镉胁迫下外源IAA对栝楼生理变化和耐镉性的影响

为探究IAA对镉胁迫下栝楼(Trichosanthes kirilowii Maxim.)生理响应及耐镉性的影响,以保定栝楼(耐镉品种)和浦江栝楼(镉敏感品种)为材料,进行土培试验,研究800μmol/L镉胁迫下不同浓度IAA(10,35,60μmol/L)对2个栝楼品种的生长特性、光合特性、抗氧化酶活性及丙二醛(MDA)含量的影响。结果表明:(1)与CK组相比,800μmol/L Cd处理下,保定和浦江栝楼的平均根长、地下部分鲜重、超氧化物歧化酶(SOD)活性、叶绿素含量、最大光化学效率(F_v/F_m)和电子传递速率(ETR)明显降低,初始荧光(Fo)、过氧化物酶(POD)、过氧化氢酶(CAT)活性及MDA含量显著增高,细胞膜脂过氧化程度加深;(2)随IAA浓度升高,保定栝楼平均根长与地下部分鲜重持续增长,最大增幅为24.83%和40.55%,而浦江栝楼则由上升转为下降趋势;(3)外源IAA加入后,2个栝楼品种幼叶叶绿素a、叶绿素b、叶绿素a+b含量、叶绿素a/b、叶绿素荧光参数中的Fv/Fm、ETR及抗氧化酶SOD、POD、CAT活性均呈现出先升后降的趋势,Fo和MDA含量先降后升,其中,35μmol/L的IAA使叶绿素a、叶绿素b、叶绿素a+b含量、叶绿素a/b、Fv/Fm、ETR及SOD、CAT活性上升到峰值,而MDA含量降低35.44%～43.23%,达到谷值;(4)由隶属函数均值耐镉性分析得知,不同浓度IAA对栝楼镉毒害的缓解程度为35μmol/L10μmol/L60μmol/L0μmol/L。由此可见,外源IAA的施加对于缓解植物镉胁迫具有一定的应用前景。     

连作植烟土壤具有显著积累特征的两种酚酸的化感效应评价

为科学评价连作植烟土壤中具有显著积累特征的两种酚酸的化感效应,丰富现有连作障碍机理,以烤烟品种K326为试验材料,基于前期田间实测结果设置不同浓度梯度(间苯三酚:0.20、1.00、5.00 mg·kg^-1,对羟基苯甲酸0.02、0.10、0.50 mg·kg^-1)进行盆栽试验。结果表明,两种酚酸物质均显著抑制了烤烟种子萌发。间苯三酚抑制了幼苗根系的生长,且浓度越大抑制作用越强;对羟基苯甲酸对幼苗根系生长的影响表现出“低促高抑”的浓度效应。随着间苯三酚浓度的增加,叶片超氧化物歧化酶(SOD)和过氧化氢酶(CAT)活性呈上升趋势,而过氧化物酶(POD)活性呈下降趋势;随着对羟基苯甲酸浓度的增加,叶片SOD、CAT和POD活性均呈先升后降趋势。此外,两种酚酸物质均降低了烤烟幼苗叶片叶绿素含量,增加了丙二醛(MDA)含量。随着处理时间的延长,胁迫作用趋势相同。由此可知,间苯三酚和对羟基苯甲酸不同程度地抑制了烤烟幼苗的生长,且存在浓度效应。间苯三酚表现为抑制作用,浓度越高抑制作用越强;对羟基苯甲酸浓度≤0.10 mg·kg^-1时表现为促进作用,>0.10 mg·kg^-1时表现为抑制作用。对羟基苯甲酸较间苯三酚具有更强的生物毒性。本研究可以丰富植烟土壤连作障碍机理,为今后连作障碍调控提供理论依据。     

