Cover crop root contributions to soil carbon in a no‐till corn bioenergy cropping system

Crop residues are potential biofuel feedstocks, but residue removal may reduce soil carbon (C). The inclusion of a cover crop in a corn bioenergy system could provide additional biomass, mitigating the negative effects of residue removal by adding to stable soil C pools. In a no‐till continuous corn bioenergy system in the northern US Corn Belt, we used ¹³CO₂ pulse labeling to trace plant C from a winter rye (Secale cereale) cover crop into different soil C pools for 2 years following rye cover crop termination. Corn stover left as residue (30% of total stover) contributed 66, corn roots 57, rye shoots 61, rye roots 50, and rye rhizodeposits 25 g C m^?2 to soil. Five months following cover crop termination, belowground cover crop inputs were three times more likely to remain in soil C pools than were aboveground inputs, and much of the root‐derived C was in mineral‐associated soil fractions. After 2 years, both above‐ and belowground inputs had declined substantially, indicating that the majority of both root and shoot inputs are eventually mineralized. Our results underscore the importance of cover crop roots vs. shoots and the importance of cover crop rhizodeposition (33% of total belowground cover crop C inputs) as a source of soil C. However, the eventual loss of most cover crop C from these soils indicates that cover crops will likely need to be included every year in rotations to accumulate soil C.     

Pathogenicity of Rhizoctonia solani and Phytophthora nicotianae to Brassicaceae cover crops

Soil-borne diseases can reduce nursery crop performance and increase costs to nursery producers. In particular, soil-borne diseases caused by Phytophthora nicotianae and Rhizoctonia solani are the most economically important problems of Southeastern United States nursery producers. Methyl bromide was widely used as a standard treatment for management of soil-borne diseases until the implementation of the Montreal protocol. Since then, many chemical and non-chemical soil-borne disease management methods have been tested, but are not yet providing effective and consistent results like methyl bromide. Cover crops that belonged to the Brassicaceae family can be incorporated into the soil to control soil-borne diseases and this process is widely known as biofumigation. But, the use of Brassicaceae cover crops has not been widely explored as a method of controlling soil-borne diseases in woody ornamental nursery production. The objective of this study was to evaluate Brassicaceae cover crops for susceptibility to most destructive soil-borne pathogens of nursery production, P. nicotianae and R. solani, to identify effective cover crops that can be used in the biofumigation process in woody ornamental nursery production. Brassica species intended to be used in the fresh market or biofimigation were screened for their susceptibility to R. solani and P. nicotiane in an environmentally controlled greenhouse. At the end of experiments, plant growth data (plant height, width and fresh weight), total damping-off were recorded, and cover crop root systems were assessed for disease severity using a scale of 0–100% roots affected. Among the tested 15 cover crops in the Brassicaceae family, oilseed radish (Raphanus sativus L.), yellow mustard “White Gold” (Sinapis alba L.), turnip “Purple Top Forage” (Brassica rapa L.), arugula “Astro” (Eruca vesicaria (L.) Cav. ssp. sativa (Mill.) Thell.), mighty mustard^® “Pacific Gold” (B. juncea (L.) Czern.), brown mustard “Kodiak” (B. juncea (L.) Czern.), rape “Dwarf Essex” (B. napus L.) and mustard green “Amara” (B. carinata A. Braun) showed numerically lower root rot disease severity and total damping-off in topsoil which had pre-existing populations of R. solani or P. nicotinanae compared to other cover crops. Since these above mentioned Brassicaceae crops shows the ability to withstand the higher disease pressure from R. solani and P. nicotinanae under the greenhouse conditions they can be used in the further experiments to evaluate their ability in biofumigation. Further research is necessary to evaluate the performance of these cover crops under the field conditions.     

