Diversity of Rhizosphere Soil Arbuscular Mycorrhizal Fungi in Various Soybean Cultivars under Different Continuous Cropping Regimes

Recent studies have shown that continuous cropping in soybean causes substantial changes to the microbial community in rhizosphere soil. In this study, we investigated the effects of continuous cropping for various time periods on the diversity of rhizosphere soil arbuscular mycorrhizal (AM) fungi in various soybean cultivars at the branching stage. The soybean cultivars Heinong 37 (an intermediate cultivar), Heinong 44 (a high-fat cultivar) and Heinong 48 (a high-protein cultivar) were seeded in a field and continuously cropped for two or three years. We analyzed the diversity of rhizosphere soil AM fungi of these soybean plants at the branching stage using morphological and denaturing gradient gel electrophoresis (DGGE) techniques. The clustering analysis of unweighted pair-group method with arithmetic averages (UPGMA) was then used to investigate the AM fungal community shifts. The results showed that increasing the number of years of continuous cropping can improve the colonization rate of AM fungi in different soybean cultivars at the branching stage. The dominant AM fungi in the experimental fields were Funneliformismosseae and Glomus spp. The number of years of continuous cropping and the soybean cultivar both had obvious effects on the diversity of AM fungi, which was consistent with the results of colonization rate analysis. This study establishes a basis for screening dominant AM fungi of soybean. In addition, the results of this study may be useful for the development of AM fungal inoculants.     

Marigold,Castor Bean,and Chrysanthemum as Controls of Meloidogyne incognita and Pratylenchus alleni

Root and soil populations of Meloidogyne incognita were significantly fewer from marigold, castor bean, and chrysanthemum than from tomato roots and soil, but not from fallow soil. Root populations of Pratvlenchus alleni were significantly fewer from marigold, castor bean, and chrysanthemum than from tomato: marigold had the fewest. Root populations of M. incognita and P. alleni from tomato simultaneously cultivated with marigold, castor bean, and chrysanthemum were significantly fewer than from tomato cultivated alone. Aborted giant cells and dead M. incognita (larvae and females) were observed in roots of marigold and castor bean, but not in chrysanthemum or tomato. Significantly more males than females occurred in castor bean roots. lnfcction sites of P. alleni appeared normal in all hosts. Thin-layer and column chromatography of alcoholic extracts from castor bean revealed no nematicidal thiophenc derivatives.     

Solute Leakage in Soybean Seedlings under Various Heat Shock Regimes

The leakage of solute from intact seedlings during incubationunder various heat shock (HS) regimes was studied. ContinuousHS at 40?C did not induce leakage of amino acids, soluble sugarsand electrolytes into the incubation medium, when compared withcontrol incubation at 28?C. Continuous HS at 45?C (lethal treatment)caused leakage to increase continuously and linearly duringa 5-h treatment period. However, brief HS at 47.5?C, (lethaltreatment), unlike continuous HS at 45?C, induced leakage ata slower rate which reached a plateau within 2 to 3 h at 28?C.Preincubation for 2 h at 40?C completely prevented the leakagecaused by the brief HS at 47.5?C, but not that caused by continuous45?C HS. The amount of leakage during 2 h of incubation at 45?Cwas reduced to half by 30 min preincubation at 40?C and wasreduced to a minimal level by 1-h preincubation. Greater reductionof leakage at 45?C HS was observed when an additional 4 h ofincubation at 28?C immediately followed the 40?C preincubation.These results and previous findings (Lin et al. 1984) indicatethat the synthesis and accumulation of HS proteins (HSPs) areimportant for preventing HS-induced leakage from the cells.One of the HSPs, 15 kD in size appeared to be associated withthe plasma membrane. (Received February 12, 1985; Accepted August 30, 1985)     

