黄瓜种子及其萌发期的化感作用研究

以大白菜、萝卜、番茄和黄瓜种子为受体,采用实验室培养皿种子发芽生物测试法研究了黄瓜种子浸提液、种子萌发、胚根和芽苗分泌物、芽苗腐解物和芽苗浸提液的化感效应。结果表明:(1)黄瓜种子浸提液对大白菜、萝卜、番茄和黄瓜种子萌发均有化感抑制作用,即黄瓜种子内含有某些化感抑制物质。(2)在水浸提过的黄瓜种子萌发过程中,它不仅对其近邻套种的大白菜、萝卜和番茄种子萌发产生化感抑制作用,而且其胚根和芽苗分泌物对后茬播种的4种蔬菜种子发芽也表现出不同程度的化感抑制作用;黄瓜芽苗腐解物和芽苗水浸提液也对各受体蔬菜种子发芽与生长产生不同程度的化感抑制作用,且随着腐解芽苗量的增加或浸提液浓度的升高,各受体蔬菜种子的发芽指标值、化感效应指数值和综合效应值随之降低。(3)黄瓜种子浸提液及芽苗各器官的化感物质对黄瓜种子的萌发与生长产生了自毒作用,且黄瓜芽苗腐解物、芽苗浸提液、胚根及芽苗分泌物对受体黄瓜的自毒作用均为最大。研究发现,黄瓜种子浸提液、种子萌发时期以及芽苗各器官的化感物质主要是通过抑制受体胚根的生长而起化感抑制作用,即受体蔬菜种子胚根对化感效应最为敏感;因黄瓜种子及萌发期释放化感物质的途径有所不同,导致受体大白菜、萝卜、黄瓜和番茄的化感响应也不相同;在黄瓜种子萌发和芽苗生长的早期,化感物质即开始在芽苗体内进行合成与积累,一部分可通过胚根和芽苗分泌途径释放到环境中,另一部分可通过芽苗腐解途径释放化感物质,并对受体蔬菜种子萌发与生长表现出较强的化感抑制作用。     

Puccinia xanthii forma specialis ambrosia-trifidae

The rustPuccinia xanthii forma specialisambrosia-trifidae attacks leaves of giant ragweed,Ambrosia trifida L., significantly reducing seed and pollen production, seed weight and seedling vigor. In tests, teliospores did not infectA. artemisiifolia L.,Xanthium strumarium L., and cultivated varieties of chrysanthemum, safflower, sunflower, cosmos, marigold, lettuce, endive, dahlia, zinnia, gloriosa daisy, aster, scabiosa, and gallardia. This fungus commonly attacksA. trifida in eastern and central North America, and it should be of value for the biological control ofA. trifida where introduced in Eurasia.

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Puccinia xanthii forma specialis ambrosia-trifidae

     

Quantitative trait loci associated with seed and seedling traits in Lactuca

Seed and seedling traits related to germination and stand establishment are important in the production of cultivated lettuce (Lactuca sativa L.). Six seed and seedling traits segregating in a L. sativa cv. Salinas x L. serriola recombinant inbred line population consisting of 103 F8 families revealed a total of 17 significant quantitative trait loci (QTL) resulting from three seed production environments. Significant QTL were identified for germination in darkness, germination at 25 and 35°C, median maximum temperature of germination, hypocotyl length at 72 h post-imbibition, and plant (seedling) quality. Some QTL for germination and early seedling growth characteristics were co-located, suggestive of pleiotropic loci regulating these traits. A single QTL (Htg6.1) described 25 and 23% of the total phenotypic variation for high temperature germination in California- and Netherlands-grown populations, respectively, and was significant between 33 and 37°C. Additionally, Htg6.1 showed significant epistatic interactions with other Htg QTL and a consistent effect across all the three seed production environments. L. serriola alleles increased germination at these QTL. The estimate of narrow-sense heritability (h²) of Htg6.1 was 0.84, indicating potential for L. serriola as a source of germination thermotolerance for lettuce introgression programs.     

