryza sativa L. seedlings

(pp. 41–46)

Silicon availability in 36 commercial nursery bed soils was evaluated by four methods the phosphate buffer (pH 6.2, 40 mmol L^?1), incubation, supernatant and acetate buffer (pH 4.0, 1 mol L^?1) Methods. The influence of silicon availability in the nursery bed soils on the silicon uptake of rice Oryza sativa L. cv. Hitomebore seedlings and the effect of silicon fertilizer application were examined in a glass house in 2002.

The results revealed that the best correlation between silicon content in rice seedlings and available silicon in soils was obtained with the phosphate buffer-solution method (r = 0.86). More precise evaluation of available silicon was achieved by grouping soils based on these phosphate absorption coefficients (PAC). The correlation coefficients between silicon content in rice seedlings and available silicon in soils were 0.92 and 0.72 for volcanic soils (PAC > 1500) and non volcanic soils (PAC < 1500), respectively.

We concluded that the phosphate buffer method is the most easily adjusted method for estimation of silicon availability in nursery bed soils, and silicon fertilizers should be applied when silicon availability in non-volcanic nursery bed soils goes below 200 mg kg^?1, whereas the level is less than 350 mg kg^?1 in volcanic soils.     

Phosphate Diffusion Coefficients in Soil as Affected by Bulk Density and Water Content

Phosphate diffusion coefficients (D_e) were determined by the quantity of P that diffused from a soil block with P addition into a soil block without P addition. To compare the results with theoretical concepts and to quantify the influencing factors, D_e was also calculated using the equation of Nye (1968). This equation takes into account the P diffusion coefficient in water, D₁, the volumetric water content, θ, the impedance factor, f, and the buffer power, b, of the soil. The results show that D_e strongly depends on volumetric water content whereas the effect of bulk density on D_e values was relatively small. If the weighted average buffer power was used, calculated D_e values were in good agreement with measured values at higher soil moisture contents. At θ < 0.22 g cm^?3 the measured values for D_e were smaller than the calculated. This effect is attributed to incomplete contact between the two soil blocks. The only small influence of bulk density on D_e is caused by the fact that bulk density affects both θ and b in a way which compensates each other.     

Cadmium accumulation and bioavailability in Coontail (Ceratophyllum demersum L.) plants

The aquatic vascular plant (Ceratophyllum demersum L.) was investigated as a potential biological filter for removal of Cd from wastewaters. Plants were grown in and harvested weekly from 0.10 M Hoagland nutrient solutions containing concentrations of Cd from 0.01 to 1.03 μg Cd mL^?1. Tissue Cd was positively correlated to increased concentrations of Cd in solution. Concentration factors (CFs) of Cd in plants after one week were 13.3 for the 0.01 μg Cd mL^?1 treatment; 451.4 for plants treated with 0.04 μg Cd mL^?1, and 506.5 for plants treated with 1.03 μg Cd mL^?1. Plants treated with 0.01 μg Cd mL^?1 sustained tissue Cd concentrations almost 9-fold over those at week 1. However, after 5 weeks tissue Cd concentration in plants exposed to 1.03 μg Cd mL^?1 had decreased 97% compared to the week 1 concentration. Growth measurements of dry weight, stem lengths, and lateral shoot growth were nagatively correlated to increased Cd treatments. Our results suggest that Coontail exposed to very low Cd concentrations (0.01 μg Cd mL^?1) can take up and accumulate Cd. However, plants exposed to Cd at 0.04 μg Cd mL^?1 or above did not accumulate Cd past one week.     

Improvement of wheat and maize crops by inoculating Aspergillus spp. in alkaline soil fertilized with rock phosphate

Aspergillus tubingensis and A. niger were isolated from the landfills of rock phosphate mines and tested for their efficacy to solubilize rock phosphate (RP), and improve plant growth and phosphate (P) uptake by plants grown in soil amended with RP. The results showed that they effectively solubilized RP in Pikovskaya's (PKV) liquid medium and released significantly higher amounts of P into the medium. A. tubingensis solubilized and released 380.8 μg P mL^?1, A. niger showed better efficiency and produced 403.8 μg P mL^?1. Field experiments with two consecutive crops in alkaline agricultural soil showed that inoculation of these fungi along with RP fertilization significantly increased yield and nutrient uptake of wheat and maize plants compared with control soil. P uptake by wheat and maize plants and the available P increased significantly in the RP-amended soil inoculated with fungi compared with control. These results suggest that the fertilizer value of RP can be increased, especially in alkaline soils, by inoculating P-solubilizing fungi.     