Induction and origin of adventitious roots from chimeras of <Emphasis Type="Italic">Brassica juncea</Emphasis> and <Emphasis Type="Italic">Brassica oleracea</Emphasis>

Adventitious roots were induced from shoots and leaves of the chimera plant TCC (LI-LII-LIII = TCC; T = Tuber mustard, C = Red Cabbage), previously developed by in vitro grafting of tuber mustard (Brassica juncea) and red cabbage (B. oleracea). The regeneration frequency of adventitious roots from TCC shoots and leaf sections was markedly higher than that obtained from the parents TTT (tuber mustard) and CCC (red cabbage). Moreover, levels of α-naphthaleneacetic acid in the culture medium had lower effects on rooting efficiency of TCC chimeras compared to those of TTT and CCC. The number and fresh weight of adventitious roots per TCC shoot, 13.11 roots and 0.274 g, respectively, were also significantly higher than those of the parents. This demonstrated that replacing the histogenic LI layer (the outermost apical cell layer) with a different genotype might improve adventitious root induction capability of these vegetative tissues due to likely synergistic effects between LI and the other two histogenic layers, LII and LIII. Following polymerase chain reaction analysis and histological investigation, it was found that these adventitious roots originated from the LIII histogenic layer.     

土壤灭菌-生物有机肥联用对连作马铃薯及土壤真菌群落结构的影响

甘肃省中部沿黄灌区是全国重要的加工型马铃薯生产基地,然而因集约化生产带来的连作障碍问题已经严重影响到当地马铃薯种植业的健康发展。结合田间试验和相关的室内分析,从马铃薯块茎产量和品质、植株生理特征和土壤真菌群落结构等角度,初步评估土壤灭菌和生物有机肥联用(Ammonia Disinfection plus Bio-organic Fertilizer Regulation,ABR)对马铃薯连作障碍的防控效果。同对照相比,ABR处理的块茎产量和商品薯率分别显著增加约71.1%—152.1%和39.2%—53.3%,但块茎化学品质变化不大。ABR处理叶绿素含量和根系活力较CK均显著增加,而叶片和根系丙二醛含量显著下降。PCR-DGGE分析发现,ABR处理显著影响了马铃薯连作土壤的真菌群落结构,表现为真菌群落的多样性指数较CK相比显著下降。ABR处理还有效抑制了土传病害的滋生,植株发病率和收获后的病薯率较CK分别显著下降约67.2%—82.2%和69.1%—70.5%。采用Real-time PCR评估连作土壤中3种优势致病真菌的数量变化,显示ABR处理下立枯丝核菌、茄病镰刀菌和接骨木镰刀菌的数量在生育期内较CK均有不同程度的下降。综合来看,土壤灭菌和生物有机肥联用技术在防控甘肃省中部沿黄灌区马铃薯连作障碍上具有较大的应用潜力,而对土传病害的抑制和微生物群落结构的改善是其主要的作用机理。     

Dieback of Passion Fruit in Surinam

In Surinam, the commercial cultivation of the yellow passion fruit (Passiflora edulis f. flavicarpa) is difficult due to the occurrence of dieback. Symptoms referred to as dieback include a decrease in elongation of the shoot end internodes after a period of normal growth leading to wilting and death of the shoots. Fruits from plants showing dieback symptoms are much smaller than those from healthy plants. From shoots with dieback symptoms, three fungi were isolated including Colletotrichum gloeosporioides. However, inoculation experiments with these fungi on shoots of vigorously growing plants were negative, even after wound inoculation. It appeared that plants with dieback symptoms had a poorly developed root system, From these roots Fusarium solani was isolated, which appeared to be highly pathogenic to roots of the yellow passion fruit. After inoculation of the roots of 3-month-old plants, roots became infected and the aerial plant parts showed typical dieback symptoms. Plants with their root system reduced either by inoculating with F. solani or by clipping, and subsequently inoculated with C. gloeosporioides on the aerial parts 2 weeks later, showed dieback symptoms and infection by C. gloeosporioides in shoots with these symptoms. Thus, a badly functioning root system, for example caused by infection of F. solani leads to dieback and predisposes plants to infection by C. gloeosporioides. The latter fungus itself is not a primary pathogen of shoots of the yellow passion fruit in Surinam.     