氮素形态对小麦生长中后期保护酶等生理特性的影响

在水培条件下,探究了氮素形态对小麦营养生殖并进生长时期保护酶的影响。结果表明:铵、硝态氮单供和混合供应对小麦保护酶活性影响不同。在铵、硝态氮混合供应下,叶片SOD、CAT,根系SOD活性最高,同时叶片、根系可溶性蛋白含量最大,根系活力和根系表面K3Fe(CN)6还原活性最高,但MDA含量最高;单供硝态氮时叶片POD活性最高,根系MDA含量最高;单供铵态氮时根系POD、CAT活性最高。单供尿素时叶片仅CAT活性高于铵和硝态氮混合供给,但同时叶片MDA含量也高于铵和硝态氮混合供给。叶片以SOD、CAT和POD为主清除自由基,而根系以SOD和POD为主清除自由基。铵和硝态氮混合供给下,小麦成熟后,地上部干重,籽粒产量最大,根冠比最小。     

看麦娘根系对小麦根水提液化感作用的生理响应

为探讨小麦化感作用机理,以强化感小麦‘115/青海麦’、‘92L89’和弱化感小麦‘抗10103’材料,设置小麦根水提液浓度为0%、0.2%、1.0%和5.0%的水培试验,测定了看麦娘根系对小麦根水提液化感作用的生理响应。结果表明,小麦根水提液处理显著抑制了看麦娘根的生长,根鲜重抑制率随处理浓度的升高而增大,强化感小麦的抑制率高于弱化感小麦。当处理浓度达5.0%时,不同化感小麦间无显著差异。‘115/青海麦’、‘92L89’和‘抗10103’根水提液处理后,看麦娘的根系活力的抑制率分别为52.0%~59.6%、46.5%~55.0%和27.2%~44.7%,但前二者间无显著差异;看麦娘根系中可溶蛋白含量、SOD、POD、CAT活性及MDA含量显著升高;SOD和POD活性大小均表现为‘115/青海麦’‘92L89’≈‘抗10103’;CAT活性随处理浓度的升高显著增大,但不同品种小麦间无显著差异。‘115/青海麦’、‘92L89’和‘抗10103’根水提液处理的看麦娘根系MDA含量依次是对照的10.9~25.5倍、5.9~24.2倍和1.2~6.8倍。小麦化感作用引起看麦娘根系细胞膜脂氧化胁迫,并诱导看麦娘根系抗氧化物质类黄酮和总酚含量的合成。可见,降低根系活力、增强保护酶系统活性及抗氧化物质代谢是看麦娘应答小麦化感作用的生理响应。     

看麦娘叶片对化感小麦根水提液的生理响应

为阐明小麦化感抑草的生理机制,选择强化感小麦‘115/青海麦’、‘92L89’和弱化感小麦‘抗10103’,通过添加浓度为0.2%、0.5%和1.0%的小麦根水提液进行水培试验3周后,测定了各处理看麦娘的鲜重,分析叶片中叶绿素(SPAD值)、可溶蛋白、MDA、类黄酮、总酚的含量以及SOD、POD、CAT活性。结果表明,水提液处理显著抑制了看麦娘的生长,抑制率在不同处理浓度及小麦品种间均存在显著差异,强化感小麦的抑制率显著高于弱化感小麦。在处理浓度范围内,不同小麦根水提液的抑制率大小依次为‘115/青海麦’(24.7%~74.3%)‘92L89’(15.7%~71.6%)‘抗10103’(13.8%~61.4%);0.2%、1.0%和5.0%水提液处理的抑制率大小依次为13.8%~24.7%、41.7%~66.4%和61.4%~74.2%。看麦娘叶绿素含量(SPAD值)随处理浓度增大显著降低,可溶蛋白含量,SOD、POD、CAT活性,MDA、类黄酮含量随处理浓度增大显著升高,强化感小麦对看麦娘的生理刺激作用高于弱化感小麦。1.0%‘115/青海麦’及5.0%各小麦水提液处理的看麦娘总酚含量高于对照。可见,小麦化感胁迫提高了看麦娘的保护酶系统活性,增强了抗氧化物质代谢,但显著增强了细胞膜脂质过氧化和叶绿素降解,不利于靶标植物看麦娘的生长。     

