Differential atrazine interference with the Hill reaction of isolated chloroplasts from Chenopodium album L. biotypes

Atrazine [2-chloro-4-(ethylaminol-6-(isopropyl-amino)-s-triazine] resistant biotypes of lamb's quarters (Chenopodium athum L) were reported in the maize growing areas of Ontario, where the herbicide had been used fur a number of years. Field samples from four locations proved tolerant to higber than recommended rates of atrazine in controlled environment screening trials. A resistant biotype was not killed with up to 40 kg/ha atrazine. Diuron at 5 x10-5 M inhibited the Hill reaction with isolated chloroplasts of resisiant and susceptible biotypes of lamb's-quarters. However, with 10-4 M atrazine, the photochemical activity was inhibited in chloroplasts isolated from the susceptible biotype but not in chloroplasts from the resisiant biotype. With maize chloroplasts, inhibition with 10-4 M atrazine was the same as with the susceptible biotype of lamb's-quarters. These studies suggested that a new mechanism of intraspecific resistance in lamb's quarters to atrazine was involved, other than differences in uptake, translocation and metabolism reported with interspecific comparisons involving the s-triazines and other herbicides, It was also concluded that atrazine and diuron did not have precisely the same mechanism of action as photosynthetic inhibitors with lamb's-quarters, and that external and or internal structure or function of chloroplasts in relation to atrazine inhibition can vary significantly even in biotypes of the same species.     

Etude sous conditions contrôlées de la concurrence entre le chénopode blanc (Chenopodium album (L.)) et la variété de maïs INR A 258, en fonction du stade de développement du chénopode

Dans Jes conditions contrôlées d'une salle climatisée, la concurrence entre le chénopode blanc (Chentopdium album) et de jeunes plantules de maïs (variété INRA 258) a éléétudiée en fonetion du stade de développcment du chénopode. sur sol et sur solution nutritive, Des eftets depressifs croissants du cbénopode sur e maïs en fonction du stade de développement atteint par le chénopode au début de chuque essai ont élé mis en évidence: ceseffets sont plus précoces sur solution nutritive que sur sol. II csi suggéré que des effets non compétitifs peuvent contribucr à la concurrence exercée par le chénopode blane sur le maïs. Study under controlled conditions of the competition between Chcnopodium album (L.) I and the rariety of maize INRA 258 in relation to the stage of development of Chcnopodium album In the controlled conditions of a growth chamber, the competition between Chenopodium album and young plants of maize (variety INRA 258) was studied, in relation to the stage of development of C. album in soil and in nutrient solution Increasing depressive effects on the maize in relation to the stage of development of C. album at the start of each test were noted earlier These effects were shown in nutrient solutions rather than in soil. It is suggested that non-competitive effects can contribute to the influence exercised by C. album on the maize Untersuchungen zur Konkurrcnzwirkung von Chenopodium album (L.) auf die Maissorte INRA 258 unter Kontrollierten Bedingungen in Zusamenhang mit dem Entwickhungsstadium ran Chenopodium album Es wurden Untersuchungen zur Konkurrenz zwischen Chenopodium album und jungen Maispflanzen (Sorte INRA 25K) in Klimakammern durchgeführL Die Pflanzen wuchsen in Nährlösung bzw. im Bo den Das Ziel der Untersuchungen war, den Einfluss des Entwicklungsstadiums von C. album festzustellen. Mit zunehmendem Entwicklungssiadium der C. album PManzen wurde der Mais deutlich stärker untcrdruckt Diese lilfckte waren in Nährlösung Prüher ausgeprägt als im Boden. Is wird angenommen. dass cinige nicht auf einer Konkurrcnzwirkung beruhenden Effekte zum Einfluss von C. Album auf Mais beitragen können     

Emerging foliar and soil‐borne pathogens of leafy vegetable crops: a possible threat to Europe