Sex Determination and Sex Ratios in Crocodylus palustris

Incubation temperature determines sex in the mugger crocodile,Crocodylus palustris. Exclusively females are produced at constanttemperatures of 28.0°C through 31°C. At 32.5°C,only males are produced. Both sexes are produced in varyingproportions at 31.5, 32.0, and 33.0°C. Embryo survival isnot affected within this range, but developmental rate and totalincubation time are strongly temperature dependent. In naturalnests laid in breeding enclosures, cool incubation temperaturesproduced only females whereas males were produced only in warmnests. Clutch sex ratios were female or male biased. Yearlysex ratios (=percent male) varied from 0.05 to 0.58; overallsex ratio during six nesting seasons was 0.24 (1 male: 3 females).Sex ratio and incubation time vary with nest location and temperaturein a manner consistent with the constant temperature results.Incubation time decreases with increasing incubation temperature,and is an accurate predictor of sex ratio in the field and laboratory. To date, temperature-dependent sex determination (TSD) has beenreported in five species of Crocodylus and in three speciesof Alligatorinae; but the TSD patterns in these groups differ.The TSD pattern of C. palustris is similar to that of C. porosus.Nesting in C. palustris is synchronized with the seasonal availabilityof thermal regimes suitable for incubation. Resultant sex ratiosare a consequence of when and where eggs are laid. Early nestsare located in warm, sunny sites; in contrast, late season nestsare located in the shade. An egg transplant experiment demonstratedthat sex ratios could be altered by simple manipulations ofnest temperatures in the field. The adaptive significance ofTSD in crocodilians may relate to the influence of incubationtemperature on various hatchling attributes, particularly growth.     

The Effects of Temperature on Pratylenchus scribneri and P. alleni Populations on Soybeans and Tomatoes

In soil temperature tests, rates of Pratylenchus scribneri and P. alleni reproduction were measured at various lemperatures on ''Clark 63'' and ''Cutler 71'' soybeans and ''Rutgers'' tomatoes. Recovered P. scribneri equaled or exceeded initial inoculum levels at temperatures of 27.5 C or higher on soybeans, and at 20 C or higher on tomatoes. Population increases were greatest at 3.5 C on both hosts. Populations increased on soybeans, but not on tomatoes, when soil temperature was raised from 25 to 35 C for either 3 or 9 days. Recovered P. alleni were less than the initial inoculum at 27.5 C but higher at 32 and 37.5 C and at 27.5 C on tomatoes, the lowest temperature tested for this nematode. In the field, soil temperatures 10 cm deep in eastern Kansas soybean growing areas reach 27.5 C only occasionally and for relatively short periods, which probably explains the relatively low and variable populations of P. scribneri and P. alleni on soybeans there.     

Seasonal Populations of Pratylenchus penetrans and Meloidogyne hapla in Strawberry Roots

Strawberry roots were sampled through the year to determine the populations and distribution of Pratylenchus penetrans and Meloidogyne hapla. Three strawberry root types were sampled—structural roots; feeder roots without secondary tissues; and suberized, black perennial roots. Both lesion and root-knot nematodes primarily infected feeder roots from structural roots or healthy perennial roots. Few nematodes were recovered from soil, diseased roots, or suberized roots. Lesion nematode recovery was correlated with healthy roots. In both 1997 and 1998, P. penetrans populations peaked about day 150 (end of May) and then declined. The decline in numbers corresponded to changes in total strawberry root weight and root type distribution. The loss of nematode habitat resulted from loss of roots due to disease and the transition from structural to suberized perennial roots. Meloidogyne hapla juvenile recovery peaked around 170 days (mid June) in 1997 and at 85, 147, 229, and 308 days (late March, late May, mid August, and early November, respectively) in 1998. There appear to be at least four generations per year of M. hapla in Connecticut. Diagnostic samples from an established strawberry bed may be most reliable and useful when they include feeder roots taken in late May.     

Peripheral Coding of Sex Pheromone Blends with Reverse Ratios in Two Helicoverpa Species