蚯蚓肥对温室番茄植株抗氧化能力的影响

研究蚯蚓肥对温室番茄植株体内抗氧化能力的调节作用,为提高设施栽培条件下蔬菜抗病、抗逆能力提供理论依据和技术支撑。温室内盆栽试验,设置对照(CK)、化肥(CF)、牛粪(CM)和蚯蚓肥(EM)4个处理。观测植株表型性状(株高、茎粗、SPAD值和叶面积),并分别于番茄定植后25 d、50 d和75 d采集植株功能叶片和根系,测定防御酶活性(过氧化氢酶,CAT;过氧化物酶,POD;多酚氧化酶,PPO和超氧化物歧化酶,SOD)、根系活力及丙二醛含量(MDA)。结果表明:(1)蚯蚓肥可显著增加番茄叶面积和根系活力,对株高、茎粗、根系鲜重和叶片SPAD值以及根系丙二醛含量无显著影响;随生育期的延长,蚯蚓肥处理的根冠比呈上升趋势。(2)根系和叶片防御酶活性变化趋势相似,随生育期的延长,CAT活性呈下降趋势,POD活性呈上升趋势,PPO和SOD活性呈先升高后下降趋势。蚯蚓肥可提高番茄的CAT和POD活性,EM处理较其它处理增加21.63%～511.2%;EM处理的PPO活性较CM处理增加54.6%～163.9%,CF处理较CM和EM处理的SOD活性降低23.32%～91.67%。(3)相关性分析表明,番...     

外源脱落酸对铝胁迫下栝楼叶绿素荧光及生理活性的影响

通过比较不同栝楼品种的耐铝性差异,确定最佳缓解浓度,选取耐铝性有差异的保定栝楼和浦阳栝楼为试验材料,设置6组不同浓度的Al~(3+)和脱落酸,研究Al~(3+)和脱落酸的互作对2个栝楼品种植株的根长、叶绿素荧光特性、根系活力、MDA含量、抗氧化酶活性和抗氧化剂的含量。结果表明:(1)保定栝楼和浦阳栝楼在铝胁迫和外源脱落酸的作用下,各指标体现出相似的变化趋势,但是变化程度和敏感性显露出了一定的区别,保定栝楼具有更强的耐铝能力。(2)铝胁迫使得2个品种栝楼的根长、根系活力、叶绿素荧光参数中的F_v/F_m、F_m和ETR、抗氧化酶SOD、POD、CAT以及抗坏血酸-谷胱甘肽(AsA-GSH)循环系统中AsA、GSH含量都有所抑制,而F_0、MDA和DAsA上升。(3)外源脱落酸加入后,2个品种的抗氧化酶以及AsA、GSH均显现了先上升后下降的趋势,MDA持续降低,F_0和DAsA呈现出由降而升的趋势,其中5mg/L的外源脱落酸使栝楼的根长、根系活力、F_v/F_m、F_m、ETR、SOD、POD、CAT以及AsA、GSH上升到峰值,MDA下降,F_0和DAsA降到谷值。(4)经过隶属函数和耐铝系数的分析,2个品种都能得出相同的结果,铝胁迫下各浓度外源脱落酸对2个品种的缓解程度为5 mg/L7.5 mg/L2.5 mg/L0mg/L10mg/L。保定栝楼的耐铝能力强于浦阳栝楼,低浓度的外源脱落酸可以缓解铝胁迫,5 mg/L浓度的外源脱落酸缓解铝胁迫效果最佳,在生产实际中要选择耐铝性较强的栝楼进行栽培,并辅以适当浓度的外源脱落酸。     