Italy is one of the leading countries for the production and consumption of ready‐to‐eat salads. This has led to a steady increase in the area used over the last decade for the growth of leafy vegetables under intensive systems. The recent new diseases that affect lettuce, wild and cultivated rocket, lamb's lettuce, spinach and basil are reported here: Plectosphaerella cucumerina on wild rocket, endive and lamb's lettuce, Fusarium equiseti on wild and cultivated rocket and lettuce, Myrothecium verrucaria on spinach and wild rocket, Myrothecium roridum on lamb's lettuce, Allophoma tropica on lettuce and Alternaria sp. on basil and rocket are among the new foliar pathogens in Italy. Among the soil‐borne pathogens, Pythium aphanidermatum, Pythium irregulare and Pythium Cluster B2a have recently been isolated in Italy on spinach and Swiss chard, lamb's lettuce and lettuce, while Fusarium oxysporum f. sp. lactucae, which causes lettuce wilt, is gradually spreading to new countries. Some of the new pathogens have been found to be transmitted by seed and typical of tropical climate, and are thus favoured by the increases in temperature linked to climate change. The globalization of markets, climate change and intensive cultivation are among the factors responsible for the proliferation and spread of some of these new pathogens that are ‘alien’ to Italian production systems.     

SHOOT ZONE UPTAKE OF SOIL-APPLIED HERBICIDES*

Summary. Studies were conducted to determine the effects of herbicide placement at different zones of maize (Zea mays L.) and pea (Pisum sativwm L.) shoots below the soil surface after emergence. Soil was removed from around the shoots and replaced with herbicide-treated soil. A wax barrier ensured separate exposure of the zones to treated soil. EPTC, chlorpropham, propham and sulfallate did not affect pea shoot growth, but in maize the shoot zone adjacent to the crown root node was extremely sensitive. Treatment in this area markedly reduced growth and severely inhibited the crown roots. The difference in susceptibility between these species may he due to the location of the growing point relative to the treated soil. Shoots of maize and pea were sensitive to diuron. In maize the shoot adjacent to the crown root node and the tissue of the first internode were the most susceptible. In pea the- uppermost shoot (beneath the soil surface) was the most sensitive. Trifluralin did not affect growth of maize and pea when placed in the shoot zone after emergence, although the crown roots of maize were severely inhibited. Naptalam, dalapon and 2,4-D did not affect growth of maize under similar conditions, and of these only 2,4-D reduced growth of pea. Zone d'abiorption des tiges pour les herbicides appliqués sur h sol     

解淀粉芽胞杆菌菌株HRH317对感染串珠镰孢菌玉米幼苗伏马毒素B1含量及相关防御酶活性的影响

为探讨解淀粉芽胞杆菌Bacillus amyloliquefaciens菌株HRH317对感染串珠镰孢菌Fusarium moniliforme玉米幼苗产生伏马毒素B_1(FB_1)的影响,采用牛津杯法测定菌株HRH317对串珠镰孢菌的抑制活性,并通过浸种处理进行盆栽试验,应用高效液相色谱技术对生长至3叶期后不同时间玉米幼苗叶片中FB_1含量进行测定,同时于室内测定玉米幼苗叶片防御酶超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、苯丙氨酸解氨酶(PAL)和过氧化物酶(POD)的活性。结果表明:解淀粉芽胞杆菌菌株HRH317能明显抑制串珠镰孢菌生长,抑菌圈直径平均可达33.31 mm;玉米幼苗生长至3叶期后1～6 d,菌株HRH317能有效抑制玉米植株体内FB_1含量,经串珠镰孢菌分生孢子悬浮液与菌株HRH317菌悬液1∶1混合液处理玉米种子后,对幼苗中FB_1的抑制率为59.20%～75.70%;而玉米种子先接种菌株HRH317菌悬液后接种串珠镰孢菌分生孢子悬浮液处理对幼苗中FB_1的抑制率为76.77%～88.10%。且这2种处理中幼苗叶片的SOD、CAT、PAL和POD活性均较对照有不同程度提高,其峰值是对照的1.24～5.45倍。表明解淀粉芽胞杆菌菌株HRH317可通过抑制FB_1产生来降低串珠镰孢菌对玉米幼苗的侵害,同时能诱导玉米植株体内防御酶活性的表达而增强其系统抗性,在防治玉米穗腐病方面具有潜在的应用价值。     

Uptake and phytotoxicity of chlorsulfuron in Zea mays L. in the presence of 1,8-naphthalic anhydride

