Effect of intercropping maize and soybeans on Striga hermonthica parasitism and yield of maize

Striga hermonthica a major biotic constraint to cereal production can be controlled by trap crops. Soybean cultivars vary in ability to stimulate suicidal germination of the weed. An experiment was conducted to select soybean (Glycine max) varieties with the ability to stimulate germination of S. hermonthica seeds. Experiments were conducted with strigol Nijmegen 1® (GR 24), a synthetic stimulant, as a check. In the pot and field experiments, maize (variety WH507) was intercropped with soybeans. Variation occurred among soybean varieties in inducing germination of S. hermonthica. The relative germination induction by soybean varieties ranged from 8% to 66% compared to 70% for synthetic stimulant check. Varieties TGX1448-2E, Tgm 1576, TGX1876-4E and Tgm 1039 had the highest relative germination. Soybean varieties TGX 1831-32E, Tgm944, Tgm 1419 and Namsoy4m had high stimulation but low attachment. Intercropping maize with soybeans in the field led to a low S. hermonthica count and high maize yield.     

Over-expression of AGPase genes enhances seed weight and starch content in transgenic maize

Cereal crops accumulate starch in the seed endosperm as an energy reserve. ADP-glucose pyrophosphorylase (AGPase) plays a key role in regulating starch biosynthesis in cereal seeds. The AGPase in the maize endosperm is a heterotetramer of two small subunits, encoded by Brittle2 (Bt2) gene, and two large subunits, encoded by the Shrunken2 (Sh2) gene. The two genes (Bt2, Sh2) from maize were introduced into two elite maize inbred lines, solely and in tandem, and under the control of endosperm-specific promoters for over-expression. PCR, Southern blotting, and real-time RT-PCR analysis indicated that the transgenes were integrated into the genome of transgenic plants and were over-expressed in their progeny. The over-expression of either gene enhanced AGPase activity, seed weight and starch content compared with the WT, but the amounts were lower than plants with over-expression of both Bt2 and Sh2. Developing seeds from co-expression transgenic maize plants had higher cytoplasmic AGPase activity: the 100-grain weight increased 15% over the wild type (WT), and the starch content increased to over 74% compared with the WT of 65%. These results indicate that over-expression of the genes in transgenic maize plants could improve kernel traits. This report provides a feasible approach for increasing starch content and seed weight in maize.     

玉米-大豆间作和施氮对玉米产量及农艺性状的影响

为研究玉米-大豆间作模式和施氮水平对玉米产量、主要农艺性状及生长动态的影响,进行2个种植模式(玉米单作和玉米-大豆间作)和2个施氮水平(0 kg/hm2,150 kg/hm2)的双因素随机区组试验,以期揭示施氮和间作对玉米产量的影响规律,为提高玉米-大豆间作系统产量提供一定的理论依据。研究结果表明:(1)与不施氮相比,施氮显著增加了春秋两季间作玉米产量,分别达到23.81%和40.99%。施氮处理下的间作玉米地上部生物量较不施氮提高了29.91%,单作模式下显著提高了40.34%,两者差异均达到显著水平。(2)与不施氮相比,施氮150 kg/hm2条件下春玉米单作和间作模式百粒重分别提高了18.92%和19.23%,秋玉米单作和间作模式百粒重分别提高了31.03%和32.75%,差异均达到显著水平。与不施氮相比,施氮150 kg/hm2条件下,单作和间作模式均显著提高秋玉米穗长。与不施氮相比,施氮150 kg/hm2条件下,单作秋玉米的穗粗提高了18.67%,差异显著。(3)施氮和间作均能促进玉米干物质累积、提高株高和叶绿素(SPAD值),且表现为施氮效果高于间作效果。总体来看,种植模式和施氮水平对玉米产量、主要农艺性状和生长动态均有一定影响,且施氮效果优于间作效果。由于土壤具有一定的供氮能力,而间作豆科能为玉米供给一定量的氮素,故对于春玉米而言,施氮效果仅在百粒重中表现,随着土壤原有氮素被玉米吸收利用减少后,供氮能力下降,在秋玉米中施氮效果显著提高。     