Soil aggregate sequestration of cover crop root and shoot-derived nitrogen

Cover crop roots and shoots release carbon (C) and nitrogen (N) compounds in situ during their decomposition. Depending upon the season, these C and N compounds may be sequestered, the C may be respired or the N may be leached below the root zone. A field study was established to identify the contributions of cover crop root and shoot N to different regions within aggregates in the A_p horizon of a Kalamazoo loam soil. Fall-planted rye plants (Secale cerealeL.) were labeled the next May with foliar applications of solutions containing 99% atom (¹⁵NH₄)₂SO₄. Isotopic enrichment of soil aggregates ranging from 2.0 to 4.0, 4.0–6.3 and 6.3–9.5 mm across was determined following plant residue applications. Concentric layers of aggregates were removed from each aggregate by newly designed meso soil aggregate erosion (SAE) chambers. Non-uniform distributions of total N and recently derived rye N in soil macroaggregates, across time, suggested that the formations and functions of macroaggregates are very dynamics processes and soil aggregates influence where N is deposited. Early in the season, more ¹⁵N migrated to the interior regions of the smallest aggregates, 2–4 mm across, but it was limited to only surfaces and transitional regions of the larger aggregates, 6.3–9.3 mm across. Exterior layers of aggregates between 6.0 and 9.5 mm retained 1.6% of the N_{derived from roots} in July 1999, which was three times more than their interior regions. This was slightly greater than the % N_{derived from shoot.} One month later, as the maize root absorption of N increased rapidly, % N_{derived from roots} and % N_{derived from shoot} were nearly equal in exterior layers and interior regions of soil aggregates. This equilibrium distribution may have been from either greater diffusion of N within the aggregates and/or maize root removal form aggregate exteriors. Results supported that most of roots grew preferentially around surfaces of soil aggregates rather than through aggregates. Cover crop roots contributed as much N as cover crop shoots to the total soil N pool. Subsequent crops use N from the most easily accessible zones of soil structure, which are surfaces of larger soil aggregates. Therefore maintaining active plant roots and aggregated soil structure in the soil enhances N sequestration and maximize soil N availability. These studies suggest that the rapid and perhaps bulk flow of soil N solutions may bypass many of the central regions of soil aggregates, resulting in greater leaching losses.     

Variation in structure and plant regeneration of Agrobacterium rhizogenes transformed and control roots of the potato cv. Bintje

Agrobacterium rhizogenes transformed and control roots of the tetraploid potato cv. Bintje were compared. Transformed roots were obtained after infection by A. rhizogenes 15834 or 1855. Both in leaf and stem segments, more roots were formed at the basal side of the segments, indicative for a polarity in root formation. As compared to control roots the transformed roots are characterized by smaller and more densely stained cells, a zone of cell division, and smaller statoliths. These characteristics are correlated with vigorous growth, high branching incidence and diminished geotropism. The plant regeneration procedure according to Ooms et al. [1] was modified. The transformed roots required less 2,4-D than control roots for the induction of shoot-competent calli. The callus and shoot induction phases were reduced from 8 and 6 weeks to 3 and 3 weeks, respectively. Upon induction, 25%, 58% and 61% of the root clones originating from tuber, stem and leaf, respectively, produced shoots, whereas all of the control roots produced shoots. Shoot outgrowth occurred on liquid MS medium in the absence of hormones.Abbreviations Ri-root Agrobacterium rhizogenes transformed root - BAP benzylaminopurine - IAA indoleacetic acid - GA₃ gibberellic acid - NAA naphthaleneacetic acid - 2,4-D 2,4 dichlorophenoxyacetic acid     

Development of formulations based on Streptomyces rochei strain PTL2 spores for biocontrol of Rhizoctonia solani damping-off of tomato seedlings

Rhizoctonia solani is one of the most problematic soil-borne pathogenic fungi for several crop cultures worldwide. This study highlights the effectiveness of high-antagonistic Streptomyces rochei strain PTL2, isolated from root tissues of Panicum turgidum, in controlling the R. solani damping-off and growth promotion of tomato (cv. Marmande) seedlings. The isolate PTL2 was characterised for in vitro biocontrol and plant growth-promoting traits. It exhibited remarkable positive results in all trials, including production of hydrogen cyanide, siderophores, 1-aminocyclopropane-1-carboxylate deaminase and phytohormones, chitinolytic activity and inorganic phosphate solubilisation. PTL2 spores were formulated as wettable talcum powder, sodium alginate pellets and sodium alginate-clay pellets. Their abilities in the biocontrol of R. solani and plant growth promotion were investigated in autoclaved and non-autoclaved soils. Talcum powder and sodium alginate pellets significantly reduced the damping-off severity index compared to a positive control. The talcum powder exhibited the highest protective activity, reducing the disease incidence from 89.3% to 14.1%, whereas chemical seed treatment with Thiram^® provided a disease incidence of 16.7%. Furthermore, the talc-based powder formulation resulted in greatest increases in the root length, shoot length and dry weight of seedlings. The interesting biocontrol potential and growth enhancement of tomato seedlings open up promising perspectives for the possible application of talcum powder formulation based on PTL2 spores in crop improvement.     