锌铬复合污染对水稻根系抗氧化酶活性的影响

为了研究水稻在金属复合污染条件下的适应过程,该文就成都平原两种常见的重金属污染元素(锌、铬)对土壤复合处理后,进行水稻盆栽试验.结果表明,锌铬复合污染条件下,在水稻不同生育期,水稻根系3种抗氧化酶(SOD,POD和CAT)活性随锌、铬浓度的增加而表现出不同的变化趋势.在水稻分蘖期,随铬浓度的增加,水稻根系SOD活性和POD活性呈先降后升的变化趋势;随锌浓度的增加,水稻根系SOD活性和POD活性呈升高的趋势;而水稻根系CAT活性则随锌、铬浓度的增加呈一定的降低趋势.在水稻孕穗期,随锌、铬浓度的增加,水稻根系SOD活性呈先降后升的变化趋势,POD活性及CAT活性则呈降低的趋势.在水稻灌浆结实期,水稻根系SOD、POD及CAT活性均呈一定的降低趋势.水稻籽粒产量随锌、铬浓度的增加呈降低的趋势.锌、铬浓度对水稻籽粒产量产生了复合效应,并与水稻籽粒产量有极显著的线性回归关系.这表明水稻通过调节自身的生理代谢能提高对锌铬复合污染的生态适应性,这能为培育适合重金属污染地区生长的水稻品种提供理论基础.     

As对谷子萌发、幼苗生长及抗氧化酶系统的影响

为探究As对谷子苗期生长的影响及谷子生长的临界浓度,设置As浓度梯度分别为0,2,4,8,10mg/kg,以石英砂为基质,通过培养试验,分析探讨不同浓度As对谷子萌发、苗期生长与抗氧化酶系统的影响。研究表明:低浓度的As(4mg/kg)促进谷子萌发,高浓度的As(≥4mg/kg)抑制谷子萌发。当As浓度为2 mg/kg时,谷子的发芽势、发芽率和发芽指数在所有处理中最高,其值分别为18.67%,32.00%和19.00%。低浓度的As对谷子幼苗生长有促进作用,高浓度对其有抑制作用,当As浓度为2mg/kg,谷子的株高、根长和地上部鲜重与地下部鲜重都达到最大值,分别为15.46,7.34cm和0.64,0.16g,随着As浓度梯度增加,地下部鲜重/地上部鲜重先增加后降低,As对根系的促进与抑制作用大于地上部,谷子幼苗根部的As含量均明显高于地上部,且TF值逐渐减小;当As浓度4mg/kg时,As能促进谷子叶绿素的合成,在As浓度为2 mg/kg时,谷子的叶绿素a、叶绿素b和叶绿素总量显著高于对照28.38%,51.60%,37.06%,As浓度高于4mg/kg时,谷子的叶绿素合成受到抑制;随着As浓度梯度的增加,谷子幼苗的SOD活性不断增强,POD与CAT活性呈先增加后减小的趋势,但均高于对照,As浓度4mg/kg后,CAT活性开始减小;As浓度8mg/kg后,谷子幼苗的POD活性开始减小,表明低浓度的As(4mg/kg)对谷子萌发及幼苗生长有促进作用,高浓度的As(≥4mg/kg)抑制谷子萌发和幼苗生长,且根系对As胁迫的反应更为敏感,抗氧化酶系统(POD和CAT)活性降低。研究可为谷子的抗污染栽培和无公害生产提供科学依据。     

连作对花生根系分泌物化感作用的影响

采用连续收集法提取连作5 年、3 年和轮作处理的花生结荚期根系分泌物, 研究其对土壤微生物及花生种子发芽、幼苗生长发育和细胞膜过氧化的化感作用及连作对花生根系分泌物化感作用的影响。结果表明,花生结荚期根系分泌物对花生根腐镰刀菌36194 菌丝的生长、叶片超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)活性及丙二醛(MDA)含量存在促进作用, 对固氮菌14046 的生长, 花生种子胚根的伸长、幼苗的苗高、茎叶鲜重、根系鲜重、叶片叶绿素含量等有抑制作用, 促进和抑制作用均随根系分泌物添加浓度和连作年限的增加呈增强趋势。连作花生结荚期根系分泌物化感物质在土壤中的累积, 很可能是导致花生连作障碍的原因之一。     