The sites of uptake of chlorsulfuron in maize (Zea mays L.) were investigated at three different growth stages. Exposure of seedling roots, or shoots separately, to herbicide-treated sand over 4 days resulted in inhibition of both roots and shoots. Exposure of seedling roots to chlorsulfuron-treated soil over 21 days severely inhibited both roots and foliage, while separate shoot exposure also reduced both foliage and root growth. After plant emergence, exposure of the crown root node, growing point and lower stem to treated soil reduced foliage and root growth, but exposure of the shoot above the growing point caused only slight inhibition of foliage and had no effect on roots. The herbicide safener 1,8-naphthalic anhydride (NA) applied as a dust (10 g kg^?1 seed weight), or as a 50 mg 1^?1 suspension in water to maize seeds, reduced the root inhibition by chlorsulfuron in 4-day-old seedlings. NA completely prevented both foliage and root injury when chlorsulfuron was placed in soil in the shoot zone before emergence, or in the shoot zone below the soil surface after plant emergence. NA slightly decreased injury to foliage, but not to roots when chlorsulfuron was placed in soil in the root zone before emergence. NA seed treatment protected both roots and foliage against injury from foliarly applied chlorsulfuron. Plants were also protected when a suspension of NA in water was sprayed on the foliage seven days before chlorsulfuron. When a mixture of NA and chlorsulfuron was applied to foliage, root injury was reduced more than foliage injury.     

Root growth and the relative importance of root and shoot competition in interactions between rice (Oryza sativa) and Echinochloa crus-galli

