苗期甜菜根系分泌物的分布特征研究

根系分泌物的量及分布特征直接影响着根际微生物的种群、结构和功能,进而间接影响着根际养分的有效性和作物的生长。本研究通过砂培盆栽培养,收集得到甜菜苗期的根系分泌物,利用高效液相色谱测定了根系分泌物中的3大类物质,包括土壤中常见的17种氨基酸、10种有机酸和4种糖类物质,并分析得到了到甜菜根表面不同距离的各种氨基酸、有机酸和糖类物质的含量及其分布特征。结果表明：总的来看,有机氮高效品种‘KWS8138’根系分泌物含量普遍高于有机氮低效品种‘Beta176’,距离根表面0~5 mm范围内降幅较大,随后逐渐趋于平缓。总氨基酸含量、甘氨酸、丙氨酸、丝氨酸、精氨酸和酪氨酸变化趋势相似。各种糖类物质的含量变化顺序为：葡萄糖>果糖>蔗糖>半乳糖。有机酸中草酸和甲酸的含量变化也是距离根表面越远越低缓。因此,沿甜菜根表面形成了根系分泌物的分布梯度,在为根际微生物提供能源和碳源的同时,对植物根际养分的有效吸收利用提供理论参考。     

玉竹连作对土壤微生物区系的影响

连作导致玉竹产量及品质下降,严重威胁玉竹的生产,对玉竹连作模式下相关土壤生态指标进行研究,对于认识和克服连作障碍具有十分重要的意义。为揭示玉竹连作对根际土壤微生态的影响,采用稀释平板测数法和最大或然数法研究了玉竹连作3年根际土壤和种植1年玉竹的根际土壤3大类群土壤微生物及主要功能微生物的数量变化。结果表明:连作对玉竹根际土壤微生物区系构成明显影响,其细菌数量较对照土壤降低了12.4%,放线菌数量降低了13.2%,真菌数量则增加了239%,好氧自生固氮菌数量降低了36.3%,氨化细菌数量降低了48.6%,硝化细菌数量变化不大,有机磷细菌数量增加了12.1%,无机磷细菌数量降低了71.1%。说明连作不利于玉竹根际土壤微生态质量的维持。     

根际通电栽培对植物工厂生菜生长及叶烧病发生的影响

针对人工光植物工厂内由钙缺乏而引起的叶菜叶烧病频发的问题,以较易发生叶烧病的生菜作为试验材料,采用在生菜根际建立电压为-15kV全天工作的通电系统,使生菜植株带负电,并在地上部生长空间形成正向电场的方法,设置根际通电处理为试验组,无通电处理为对照组,探究根际通电栽培对生菜生长、钙离子吸收以及叶烧病发生的影响。结果表明:1)对照组的生菜植株在第13天发生叶烧病现象,而试验组的生菜则在第15天出现叶烧病,且在第20天叶烧病的发生率降低了46.9%;2)移栽第20天,与对照组相比,试验组生菜植株的地上部和地下部鲜质量分别提高了6.3%和30.9%,干质量分别提高了17.2%和22.4%;3)移栽第10天,试验组生菜的根系活力和Ca~(2+)吸收能力分别较对照组提高了133.3%和108.9%;4)移栽第20天,试验组生菜的气孔导度、蒸腾速率和胞间二氧化碳浓度分别较对照组提高了46.2%、49.0%和5.2%,生菜光合产物的积累也得到提高。综上,在植物工厂综合环控条件下,根际通电栽培能显著改善生菜的生长发育,并能有效地减缓叶烧病的发生。     

解淀粉芽孢杆菌LJ1诱导黄瓜抗白粉病的研究

解淀粉芽孢杆菌LJ1是从土壤中分离得到的一株对黄瓜白粉病具有较好防效的生防细菌。田间试验发现,用LJ1发酵上清100倍稀释液喷施黄瓜幼苗,在施药后14 d时其对黄瓜白粉病的防效可达83.45%。为研究LJ1防治病害的作用机制,用LJ1发酵上清100倍稀释液喷施黄瓜幼苗,测定黄瓜叶片中的超氧化物歧化酶(SOD)、多酚氧化酶(PPO)、苯丙氨酸解氨酶(PAL)等与诱导抗病性相关的酶活性和信号分子水杨酸含量的变化,并检测了苗期根围土壤中真菌的动态。结果显示,经过LJ1发酵液处理后3种酶的活性和水杨酸的含量在不同时间点均有一个骤增的过程,其活性显著高于对照,并且7 d后土壤中的可培养真菌数量急剧减少。说明LJ1发酵液中有诱导黄瓜产生抗病性的物质,并且诱导后分泌的抗性物质对真菌具有广谱性。     

