Changes in penetration resistance of Ultisols from southern China as affected by shearing

Shear stresses and soil properties modified due to stress play an important role during formation of seals in a series of rainfall events and during tillage. The objectives of the study were to evaluate the effects of the penetrometer geometry on the penetration resistance as affected by shearing under different initial soil conditions and to use the information on soil strength to elucidate shearing process. Nine homogenous air-dried soils (<2 mm) were sprayed and stored so as to obtain equilibrium soil water contents. The moist soils were sheared by horizontal displacement of layers of soil particles/aggregates in between hands in one direction. The soil cores were prepared with comparable bulk density before the measurement of maximum penetration resistance (P_max) with a small flat tip and a cone tip penetrometers. At a wide range from 0.05 to 6.2 MPa, P_max was linearly correlated between the small flat tip and the cone tip penetrometers. The conversion ratio was higher under the saturation condition irrespective of the shearing effect. The penetrometer with the small flat tip was more sensitive for the weak soils. Shearing generally increased P_max in most cases, but it decreased P_max for some sandy soils under both saturated and unsaturated conditions and for a clayey soil under the saturated condition. The soil consisting of swelling clay exerted a decrease in P_max. Rearrangement and/or sliding of particles/aggregates and increase in soil suction during shearing were attributed to the increase in P_max. Increase in porosity due to the aggregation during shearing was ascribed to the decrease in P_max. In addition, it was shown that agricultural cultivation resulted in a reduction in soil strength.     

不同供磷状况下CO2浓度升高对番茄根系生长及养分吸收的影响

采用培养试验研究了磷缺乏与正常供磷条件下,CO₂浓度由350μL/L升高至800μL/L苗期番茄的生物量、根系特征和不同器官N、P、K养分含量的变化。结果表明,无论缺磷与否,CO₂浓度升高均能显著增加番茄地上部及根系的干物质积累量,提高根冠比。在磷缺乏条件下,CO₂浓度升高对番茄根系生长的促进主要表现为增加根系的体积和表面积;而在磷正常供应条件下主要表现为同时增加根体积和分根数,有利于形成强壮的根系。在两种供磷水平下,CO₂浓度升高对番茄各器官的N、P、K含量产生不同的稀释效应,但N、P、K总积累量却随CO₂浓度升高而显著增加;而且CO₂浓度与供P水平对番茄植株的N、P、K积累量具有极显著的正交互效应。     

抗病性不同大豆品种根面及根际微生物区系的变化 Ⅰ.非连作大豆(正茬)根面及根际微生物区系的变化

采用平板计数法测定了3个抗病性不同的大豆品种在生育期内根面和根际微生物区系的变化情况,并应用荧光计数法直接测定了根际细菌和真菌的生物量。结果表明,土体的微生物种类最丰富、根际的次之、根面的较单一。播种后从三叶期到鼓粒初期,根面和根际的可培养细菌总数随生育期逐渐增加,鼓粒初期达最大值,而成熟期则有明显的下降;大豆根际细菌生物量也存在相同的变化规律。抗病性不同的大豆品种其根面、根际可培养细菌总数存在差异;抗病品种大豆的根瘤重明显高于感病品种。种植一季后感病品种根际积累的病原生物(镰孢霉Fusarium.sp.和大豆胞囊线虫Heterodera.glycines的胞囊数)明显高于抗病品种。说明大豆根系分泌物对微生物具有选择性的促进或抑制作用,不同大豆品种以及同一大豆品种在不同生育时期根系分泌物的组成和数量不同,从而使大豆根面及根际形成了特定的微生物区系组成。     

RNA沉默—新型的植物病毒病害防治策略

植物RNA沉默(RNAsilencing)是植物本身固有的一种抗病毒防御系统,是对病毒在复制过程中形成双链RNA(dsRNA)的特殊反应。RNA沉默介导的dsRNA干涉病毒侵染转基因抗病毒有其不足,几种体外生产dsRNA的抗病毒体系能弥补这一不足,它们与转基因表达病原RNA不同,但仍然依赖RNA沉默机制来获取病原抗性。综述了近年来发展起来的各种启动RNA沉默的dsRNA产生策略、抗病状况和存在的问题及发展前景     

人工合成gna基因在小麦中的表达及其抗蚜虫效果研究

利用经密码子优化的、人工合成雪花莲凝集素(Galanthusnivalisagglutinin),gna基因及RbcS启动子构建了高效特异表达载体pBAC202,通过基因枪轰击小麦幼胚和幼穗,将外源gna基因导入到3个高产小麦(TriticumaestivumL.)品种中,获得42个转基因后代株系。对转基因后代植株进行了DNA点杂交、PCR及PCR-Southern验证,确认外源gna基因已成功地整合到小麦基因组中。进一步的凝集素活性检测表明,小麦叶片中表达的凝集素可使红细胞凝集,并具有正常的生物学活性。转基因植株在人工饲养条件下进行抗蚜虫(Rhopalosiphumpadi)试验,蚜口密度抑制率从19%～73%不等,部分株系抑虫效果较好。利用转基因的方式可将外源抗虫基因gna导入到小麦中,并可有效地增强小麦的抗蚜能力,为选育具抗虫特性的优质小麦提供新的途径。     