综述: 植物－土壤－微生物系统中的酚酸

Phenolic acids are very common compounds in pedosphere. The objective of this review was to summarize the current knowledge of the behaviors of phenolic acids in plant-soil-microbe system. When phenolic acids originated from leaching, decomposition and exudation of living and dead plant tissues enter soils, they can react physicochemically with soil particle surfaces and/or incorporate into humic matter. Phenolic acids desorbed from soil particle surfaces and remained in solution phase can be utilized by microbe as carbon sources and absorbed by plants. The degradation products of phenolic acids by microbe include some organic and/or inorganic compounds such as new phenolic acids. In addition, phenolic acids in soils can stimulate population and activity of microbe. Phenolic acids can inhibit plants growth by affecting ion leakage, phytohormone activity, membrane permeability, hydraulic conductivity, net nutrient uptake, and enzyme activity. Behaviors of phenolic acids in soils are influenced by other organic compounds (phenolic acids, methionine, glucose, etc.) and/or inorganic ions. The role of phenolic acids as allelopathic agents should not be neglected only based on their low specific concentrations in natural soils, because numbers and interactions of phenolic acids will increase their allelopathic activities.     

生物炭–过氧化钙复合颗粒对酚酸胁迫下番茄生长和根际微生态的调控效应

【目的】酚酸胁迫是造成番茄连作障碍的主要因素之一。本研究探讨了生物炭–过氧化钙复合颗粒(简称复合颗粒)作为土壤改良剂缓解番茄酚酸胁迫的效果和机理。【方法】供试番茄品种为千禧圣女果,供试土壤为赤红壤。盆栽试验共设5个处理：常规栽培(CK),酚酸胁迫处理(T1),添加外源酚酸的同时分别添加生物炭–过氧化钙复合颗粒(T2)、生物炭颗粒(T3)、过氧化钙颗粒(T4),其中土壤酚酸胁迫浓度为140μg/g。在番茄移栽后30和120天,测定了番茄生长和生理指标。在收获期,调查果实产量与品质,分析根际土壤理化性状,并使用高通量测序技术解析番茄根际土壤中细菌和真菌的群落结构特征。【结果】施用复合颗粒(T2处理)能够增强番茄根系活力,促进植株生长发育;单株番茄果实产量、单果重和糖酸比分别比T1处理增加了13.8%、20.1%和52.6%。与T1处理相比,T2处理番茄收获期根际土壤中残余总酚酸含量下降了44.6%,电导体(EC)值下降17.7%,pH提高0.77个单位,有机质含量增加77.4%。复合颗粒处理能够改善酚酸胁迫下番茄根际土壤微生物群落结构,使细菌多样性提高,真菌多样性降低,并有效恢复微生物群落...     

西芹根物质浸提液处理后黄瓜叶片内几种氮代谢物质含量的变化

以黄瓜品种"津春4号"为材料,在黄瓜植株第1片真叶长至宽5 cm时,使用西芹鲜根及根际区物的乙醇、丙酮和蒸馏水浸提液对黄瓜植株进行化感处理(灌根),测定处理后黄瓜叶片内几种氮代谢物质含量的变化,从氮代谢角度研究西芹鲜根及根际区物浸提液处理后黄瓜植株对黄瓜枯萎病菌的化感抑制作用机理。结果表明:处理后黄瓜叶片内可溶性蛋白质、氨、丝氨酸、精氨酸、脯氨酸与总氨基酸含量均高于对照,含量达到峰值时分别较丙酮对照增加10.58%、18.31%、6.54%、3.10%、9.64%和3.16%,而硝酸盐、甘氨酸和蛋氨酸含量则低于对照,含量达到最低值时分别较丙酮对照降低7.52%、3.41%和15.55%。对于不同西芹根物质浸提液处理,丙酮浸提液处理的氮代谢物质含量变化最显著,乙醇浸提液处理次之,蒸馏水浸提液处理变化最低,西芹鲜根浸提液处理的氮代谢物质含量变化高于西芹根际区物浸提液处理。     