Distribution of Fusarium Molds and Fumonisins in Dry-Milled Corn Fractions

The distribution of Fusarium molds and fumonisins was determined in commercial and experimental dry-milled corn fractions. Fusarium infection of the commercial whole corn samples ranged from 10 to 28%; F. moniliforme was the predominant species. Fusarium counts in corn fractions were <100 colony-forming units (CFU)/g in flaking grits, <100 - 6.4 × 10⁴ CFU/g in bran, <100 − 1.6 × 10⁴ CFU/g in germ, and <100 − 2.7 × 10³ CFU/g in flour. Fumonisin concentrations were ≤0.1 μg/g in flaking grits, 0.2–1.1 μg/g in flour, 0.1–2.0 μg/g in germ, and 1.5–3.2 μg/g in bran. Yellow, blue, and white dent corns naturally contaminated with varying levels of fumonisins (25.4, 3.9, and 0.3 μg of fumonisin B₁ per gram) and Fusarium molds (3.9 × 10⁶, 8.0 × 10⁵, and 2.6 × 10⁴ CFU/g) were experimentally dry milled with a horizontal drum degermer. Number 5 grits contained significantly lower Fusarium counts and fumonisin concentrations than the whole kernel corn. Fusarium counts and fumonisins increased as grit size decreased, and high Fusarium counts and fumonisin concentrations were found in germ, bran, and fines.     

Soil Respiration,Nitrification, and Denitrification in a Wheat Farmland Soil under Different Managements

A field experiment was conducted during the 2010 to 2011 winter wheat–growing season to understand the soil respiration (R_s ), nitrification, and denitrification rates in winter wheat farmland soil under no-tillage (NT) treatment with rice straw incorporation. The experimental treatments include NT, NT with rice straw covers on the surface (NTS), conventional tillage (CT), and CT with straw incorporation (CTS). No-tillage and straw incorporation treatments did not change the seasonal patterns of R_s , gross nitrification (Gn), and denitrification (D) rates compared with CT. Compared with the CT treatment, the NT, NTS, and CTS treatments significantly reduced R_s (P < 0.01), and the NT and NTS treatments significantly increased Gn and D (P < 0.01). CTS also significantly increased Gn (P < 0.01) but had no significant effect on D (P > 0.05). Further analysis showed that the temperature sensitivity of soil respiration (Q ₁₀) of CT, NT, NTS, and CTS were 4.26, 1.86, 3.25, and 2.36, respectively. Our findings suggest that, compared with CT, the NT and straw incorporation treatments reduced R_s and Q ₁₀ and increased Gn and D.     

Quantifying soil respiration in response to short-term tillage practices: a case study in southern Turkey

Abstract

The study aimed at quantifying the rates of soil CO₂ efflux under the influence of common tillage systems of moldboard plow (PT), chisel plow (CT), rotary tiller (RT), heavy disc harrow (DT), and no-tillage (NT) for 46 days in October and November in a field left fallow after wheat harvest located in southern Turkey. The NT and DT plots produced the lowest soil CO₂ effluxes of 0.3 and 0.7 g m^?2 h^?1, respectively, relative to the other plots (P < 0.001). Following the highest rainfall amount of 87 mm on the tenth day after the tillage, soil CO₂ efflux rates of all the plots peaked on the 12th day, with less influence on soil CO₂ efflux in the NT plot than in the conventional tillage plots. Soil evaporation in NT (64 mmol m^?2 s^?1) was significantly lower than in the PT (85 mmol m^?2 s^?1) and RT (89 mmol m^?2 s^?1) tillage treatments (P < 0.01). The best multiple-regression model selected explained 46% of variation in soil respiration rates as a function of the tillage treatments, soil temperature, and soil evaporation (P < 0.001). The tillage systems of RT, PT, and CT led, on average, to 0.23, 0.22, and 0.18 g m^?2 h^?1 more soil CO₂ efflux than the baseline of NT, respectively (P≤0.001).     