Competition between rice (Oryza sativa) and Echinochloa crus-galli (L.) Beauv. (barnyard-grass or cockspur) was studied experimentally in the field in Sri Lanka. By growing rice in bags sunk in the paddy soil such.that the roots of the plants were either separated from or free to mingle with those of neighbouring weeds, or by growing rice in the same bags but in the absence of weeds, it was possible to calculate the relative importance of interference below ground (root competition) and above ground (shoot competition). Three rice varieties with different above-ground morphology were inhibited to different extents by E. crus-galli but in each case root competition was more important than shoot competition. In the variety that was most sensitive to root competition, Bg 400–1, inhibition of total growth due to root competition increased with increasing weed density. When grown in monocultures in pots in the field, the two species exhibited similar growth rates whether or not the soil was fertilized, but in 1:1 mixtures growth of E. crus-galli was greater, particularly if fertilizer was added, as this promoted the growth of the weed, while having no effect on the rice. In mixture, relative yield totals were close to 1.0, with the relative yield of E. crus-galli being consistently greater than 0.5, particularly with added nutrients, while the relative yield of rice was always less than 0.5. The relative crowding coefficient for rice with respect to E. crus-galli (K_eb), when based on total plant dry weight, was lower in fertilized than in non-fertilized soil, falling in the former case from 0.4 to 0.2 during the experiments Kroot_rb, based on root dry weight, decreased with increasing soil depth and was lower in fertilized than in non-fertilized soil, most notably after 62 days of growth, when root dry weight was at its maximum. It is concluded that inhibition of root growth of rice, leading to a reduced ability to obtain resources from the soil, was the major factor contributing to the decline in the growth of rice in the presence of E. crus-galli. Croissance racinaire et importance relative de la compétition racinaire et aérienne dans les interactions entre le riz (Oryza sativa) et le panic (Echinochloa crus-galli) La competition entre le riz (Oryza sativa) et le panic (Echinochloa crus-galli) a étéétudiée dans une expérimentation de plein champ au Sri Lanka. En cultivant du riz dans des sachets immergés dans la riziére de telle façon que ses racines soient soil séparées, soil libres de se mé1anger avec celles des adventices voisines, ou en cultivant du riz dans des sachets analogues mais en 1'absence de mauvaises herbes, il est possible de calculer 1'importance relative de 1'interférence sous le sol (compétition racinaire) et de celle au dessus du sol (compétitionaérienne). 3 variétés de riz ayant une morphologie aérienne différente ont été inhibées par le panic à des niveaux différents mais chaque fois la compétition racinaire a été plus importante que l'aérienne. Chez la variété la plus sensible à la compétition racinaire, Bg 400–1, le ralentissement de la croissance totale due à la compétition racinaire augmenta avec l'augmentation de la densité de mauvaises herbes. Quand elles sont cultivées en pots dans les champs, les deux espèces ont des taux de croissance similaires que le sol soit ou non fertilisé, mais dans un mélange 1/1 la croissance du panic est plus forte surtout si l'on ajoute de l'engrais puisque celui-ci favorise la croissance de l'adventice sans avoir d'effet sur le riz. En mélange, le total des poids relatifs était très proche de 1, avec un poids relatif de panic conséquemment plus grand que 0,5, surtout dans le cas d'adjonction d'engrais, tandis que celui du riz était toujours inférieur à 0,5. Le coefficient de tassement relatif du riz en regard du panic (K_rb), basé sur le poids sec de la plante entière était plus faible dans les sols fertilisés que dans les autres, tombant de 0,4 à 0,2 pendant l'expérimentation. KracinC_rb, basé sur le poids sec de racine, diminue avec l'augmentation de la profondeur du sol et est plus bas dans les sols fertilisés que dans les autres, surtout après 62 jours de croissance quand le poids sec de racine est à son maximum. Il est conclu que le ralentissement de la croissance des racines de riz, aboutissant à une capacité moindre à prélever les ressources du sol, était le facteur principale de la réduction de la croissance du riz en présence de panic. Wurzelwachstum von und Wurzel- sowie Sproßkonkurrenz zwischen Reis (Oryza sativaj und Echinochloa crus-galli In Freilandversuchen in Sri Lanka zur Wurzelund Sproßkonkurrenz zwischen Reis (Oryza sativa L.) und Hühnerhirse (Echinochloa crus-galli (L.) Beauv.) wurde Reis so in Beuteln gepflanzt, daß seine Wurzeln sich entweder getrennt oder mit denen des Unkrauts zusammen wachsend entwickeln konnten. Andere Parzellen blieben unkrautfrei. E. crus-galli konkurrierte mit 3 Reissorten von unterschiedlicher Sproßmorphologie verschieden stark, doch die Wurzelkonkurrenz war immer bedeutender als die Sproßkonkurrenz. Bei der gegen Wurzelkonkurrenz empfindlichsten Sorte “Bg 400–1” war die Gesamthemmung auf die mit der Unkrautdichte zunehmende Wurzelkonkurrenz zurückzuführen. In Monokultur hatten beide Arten unabhängig von der Düngung ähnliche Wachstumsraten, aber in l:l-Mischkulturwuchs E. crus-galli stärker, besonders bei Düngung, die auf den Reis wirkungslos blieb. In Mischkultur lag der relative Gesamtertrag nahe 1,0, wobei der Ertrag von E. crus-galli stets über 0,5, der des Reises jedoch immer unter 0,5 lag. Für die Wechselbeziehung zwischen Reis und E. crus-galli auf die Gesamttrockenmasse wurde ein ‘crowding coefficient’ (K_rb) für Reis berechnet, der mit Düngung kleiner war als ohne. K_rb nahm während des Versuchs von 0,4 auf 0,2 ab. Der auf die Wurzeltrockenmasse bezogene ‘crowding coefficient’ (Kroot_rb) nahm mit zunehmender Bodentiefe ab. Kroot_rb war niedriger in gedüngtem verglichen mit ungedüngtem Boden. Dieser Unterschied war nach 62 Tagen Wachstum besonders deutlich, als die Wurzeltrockenmasse ein Maximum erreichte. Die Hemmung des Wurzelwachstums des Reises wurde als wichtiger Faktor für seine Wachtum-shemmung in der Gegenwart von E. crus-galli angesehen, weil sie die Fähigkeit des Reises zur Nährstoffaufnahme aus dem Boden verringert.     

Cropping systems modify soil biota effects on wheat (Triticum aestivum) growth and competitive ability

Plants alter soil biota which subsequently modifies plant growth, plant–plant interactions and plant community dynamics. While much research has been conducted on the magnitude and importance of soil biota effects (SBEs) in natural systems, little is known in agro‐ecosystems. We investigated whether agricultural management systems could affect SBEs impacts on crop growth and crop–weed competition. Utilising soil collected from eight paired farms, we evaluated the extent to which SBEs differed between conventional and organic farming systems. Soils were conditioned by growing two common annual weeds: Amaranthus retroflexus (redroot pigweed) or Avena fatua (wild oat). Soil biota effects were measured in wheat (Triticum aestivum) growth and crop–weed competition, with SBEs calculated as the natural log of plant biomass in pots inoculated with living soil divided by the plant biomass in pots inoculated with sterilised soil. SBEs were generally more positive when soil inoculum was collected from organic farms compared with conventional farms, suggesting that cropping systems modify the relative abundance of mutualistic and pathogenic organisms responsible for the observed SBEs. Also, as feedbacks became more positive, crop–weed competition decreased and facilitation increased. In annual cropping systems, SBEs can alter plant growth and crop–weed competition. By identifying the management practices that promote positive SBEs, producers can minimise the impacts of crop–weed competition and decrease their reliance on off‐farm chemical and mechanical inputs to control weeds, enhancing agroecosystem sustainability.     