Sorption–desorption of phenolic acids as affected by soil properties

Phenolic acids have been implicated in the process of allelopathy and are, therefore, of interest in plant management as a basis for new herbicide structures. The potential bioavailability of phenolic acids is controlled by sorption–desorption processes in soil. Sorption–desorption of p-coumaric acid (4-hydroxycinnamic acid), ferulic acid (3-methoxy-4-hydroxycinnamic acid), veratric acid (3,4-dimethoxybenzoic acid), vanillic acid (3-methoxy-4-hydroxybenzoic acid), and p-hydroxybenzoic acid (4-hydroxybenzoic acid) was characterized on soils with varying physicochemical properties. The phenolic acids sorbed quickly (<8 h) and in high proportions to the amount applied (average 84% of applied was sorbed). Sorption was irreversible with the batch desorption method used (0.01 N CaCl₂ extraction). Pretreatment of soils to remove organic matter and free metal oxides from the soils decreased sorption, particularly in soils with free oxides removed. Statistical analysis suggested that sorption of p-coumaric and ferulic acids was correlated with soil clay content and veratric acid sorption was correlated with several soil factors. In contrast, no consistent relationship between soil characteristics and vanillic and p-hydroxybenzoic acid sorption was found. Based on the results of these experiments, i.e. the high reactivity of the phenolic acids, it is unlikely that these chemicals would be transported far from their point of origin, limiting their range of influence. It appears that, for phenolic acids to have allelopathic effects on plants, they would have to persist in the soil for long periods, resulting in a buildup of the chemical to high enough levels so that sufficient chemical would be in solution to cause the allelopathic effects, even though they may be strongly sorbed at lower concentrations.     

Sorption of benzoic acid onto soil colloids and its implications for allelopathy studies

The fate of allelochemicals in the soil environment largely determines the expression of allelopathy in the natural environment. In allelopathy research, the sorption of allelochemicals onto soil particles has been less well studied than their degradation. A study was carried out to evaluate the growth of cucumber (Cucumis sativus var Marketmore 76) and radish (Raphanus sativus var Crimson giant) in soil amended with 1, 5, 10 and 20 mg l^–1 benzoic acid as model allelopathic substance. Growth of both cucumber and radish was not inhibited in soil amended with benzoic acid. A labeled study indicates that sorption of benzoic acid onto soil particles increases with concentration. Benzoic acid isotherms of both soils were non-linear, with an N value of 0.875 for a garden soil and 0.891 for a garden soil + sand, and they may explain the reason for the limited allelopathic effect of benzoic acid at concentrations often recorded in natural soil.     

Potassium uptake of rye-grass (Lolium perenne) and red clover (Trifolium pratense) as related to root parameters

Summary Rye-grass (Lolium perenne) is known to be a strong competitor to red clover (Trifolium pratense) for soil K⁺ under conditions of low K availability in the soil. The objective of this study was to clarify whether this competitive behaviour of the two species can be explained by root morphology. Total K⁺ uptake ofL. perenne andT. pratense was studied under field conditions in relation to root fresh weight, root density, root cation exchange capacity, root surface and root length. The soil was an Alfisol, Udalf. All root parameters, when calculated per unit soil surface (M²), were higher inL. perenne than inT. pratense. In addition,L. perenne had longer root hairs and a denser root hair system thanT. pratense. The greatest difference in root morphology between species was root length, withL. perenne roots averaging 4–6 times longer than those ofT. pratense.Significant correlations were found between the total K⁺ uptake and all root parameters examined, with highest correlationsforroot fresh weight (r,0.92***T. pratense; 0.94***L. perenne) and root length (r, 0.91***T. pratense;r, 0.93***L. perenne). Potassium uptake per unit root fresh weight, root surface and root length were all significantly higher forT. pratense than for L. perenne. Differences in the rate of K⁺ uptake between species were particularly high when expressed per unit root length. Because of its greater root length and surface area,L. perenne can take up more soil K⁺ thanT. pratense, particularly where there is a low K^– supply in the soil. Under such conditionsL. perenne will be a particularly strong competitor toT. pratense.     