The effect of soil- and tuber-borne inoculum on the incidence of potato gangrene

Under optimum growing conditions neither tuber- nor soil-borne Phoma exigua var. foveata inoculum appreciably affected stand or yield of the subsequent potato crop. Seed tubers with gangrene rots caused high levels of stem and tuber symptoms when planted in var. foveata contaminated or uncontaminated land; contaminated seed tubers with no rots also produced progeny with a high gangrene potential. Sufficient soil-borne inoculum was carried over in land that produced a gangrene affected crop in the previous year to override the effect of tuber disinfection. Effective gangrene control was achieved by a combination of tuber disinfection shortly after harvest over successive years with a 1 in 5 yr potato crop rotation. Gangrene rots usually developed through injuries to the tuber periderm, rots in other tubers being associated with pustules of powdery scab (Spon-gospora subterranea).     

类芽孢杆菌QHZ11-gfp在马铃薯植株上的定殖特征及促生效果

[背景] 生防菌在作物根系的有效定殖是其功能发挥的前提,而直观的跟踪技术和有效的定量方法是研究生防菌根系分布规律的重要工具。[目的] 研究马铃薯黑痣病病原菌立枯丝核菌（Rhizo ctonia solani） JT18的拮抗菌QHZ11在马铃薯植株上的定殖特征及对马铃薯的促生效果。[方法] 采用绿色荧光蛋白（Green Fluorescent Protein,GFP）对QHZ11进行标记,将标记菌株菌悬液、生物有机肥和无菌水分别接种至灭菌土壤,通过激光共聚焦显微技术和实时荧光定量PCR等方法观察和测定标记菌株在马铃薯植株不同部位的定殖特征、数量变化及对马铃薯的促生效果。[结果] pHAPII质粒成功导入QHZ11并可稳定遗传40代,记为QHZ11-gfp;菌株标记前后的菌落形态、生长曲线和对R.solani JT18的拮抗能力等基本一致。从第7天开始,相继在马铃薯芽上和根上发现了绿色荧光,说明QHZ11-gfp成功定殖到了马铃薯的芽、根等部位。QHZ11-gfp在根系和匍匐茎的定殖数量均呈现先升高至块茎形成期达到峰值后下降的趋势,并且在整个生育期根系的定殖数量始终大于匍匐茎。菌悬液和生物有机肥处理均显著促进了马铃薯根系的生长,并通过增加株高等农艺性状提高了块茎产量。其中,生物有机肥处理在各部位的荧光强度、定殖数量和对马铃薯的促生效果均显著优于菌悬液。[结论] QHZ11-gfp可在马铃薯植株上成功定殖并对马铃薯有良好的促生效果,将其制成生物有机肥促进了其定殖,使促生效果也更好。     

Strategy differences of two potato species in response to nitrogen starvation. Do plants have a genetic switch for nitrogen signalling?

Survival responses to nitrogen starvation are well known in micro‐organisms but little studied in plants. To construct a framework for study of the plant responses, we investigated the strategy differences of tubers from two closely related potato species. Solanum tuberosum conserves tuber nitrogen by inhibiting shoot growth, but S. phureja mobilizes tuber nitrogen to grow shoots, flowers and seeds. Genetic analysis of progeny from S. phureja–haploid S. tuberosum crosses uncovered segregation of a single dominant gene for the S. tuberosum inhibition strategy. Within S. tuberosum, haploid progeny closely resembled their tetraploid parents, suggesting strong genetic control of the inhibition. Growth of the inhibited shoots was proportional to sub‐optimal levels of added nitrate, and was triggered by exogenous gibberellic acid (GA₃). These observations support the notion that potato plants can closely tie shoot growth to ambient nitrogen levels – probably by a root–shoot nitrogen signal transduction pathway, and that this can be overridden by emergency mobilization of nitrogen reserves, perhaps by GA signalling from the tuber. Furthermore, genes for such developmental switches can be identified by classical genetic analysis of closely related species, such as S. tuberosum and S. phureja, that exhibit opposite survival strategies.     