Soil nutrient supply and biomass production in a mixed forest on a skeleton‐rich soil and an adjacent beech forest

In the natural forest communities of Central Europe, beech (Fagus sylvatica L.) predominates in the tree layer over a wide range of soil conditions. An exception with respect to the dominance of beech are skeleton‐rich soils such as screes where up to 10 broad‐leaved trees co‐exist. In such a Tilia‐Fagus‐Fraxinus‐Acer‐Ulmus forest and an adjacent mono‐specific beech forest we compared (1) soil nutrient pools and net nitrogen mineralization rates, (2) leaf nutrient levels, and (3) leaf litter production and stem increment rates in order to evaluate the relationship between soil conditions and tree species composition. In the mixed forest only a small quantity of fine earth was present (35 g l^—1) which was distributed in patches between basalt stones; whereas a significantly higher (P < 0.05) soil quantity (182 g l^—1) was found in the beech forest. In the soil patches of the mixed forest C and N concentrations and also concentrations of exchangeable nutrients (K, Ca, Mg) were significantly higher than in the beech forest. Net N mineralization rates on soil dry weight basis in the mixed forest exceeded those in the beech forest by a factor of 2.6. Due to differences in fine earth and stone contents, the volume related soil K pool and the N mineralization rate were lower in the mixed forest (52 kg N ha^—1 yr^—1, 0—10 cm depth) than in the beech forest (105 kg N ha^—1 yr^—1). The leaf N and K concentrations of the beech trees did not differ significantly between the stands, which suggests that plant nutrition was not impaired. In the mixed forest leaf litter fall (11 %) and the increment rate of stem basal area (52 %) were lower than in the beech forest. Thus, compared with the adjacent beech forest, the mixed forest stand was characterized by a low volume of patchy distributed nutrient‐rich soil, a lower volume related K pool and N mineralization rate, and low rates of stem increment. Together with other factors such as water availability these patterns may contribute to an explanation of the diverse tree species composition on Central European screes.     

Proton toxicity interferes with the screening of common bean (Phaseolus vulgaris L.) genotypes for aluminium resistance in nutrient solution

Common bean (Phaseolus vulgaris L.) proved to be very sensitive of low pH (4.3), with large genotypic differences in proton sensitivity. Therefore, proton toxicity did not allow the screening of common bean genotypes for aluminium (Al) resistance using the established protocol for maize (0.5 mM CaCl₂, 8 μM H₃BO₃, pH 4.3). Increasing the pH to 4.5, the Ca²⁺ concentration to 5 mM, and addition of 0.5 mM KCl fully prevented proton toxicity in 28 tested genotypes and allowed to identify differences in Al resistance using the inhibition of root elongation by 20 μM Al supply for 36 h as parameter of Al injury. As in maize, Al treatment induced callose formation in root apices of common bean. Aluminium‐induced callose formation well reflected the effect of Ca supply on Al sensitivity as revealed by root‐growth inhibition. Aluminum‐induced callose formation in root apices of 28 bean genotypes differing in Al resistance after 36 h Al treatment was positively correlated to Al‐induced inhibition of root elongation and Al contents in the root apices. However, the relationship was less close than previously reported for maize. Also, after 12 h Al treatment, callose formation and Al contents in root apices did not reflect differences in Al resistance between two contrasting genotypes, indicating a different mode of the expression of Al toxicity and regulation of Al resistance in common bean than in maize.     

Using arbuscular mycorrhiza to reduce the stressful effects of soil compaction on corn (Zea mays L.) growth

Soil compaction is of great importance in agriculture, because its high levels may adversely affect plant growth and the environment. Since mechanical methods are not very efficient and economical, using biological methods to alleviate the stress of soil compaction on plant growth may be beneficial. The objectives of this study were to: (1) evaluate the effects of soil compaction on corn (Zea mays L.) growth, and (2) test the hypothesis that applying arbuscular mycorrhiza (AM) with different origins can partially or completely overcome the stressful effects of soil compaction on corn growth under unsterilized and sterilized conditions. Corn was planted in unsterilized and sterilized compacted soils, while treated with three species of AM including, Iranian Glomus mosseae, Iranian Glomus etunicatum, and Canadian Glomus mosseae, received from GINCO (Glomales in vitro Collection), Canada. Plant growth variables and soil resistance parameters were determined. AM significantly increased root fresh (maximum of 94% increase) and dry (maximum of 100% increase) weights in the compacted soil. AM with different origins may improve corn growth in compacted soils, though its effectiveness is related to the level of compaction and also to the interaction with other soil microorganisms.