Effects of Simulated Oilfield Produced Water on Early Seedling Growth After Treatment in a Pilot-Scale Constructed Wetland System

Seed germination and early seedling growth bioassays were used to evaluate phytotoxicity of simulated oilfield produced water (OPW) before and after treatment in a subsurface-flow, pilot-scale constructed wetland treatment system (CWTS). Responses to untreated and treated OPW were compared among seven plant species, including three monocotyledons: corn (Zea mays), millet (Panicum miliaceum), and sorghum (Sorghum bicolor); and four dicotyledons: lettuce (Lactuca sativa), okra (Abelmoschus esculents), watermelon (Citrullus lanatus), and soybean (Glycine max). Phytotoxicity was greater in untreated OPW than in treated OPW. Exposures to untreated and treated OPW enhanced growth in some plant species (sorghum, millet, okra, and corn) relative to a negative control and reduced growth in other plant species (lettuce, soybean, and watermelon). Early seedling growth parameters indicated that dicotyledons were more sensitive to test waters compared to monocotyledons, suggesting that morphological differences between plant species affected phytotoxicity. Results indicated the following sensitivity scale for plant species: lettuce > soybean > watermelon > corn> okra≈millet >sorghum. Phytotoxicity of the treated OPW to lettuce and soybean, although concentrations of COCs were less than irrigation guideline concentrations, suggests that chemical characterization and comparison to guideline concentrations alone may not be sufficient to evaluate water for use in growing crops.     

石蒜对萝卜、黄瓜、番茄和油菜幼苗的化感效应

在室内用离体生测方法研究了石蒜水浸提物对萝卜、黄瓜、番茄和油菜的化感效应.结果表明, 石蒜水浸提物对4种植物种子的萌发和幼苗生长均具有较强的抑制作用.4种受体作物中,以番茄最为敏感,最低浓度0.0125 g·ml^-1处理下,番茄种子完全受到抑制不能萌发,油菜、萝卜与黄瓜种子的萌发抑制率分别为17.73％、14.97％和2.65％.相同浓度处理时,石蒜水浸提物对萝卜、黄瓜及油菜芽的生长抑制作用高于对根的抑制作用.采用去胚乳小麦生长法和高梁河沙法测定了石蒜甲醇浸提物对光合作用和非光合作用抑制活性.结果表明,石蒜甲醇浸提物对去胚乳小麦和高梁的生长有较强的抑制作用,对高梁芽和根生长的抑制作用高于对去胚乳小麦根和芽生长的抑制,说明石蒜对植物的化感效应主要体现在抑制非光合作用活性上,但也一定程度地抑制植物的光合作用.     

巨尾桉叶片挥发物对三种植物种子萌发及幼苗生长的化感效应

为探讨人工巨尾桉林叶片挥发物对周边农作物的化感作用,采用不同质量新鲜巨尾桉叶片及由新鲜叶片提取的桉叶油对玉米、辣椒、西红柿等三种植物种子进行处理,观测其种子萌发和幼苗生长状况。结果表明:(1)在三种被测植物中,桉树叶片挥发物对玉米种子萌发影响最小,对辣椒影响最大;(2)当叶片用量小于200g时,桉树叶片挥发物对三种植物种子萌发影响不明显,当叶片用量达400g时,能完全抑制辣椒、西红柿种子萌发,并能极显著降低玉米种子萌发(P0.01);(3)玉米幼苗芽生长随叶片用量的增加呈现先促进后抑制现象,对芽高、鲜重、干重的促进和抑制均达到显著(P0.05)或极显著水平(P0.01);(4)当叶片用量小于或等于100g时,桉树叶片挥发物对辣椒、西红柿幼苗生长影响不明显,当用量达到200g时则能极显著抑制辣椒、西红柿幼苗生长;(5)桉叶油对三种测试植物的抑制效果与叶片自然挥发物相似,且效果更显著。     

Serological detection of Cucumber mosaic virus on some important host crops in the north region of Iran

Plant virus diseases cause major losses in agricultural and horticultural products, especially in tropical and subtropical regions. The first step to manage these diseases is detecting, identifying and determining the pathogen characteristics. Cucumber mosaic disease is one of the most prevalent and damaging plant diseases in the world which is caused by Cucumber mosaic virus (CMV). Each year, this virus causes yield decreasing and substantial economic damages in its host plants worldwide including the north of Iran. In order to study and identify CMV, 935 leaf samples were collected based on typical symptoms from 10 crops (tomato, pea, tobacco, soybean, watermelon, broad bean, squash, cucumber, eggplant and lettuce) in different regions of Golestan and Mazandaran provinces (north of Iran) during 2009–2010. Suspicious samples were analysed by DAS-ELISA and polyclonal antibodies. The results showed that 275 samples (29.4%) were infected by CMV. Between these hosts, the highest and the lowest CMV infection was associated to watermelon (62.44%) and lettuce (0%), respectively. Among sampling locations, Behshahr (100%) and Minoodasht (3.47%) showed the maximum and minimum infection, respectively.     