Responses of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">niveum</Emphasis> to exogenously added sinapic acid in vitro

To assess the influence of phenolic acids from plant root exudates on soil pathogens, we studied the effect of sinapic acid added to chemically defined media on the growth and virulence factors of Fusarium oxysporum f. sp. niveum. Sinapic acid inhibited the growth and conidial formation and germination of F. oxysporum f. sp. niveum by 6.7–8.8% and 11.2–37.3%, respectively. Mycotoxin production by F. oxysporum f. sp. niveum was stimulated by 81.6–230.7%. Pectinase, proteinase, cellulase, and amylase activities were stimulated at a lower concentration of sinapic acid, while they were inhibited at a higher concentration. It is concluded that sinapic acid inhibited the growth and conidial germination of F. oxysporum f. sp. niveum and decreased the pathogenic enzymes’ activity at higher doses.     

Soil sickness of peanuts is attributable to modifications in soil microbes induced by peanut root exudates rather than to direct allelopathy

The quantity and quality of peanut yields are seriously compromised by consecutive monoculture in the subtropical regions of China. Root exudates, which represent a growth regulator in peanut–soil feedback processes, play a principal role in soil sickness. The growth inhibition of a species in an in vitro bioassay enriched with root exudates and allelochemicals is commonly viewed as evidence of an allelopathic interaction. However, for some of these putative examples of allelopathy, the results have not been verified in more natural settings with plants continuously growing in soil. In this study, the phenolic acids in peanut root exudates, their retention characteristics in an Udic Ferrosol, and their effects on rhizosphere soil microbial communities and peanut seedling growth were studied. Phenolic acids from peanut root exudates were quickly metabolized by soil microorganisms and did not accumulate to high levels. The peanut root exudates selectively inhibited or stimulated certain communal bacterial and fungal species, with decreases in the relative abundance of the bacterial taxa Gelria glutamica, Mitsuaria chitosanitabida, and Burkholderia soli and the fungal taxa Mortierella sp. and Geminibasidium hirsutum and increases in the relative abundance of the bacterial taxon Desulfotomaculum ruminis and the fungal taxa Fusarium oxysporum, Bionectria ochroleuca and Phoma macrostoma. The experimental application of phenolic acids to non-sterile and sterile soil revealed that the poor performance of the peanut plants was attributed to changes in the soil microbial communities promoted by phenolic acids. These results suggest that pathogenic fungal accumulation at the expense of such beneficial microorganisms as plant growth promoting rhizobacteria, mycorrhizal fungi induced by root exudates, rather than direct autotoxicity induced by root exudates, might represent the principal cause underlying the soil sickness associated with peanut plants. We hope that our study will motivate researchers to integrate the role of soil microbial communities in allelopathic research, such that their observed significance in soil sickness during continuous monocropping of fields can be further explored.     

Control of pitch pine seed germination and initial growth exerted by leaf litters and polyphenolic compounds

We investigated the influence of three concentrations of water extracts of three leaf litter species (pitch pine, huckleberry and white oak) and a mixture of all litters on the germination of pitch pine seeds and initial seedling growth in a microcosm experiment. All three plant species are important components of the pine barrens ecosystems in New Jersey, where it has been seen that pine seedling recruitment occurs only after stand replacing fire or in disturbed sites, where surface organic soil horizons and leaf litter have been removed. Leaf litter extracts did not influence seed germination, but significantly reduced seedling growth at high concentrations. There was little difference between the leaf litter species in growth suppression. As charcoal is a natural residue on the forest floor following fire, its influence on growth suppression was examined; it has been shown to immobilize polyphenols. Charcoal removed the inhibitory effect of leaf litter extracts and allowed the fertilizer effect of nutrients leached from the leaves to enhance seedling growth, particularly at the higher concentration of litter extract used. Responses to litter extracts were compared to four pure phenolic compounds, catchecol, p-coumaric acid, p-hydroxybenzoic acid and tannic acid. None of these compounds suppressed pine seedling growth, suggesting that these phenolics are not allelopathic to pine seedlings. The results are discussed in the context of fire as a driving factor in these oligotrophic and seasonally dry ecosystems and the interactions between nutrient supply and allelopathic chemistry of different leaf litters.