茶树根际与非根际土壤磷形态变化特征

以茶树(Camellia sinensis)根际和非根际土壤为研究对象,选取湖南省石门、临澧、桃源、长沙、安化、资兴等6县(市)的茶园为采样点,对其根际和非根际土壤的全磷、有效磷及无机磷的不同化学形态进行了分析。结果表明,茶树根际土壤全磷和有效磷含量均高于非根际土壤,有效磷在根际富集明显;土壤无机磷含量及占全磷的比例差异都很大。不同母质发育土壤的无机磷组成也不同,板页岩母质发育的根际土壤中Al-P含量最高,Fe-P其次,O-P最少。花岗岩和第四纪红色黏土发育的根际土壤Fe-P最高,Al-P其次,O-P最少。3种母质发育的非根际土壤中均为Fe-P含量最高。根际无机磷中的Al-P,Fe-P和Ca-P含量与有效磷呈极显著正相关,非根际Al-P和Fe-P与土壤有效磷显著正相关关系。根际、非根际土壤全磷和有效磷含量与pH值相关性不显著,根际、非根际土壤有效磷和全磷含量相关性极显著。     

Soil Acidity Changes in Bulk Soil and Maize Rhizosphere in Response to Nitrogen Fertilization

The capacity of nitrogen (N) fertilizers to acidify the soil is regulated principally by the rate and N source. Nitrogen fertilizers undergo hydrolysis and nitrification in soil, resulting in the release of free hydrogen (H⁺) ions. Simultaneously, ammonium (NH₄ ⁺) absorption by roots strongly acidifies the rhizosphere, whereas absorption of nitrate (NO₃ ^?) slightly alkalinizes it. The rhizosphere effects on soil acidity and plant growth in conjunction with N rate are not clearly known. To assess the impact of these multiple factors, changes in the acidity of a Typic Argiudol soil, fertilized with two N sources (urea and UAN) at two rates (equivalent to 100 and 200 kg N ha^?1), were studied in a greenhouse experiment using maize as the experimental plant. Soil pH (measured in a soil–water slurry), total acidity, exchangeable acidity, and exchangeable aluminum (Al) were measured in rhizospheric and bulk soil. Plant biomass and foliar area (FA) were also measured at the V₆ stage. Nitrogen fertilization significantly reduce the pH in the bulk soil by 0.3 and 0.5 units for low and high rates respectively. Changes in the rhizosphere (the “rhizospheric effect”) resulted in a significant increase in soil pH, from 5.9 to 6.2. The rhizospheric effect × N source interaction significantly increased exchangeable acidity in the rhizosphere relative to bulk soil, particularly when UAN was added at a low rate. Only total acidity was significantly increased by the fertilizer application rate. In spite of the bulk soil acidification, no significant differences in exchangeable aluminum were detected. Aerial biomass and FA were significantly increased by the higher N rate, but N source had no effect on them. Although changes in acidity were observed, root biomass was not significantly affected.     

Chemical Speciation of Heavy Metals in the Fractionated Rhizosphere Soils of Sunflower Cultivated in a Humic Andosol

Chemical speciation and bioaccumulation factor of iron (Fe), manganese (Mn), and zinc (Zn) were investigated in the fractionated rhizosphere soils and tissues of sunflower plants grown in a humic Andosol. The experiment was conducted for a period of 35 days in the greenhouse, and at harvest the soil system was differentiated into bulk, rhizosphere, and rhizoplane soils based on the collection of root-attaching soil aggregates. The chemical speciations of heavy metals in the soil samples were determined after extraction sequentially into fractions classified as exchangeable, carbonate bound, metal–organic complex bound, easily reducible metal oxide bound, hydrogen peroxide (H₂O₂)–extractable organically bound, amorphous mineral colloid bound, and crystalline Fe oxide bound. Iron and Zn were predominantly crystalline Fe oxide bound in the initial bulk soils whereas Mn was mainly organically bound. Heavy metals in the exchangeable form accumulated in the rhizosphere and rhizoplane soils, comprising <4% of the total content, suggesting their relatively low availability in humic Andosol. Concentrations of organically bound Fe and Mn in soils decreased with the proximity to roots, suggesting that organic fraction is the main source for plant uptake. Concentrations of Mn and Zn in the metal–organic complex also decreased, indicating a greater ability of sunflower to access Mn from more soil pools. Sunflower showed bioaccumulation factors for Zn, Fe, and Mn as large as 0.39, 0.05, and 0.04 respectively, defining the plant as a metal excluder species. This result suggests that access to multiple metal pools in soil is not necessarily a major factor that governs metal accumulation in the plant.     