Chemical Composition and Allelopathic,Phytotoxic and Pesticidal Activities of Atriplex cana Ledeb. (Amaranthaceae) Essential Oil

The chemical composition and allelopathic, phytotoxic and pesticidal activities of Atriplex cana Ledeb . (Amaranthaceae) essential oil were investigated. Nineteen compounds were identified via GC/MS, representing 82.3 % of the total oil, and the most abundant constituents were dibutyl phthalate (21.79 %), eucalyptol (20.14 %) and myrtenyl acetate (15.56 %). The results showed that volatile organic compounds (VOCs) released by A. cana significantly inhibited seedling growth of Amaranthus retroflexus L. and Poa annua L., and 80 g of fresh stems and leaves of A. cana in a 1.5 L airtight container almost completely suppressed the seed germination of both plants. Meanwhile, 5 μg/mL essential oil completely inhibited the seed germination of A. retroflexus, Medicago sativa L., P. annua and Echinochloa crusgalli L. Pesticidal testing revealed that the essential oil had strong behavioral avoidance and lethal effects on Aphis pomi DeGeer . Five microliters of essential oil/Petri dish treatment resulted in an 84.5 % mortality rate after 12 h, and the mortality rate reached nearly 100 % after 48 h. This report is the first one on the chemical composition as well as the biological activity of the essential oil of A. cana, and our results indicate that the oil is valuable in terms of being further exploited as a bioherbicide/insecticide.     

Differential effects of camptothecin and interactions with plant hormones on seed germination and seedling growth

Effects of camptothecin, a naturally occurring alkaloid, on seed germination varied from promotive to inhibitory, depending on the species used. It markedly inhibited seedling root growth but its inhibition of hypocotyl growth varied among species. Camptothecin inhibited GA₃-induced dark germination of lettuce (Lactuca sativa L.) seeds and hypocotyl elongation of seedlings. In contrast to ABA, the camptothecin inhibition of GA₃-induced germination could not be overcome by cytokinin. When seeds were germinated at 29C with a 0.5 h light treatment, little or no germination occurred in the camptothecin treatment, but addition of cytokinin overcame this inhibition.     

离体蒜苗挥发物的化感作用及其成分分析

采用半密闭容器法研究了离体蒜苗挥发物对黄瓜、番茄、萝卜的化感作用,并对其成分进行了GC-MS分析。结果表明:蒜苗挥发物对黄瓜的种子萌发及幼苗生长表现为低促高抑的双重浓度效应,对番茄和萝卜则表现出抑制作用,并且随供体蒜苗质量的增大,抑制作用增强;供体蒜苗质量低于400g时,受体植株SOD、POD、CAT、PPO、PAL等活性增强,供体质量为400g时,受体作物上述酶活性均降低;离体蒜苗挥发物的主要化学成分为二烯丙基二硫化物(23.33%)、1,3-二噻烷(18.34%)和邻苯二甲酸二丁酯(6.30%)。     

Influence of Ultraviolet Radiation on Germination and Early Seedling Growth

The influence of UV-B radiation (280–320 nm) from filtered and unfiltered FS-40 fluorescent sunlamps on germination and early seedling growth was examined for a range of vegetables (tomato, radish, cucumber, lettuce, and bean) and field crops (wheat, cotton, soybean, and millet). Continuous exposure of seeds for 3 days to 26.9 × 10⁻² W × m⁻² UV-B radiation (280–320 nm) at 25°C, had a slight effect on fresh weight of seedlings but no appreciable influence on germination percentage, or dry weight of seedlings. Extending the time of exposure to 6 days, however, resulted in abnormal seedling growth in all species but wheat. Typical responses were short, stubby roots, bronzing of the cotyledons, increased pigmentation, and abnormal curvature of the shoots.     