Effects of cotton–maize rotation on soil microbiome structure

Verticillium wilt is a disastrous disease in cotton-growing regions in China. As a common management method, cotton rotation with cereal crops is used to minimize the loss caused by Verticillium dahliae. However, the correlation between soil microbiome and the control of Verticillium wilt under a crop rotation system is unclear. Therefore, three cropping systems (fallow, cotton continuous cropping, and cotton–maize rotation) were designed and applied for three generations under greenhouse conditions to investigate the different responses of the soil microbial community. The soil used in this study was taken from a long-term cotton continuous cropping field and inoculated with V. dahliae before use. Our results showed that the diversity of the soil bacterial community was increased under cotton–maize rotation, while the diversity of the fungal community was obviously decreased. Meanwhile, the structure and composition of the bacterial communities were similar even under the different cropping systems, but they differed in the soil fungal communities. Through microbial network interaction analysis, we found that Verticillium interacted with 17 bacterial genera, among which Terrabacter had the highest correlation with Verticillium. Furthermore, eight fungal and eight bacterial species were significantly correlated with V. dahliae. Collectively, this work aimed to study the interactions among V. dahliae, the soil microbiome, and plant hosts, and elucidate the relationship between crop rotation and soil microbiome, providing a new theoretical basis to screen the biological agents that may contribute to Verticillium wilt control.     

Effect of Cercospora zeae-maydis infested maize residue on progress and spread of grey leaf spot of maize in central Uganda

The spread of grey leaf spot caused by Cercospora zeae‐maydis from infested maize residue on the soil surface and progress over time were studied under tropical conditions at two locations in central Uganda, Kabanyolo and Namulonge, using a susceptible local cultivar, LP16. Infested maize residue collected the previous season was used as the inoculum source in varying amounts; 80% coverage, 40% coverage and 0% soil coverage were used to simulate no tillage, minimum tillage and maximum tillage with complete burial of residues. In all the seasons, disease spread was adequately described by a power law model, whereas disease progress over time was equally well represented by both Gompertz and logistic models. The time factor was highly significant (P < 0.05) for disease development, indicating both increases in disease intensity over time and change in gradients with time. The amount of infested residue on the soil surface in the maize planting was highly significant (P < 0.05) for slope (b) and intercept (a) of the linearised disease gradient. The gradient parameter (b) based on the average for the four directions from the residue source ranged from ?0.96 to ?0.08 in the second cropping season of 1999, from ?2.16 to ?0.01 and from ?0.87 to ?0.03 in the first and second seasons of 2000, respectively. The distance from the infested residue significantly affected foliar disease, but the direction from the residue inoculum source did not, nor was the distance by direction interaction significant. However, interactions between amount of residue and distance were significant. Percentage leaf area affected by grey leaf spot, the areas under disease progress curves and disease intercepts decreased with distance at both locations, but rate of disease increase (r) was generally constant with distance from residue source at Kabanyolo, but not at Namulonge. The relationship between disease severity and amount of residue cover appeared to be affected by the level of background inoculum. At Kabanyolo, where little background inoculum was present, differences were detected in disease severity parameters between the no‐residue plots and residue plots, whereas at Namulonge, where high background levels existed, there was no effect of residue on disease.     