Temporal and spatial variability of the CH4 dynamics of landfill cover soils

Landfills are regarded as important sources of the atmospheric methane (CH₄), one of the major greenhouse gases. In this study we investigated the CH₄ dynamics of landfill cover soils in a long‐term field experiment. The CH₄ emission rates were low, mostly ranging from —100 to 100 μmol m^—2 h^—1, with prevailing negative values. Higher values of up to 130,000 μmol m^—2 h^—1, obtained concurrently, were due to mice burrows, connecting the reduced soil sections with the aerated ones. Thus, the appearance of spatial dissimilarity was the most important factor influencing temporal variability. Reducing the soil cover from 120 cm to at least 60 cm caused a tendency of increased CH₄ emission. The oxidation rates were also low and differed with low temporal variability from 1.0—11.9 nmol g^—1 h^—1 in 0—10 cm soil depth and 0—5.3 nmol g^—1 h^—1 in 40—50 cm, respectively. Highest rates were obtained at 25—30 % soil water content. A mapping of CH₄ concentrations over the whole landfill showed a large spatial variation with values of 3.1—343 nmol g^—1. Subsequent CH₄ emission rates were between —0.2 and 120,000 mmol m^—2 d^—1 and showed a positive correlation to the CH₄ concentrations (r = 0.993, P < 0.05). Thus, by a large scale mapping of CH₄ concentrations a low‐cost procedure is proposed to identify the hot spots of CH₄ release which should be treated with additional thick and well aerated cover soil materials.     

Analysis of factors limiting associative N2-Fixation (C2H2 reduction) with two cultivars of Sorghum nutans

A 5–10 times larger nitrogenase activity per plant (max. 8 μmol C₂H₄ h⁻¹ plant⁻¹) was found in Sorghum nutans cultivar CSV 5 compared to the cultivar IS 5218 (max. 0.9 μmol C₂H₄ h⁻¹ plant⁻¹). This significant difference was reproduced for all water regimes from 8 to 21% soil moisture. The number of chemoorganotrophic bacteria (“total cell number”) on a medium with six carbon sources was very similar for both cultivars (3−4 × 10⁸ cells g⁻¹ rhizosphere soil). On the other hand, in the “high fixing” variety CSV 5 the number of N₂-fixing bacteria was greater than in IS 5218 by a factor of 3, the number of actinomycetes was reduced ten-fold and Arthrobacter strains to one-third or one-quarter. The number of aerobic N₂-fixing bacteria growing in an O₂ gradient system was increased 100-fold. Cultivar CSV 5 has a reduced photosynthetic area (20–30%), a reduced root weight (up to 50%, depending on the water regime) and a reduced transpiration rate (30–40%) compared to cv. IS 5218. Grain production however is 10–20% greater. At all soil moisture regimes the pH in the rhizosphere of cv. CSV 5 is 0.5–1.5 pH units below the values for cv. IS 5218.     

Plant,Microbial and Soil Factors,Determining Nitrogen Fixation in the Rhizosphere

A genotype effect on associative (rhizosphere) N₂-fixation was observed with two cultivars of Sorghum bicolor (nutans) with a maximum rate of 8 μmol C₂H₄ · h^?1 · plant^?1 in one genotype compared to 0.9 μmol in the other. Characteristics of the high fixing genotype were a reduced transpiration rate, a lower number of stomata and increased root exudate production per gram root dry weight with higher concentration of dicarboxylic acids. The bacterial rhizosphere composition revealed a three times higher number of N₂-fixing bacteria, a tenfold reduction of actinomycetes and a threefold reduction of Arthrobacter associated with the high fixing cultivar compared to the low fixing genotype. From these and other plant rhizospheres two new nitrogen fixing bacteria, Pseudomonas stutzeri and Erwinia herbicola, were characterized. With the N₂-fixing bacteria Azospirillum brasilense and Klebsiella pneumoniae an enhancement of specific nitrogenase activity by aromatic compounds, for example phenolics, the herbicide alachlor and the insecticide carbofuran was demonstrated. An oscillating nitrogenase activity in Azospirillum brasilense under microaerobic conditions was found, resulting from an encystation and deencystation under those conditions. Experiments with wheat roots demonstrated that reduced oxygen tensions, essential for a maximum rhizosphere N₂-fixation, reduced root growth significantly and altered the N-metabolism of the roots.