The relative proportions of components in a pheromone blend play a major role in sexual recognition in moths. Two sympatric species, Helicoverpa armigera and Helicoverpa assulta, use (Z)-11-hexadecenal (Z11–16: Ald) and (Z)-9-hexadecenal (Z9–16: Ald) as essential sex pheromone components but in very different ratios, 97∶3 and 7∶93 respectively. Using wind tunnel tests, single sensillum recording and in vivo calcium imaging, we comparatively studied behavioral responses and physiological activities at the level of antennal sensilla and antennal lobe (AL) in males of the two species to blends of the two pheromone components in different ratios (100∶0, 97∶3, 50∶50, 7∶93, 0∶100). Z11–16: Ald and Z9–16: Ald were recognized by two populations of olfactory sensory neurons (OSNs) in different trichoid sensilla on antennae of both species. The ratios of OSNs responding to Z11–16:Ald and Z9–16:Ald OSNs were 100∶28.9 and 21.9∶100 in H. armigera and H. assulta, respectively. The Z11–16:Ald OSNs in H. armigera exhibited higher sensitivity and efficacy than those in H. assulta, while the Z9–16:Ald OSNs in H. armigera had the same sensitivity but lower efficacy than those in H. assulta. At the dosage of 10 µg, Z11–16: Ald and Z9–16: Ald evoked calcium activity in 8.5% and 3.0% of the AL surface in H. armigera, while 5.4% and 8.6% of AL in H. assulta, respectively. The calcium activities in the AL reflected the peripheral input signals of the binary pheromone mixtures and correlated with the behavioral output. These results demonstrate that the binary pheromone blends were precisely coded by the firing frequency of individual OSNs tuned to Z11–16: Ald or Z9–16: Ald, as well as their population sizes. Such information was then accurately reported to ALs of H. armigera and H. assulta, eventually producing different behaviors.     

Effects of Local Variations in Soil Moisture on Hydrophobic Deposits and Dye Diffusion in Corn Roots

The effects of local soil water content (SWC) alone on the development of hydrophobic deposits and on diffusivity of root tissues were studied in primary roots of maize seedlings whose top regions were growing at a constant rate in damp soil. Corn primary roots were grown through 30 cm-profiles of soil with pre-set dry (8% SWC), or wet (23% SWC), middle layers, and moist (15% SWC) top and bottom layers. When the root tissues in the soil of variable SWC were maturing (approximately 17% of the total root length), the tips of the roots were in damp soil several centimeters away and growing at the rate characteristic of this environment. Histochemistry and fluorescence microscopy on root sections of similar age showed that in these roots local SWC had no effect on hypodermal and endodermal suberization: both had equally developed Casparian bands and suberized lamellae. However, local SWC did alter the hydrophobic deposits in the walls of the epidermis; they increase in dry soil. To study tissue diffusivity, root portions from dry or wet soil were divided in the transverse direction, and half was dipped in sulphorhodamine G (SRG), sectioned, and observed with fluorescence optics. The other half was studied with a cryo-scanning electron microscope to observe the contents of the cortical intercellular spaces. The rate of SRG diffusion in a portion of root from dry or wet soil was independent of local SWC but was positively correlated with numbers of fluid-filled intercellular spaces. The implications of these observations for the movements of ions through the root cortex are discussed.     

Colonization of Wheat Roots by an Exopolysaccharide-Producing Pantoea agglomerans Strain and Its Effect on Rhizosphere Soil Aggregation

The effect of bacterial secretion of an exopolysaccharide (EPS) on rhizosphere soil physical properties was investigated by inoculating strain NAS206, which was isolated from the rhizosphere of wheat (Triticum durum L.) growing in a Moroccan vertisol and was identified as Pantoea aglomerans. Phenotypic identification of this strain with the Biotype-100 system was confirmed by amplified ribosomal DNA restriction analysis. After inoculation of wheat seedlings with strain NAS206, colonization increased at the rhizoplane and in root-adhering soil (RAS) but not in bulk soil. Colonization further increased under relatively dry conditions (20% soil water content; matric potential, −0.55 MPa). By means of genetic fingerprinting using enterobacterial repetitive intergenic consensus PCR, we were able to verify that colonies counted as strain NAS206 on agar plates descended from inoculated strain NAS206. The intense colonization of the wheat rhizosphere by these EPS-producing bacteria was associated with significant soil aggregation, as shown by increased ratios of RAS dry mass to root tissue (RT) dry mass (RAS/RT) and the improved water stability of adhering soil aggregates. The maximum effect of strain NAS206 on both the RAS/RT ratio and aggregate stability was measured at 24% average soil water content (matric potential, −0.20 MPa). Inoculated strain NAS206 improved RAS macroporosity (pore diameter, 10 to 30 μm) compared to the noninoculated control, particularly when the soil was nearly water saturated (matric potential, −0.05 MPa). Our results suggest that P. agglomerans NAS206 can play an important role in the regulation of the water content (excess or deficit) of the rhizosphere of wheat by improving soil aggregation.     