Microencapsulated Spodoptera frugiperda nucleopolyhedrovirus: insecticidal activity and effect on arthropod populations in maize

Excessive use of chemical insecticides to control Spodoptera frugiperda in maize (Zea mays L.) crops in Colombia has resulted in pest resistance and environmental contamination. A Colombian multicapsid nucleopolyhedrovirus (SfMNPV) was produced in laboratory reared larvae and formulated as a wettable powder by microencapsulating the viral occlusion bodies (OBs) with a methacrylic acid polymer. Microencapsulated OBs were tested for insecticidal activity in laboratory, greenhouse and field bioassays, and were then sprayed onto a commercial crop where their effect on arthropod populations was estimated. In laboratory bioassay, microencapsulated and non-microencapsulated OBs caused a larval mortality of 98 and 96%, respectively. The percent of plants showing recent feeding damage in a greenhouse experiment oscillated between 2.5 and 7.5% when OBs were applied compared to 70% observed in the control. In two field trials, the highest dose of 1.5×10¹² OBs/ha (1500 g/ha) was selected as the recommended rate for field application in maize crops, with efficacies higher than 70%. Total arthropod populations were negatively affected by chemical pesticides; whereas the OBs formulation did not affect the diversity of these organisms. Our results indicate that microencapsulated SfMNPV has the potential to be commercialised as an alternative to substitute or complement chemical insecticides in fall armyworm integrated management programmes.     

Occurrence of an isolate of maize stripe virus on sorghum in India*

A disease characterised by chlorotic stripes and bands, named sorghum stripe disease (SStD), was observed on sorghum in India with an incidence of less than 0.5% to nearly 10%. The affected plants were dwarfed and had poor or no panicle formation. This disease could be transmitted by the delphacid planthopper Peregrinus maidis to sorghum but not to Brachiaria eruciformis; Cenchrus ciliaris; Chloris barbata; Dichantium annulatum; Dichantium aristatum; Digitaria ciliaris: Dinebra retroflexa; Echinocloa colona; Eleusine coracana; Pennisetum glaucum; Pennisetum violaceum; Setaria pallida Fusca; Triticum aestivum and Zea mays. Sorghum stripe disease was shown to be caused by a tenuivirus serologically related to maize stripe virus (MStV). Virus particles were filamentous, less than 10 nm in width. The purified virus preparation contained only one polypeptide of 34 500 D. Eight species of nucleic acids, four ssRNA of 1.21, 0.87, 0.73, 0.47 ± 10⁶D and four dsRNA of 2.43, 1.69, 1.40, 0.71 ± 10⁶D, were extracted from purified virus preparations. When the four dsRNA were denatured, they migrated along with the four ssRNA species indicating that dsRNA contained duplex RNA of same molecular weight as the four ssRN A. In enzyme-linked immunosorbent assay and in electro-blot immunoassay it was evident that MStV-Sorg was serologically more closely related to the MStV isolates from Florida, Reunion and Venezuela than to a RStV isolate from Japan. The virus was named MStV-Sorg to distinguish it from MStV which readily infects maize. This is the first report of occurrence of a tenuivirus in the Indian subcontinent.     

A comparison of canopy evapotranspiration for maize and two perennial grasses identified as potential bioenergy crops

In the Midwestern US, perennial rhizomatous grasses (PRGs) are considered one of the most promising vegetation types to be used as a cellulosic feedstock for renewable energy production. The potential widespread use of biomass crops for renewable energy production has sparked numerous environmental concerns, including the impacts of land‐use change on the hydrologic cycle. We predicted that total seasonal evapotranspiration (ET) would be higher for PRGs relative to maize resulting from higher leaf area and a prolonged growing season. We further predicted that, compared with maize, higher aboveground biomass associated with PRGs would offset the higher ET and increase water‐use efficiency (WUE) in the context of biomass harvests for liquid biofuel production. To test these predictions, ET was estimated during the 2007 growing season for replicated plots of Miscanthus×giganteus (miscanthus), Panicum virgatum (switchgrass), and Zea mays (maize) using a residual energy balance approach. The combination of a 25% higher mean latent heat flux (λET) and a longer growing season resulted in miscanthus having ca. 55% higher cumulative ET over the growing season compared with maize. Cumulative ET for switchgrass was also higher than maize despite similar seasonal‐mean λET. Based on total harvested aboveground biomass, WUE was ca. 50% higher for maize relative to miscanthus; however, when WUE calculated from only maize grain biomass was compared with WUE calculated from miscanthus harvested aboveground biomass, this difference disappeared. Although WUE between maize and miscanthus differed postsenescence, there were no differences in incremental WUE throughout the growing season. Despite initial predictions, aboveground biomass for switchgrass was less than maize; thus WUE was substantially lower for switchgrass than for either maize scenario. These results indicate that changes in ET due to large‐scale implementation of PRGs in the Midwestern US would likely influence local and regional hydrologic cycles differently than traditional row crops.     