Uptake patterns of soil-applied 45Ca and 32P in some legume species as influenced by differential trifluralin placement

The effect of localized placement of trifluralin on uptake patterns of soil-applied ⁴⁵Ca in vetch (Vicia sativa L.), pea (Pisum sativum L.) and soybean (Glycine max) and ³²P in vetch and pea was investigated in two soil zones in the roots and in the shoot zone before and after plant emergence. When trifluralin was in the upper root zone severe inhibition of lateral roots occurred as well as a marked decrease in uptake of ⁴⁵Ca and ³²P from this zone. Root growth in the lower zone was unaffected, but uptake of ⁴⁵Ca and ³²P was slightly reduced. Compensatory adventitious root growth as well as a marked increase in uptake of ⁴⁵Ca and ³²P occurred in the shoot zone. Neither root growth nor uptake of ⁴⁵Ca or ³²P in the upper root zone were affected by the presence of trifluralin in the lower root region. When trifluralin was placed in the shoot zone after plant emergence, adven-titious roots on the shoots were inhibited and uptake of ⁴⁵Ca and ³²P was reduced.     

Effect of chlorsulfuron and 1, 8 naphthalic anhydride on uptake of 45Ca in maize

The effect of chlorsulfuron on uptake of ⁴⁵Ca was studied in maize (Zea mays L. cv. Earliking) plants grown from seeds dusted with 1, 8 naphthalic anhydride (NA). ⁴⁵Ca absorption in sand-grown maize was significantly decreased when chlorsulfuron was applied to the foliage but this was not so when seeds had been dusted with NA. Uptake of ⁴⁵Ca was also reduced when either root or shoot soil zones were separately exposed to chlorsulfuron. When seeds had been dusted with NA, uptake of ⁴⁵Ca from main roots was similar to that of untreated plants, but only when chlorsulfuron was localized in the shoot zone. NA did not counteract the severe reduction in ⁴⁵Ca absorption when chlorsulfuron was localized in the root zone.     

Simulation study of the competitive ability of erect, semi-erect and prostrate cowpea (Vigna unguiculata) genotypes

Ecophysiological simulation models provide a quantitative method to predict the effects of management practices, plant characteristics and environmental factors on crop and weed growth and competition. The INTERCOM interplant competition model was parameterised, calibrated by monoculture data for three cowpea (Vigna unguiculata) genotypes that differed in growth habit, common sunflower (Helianthus annuus) and common purslane (Portulaca oleracea), and used to simulate competition of cowpea cover crops with sunflower or purslane. The simulation results were compared with observations from field competition experiments in 2003 and 2004. INTERCOM more accurately simulated actual field data for the competition of cowpea genotypes and sunflower than companion field experiments for the competition of cowpea and purslane. The validated simulation model of cowpea and sunflower at two densities was used to study the effects of cowpea growth habit on final biomass production of cowpea and sunflower. The model suggested that erect growth habit was more competitive than semi‐erect and prostrate growth habit, when cowpea genotypes were grown with sunflower. Cowpea leaf area distribution was important to higher cowpea biomass production, while cowpea height growth was important to reduce sunflower biomass. Our simulation approach is suggested as a method for crop breeders to gauge the likely success of selection for competitive crops before undertaking expensive long‐term breeding experiments.     

Pathogenicity and reproductive fitness of <Emphasis Type="Italic">Pratylenchus coffeae</Emphasis> and <Emphasis Type="Italic">Radopholus arabocoffeae</Emphasis> on Arabica coffee seedlings (<Emphasis Type="Italic">Coffea arabica</Emphasis> cv. Catimor) in Vietnam