Cross-Site Soil Microbial Communities under Tillage Regimes: Fungistasis and Microbial Biomarkers

The exploitation of soil ecosystem services by agricultural management strategies requires knowledge of microbial communities in different management regimes. Crop cover by no-till management protects the soil surface, reducing the risk of erosion and nutrient leaching, but might increase straw residue-borne and soilborne plant-pathogenic fungi. A cross-site study of soil microbial communities and Fusarium fungistasis was conducted on six long-term agricultural fields with no-till and moldboard-plowed treatments. Microbial communities were studied at the topsoil surface (0 to 5 cm) and bottom (10 to 20 cm) by general bacterial and actinobacterial terminal restriction fragment length polymorphism (T-RFLP) and phospholipid fatty acid (PLFA) analyses. Fusarium culmorum soil fungistasis describing soil receptivity to plant-pathogenic fungi was explored by using the surface layer method. Soil depth had a significant impact on general bacterial as well as actinobacterial communities and PLFA profiles in no-till treatment, with a clear spatial distinction of communities (P < 0.05), whereas the depth-related separation of microbial communities was not observed in plowed fields. The fungal biomass was higher in no-till surface soil than in plowed soil (P < 0.07). Soil total microbial biomass and fungal biomass correlated with fungistasis (P < 0.02 for the sum of PLFAs; P < 0.001 for PLFA 18:2ω6). Our cross-site study demonstrated that agricultural management strategies can have a major impact on soil microbial community structures, indicating that it is possible to influence the soil processes with management decisions. The interactions between plant-pathogenic fungi and soil microbial communities are multifaceted, and a high level of fungistasis could be linked to the high microbial biomass in soil but not to the specific management strategy.     

Colonization of Wheat Root Hairs and Roots by Agrobacteria

Formation of extracellular structures in pure culture and in interaction with wheat root surface was studied by scanning and transmission electron microscopy. The effects of various factors (growth temperature as well as pretreatment of agrobacteria with kalanchoe extract, acetosyringone, and centrifugation) on formation of extracellular structures was tested. The data on Agrobacterium tumefaciens (wild-type strain C58 and mutants LBA2525 (virB2::lacZ) and LBA288 (without the Ti plasmid)) adhesion to wheat root surface and root hairs after pretreatment of agrobacteria with inducer of virulence genes (vir) acetosyringone were obtained. Formation of agrobacterial cell aggregates on wheat root hair tips was demonstrated. The proportion of root hairs with agrobacterial aggregates on the root hair tip insignificantly changed after pretreatment with acetosyringone but considerably increased after treatment of A. tumefaciens C58 and LBA2525 with kalanchoe leaf extract. The most active colonization of root hairs and formation of agrobacterial aggregates on hair root tips was observed at 22°C. The capacity of agrobacteria for adhesion on monocotyledon surface could be changed by pretreatment of bacteria with various surface-active substances. Bacterial cells subjected to centrifugation had a decreased capacity for attachment to both wheat root surface and root hairs. The relationship between the capacity for adhesion and pilus production in agrobacteria was considered.     

Susceptibility of Soybean Cultivars and Lines to Pratylenchus hexincisus

Population increase of Pratylenchus hexincisus on 41 soybean cultivars (maturity groups I-VI) and lines was tested under greenhouse conditions. After 3 months, P. hexincisus was recovered from the roots of all plants tesled. Final populations of P. hexincisus per pot were larger than the initial population in 13 cultivars. Pathogenicity of P. hexincisus on five soybean cultivars representing maturity groups (I-V) was demonstrated under greenhouse conditions. An inoculmn of 5,000 P. hexineisus/plant significantly decreased the root and shoot biomass of all five soybean cultivars after 3 months.     