玉米早期根系构型及其生理特性对土壤水分的响应

为了探明玉米早期根系结构及其对土壤水分的生理响应,揭示玉米幼苗的抗旱机理,以蠡玉18为材料,采用盆栽试验,设置轻度胁迫(LS)、中度胁迫(MS)、重度胁迫(SS)和正常供水(CK)4个水分处理,系统研究从播种开始持续水分处理对夏玉米苗期根系形态结构及活力、保护酶系统及生理调节物质的影响。结果表明:随着水分胁迫程度的加剧,玉米根长、根表面积、根体积和根干重等各形态指标较CK下降幅度逐渐增大,不同水分胁迫使夏玉米苗期根系结构存在差异。轻度和中度胁迫显著增加了细根(0.05—0.25 mm)根长和根表面积比例,重度水分胁迫显著降低粗根(0.50 mm)根长与根表面积比例。玉米苗期根冠比、根系活力和丙二醛(MDA)含量随水分胁迫程度的增强而上升,随着胁迫时间的延长,根冠比逐渐降低。根系可溶性蛋白含量随土壤水分含量的下降而下降,MS、SS处理较CK显著降低(P0.05)。夏玉米根系中SOD对水分胁迫较CAT、POD更敏感,轻度水分胁迫下主要依赖CAT、中度水分胁迫下主要依赖POD、重度水分胁迫下主要依赖SOD来降低氧化伤害;且重度胁迫下,随着胁迫时间的延长保护酶活性下降。苗期玉米通过增加根冠比、增强根系活力和不同保护酶活性及降低可溶性蛋白等渗透调节物质来协同减少水分胁迫的危害。     

Dynamics of infestation and losses of stored maize due to the larger grain borer (Prostephanus truncatus Horn) and maize weevils (Sitophilus zeamais Motschulsky)

The dynamics of infestation and losses of stored maize due to the larger grain borer (Prostephanus truncatus Horn) and maize weevils (Sitophilus zeamais Motschulsky) were investigated in Tanzania. In single species populations, significantly higher maize damage (P ≤ 0.05) was caused by P. truncatus (23.3%) compared to S. zeamais (6.4%). Weight losses caused by P. truncatus were higher (12.7%) than for S. zeamais (6.7%) under similar conditions. A higher proportion of P. truncatus in the initial infestation population led to significantly higher (≤0.05) maize damage compared to equal or higher proportions of S. zeamais. Irrespective of the initial number of S. zeamais, population size increased to levels higher than for P. truncatus. Maize losses in unimproved storage structures were >80% compared to <2.5% in improved structures under natural infestation. Inter-specific competition appeared to influence the dynamics of infestation, damage levels and weight losses. The first two months of storage were critical for damaging insect populations to build up.     

Host-plant mediated effects of transgenic maize on the insect parasitoid Campoletis sonorensis (Hymenoptera: Ichneumonidae)

Determining the impact of genetically modified (GM) crops on beneficial organisms is an important aspect of the environmental risk assessment of GM crops. In the present study, the impact of Bt maize expressing Cry1Ab on the development and behaviour of the parasitoid Campoletis sonorensis was compared to individuals reared on hosts fed conventionally bred plants partially resistant to the European corn borer (Ostrinia nubilalis Hübner) and on susceptible maize hybrids. Adult parasitoids reared on Bt maize-fed Spodoptera frugiperda larvae were significantly smaller (15–30%) than those reared in hosts fed either of the conventional maize hybrids. The magnitude of this effect was dependent on the size of the host at oviposition and its subsequent growth rate. The development time of C. sonorensis was not affected by the maize treatment. In choice tests, female parasitoids displayed no preference for hosts fed a specific maize hybrid. No Cry1Ab was detected within adult parasitoids.     