The pathogenicity and reproductive fitness of Pratylenchus coffeae and Radopholus arabocoffeae from Vietnam on coffee (Coffea arabica) seedlings cv. Catimor were evaluated in greenhouse experiments. The effect of initial population densities (Pi = 0, 1, 2, 4, 8, 16, 32, 64, 128, and 256 nematodes per cm³ soil) was studied for both species at different days after inoculation (dai). The data were adjusted to the Seinhorst damage model Y = m + (1-m).z^Pi-T. Tolerance limit (T) for P. coffeae was zero for the height and the diameter of the coffee plants. For the diameter, the T-value for R. arabocoffeae was 25.6 for 30 and 60 dai and 12.8 for 90 and 120 dai. After 4 months T was zero. The low tolerance limits indicate that Arabica coffee is highly intolerant to both nematode species. At the end of the experiment (180 dai), all plants were infected and most were dead when inoculated with R. arabocoffeae at initial densities of 32, 64, 128 and 256 nematodes/cm³ soil. For P. coffeae plant death was already observed at the lowest inoculation densities. Growth of coffee was reduced at all inoculation levels for both species. Pratylenchus coffeae and R. arabocoffeae caused intense darkening of the roots, leaf chlorosis and a strong reduction of root and shoot growth. It was observed that P. coffeae mainly destroyed lateral roots rather than tap roots, whereas R. arabocoffeae reduced tap root length rather than the lateral roots. At the lowest inoculum densities, the reproduction factor of P. coffeae was 2.38 and 2.01 for R. arabocoffeae, indicating that arabica coffee is a host for both species. Plant growth as expressed by shoot height and shoot and root weight measured 60 dai was negatively correlated with nematode (both species) density as expressed by the geometric mean of nematode numbers at 30 and 60 dai.     

干旱胁迫下两种芽胞杆菌对玉米幼苗促生作用研究

供试玉米品种为‘先玉335’（XY335）,供试菌株为NECC11322(Bacillus subtilis)枯草芽胞杆菌、NECC11324(B. megaterium)巨大芽胞杆菌。试验设4组处理：Con（无菌水浸种,干旱胁迫）,C1（无菌水浸种,正常供水）,C2（NECC11322菌液浸种,干旱胁迫）,C3（NECC11324菌液浸种,干旱胁迫）,测定了浸种后盆栽玉米幼苗的生长状况、抗性生理指标及植株的N、P、K含量。结果表明：在干旱胁迫下接种NECC11322、NECC11324后,玉米幼苗叶片及根系SOD、POD、CAT、APX活性较Con均不同程度升高,其中接种NECC11322根系POD活性增长最为显著,较Con增长67.78%;植株叶片及根系可溶性蛋白含量、可溶性糖含量较Con均呈上升趋势,其中接种NECC11322叶片可溶性糖含量增长最为显著,较Con增长152.10%;与Con相比,植株叶片及根系全氮、全磷、全钾养分含量均有所提升,接种NECC11322后叶片全氮含量升高最为显著,较Con增长88.47%;干旱胁迫下接种两种芽胞杆菌后,植株叶片及根系丙二醛含量较Con均显著降低,接种NECC11322[JP2]后根系降幅最为显著,较Con降低51.03%。综上可知,干旱胁迫下接种两种芽胞杆菌均可降低干旱胁迫对玉米幼苗生长的抑制,其中NECC11322菌株更具有抗旱性。通过提高保护酶活性、可溶性蛋白、可溶性糖含量、叶绿素含量以及植株养分含量提高玉米幼苗抗旱性,促进玉米幼苗生长。     

Mining the record: historical evidence for climatic influences on maize –Abutilon theophrasti competition

Variations in climate are widely recognized as central factors governing the competitive balance in mixed‐species plant communities. In agricultural systems, highly variable patterns of crop yield reduction as a function of weed density have been documented across sites and among years at the same site for several crop–weed combinations. This variation is typically attributed to contrasting environmental conditions. Despite broad acknowledgement of their importance, experimental and temporal limitations have constrained the investigation and systematic understanding of environmental controls on the dynamics of competition. For several well‐studied crop–weed associations, aggregating historical data from similar competition experiments provides an opportunity to explore interference relationships over an array of conditions. In this study, 19 site‐years of maize –Abutilon theophrasti (velvetleaf) data were compiled and the weather characterized (i.e. average ambient temperature and moisture regime) for discrete portions of each growing season. These features were then related to patterns of maize yield loss from A. theophrasti interference at high weed densities. Results of this analysis suggest that temperatures following establishment, together with the presence or absence of water stress during the maize crop's exponential growth phase, account for over 60% of the observed variation in relative yield loss.     