Zinc fertilisation increases tolerance to Rhizoctonia solani (AG 8) in Medicago truncatula

The effect of Zn fertilisation on tolerance of Medicago truncatula to infection by the root-rotting pathogen Rhizoctonia solani (AG 8) was studied in a field survey and in two experiments in controlled conditions. From the field survey, the concentration of Zn in the shoots of medics was found to be inversely related to the severity of disease on the root. Overall, the addition of Zn to Zn-deficient soil in controlled environment experiments resulted in reduced yield loss in the presence of R. solani, a reduction in disease score and no change in the concentration of nutrients in the shoots. However, under Zn deficiency, increasing levels of added R. solani resulted in significant yield loss, an increase in disease score and a reduction in concentration of Zn in the roots. This occurred despite a decrease in the number of infection sites caused by the fungus on the root and a lower amount of R. solani DNA extracted in medics deficient in Zn compared with plants supplied with Zn. While plants supplied with Zn were able to maintain a stable concentration of Zn in the shoots, the concentration of Zn in the roots also declined with increasing levels of R. solani. In conclusion, Zn application does not directly inhibit infection by R. solani, nor reduce its pathogenicity, but it does strongly increase root growth. The net result is that Zn-sufficient plants are more tolerant to the effects of root pruning by the fungus than Zn-deficient plants.     

Isolation and Pathogenicity of Rhizosphere Fungi of Cocoyam in Relation to Cocoyam Root Rot Disease

Cocoyam is the second most important staple crop of Cameroon and root rot is a destructive disease of this plant. Pythium myriotylum (Pm), Fusarium solani (Fs), and Rhizoctonia solani (Rs) were isolated from the rhizosphere of root rot affected cocoyams and from the soil of a cocoyam experimental field plot temporarily devoid of same in Mamu, Cameroon. Pm was isolated from the above soil by the cocoyam leaf disc baits. Fs and Rs were also isolated from the same soils by the water dilution method and from the roots of diseased cocoyams but were always associated with mycelial growth of Pm. Pathogenicity of Pm and in combinations with Fs or Rs or Fs + Rs all developed cocoyam root rot disease (CRRD) symptoms on 3– and 7–month old cocoyam plantlets 2–7 days after inoculation. Symptoms included rotted roots and wilting with general chlorosis of inoculated plantlets. No symptoms of CRRD were noted on cocoyam plantlets inoculated with Fs, Rs, Fs + Rs, and distilled water. Results indicated that CRRD is not caused by several pathogens but only by Pm. Pm isolates from the soils and roots of diseased cocoyams and those maintained in the ROTREP laboratory have significantly bigger diameter of mycelial colony growth in 24 h–period at 31 °C on lima bean sucrose agar, V–8 juice sucrose agar, and potato sucrose agar than on potato dextrose agar and 2 % water agar. The cocoyam plantlets were raised axenically from tissue culture of explants in the laboratory.     

Effect of Alternaria solani culture filtrate on adventitious shoot regeneration in potato

Summary The effect of Alternaria solani culture filtrate on adventitious shoot regeneration from tuber discs was evaluated using five potato cultivars, which were selected based on their field reaction to Alternaria solani and which represented a range of disease reactions. The culture filtrate stimulated regeneration, a response that could prove to be very useful in the wider utilization of transformation and in vitro selection technology.Research conducted at the Scottish Crop Research Institute during a transfer of work of the senior author     

Effect of potato used as a trap crop on potato cyst nematodes and other soil pathogens and on the growth of a subsequent main potato crop