Biomass yield and nitrogen fixation of legumes monocropped and intercropped with rye and rotation effects on a subsequent maize crop

The research is focused on an ecologically sound and highly productive cultivation system for fodder and/or biomass for thermal power generation on the basis of winter legumes and maize as subsequent summer crop, managed without additional nitrogen fertiliser. Therefore the yield of biomass and N-fixing capacity of a winter pea (Pisum sativum L.) and crimson clover (Trifolium incarnatum L.) monocropped and intercropped with rye (Secale cereale L.) were examined for five years in a field trial. In mid-June above-ground biomass of winter crops was removed and maize transplanted. The winter crops achieved maximum dry matter yield about three to five weeks before maturity. Mixed stands yielded more biomass than pure stands and exhibited greater yield stability. The relative advantage of intercropping, expressed as land equivalent ratio (LER), determined for intercropped winter pea/rye were 1.1 to 1.2 and for crimson clover/rye 1.3. At maturity, the amount of fixed nitrogen ranged between 178 kg N for crimson clover and 242 kg N ha^-1 for winter pea, respectively. At the end of anthesis (middle of June, harvesting stage for silage fodder) 75% and 88% of the total fixed nitrogen was achieved, for clover and pea, respectively. In intercropping the amount of fixed nitrogen was lower than in pure stands due to a lower seed density of the legume; however, the N-fixing efficiency was greater than in pure stands. N-release of the winter pea in a pure stand produced a maximum yield in maize (Zea mays L.) without additional N-fertiliser. An additional N mineral fertilisation of 75 to 150 kg N and 75 to 225 kg N was necessary to achieve maximum yields in maize following intercropped winter pea and crimson clover, respectively. Legumes in mixed stands with rye resulted in lower amounts of residual nitrogen after maize harvest. The beneficial effect of legumes on maize can be divided into N-effects and rotation effects. Both effects were positive regarding winter pea. The rotation effect of crimson clover in pure stands on maize was negative. Allelopathic effects and the high sensitivity of crimson clover to mineral nitrogen in the soil, released by residues of the preceding crop, winter rape (Brassica rapa L.), were discussed as the reason for this observation. The combination of the winter pea in pure stand and maize achieved the highest total biomass yield from winter and summer crops, unfertilised (156 dt ha^-1 dry). The combinations of intercropped legumes and maize produced biomass yields of 142 to 145 dt ha^-1. Because winter pea is highly susceptible to lodging, intercropping with low seed density of rye is recommended (3/4 winter pea, 1/4 rye). The rye crop prevents lodging by providing support and high rates of N-fixation are achieved with high seed density of pea. Intercropping with crimson clover and rye should be based on high seed densities of legumes, too because rye is highly competitive within those mixtures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impacts of Bt maize inoculated with rhizobacteria on development and food utilization of Mythimna separata