Responses of annual bluegrass (Poa annua L.) and creeping bentgrass (Agrostis palustris Huds.) seedlings to nitrogen,phosphorus and potassium†

The growth response to fertilizer under different soil texture (light clayey soil and composite soil consisting of the light clayey soil and sand), and the timing of N fertilization (early summer and fall fertilization) were determined for annual bluegrass (Poa annua L.) and creeping bentgrass (Agrostis palustris Huds., var. Penncross) in glasshouse experiments. At the three‐leaf stage, both plant species were treated with 30, 100, 300, 1,000, 3000 or 10 000 p.p.m. (w/v) of N, P₂O₅ and K₂O equivalent to ammonium sulfate (N), calcium superphosphate (P) and potassium chloride (K). Three weeks after fertilization, the dry weight of shoots or roots was measured. Both plant species were more sensitive to N than to P or K; however, creeping bentgrass required a higher rate of N (1000 p.p.m.) for maximum shoot and root growth than did the weed (300 p.p.m., N). Shoot growth of both plant species was strongly affected by the timing of N fertilization and soil texture; their shoot responses to N increased in the order: composite soil > light clayey soil, and early summer N fertilization > fall N fertilization, and this tendency was especially marked for creeping bentgrass. These results suggest that the response of annual bluegrass to fertilizer is similar to that of creeping bentgrass; however, the two plants are different in regard to the N requirement or their relationship between response to N and application timing and/or soil texture.     

Management strategies and distribution of Aphanomyces root rot of alfalfa (Medicago sativa), a continuing threat to forage production in the United States

Alfalfa (Medicago sativa) is one of several legumes that is affected by Aphanomyces root rot (ARR) caused by Aphanomyces euteiches. Symptoms of ARR on alfalfa seedlings include a yellow-grey discolouration of roots, rotting and loss of lateral roots, stunted growth, chlorotic foliage and reduction of nitrogen-producing nodules on roots. Infection can also occur on adult plants leading to loss of lateral roots and nodules. At the seedling stage, ARR decreases alfalfa stand establishment, and field longevity is reduced when adult plants are infected. A. euteiches is an oomycete pathogen that has motile zoospores and thick-walled oospores that can survive for many years in soil. Two races are currently recognized by pathogenicity on differential alfalfa check cultivars. Most alfalfa cultivars contain race 1 resistance, but there is an increasing development of cultivars with resistance to race 2. Management strategies include planting resistant cultivars, avoiding planting in fields with poor drainage and rotating crops with nonhost plants.     

The hotter the weather,the greater the infestation of Portulaca oleracea: opportunistic life‐history traits in a serious weed

Portulaca oleracea, an r‐strategist, is one of the world's most troublesome weeds. During hot seasons, P. oleracea frequently becomes monodominant in choy sum (Brassica parachinensis) fields in Guangzhou city, southern China. Here, we studied the seasonal dynamics of P. oleracea's germinable soil seedbank, population density and above‐ground biomass in choy sum fields that had been cultivated continuously for several years. Using P. oleracea seeds collected from these fields, we tested seed dormancy, survival and germination, seedling growth and generation time. Portulaca oleracea occurred at high levels during the hot season, but its occurrence was low, and the germinable soil seedbank was much greater during the cold season. The weed's opportunistic characteristics allowed it to avoid freezing and to proliferate during optimal conditions. Portulaca oleracea's generation time was very flexible, as short as 31 days during the hot season, but longer than 100 days during the colder season. Seed dormancy tended to be shorter when the seeds were stored at a higher temperature. At a temperature of 35°C, both seed germination and seedling growth showed advantages over those of choy sum. Storage for one year at a temperature of ?20°C or burial in a paddy field did not significantly reduce P. oleracea seed germination. Nevertheless, seed storage at a temperature of 15°C and soil coverage of 0.5 cm on top of the seeds significantly constrained seed germination. ‘Stale seedbed’ and/or coverage of the surface with soil are recommended during the hot season. However, rotation of rice and upland crops is not an efficient method for managing Portulaca oleracea infestation.     