Tillage Management for Cotton in Southeastern Coastal Soils during Dry Years

With rising energy costs, expensive deep tillage needs to be reevaluated. In 2002 and 2003, tillage treatments were evaluated for effectiveness in increasing cotton yield when noninversion deep tillage was either performed annually or not. Tillage treatments included a nontilled control, a straight-legged subsoil shank with bedding, and strip tillage with each of the following: a straight-legged subsoil shank, a Paratill, and a Terra-Max. In 2003, treatments were split with half the plots tilled and half not. No-tillage treatment significantly reduced penetration resistances better than others. Tillage decreased penetration resistance and improved yield but differences were significant only half the time. Treatments not tilled in the second year did not have significantly reduced penetration resistance because of a lack of recompaction during a dry first growing season. Tilling the second year improved yield marginally. Producers need to decide whether to till after a dry year on a case-by-case basis.     

Solubilization and Acquisition of Phosphorus from Sparingly Soluble Phosphorus Sources and Differential Growth Response of Brassica Cultivars Exposed to Phosphorus-Stress Environment

Phosphate (Pi), the fully oxidized and assimilated form of phosphorus (P), influences virtually all developmental and biochemical processes in plants; however, its availability and distribution are widely heterogeneous. Paradoxically, although total P is abundant in lithosphere, elusive soil chemistry of Pi renders the element the most dilute and the least mobile in natural and agricultural ecosystems, resulting in P deprivation due to its low mobility and high fixation capacity in the soil. Nonmycorrhizal Brassica does not produce specialized cluster/dauciform roots but is an effective P user compared to other crops. Using a soil low in P (Mehlich 3–extractable P) with or without P fertilization, Brassica cultivars showed substantial genetic diversity in P-utilization efficiency (PUE), P efficiency (PE), P-efficiency ratio (PER), and P-stress factor (PSF). Cultivars producing greater root biomass accumulated greater total P contents, which in turn was related negatively to PSF and positively to shoot and total biomass. Plant survival and reproduction rely on efficient strategies in exploring culture media for P. Acquisition of orthophosphate from extracellular sparse P sources may be enhanced by biochemical rescue strategies such as copious H⁺ efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H⁺-ATPase and anion channels triggered by P starvation. The P-starvation-induced solution pH changes due to H⁺ efflux, and carboxylates exudations were estimated by low-P-tolerant and low-P-sensitive cultivars in solution culture experiments. Low-P-tolerant cultivars showed more decrease in pH compared to low-P-sensitive cultivars when cultivars were grown under a P-stress environment induced by using sparingly soluble P sources (rock phosphate and tricalcium phosphate). The P contents of cultivars were inversely related to decrease in culture media pH. Low P-tolerant cultivars presented enhanced H⁺-efflux and total carboxylates exudations compared to low-P-sensitive cultivars, resulting in more rhizosphere acidification to scavenge Pi, evidencing their adaptability to P starvation. These elegant P-stress-induced rescue strategies by tested cultivars provided the basis of enhanced P solubilization and acquisition of P from sparingly soluble P sources to combat P-starved environments.     

Nitrogen and/or phosphorus fertilization effects on organic carbon and mineral contents in the rhizosphere of field grown sorghum

The effects of nitrogen (N) and/or phosphorus (P) fertilizers on the nutritional status in the rhizosphere were studied by monitoring throughout the growth period the concentrations of organic carbon (C), inorganic N, NaHCO₃ extractable P, exchangeable K, Ca, and Mg in sorghum (Sorghum bicolor L. Moench) down in an Alfisol field, and of all these elements except for extractable P, and exchangeable Ca in a Vertisol field in semi-arid tropical India. These concentrations were compared between the rhizosphere soil and bulk soil of sorghum grown in both fields.

Organic C content of the rhizosphere soil increased with plant age and was significantly higher than that in the bulk soil throughout the growth of sorghum, but it was not affected by the rates of N or P fertilizer. Inorganic N concentration in the rhizosphere soil was significantly higher than that in the bulk soil until maturity in sorghum. The content of available P in the rhizosphere soil was significantly higher than in the bulk soil after the middle of the growth stage. Its average concentration in the rhizosphere soil across growth stages was significantly higher than in the bulk soil, which contradicts the observation in many reports that there is a depletion of P in the rhizosphere soil. The concentration of three exchangeable cations, K, Ca, and Mg, showed different patterns in the rhizosphere and the bulk soils. The concentration of K was almost constantly higher in the rhizosphere soil than in the bulk soil, Ca concentration was not different between the two soils, and Mg concentration was significantly higher in the bulk soil than in the rhizosphere soil. The reasons for these discrepancies cannot be explained at present. The concentrations of these cations were not affected by the rate of N or P fertilizer except for Mg at a later growth stage. The differences between rhizosphere and bulk soils in Alfisol were similar to those in Yertisol with respect to the concentration of organic C, inorganic N, and exchangeable K and Mg.