The current trend of increasing proportion of cultivation of transgenic Bt crops is pushing towards dramatic destabilization of the agroecosystem, thus raising severe concerns about the sustainability of transgenic Bt crops as an effective management tool for the control of target insect pests in the future. Rhizobacteria is the key biological regulator to ameliorate soil‐nitrogen utilization efficiency of crop plants, especially transgenic Bt crops. A laboratory study quantified the impacts of transgenic Bt maize (Line IE09S034 with Cry1Ie vs. non‐Bt maize cv. Xianyu335) inoculated with Azospirillum brasilense (AB) and Azotobacter chroococcum (AC) on the growth, development and food utilization of a target lepidopteran insect, Mythimna separata. The results showed that the inoculation of rhizobacteria significantly prolonged the larval lifespan and pupal duration, increased RCR and AD, reduced pupal weight, pupation rate, fecundity, RGR, ECD and ECI, and shortened adult longevity of M. separata fed on transgenic Bt maize, while exact opposite trends were found in these measured indexes of growth, development and food utilization for M. separata fed on non‐Bt maize inoculated with AB and AC compared with the buffer control in both years. Thus, the results clearly depicted that the inoculation of rhizobacteria had opposite influences on the growth, development and food utilization of M. separata fed on transgenic Bt maize. Presumably, rhizobacteria inoculation can be used to stimulate plant–soil‐nitrogen uptake and promote plant growth for transgenic Bt maize and non‐Bt maize, simultaneously increasing Bt toxin production and enhancing resistance efficiency against target lepidopteran pests for transgenic Bt maize.     

Relationships between dynamics of nitrogen uptake and dry matter accumulation in maize crops. Determination of critical N concentration

The concept of critical nitrogen concentration(%N _c) has been proposed as the minimum%N in shoots required to produce the maximum aerial biomassat a given time. Several authors have shown that%N _c declines as a function of aerial biomassaccumulation (W) and the %N _c –W relationship has been proposed as a diagnostic tool of N statusin different crops, excluding maize. From data obtained in five nitrogenfertilisation experiments in irrigated maize crops, 26 critical data-pointswere selected with a precise statistical procedure. An allometric relationwas fitted and a critical %N−W relationshipmodel is proposed in maize as: If W < 1 t ha^-1%N _c = 3.40 If 1 t ha^-1≤ W ≤ 22 t ha^-1%N _c = 3.40(W)^−0.37 The model is applicable to maize crop development between emergenceand silking + 25 days. The model was tested and validated with dataobtained in a network of 17 N fertilisation experiments conducted inFrance under contrasting pedoclimatic conditions. In only nineout of 280 data-points (3.2%), the plant N status was mispredictedwhen ±5% error around %N _c wasallowed. A critical N uptake model (Nu_c, kg Nha^-1) is proposed as Nu_c = 34 (W)^0.63 A comparison between Nu_c and N uptake observedin N treatments giving the maximal grain yields has shown that maizecrops assimilate at least 30 kg N ha^-1 in a storage N poolat the silking stage. The significance of the critical%N−W and Nu−W relationships is discussed in relation to theoretical models proposed inwhole plant ecophysiology. Different relationships calculated betweenleaf area index and aerial biomass accumulation, and between N uptakeand leaf area were consistent with previous results for other crops.This strengthens the interest of the critical%N−W relationship for use as diagnostictool of nitrogen status in maize crops. This revised version was published online in June 2006 with corrections to the Cover Date.     

大豆幼苗对套作玉米遮荫环境的光合生理生态响应

以2个耐荫性不同的大豆品种为材料,田间试验设置大豆单作和玉米-大豆套作2个种植模式处理,研究不同耐荫性大豆品种的幼苗光合生理生态特性对套作玉米遮荫环境的响应。结果表明:1)玉米-大豆套作模式中,玉米遮荫显著降低大豆冠层的光合有效辐射,导致大豆幼苗光合速率、气孔导度、蒸腾速率显著下降(P0.05),分别达37.9%、54.2%和42.4%,但品种间无显著差异;而胞间二氧化碳浓度和Fv/Fm无显著变化,且光合速率的下降与气孔导度存在显著相关关系,光合速率下降主要是由气孔限制和CO2同化过程中能量不足所致;2)玉米遮荫显著降低大豆幼苗叶面积指数、叶片碳含量、叶片和根系干重及总生物量,且品种间差异显著,相关性分析显示,叶面积指数下降是导致生物量减少的主要原因;3)玉米遮荫环境中,大豆幼苗的叶片叶绿素和氮素含量提高以增强光捕获能力,但它们并不能补偿因光截获面积降低而引起的光截获量下降。     