豚草和三裂叶豚草不同植株部位种子萌发与入侵扩散关系

豚草(Ambrosia artemisiifoliaLinn.)和三裂叶豚草(A. trifidaLinn.)是我国恶性入侵物种,分布常呈高密度单一种群,且结实量庞大。探讨二者不同植株部位种子萌发(休眠)与扩散特点,对了解二者入侵机制具有重要意义。以新疆伊犁新源县发生的豚草和三裂叶豚草为材料,在种子成熟期,根据植株高度、枝条长度,按比例从上到下分为9个部位,对不同植株部位种子的形态特征、数量和萌发特性进行比较,分析这两种植物不同植株部位种子萌发与扩散的共性和差异性,研究二者种群密度调节和入侵的关系。结果表明:1)两个物种内不同植株部位间种子的长、宽、百粒重无显著差异,但三裂叶豚草种子的长度和宽度分别是豚草的2—3倍,百粒重高7倍。结合两个物种在伊犁地区分布差异,认为种子大小是两个物种分布区域性差异的原因之一。2)豚草和三裂叶豚草植株外部的上顶、中顶、上中部位种子数占植株总种子数量的50%,中中、下顶占比约23%,而下部的上基、中基、下中、下基的种子数占比约27%,表明当年生产的种子有近73%的比例具有远距离扩散的潜力。3)豚草和三裂叶豚草不同植株部位种子的萌发率具有上端中端下端的趋势;初始萌发时间为下端中端上端;萌发持续时间为上端中端下端。这种萌发方式避免了同一生长季大批种子同时萌发有可能导致高密度死亡的风险。基于上述研究分析,认为豚草和三裂叶豚草不同植株部位种子具有不同的适应功能。其中,上部所产生的种子具较强的扩散能力和低休眠性,有利于两物种快速占据新生境并扩大种群;而中、下部位的种子在母株周围就近扩散,翌年萌发率低,缓解了种群竞争。豚草和三裂叶豚草不同植株部位生产的种子特性和萌发差异是两个物种进行种群密度调节和扩散入侵的重要原因。     

Differential allelopathic expression of bark and seed of Tamarindus indica L.

Allelopathic performance of the bark and seed of Tamarindus indica L. tree was evaluated through bioassay-guided studies using seven common agronomic crops (asparagus, cucumber, lettuce, radish, sesame, tomato and welsh onion) and seven weed species (barnyard grass, Chinese milk vetch, perennial ryegrass, phacelia, timothy grass, white clover and wild ginger) under laboratory conditions. As demonstrated by a sandwich method, the bark of the tamarind tree caused strong growth inhibition (compared to the corresponding controls) in both radicles and hypocotyls of the species tested, and the inhibitory effect was highest in barnyard grass (52–65%) and lowest in welsh onion (19–13%). The crude-water soluble extracts of bark at different concentrations (1, 5 and 10%) (w/v) exhibited a strong growth inhibition in all the plant species tested, and a proportional increase in the percentage of growth inhibition was observed with an increase in the concentrations of the extracts. The magnitude of inhibition in weed species was higher (5–60%) than those of agronomic crop species (3–40%). The growth of all the weed species tested was strongly inhibited (17–56%), while the agronomic crop species showed both inhibited (5–21%) and stimulated (5–27%) growth due to the effect of crude-water soluble exudates of tamarind seed. Among the agronomic crop species tested, lettuce (22–27%) followed by radish (20–25%) and sesame (5–8%) showed stimulatory growth with the crude-water soluble exudates of seed. In the pot culture experiments using four agronomic crops (lettuce, radish, tomato and cucumber) and two weed species (barnyard grass and white clover), spraying of crude-water soluble extracts of tamarind seed-coat at three different concentrations (1, 5 and 10%) (w/v) showed that the growth of lettuce (35–62%) and radish (32–56%) was stimulated, while all other species tested showed growth inhibition (29–61%). When the spraying of crude extracts of seed-coat was turned off, the growth of both lettuce and radish continued to be stimulated (4–7%) and all other previously inhibited species recovered well, the recovery percentage ranging between 78 and 82%. However, when spraying of crude extracts of seed-coat was continued, growth increased (10–14%) in lettuce and radish, and reduced (37–76%) in four other species tested. The inhibitory or stimulatory effects of the crude extracts on agronomic crop and weed species were higher in the radicle than the hypocotyl and reached a peak with 10% (w/v) concentrations. These results clearly demonstrated the differential allelopathic effects (inhibitory and excitatory) of bark and seed of tamarind tree in the species tested. Thus, it is evident that these two organs contain certain biologically active true growth regulator(s) and are either additively or synergistically involved in the plant-specific expression, particularly by the seed-coat.     