Mycorrhizal and Endophytic Fungal Colonization in Arrhenatherum elatius L. Roots According to the Soil Contamination in Heavy Metals

The aim of this work was to jointly study non-mycorrhizal (dark septate fungi) and mycorrhizal (arbuscular mycorrhizae) colonization along a large range of heavy metal pollution in soil in order to determine the effective contribution of each type of endophytes in relation to heavy metal uptake and tolerance. Hence, eight sites were chosen in the mining area of northern France with respect both to a large range of heavy metal contamination (Cd, Pb, Zn) and monospecific colonization by Arrhenatherum elatius. Root colonization with both arbuscular mycorrhizae (AM) and dark septate fungi (DSF) as well as spore density in rhizospheric soil were estimated in relation to soil characteristics. Mycorrhizal infestation (hyphae, arbuscules and vesicles) was adversely affected by soil pollution almost to exclusion. The intensity of colonization with DSF was very low in presence of AM in non-contaminated soils but higher in polluted soils. The effect of the fungal colonization on the heavy metal tolerance of Arrhenatherum elatius is discussed.     

Soil Tillage Management Affects Maize Grain Yield by Regulating Spatial Distribution Coordination of Roots,Soil Moisture and Nitrogen Status

The spatial distribution of the root system through the soil profile has an impact on moisture and nutrient uptake by plants, affecting growth and productivity. The spatial distribution of the roots, soil moisture, and fertility are affected by tillage practices. The combination of high soil density and the presence of a soil plow pan typically impede the growth of maize (Zea mays L.).We investigated the spatial distribution coordination of the root system, soil moisture, and N status in response to different soil tillage treatments (NT: no-tillage, RT: rotary-tillage, SS: subsoiling) and the subsequent impact on maize yield, and identify yield-increasing mechanisms and optimal soil tillage management practices. Field experiments were conducted on the Huang-Huai-Hai plain in China during 2011 and 2012. The SS and RT treatments significantly reduced soil bulk density in the top 0–20 cm layer of the soil profile, while SS significantly decreased soil bulk density in the 20–30 cm layer. Soil moisture in the 20–50 cm profile layer was significantly higher for the SS treatment compared to the RT and NT treatment. In the 0-20 cm topsoil layer, the NT treatment had higher soil moisture than the SS and RT treatments. Root length density of the SS treatment was significantly greater than density of the RT and NT treatments, as soil depth increased. Soil moisture was reduced in the soil profile where root concentration was high. SS had greater soil moisture depletion and a more concentration root system than RT and NT in deep soil. Our results suggest that the SS treatment improved the spatial distribution of root density, soil moisture and N states, thereby promoting the absorption of soil moisture and reducing N leaching via the root system in the 20–50 cm layer of the profile. Within the context of the SS treatment, a root architecture densely distributed deep into the soil profile, played a pivotal role in plants’ ability to access nutrients and water. An optimal combination of deeper deployment of roots and resource (water and N) availability was realized where the soil was prone to leaching. The correlation between the depletion of resources and distribution of patchy roots endorsed the SS tillage practice. It resulted in significantly greater post-silking biomass and grain yield compared to the RT and NT treatments, for summer maize on the Huang-Huai-Hai plain.     

Vertical Distribution of Plant-parasitic Nematodes in Sandy Soil under Soybean

Vertical distribution of five plant-parasitic nematodes was examined in two north Florida soybean fields in 1987 and 1988. Soil samples were collected from 0-15 cm, 15-30 cm, and 30-45 cm deep at each site. Soil at the three depths consisted of approximately 96% sand. More than 50% of Belonolaimus longicaudatus population densities occurred in the upper 15-cm soil layer at planting, but the species became more evenly distributed through the other depths as the season progressed. Criconemella sphaerocephala was evenly distributed among the three depths in one field but was low (< 20% of the total density) in the upper 15 cm at a second site. Maximum population densities of Pratylenchus brachyurus were observed at 15-30 cm on most sampling dates. Vertical distributions of Meloidogyne incognita and Paratrichodorus minor were erratic and showed seasonal variation. A diagnostic sample from the upper 0-15 cm of these soybean fields revealed only a minority of the populations of most of the phytoparasitic species present.     