Lack of prevention of chlorsulfuron-induced inhibition by amino acids

The amino acids valine and isoleucine, which had been indicated by Ray (1984) as antidotes against growth inhibition of pea plants induced by chlorsulfuron, were ineffective in preventing the damage induced by chlorsulfuron on the plasma membrane of maize roots as measured by K⁺ uptake. In addition, in seedlings of two corn genotypes and three pea genotypes grown in culture solution containing chlorsulfuron, growth inhibition was not alleviated by valine and isoleucine. However, in agreement with Ray's results, acetolactate synthase, the key enzyme of the biosynthesis of valine and isoleucine, was inhibited in vitro by chlorsulfuron in both maize and pea. It is suggested that the chlorsulfuron mechanism of action involves several sites.     

Colonization of <Emphasis Type="Italic">Arabidopsis</Emphasis> roots by <Emphasis Type="Italic">Trichoderma atroviride</Emphasis> promotes growth and enhances systemic disease resistance through jasmonic acid/ethylene and salicylic acid pathways

Trichoderma spp. are common soil fungi used as biocontrol agents due to their capacity to produce antibiotics, induce systemic resistance in plants and parasitize phytopathogenic fungi of major agricultural importance. The present study investigated whether colonization of Arabidopsis thaliana seedlings by Trichoderma atroviride affected plant growth and development. Here it is shown that T. atroviride promotes growth in Arabidopsis. Moreover, T. atroviride produced indole compounds in liquid cultures. These results suggest that indoleacetic acid-related indoles (IAA-related indoles) produced by T. atroviride may have a stimulatory effect on plant growth. In addition, whether colonization of Arabidopsis roots by T. atroviride can induce systemic protection against foliar pathogens was tested. Arabidopsis roots inoculation with T. atroviride provided systemic protection to the leaves inoculated with bacterial and fungal pathogens. To investigate the possible pathway involved in the systemic resistance induced by T. atroviride, the expression profile of salicylic acid, jasmonic acid/ethylene, oxidative burst and camalexin related genes was assessed in Arabidopsis. T. atroviride induced an overlapped expression of defence-related genes of SA and JA/ET pathways, and of the gene involved in the synthesis of the antimicrobial phytoalexin, camalexin, both locally and systemically. This is the first report where colonization of Arabidopsis roots by T. atroviride induces the expression of SA and JA/ET pathways simultaneously to confer resistance against hemibiotrophic and necrotrophic phytopathogens. The beneficial effects induced by the inoculation of Arabidopsis roots with T. atroviride and the induction of the plant defence system suggest a molecular dialogue between these organisms.     

Comparative life history characteristics of the mite predator Neoseiulus cucumeris (Oudemans) (Acari: Phytoseiidae) on mite and pollen diets

The food diversification strategy that can offer suitable nutrients as a diet for the natural enemies may possibly help to decrease pest populations by boosting the efficiency of predators. Therefore, my objective was to evaluate the effects of plant and animal diets on biological parameters of the phytoseiid mite Neoseiulus cucumeris (Oudemans). This predator that is a natural enemy of mites was reared on plant pollens (maize (Zea mays L.), sweet pepper (Capsicum annuum L.), mungbean (Vigna radiata L.), cucumber (Cucumis sativus L.), tomato (Lycopersicon esculentum Mill.), and rose (Rosa multiflora Thunb.), offered alone, and in combination with the prey mite Tyrophagus putrescentiae (Schrank). Results showed that N. cucumeris fed, developed and reproduced better when it was provided with plant and animal diets together, compared to pollens offered alone. The predator showed the most efficient capability for completion of life parameters (development, survival, fecundity, oviposition, longevity and instantaneous rate of increase) with maize and mungbean ingested pollens in combination with T. putrescentiae. The least competency was observed on cucumber and rose pollens used separately. The lowest developmental time of the predator from neonate larva to adult emergence, the highest immature survivorship, higher fecundity, long oviposition period, and the highest adult female's life span were observed when offered maize pollens along with T. putrescentiae compared to other treatments. Accordingly, the instantaneous rate of population increase (r_i) day^?1 was surpassed by N. cucumeris for the combination of pollen and T. putrescentiae diets than pollens utilized alone. It is determined that N. cucumeris can reproduce and develop more successfully on a food complex consisting of pollen and animal diets than any single diet. These findings reveal that pollen can be used by N. cucumeris as a suitable alternative food in periods when prey in the field is sparse or absent. Moreover, such propensity of predators to feed on food from plant origin increases their survival during periods when prey is locally scarce, but natural enemies might occur at the expense of an ability to utilize food as efficiently as specialized predators to boost the management strategies of pest species in field's ecosystem.