NaCl胁迫下亚精胺对番茄种子萌发及幼苗抗氧化系统的影响

采用盆栽试验,研究了NaCl胁迫下外源亚精胺（Spd）浸种处理对“白果强丰”番茄种子萌发、幼苗生长及叶片抗氧化系统的影响.结果表明：Spd浸种处理的番茄种子发芽率（G_r）、发芽指数（G_i）和活力指数（V_i）均显著增加,分别比单纯NaCl处理增加了104.90%、142.31%和122.22%,而番茄种子的平均发芽时间（MGT）缩短了4.97%;Spd浸种处理提高了NaCl胁迫下番茄幼苗叶片超氧化物歧化酶（SOD）、过氧化物酶（POD）和过氧化氢酶（CAT）活性,降低了O₂^-·产生速率和过氧化氢（H₂O₂）含量,进而降低了丙二醛（MDA）含量和质膜电解质渗漏率,增加了叶片可溶性蛋白质含量,促进了番茄幼苗生长.表明Spd浸种促进了NaCl胁迫下番茄种子的萌发及幼苗生长,增强了植株的抗盐性.     

Effect of distillery effluent on seed germination in some vegetable crops

A laboratory experiment was conducted to study the effect of different concentrations (0%, 5%, 10%, 15%, 20%, 25%, 50%, 75% and 100%) of distillery effluent (raw spent wash) on seed germination (%), speed of germination, peak value and germination value in some vegetable crops: tomato, chilli, bottle gourd, cucumber and onion. The distillery effluent did not show any inhibitory effect on seed germination at low concentration except in tomato, but in onion the germination was significantly higher (84%) at 10% concentration as against 63% in the control. Irrespective of the crop species, at highest concentrations (75% and 100%), complete failure of germination was observed. The speed of germination, peak value and germination value also followed a similar trend. We found that a concentration of 5% was critical for seed germination in tomato and bottle gourd, and 25% in the rest of the crops. Based on the tolerance to distillery effluent, the crops studied have been arranged in the following order: cucumber > chilli > onion > bottle gourd > tomato. We conclude that the effect of the distillery effluent is crop-specific and due care should be taken before using the distillery effluent for pre-sowing irrigation purposes.     

Chemical Composition and Phytotoxic Effects of Essential Oils Obtained from Ailanthus altissima (Mill.) Swingle Cultivated in Tunisia

Ailanthus altissima Mill. Swingle (Simaroubaceae), also known as tree of heaven, is used in the Chinese traditional medicine as a bitter aromatic drug for the treatment of colds and gastric diseases. In Tunisia, Ailanthus altissima is an exotic tree, which was introduced many years ago and used particularly as a street ornamental tree. Here, the essential oils of different plant parts of this tree, viz., roots, stems, leaves, flowers, and samaras (ripe fruits), were obtained by hydrodistillation. In total, 69 compounds, representing 91.0–97.2% of the whole oil composition, were identified in these oils by GC‐FID and GC/MS analyses. The root essential oil was clearly distinguishable for its high content in aldehydes (hexadecanal ( 1 ); 22.6%), while those obtained from flowers and leaves were dominated by oxygenated sesquiterpenes (74.8 and 42.1%, resp.), with caryophyllene oxide ( 4 ) as the major component (42.5 and 22.7%, resp.). The samara oil was rich in the apocarotenoid derivative hexahydrofarnesyl acetone ( 6 ; 58.0%), and the oil obtained from stems was characterized by sesquiterpene hydrocarbons (54.1%), mainly β‐caryophyllene (18.9%). Principal component and hierarchical cluster analyses separated the five essential oils into four groups, each characterized by the major oil constituents. Contact tests showed that the germination of lettuce seeds was totally inhibited by all the essential oils except of the samara oil at a dose of 1 mg/ml. The flower oil also showed a significant phytotoxic effect against lettuce germination at 0.04 and 0.4 mg/ml (?55.0±3.5 and ?85.0±0.7%, resp.). Moreover, the root and shoot elongation was even more affected by the oils than germination. The inhibitory effect of the shoot and root elongation varied from ?9.8 to ?100% and from ?38.6 to ?100%, respectively. Total inhibition of the elongation (?100%) at 1 mg/ml was detected for all the oils, with the exception of the samara oil (?74.7 and ?75.1% for roots and shoots, resp.).     