A field experiment in which main‐crop potatoes were grown every other year was conducted on a sandy soil from 1994 to 1999. The aim of the experiment was to control soil‐borne pathogens of potato with ecologically sound methods. Potato grown as a trap crop from the end of April to the end of June (8 wk) was used to control potato cyst nematodes (PCN) (Globodera pallida), and its effects on other important soil pathogens and on the growth of a subsequent potato crop were also assessed. Additional experimental treatments were a potato crop from which the haulm was removed and a green manure crop. Three potato cultivars with different degrees of resistance to PCN were grown as the main crop. Duplicate sets of the experiment were run concurrently. The PCN were effectively controlled by the potato trap crop. When a highly resistant potato cultivar was grown as a main crop after the trap crop, the post‐harvest soil infestation was very low. When a moderately resistant cultivar was grown after the trap crop the soil infestation also remained low. When the trap crop was alternated with a susceptible potato cultivar as a main crop, soil infestation increased slightly, but the degree of control when compared with no trap crop averaged 96%. Soil infestation with root‐knot nematodes (mainly Meloidogyne hapla) increased when potato was grown as a trap crop, but soil infestation with the root‐lesion nematode Pratylenchus crenatus was not affected. Stem canker caused by Rhizoctonia solani was not affected by the trap crop but black scurf (sclerotia of R. solani) on tubers was reduced. Soil infestation with Verticillium dahliae declined in one of the duplicate sets of the experiment but not in the other. However, stem infections by V. dahliae were significantly decreased in both sets, although the effect depended on the PCN‐resistance level of the potato cultivar. When a highly resistant potato cultivar was grown Verticillium stem infections were not significantly affected, they were decreased with a moderately resistant cultivar but the decrease was most pronounced with a PCN‐susceptible cultivar. Senescence of a following potato crop was not influenced by the trap crop when a highly PCN‐resistant cultivar was grown, but it was delayed in the case of a moderately resistant or a susceptible cultivar, resulting in higher tuber yields for those cultivars. The experiment proved that a trap crop can be an alternative to chemical soil disinfection but, for several reasons, the potato itself is not an ideal crop for this purpose; a trap crop other than potato must be developed.     

Tuberization in Potato at High Temperatures: interaction between Shoot and Root Temperatures

Tuber formation in intact potato plants (Solanum tuberosum L.cv. Sebago) was reduced by high shoot or root temperatures andstrongly inhibited when both were high. When both the shootand root temperatures were high, disbudding strongly promotedtuberization. There was a smaller increase with warm roots andcool shoots, but no response with warm shoots and cool roots.When both the shoots and roots were cool, disbudding reducedtuberization. Exogenous GA₃, effectively substituted for thebuds at high temperatures, completely preventing tuberization.In apical cuttings, removal of the terminal bud, but not theroots, reduced the inhibitory effects of high temperatures ontuberization. The experiment indicates that tuber productionmay be controlled by at least three factors: a promoter, whichis not assimilate, produced by the buds at cool temperatures;an inhibitor, derived from the buds, but dependent on warm roottemperatures for its formation; and a second inhibitor derivedfrom the mature leaves and produced in response to warm shoottemperatures. Solanum tuberosumL, potato, tuberization, temperature, disbudding, gibberellic acid     

甘肃省中部沿黄灌区轮作和连作马铃薯根际土壤真菌群落的结构性差异评估

甘肃省中部沿黄灌区是西北地区乃至全国重要的加工型马铃薯生产基地,然而因集约化种植带来的连作障碍问题已经严重影响到当地马铃薯种植业的可持续发展。采用大田试验与PCR-DGGE(Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis)技术相结合的方法,并通过真菌的18S r DNA序列分析,评估轮作(未连作)和连作条件下马铃薯根际土壤真菌群落在组成结构上的差异,以期为甘肃省中部沿黄灌区马铃薯连作的土壤障碍机理研究提供新证据。结果表明,同轮作相比,连作显著降低了马铃薯块茎产量和植株生物量,并且随着连作年限的延长,连作障碍也愈加严重。长期连作(6a)也导致马铃薯根冠比显著增加和植株收获指数的显著下降。在根际土壤真菌的种群数量和多样性上,连作和轮作间无显著差异,但在群落组成结构上差异明显。真菌18S r DNA测序分析进一步表明,马铃薯连作较轮作相比增加了Fusarium sp.和Fusarium solani以及Verticillium dahliae的种群或个体数量,而这些真菌是导致马铃薯土传病害的主要致病菌类型。根际土壤真菌群落组成结构的改变特别是与土传病害有关的致病菌滋生可能是导致当地马铃薯连作障碍的重要原因。     

Hyperaccumulation of Lead by Roots, Hypocotyls, and Shoots of Brassica juncea

The effects of different concentrations of lead nitrate (10^–5 to 10^–3 M) on root, hypocotyl, and shoot growth of Indian mustard (Brassica juncea L. var. megarrhiza), and the uptake and accumulation of Pb²⁺ by its roots, hypocotyls, and shoots were investigated. Lead had no significant inhibitory effect on the root growth at concentrations of 10^–5 to 10^–4 M during the entire treatment, while at 10^–3 M, Pb slightly inhibited the root and shoot growth. B. juncea has ability to take up Pb from solutions and accumulate it in its roots, and transport and concentrate it. The Pb contents in the parts of plants treated with 10^–3 M Pb were greater than those of untreated plants, by factors of 230 in the roots, 170 in the hypocotyls, and 3 in the shoots.     