Photosynthesis and Chlorophyll Fluorescence in Two Hybrids of Sorghum under Different Nitrogen and Water Regimes

In two hybrids of sorghum (Sorghum bicolor Moench.), C51 and C42, high nitrogen concentration (HN) increased net photosynthetic rate (PN), stomatal conductance (gs), and transpiration rate (E) of well watered (HW) plants. Water stressing (LW plants) resulted in low PN, gs, and E in both hybrids, but the values were still higher in HN plants as compared to low nitrogen-grown (LN) plants. Intercellular CO2 concentration (Ci) increased in droughted plants. This increase was much higher in LN plants as compared to HN plants. Instantaneous water use efficiency was lower in LN plants as a consequence of a greater effect of water stress on photosynthesis. Leaf water potential was reduced by water stress in all treatments. Analysis of chlorophyll a fluorescence at room temperature showed that photosystem 2 (PS2) was rather tolerant to the water stress imposed. Water stress caused a slight decrease in the efficiency of excitation capture by open PS2 reaction centres (Fv/Fm). The in vivo quantum yield of PS2 photochemistry (PS2) and the photochemical quenching coefficient (qP) were slightly reduced, while the nonphotochemical quenching coefficient (qN) was increased under the water stress. However, in hybrid C42 these characters were little or not affected by the water stress.     

Effects of Soil Nutrients and Moisture on Root/Shoot Ratios in Lolium perenne L. and Trifolium repens L

The dry weights of foliage and roots (excluding stembases) werecompared in a 3³x2² factorial experiment with Lolium perenneL. and Trifolium repens L. grown in pots with three nitrogen(0–300 ppm N), phosphorus (0–300 ppm P), and soilmoisture (pF 3.2, pF 2.4, and waterlogged) treatments, eachduplicated. Root/shoot ratios were influenced significantly (P < 0.001)by N, P, and W (soil moisture), but these main effects werequalified by highly significant first-order interactions andsecond-order interactions which included N and P. The lowestR/S ratios occurred where N, P, and W were all non-limiting,and the highest where there was an imbalance of these factors. The explanation suggested is that a decrease in the availabilityof N, P, or W causes an increase in the relative weight of roots,but in the presence of N, P, or W deficiency an increase inany one of these factors causes a further increase in the R/Sratio by increasing the relative efficiency of the foliage.     

Biased Sex Ratios in Plants: Theory and Trends

This paper examines the history of sex ratio theory and the effects of multiple variables on individual and population sex ratios. It also provides examples where plants have been used to test major predictions of sex ratio theory. Then, using over 200 studies from the literature, dioecious plant species are categorized based on their life form, pollination agent, fruit dispersal agent, and sex ratio. A loglinear analysis is used to look at possible correlations between the sex ratio of a population and other life history characteristics. These data are used to examine the predictions made by De Jong et al. (Journal of Evolutionary Biology 15:7, 2002), that relative pollen and seed dispersal distances can be used to predict sex ratio bias. Despite the limited sample size, strong relationships are still observed. 93% of insect pollinated dioecious vines that have biotically dispersed fruit have male-biased sex ratios. Conversely, 61% of shrubs that are wind pollinated and have abiotic fruit dispersal have female-biased sex ratios.     

Dispersion and Distribution of Pratylenchus scribneri and Hoplolaimus galeatus in Soybean Fields

Examination of dispersional characteristics of Pratylenchus scribneri and Hoplolaimus galeatus indicated that there were patches within soybean fields in which both survival and reproduction wexe enhanced in spite of apparent homogeneity of soil type and topography. Treatment with carbofuran reduced the patchiness (or increased the dispersion) for H. galeatus while it had the opposite effect for P. scribneri. P. scribneri was less highly dispersed in conventional tillage plots than in the zero tillage plots. Populations from quadrats contained entirely within the patches could be described by the normal distribution (in the case of P. scribneri) or by the Poisson distribution (in the case of H. galeatus), while populations from quadrats contained entirely outside the patches could be described by the Poisson distribution for both nematodes. None of the distributions tested (Poisson, normal, negative binomial, Neyman''s) gave an adequate fit when populations from both inside and outside the patches were considered together. In all instances, log₁₀ and ln transformations reduced the goodness of fit of the data to all of the distributions tested. Even with logarithmic transformations, parametric statistics were not appropriate for analysis of data in most instances.