Survival strategies of Dalbulus maidis during maize off‐season in Brazil

Despite the importance of Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) as a vector of maize‐stunting pathogens, it is not understood how this leafhopper survives the maize off‐season in regions where overwintering hosts do not occur. We investigated migration and the use of alternate hosts as possible survival mechanisms for D. maidis during maize off‐season in Brazil. Dalbulus maidis populations were monitored with yellow sticky cards for 16–29 months in Anastácio (Mato Grosso do Sul State), in two farms with perennial pastures (Pasture1 and Pasture2), where maize had not been planted for >5 years, in a subsistence farm >20 km distant, where maize was annually planted (spring) (Maize1), and in Piracicaba (São Paulo State), where maize was grown year round (Maize2). RAPD‐PCR analysis of leafhoppers sampled on maize in two plots (Maize1 and Pasture1) at 15–20 and 110–120 days after germination was performed. Dalbulus maidis was trapped in the maize plots of all areas, but not in weedy or woody vegetation adjacent to the plots. Higher numbers were trapped throughout the year in Piracicaba, where maize was continuously grown under irrigation, and in the subsistence farm of Anastácio, where volunteer maize plants were available for long periods in the maize off‐season. In Anastácio farms, some population peaks were recorded in the absence of maize from midwinter to early spring, especially after soil plowing. RAPD‐PCR analysis showed that D. maidis populations sampled were genetically similar. Our data suggest that D. maidis uses a mixed strategy to survive the over‐season period in Brazil, in which part of the population overwinters locally on volunteer maize plants or nearby irrigated maize crops, whereas the other individuals migrate to colonize new maize crops in distant areas or regions. We hypothesize that immigrant D. maidis uses the contrast between plowed and vegetated soil as a visual cue for locating new maize crops.     

High-throughput retrotransposon-based genetic diversity of maize germplasm assessment and analysis

Maize is one of the world’s most important crops and a model for grass genome research. Long terminal repeat (LTR) retrotransposons comprise most of the maize genome; their ability to produce new copies makes them efficient high-throughput genetic markers. Inter-retrotransposon-amplified polymorphisms (IRAPs) were used to study the genetic diversity of maize germplasm. Five LTR retrotransposons (Huck, Tekay, Opie, Ji, and Grande) were chosen, based on their large number of copies in the maize genome, whereas polymerase chain reaction primers were designed based on consensus LTR sequences. The LTR primers showed high quality and reproducible DNA fingerprints, with a total of 677 bands including 392 polymorphic bands showing 58% polymorphism between maize hybrid lines. These markers were used to identify genetic similarities among all lines of maize. Analysis of genetic similarity was carried out based on polymorphic amplicon profiles and genetic similarity phylogeny analysis. This diversity was expected to display ecogeographical patterns of variation and local adaptation. The clustering method showed that the varieties were grouped into three clusters differing in ecogeographical origin. Each of these clusters comprised divergent hybrids with convergent characters. The clusters reflected the differences among maize hybrids and were in accordance with their pedigree. The IRAP technique is an efficient high-throughput genetic marker-generating method.

     

Similarity of maize and sorghum genomes as revealed by maize RFLP probes

Summary Densely saturated genetic maps of neutral genetic markers are a prerequisite either for plant breeding programs to improve quantitative traits in crops or for evolutionary studies. cDNA and genomic clones from maize were utilized to initiate the construction of a RFLP linkage map in Sorghum bicolor. To this purpose, an F₂ population was produced from starting parental lines IS 18729 (USA) and IS 24756 (Nigeria) that were differentiated with regard to many morphological and agronomical traits. A total of 159 maize clones were hybridized to the genomic DNA of the two parents in order to detect polymorphism: 154 probes hybridized to sorghum and 58 out of these were polymorphic. In almost all of the cases hybridization patterns were similar between maize and sorghum. The analysis of the segregation of 35 polymorphic clones in an F₂ population of 149 individuals yielded five linkage groups. The three principal ones recall regions of maize chromosomes 1, 3 and 5: in general, colinearity was maintained. A possible inversion, involving a long region of maize chromosome 3, was detected. Simulations were also performed to empirically obtain a value for the lowest number of individuals of the F₂ population needed to obtain the same linkage data.Prof. E. Ottaviano, to whom this paper is dedicated, suddenly died on June 7^th, 1991     