类芽孢杆菌QHZ11对马铃薯黑痣病的生防效果

[背景] 马铃薯黑痣病是由立枯丝核菌（Rhizoctonia solani）引起的一种典型土传病害,目前该病害生物防治的菌种资源比较有限,相应菌株生防机制的研究更是缺乏。[目的] 明确马铃薯黑痣病病原菌立枯丝核菌（R. solani） JT18的拮抗菌QHZ11对马铃薯黑痣病的生防效果,揭示QHZ11对黑痣病的部分防治机理。[方法] 在灭菌土壤中分别接种R. solani JT18（CK）,R. solani JT18和普通有机肥（Organic Fertilized,OF）,R.solaniJT18和氨基酸有机肥（AA+OF）及R. solani JT18和QHZ11生物有机肥（BOF11）,结合实时荧光定量PCR （Real-Time Fluorescence Quantitative PCR,RT-qPCR）等方法,研究马铃薯全生育期不同处理R.solaniJT18在马铃薯根际和植株不同部位的数量变化及拮抗菌QHZ11与R.solaniJT18的数量消长规律,同时比较不同处理黑痣病的病情指数及相应的防效。[结果] RT-qPCR结果表明,随马铃薯生育进程的推进,马铃薯根际、根系和匍匐茎R.solaniJT18的数量在各处理中均呈现先升高至块茎膨大期到达峰值后下降的趋势,而且各部位R.solaniJT18的数量为CK>OF>AA+OF>BOF11且根际>根系>匍匐茎;拮抗菌QHZ11的数量变化趋势与R.solaniJT18相同,但峰值在块茎形成期,并且同时期同一部位QHZ11的定殖数量均显著高于R.solaniJT18,甚至高出2个数量级,说明QHZ11占用了一定的营养资源和生态位点,严重抑制了R.solaniJT18的生长和繁殖。病情结果表明：CK病情指数最高,OF、AA+OF和BOF11处理均显著低于CK,其中BOF11处理发病最轻;生防结果则相反,为BOF11>AA+OF>OF处理,说明普通有机肥、氨基酸有机肥及生物有机肥均可不同程度地防治马铃薯黑痣病,其中以生物有机肥效果最显著。[结论] QHZ11以有机肥为载体施入土壤后,可以通过在马铃薯根际及植株不同部位竞争营养和生态位点,从而有效抑制黑痣病病原菌R.solaniJT18的生存和繁殖,起到显著的生防效果,这对QHZ11生物有机肥的应用和推广具有重要意义,并为进一步研究QHZ11的生防机制奠定了基础。     

Developmental Regulation of Susceptibility and Tolerance to Fusarium Root Rot in Beans

The soil-borne fungus, Fusarium solani f. sp. phaseoli, attacks roots and hypocotyls of bean (Phaseolus vulgaris) plants causing a devastating disease called root and foot rot. In a study of the host-pathogen relationship it was found that young bean roots, with the radicle just emerging, were highly tolerant to the pathogen, whereas older bean seedlings, with a fully developed root system, were completely susceptible. Investigations by low-temperature scanning electron microscopy demonstrated that significantly fewer spores and hyphae were present on the root surface of young bean seedlings as compared to older ones. A similar pattern of attachment was found when bean roots were inoculated with spores of F. solani f. sp. pisi, a related pathogen causing disease on peas but not on beans. Light microscopic studies showed that F. solani f. sp. pisi did not penetrate the root but rapidly formed thick-walled resting spores on the root surface. F. solani f. sp. phaseoli on the other hand quickly penetrated the root and formed an extensive network of fungal hyphae. These results demonstrate that the ability of fungal propagules to adhere to and to penetrate host tissues are two distinct processes. Furthermore, the data indicate that young bean roots lack a surface component necessary for attachment of fungal spores which may help explain their tolerance to Fusarium root rot.