多裂骆驼蓬提取物对黄瓜种子萌发和幼苗生理特性的影响

通过室内培养和盆栽土培试验研究了多裂骆驼蓬提取物对黄瓜种子萌发和幼苗生长及生理特性的影响。结果表明,多裂骆驼蓬总生物碱提取液、水溶性生物碱提取液和脂溶性生物碱提取液浸种均抑制黄瓜种子萌发过程中淀粉酶、蛋白酶和脂肪酶活性,种子活力和萌发速率降低,呼吸速率减慢;幼苗生长过程中根系活力、硝酸还原酶活性升高,叶绿素含量增加,超氧化物歧化酶（SOD）和过氧化物酶（POD）活性提高。说明用多裂骆驼蓬提取液浸种能够促进黄瓜幼苗生长,有利于培育壮苗。     

Inhibition of seed germination by extracts of bitter Hawkesbury watermelon containing cucurbitacin, a feeding stimulant for corn rootworm (Coleoptera: Chrysomelidae)

Cucurbitacins are feeding stimulants for corn rootworm used in baits to control the adults of this insect pest. Corn rootworm larvae also feed compulsively on cucurbitacins. Cucurbitacins are reported to be gibberellin antagonists that may preclude their use as seed treatments for these soil-dwelling insects. The crude extract of a bitter Hawkesbury watermelon containing cucurbitacin E-glycoside significantly inhibited germination of watermelon, squash, and tomato seeds. Although the germination of corn seed was not significantly inhibited, root elongation was inhibited by crude extracts, but not by high-performance liquid chromatography-purified cucurbitacin E-glycoside. Therefore, the effects of the major components in the bitter watermelon extract (e.g., sugars) on seed germination and root elongation were determined. Pure sugars (glucose and fructose), at concentrations found in watermelon extract, mimicked the inhibition of seed germination and root elongation seen with the crude bitter Hawkesbury watermelon extract. Removal of these sugars may be necessary to use this extract as a bait for corn rootworm larvae as a seed or root treatment.     

Allelopathic effect of <Emphasis Type="Italic">Nepeta meyeri</Emphasis> Benth. extracts on seed germination and seedling growth of some crop plants

In order to evaluate the allelopathic potential of Nepeta meyeri Benth., the effects of aqueous extracts (0.125, 0.25, 0.5, 1, 2.5 and 5%) prepared from roots and leaves of N. meyeri were studied on the seed germination and seedling growth of several economically important crops (barley, wheat, canola, safflower, and sunflower). Both the root and leaf extracts of N. meyeri caused a general phytotoxic effect on the seed germination and seedling growth of barley and sunflower at all concentrations. However, both the root and leaf extracts significantly increased the seedling growth of wheat, especially at the lower concentrations 0.125, 0.25 and 0.5%, whereas the higher concentrations had a neutral effect. The seed germination and the seedling growth of canola and safflower were also generally improved by both extracts, especially at lower concentrations. However, these advantages were not observed at higher concentrations, at which the extracts mostly had a phytotoxic effect on canola and safflower. The Allelopathic activity of N. meyeri depended on whether the extract was derived from the leaf or root parts of the plant. The maximum allelopathic effect occurred with leaf extracts. The results demonstrate that the aqueous extracts from N. meyeri have allelopathic potential and should be evaluated as an allelopathic species, presenting a risk or advantage to seed germination and seedling growth of crop or weed plants.     

Differential inactivation of seed exudate stimulation of Pythium ultimum sporangium germination by Enterobacter cloacae influences biological control efficacy on different plant species

This study was initiated to understand whether differential biological control efficacy of Enterobacter cloacae on various plant species is due to differences in the ability of E. cloacae to inactivate the stimulatory activity of seed exudates to Pythium ultimum sporangium germination. In biological control assays, E. cloacae was effective in controlling Pythium damping-off when placed on the seeds of carrot, cotton, cucumber, lettuce, radish, tomato, and wheat but failed to protect corn and pea from damping-off. Seeds from plants such as corn and pea had high rates of exudation, whereas cotton and cucumber seeds had much lower rates of exudation. Patterns of seed exudation and the release of P. ultimum sporangium germination stimulants varied among the plants tested. Seed exudates of plants such as carrot, corn, lettuce, pea, radish, and wheat were generally more stimulatory to P. ultimum than were the exudates of cotton, cucumber, sunflower, and tomato. However, this was not directly related to the ability of E. cloacae to inactivate the stimulatory activity of the exudate and reduce P. ultimum sporangium germination. In the spermosphere, E. cloacae readily reduced the stimulatory activity of seed exudates from all plant species except corn and pea. Our data have shown that the inability of E. cloacae to protect corn and pea seeds from Pythium damping-off is directly related to its ability to inactivate the stimulatory activity of seed exudates. On all other plants tested, E. cloacae was effective in suppressing damping-off and inactivating the stimulatory activity of seed exudates.