Diversity and seasonal phenology of aboveground arthropods in conventional and transgenic maize crops in Central Spain

One of the major concerns regarding the release of Bt maize is its potential negative impact on non-target organisms present in this crop. In this paper, we compare the temporal phenology and community structure of the aboveground arthropods in commercial Bt maize fields in Central Spain with those of conventional maize crops, with or without an insecticide (imidacloprid) seed treatment, over a period of three years. Spiders, harvestmen, centipedes, ground beetles, rove beetles, carrion beetles, click beetles, earwigs and damsel bugs were captured in pitfall traps every year in sufficient number to provide meaningful phenological data. One predator spider and three omnivorous species of ground beetles have been consistently present in the maize fields: Pardosa occidentalis, Poecilus cupreus, Pseudophonus rufipes and Pseudophonus griseus, respectively. Rove beetles were caught to a lesser extent, with three dominant species: Acrotona aterrima, Philonthus varians and Platystethus nitens. The variability in activity–density patterns of the aboveground fauna was mainly influenced by the year, but no detrimental effects could be attributed to Bt maize. The only exception being the changes detected in rove beetles, although these differences were transitory and varied from year to year. No changes in species richness and diversity indices for spiders and ground beetles resulted from treatments. However, imidacloprid-treated maize caused a reduction in species richness of rove beetles, even though the abundance of the main species was not reduced. Our results suggest that Bt maize could be compatible with natural enemies that are common in maize fields in Spain.     

Effects of Canavalia ensiformis and Mucuna pruriens intercrops on Pratylenchus zeae damage and yield of maize in subsistence agriculture

Host status of four leguminous cover crops [Canavalia ensiformis (L.) DC. (Jack bean), Crotalaria ochroleuca G. Don (Sunnhemp), Lablab purpureus L. (Hyacinth bean) and Mucuna pruriens (L.) DC. (velvet bean)] to Pratylenchus zeae Filipjev and effects of intercropping C. ensiformis and M. pruriens with Pan5195, H627 and Emap11 maize cultivars on P. zeae population and disease severity on maize were determined in greenhouse and field tests. Pratylenchus zeae significantly (P<0.05) reduced growth of C. ochroleuca by 36% but had no effect on C. ensiformis, M. pruriens and L. purpureus. While C. ensiformis, M. pruriens and L. purpureus reduced P. zeae population, C. ochroleuca increased it. In the greenhouse test, intercropping maize with C. ensiformis significantly (P<0.05) improved maize growth by up to 34%, Nematode populations in the roots of maize intercropped with either C. ensiformis or M. pruriens were significantly (P<0.05) reduced by up to 32% while nematode disease severity in these intercropping systems was reduced by up to 26%. In the field test, intercropping Emap11, Pan5195 and H627 with C. ensiformis significantly (P<0.05) increased maize grain yield by 190, 29 and 22%, respectively. Intercropping H627 with M. pruriens significantly (P<0.05) increased maize grain yield by 12%, but grain yields of Pan5195 and Emap11 declined by 79 and 40%, respectively. Root necrosis and soil nematode populations in the C. ensiformis–maize intercrop declined by up to 50 and 30%, respectively. Under pure maize stands, soil nematode populations increased by up to 35% in 90 days relative to the initial nematode population of three nematodes g⁻¹ of fresh soil. Section Editor: P. A. M. Bakker