Short Root Densities and Surface Phosphatase Activities of Ectomycorrhizal Morphotypes in a Slash Pine Plantation

Abstract

Mineralization is the dominant process controlling soil-solution P in the Spodosols of the southeastern United States. Pine trees growing in these soils are typically colonized by ectomycorrhizal (EM) fungi that are known to produce phosphatases. Little, however, is known of the dynamics of EM short roots or phosphatase activity in tree plantations. To address this question, short root densities, EM morphotypes, and associated surface acid phosphomonoesterase in a 12-year-old Pinus elliottii plantation in northern Florida were evaluated. The density of total (living and dead) short roots changed little from February through June, with a mean of 7.6 cm³ soil. The majority of the short roots, however, were inactive or dead with only 14 to 38% appearing viable upon visual inspection. The majority of the viable short roots were mycorrhizal. The most abundant morphotypes were formed by Cenococcum and Thelephora but these had low phosphatase activity. In contrast, less frequently observed morphotypes had substantially higher rates of enzyme production and these may play an important role in sustainable P nutrition of plantation trees.     

Root Exudates of Rice Cultivars Affect Rhizospheric Phosphorus Dynamics in Soils with Different Phosphorus Statuses

Exudation of organic acids by the roots of three rice cultivars grown in three soils of different phosphorus (P) statuses, and their impacts on the rhizospheric P dynamics and P uptake by the rice plants, were investigated. Quantum root exudates from all the rice cultivars were significantly greater at 21 days after transplantation than at panicle initiation or flowering stages. Malic acid was the most predominant organic acid present in the rice root exudates (10.3 to 89.5 μmol plant^?1 d^?1), followed by tartaric, citric, and acetic acids. Greater exudation of organic acids from rice grown in P-deficient soil by all the rice cultivars suggested response of rice plant to P stress. Results indicate that the release of organic acids in the root exudates of rice plants can extract P from strongly adsorbed soil P fraction, thereby increasing native soil P utilization efficiency and ensuring adequate P nutrition for the growing rice plants.     

Morphological Basis of Resistance in Cotton to the WhiteflyBemisia Tabaci

The morphological basis of resistance to the whiteflyBemisia tabaci Genn. (Aleyrodidae: Hemiptera) was studied. The plant characters examined were leaf area, thickness of leaf lamina, hair density, hair length, angle of insertion of leaf hair, and density of gossypol glands. Hair density and leaf thickness were positively correlated with the population ofB. tabaci, and a positive correlation was obtained between the adult whitefly population and gossypol glands on stem internodes. Cotton genotype USA-22 (sparsely hairy) was found to be more tolerant toB. tabaci than was genotype USA-13 (velvety hairy). The use of thinner and glabrous leaved cotton varieties is suggested to minimize the whitefly menace in cotton.     

Potato plant response to seed tuber bacterization in the field in various rotations

The effects of a seed tuber treatment with antagonistic isolates of fluorescentPseudomonas spp. were investigated on potato plants from 1981 to 1984. The experimental plots were located in fields in short and long rotations of potato. The short rotations are characterized by serious yield reductions which are caused by unknown microbial factors. The reductions varied from 30% in 1982 to only 3% in 1983 in the 3-year rotations. A statistically significant increase in yield (four to five months after planting) of ware potatoes varying from 9 to 20% was obtained in these plots through tuber bacterization, but only in 1981. In 1982 and 1983 initially significant improvements in shoot or tuber weight of seed potatoes were no longer detectable at ware potato harvest at the end of the growing period. Seed tuber bacterization had no effect on tuber yield in long rotations. Initial colonization of basal root parts by 53×10⁴ colony forming units (cfu) of antibiotic-resistant mutants per gram of root (fresh) dropped significantly to 20×10⁴ cfu per gram after three months. The bacterization effect on tuber yield depended on the development of harmful microbial activity and of introduced antagonists during the growing period. Seed tuber bacterization is more promising for seed potatoes than for ware potatoes in short rotations, the latter being harvested two months later.Samenvatting De invloed van pootgoedbehandeling met antagonistische isolaten van fluorescerendePseudomonas-soorten op de aardappelteelt, werd onderzocht in de periode van 1981 tot en met 1984. De proefvelden maakten deel uit van zowel ruime als nauwe rotaties met aardappelen. Kenmerkend voor de nauwe rotatie is, dat de opbrengst aanzienllijk gereduceerd wordt als gevolg van de aanwezigheid van nog onbekende microbiële factoren. Deze opbrengstverlaging varieerde van 30% in 1982 tot slechts 3% in 1983 in de 3-jarige rotaties. Pootgoedbacterisatie had in deze proefvelden een significante toename van de eindopbrengst (vier tot vijf maanden na pootdatum) van consumptieaardappelen tot gevolg, die varieerde van 9 tot 20%, echter allen in 1981. In 1982 en 1983 werd het effect van bacterisatie ook in de loop van de groeiperiode onderzocht. Aanvankelijk significante toenames van zowel spruit-als knolgewicht waren aan het einde van het groeiseizoen niet meer aantoonbaar. Pootgoedbacteristie bleek geen effect te hebben op aardappel in ruimte rotaties. Aanvankelijk werden de basale wortelgedeelten gekoloniseerd door antibioticum-resistente mutanten met 53×10⁴ kolonievormende eenheden (kve) per gram wortel(vers); dit aantal liep (drie maanden na pootdatum) echter significant terug tot 20×10⁴ kve per gram. Het effect van bacterisatie op de eindopbrengst werd bepaald door de ontwikkeling van de schadelijke microbiële activiteit en de ontwikkeling van de geïntroduceerde antagonisten tijdens het groeiseizoen. Pootgoedbacterisatie in nauwe rotaties biedt meer mogelijkheden voor de teelt van pootaardappelen dan die van consumptieaardappelen, die geruime tijd later geoogst worden.     

沙冬青根系的研究

沙冬青为轴根冬植物，主根粗壮，侧根发达。根系分布与土壤基质条件密切相关，基主体部分集中在０－３０ｃｍ土层内，根瘤位于侧根末端。冠幅平均为０．７６ｍ＾２的沙冬青，单株平均根系干重２３１．６ｇ，体积４１１．１ｃｍ＾３、表面积１３７７５ｃｍ＾２。     

Differential phytotoxicity of glyphosate in maize seedlings following applications to roots or shoot

The transport and differential phytotoxicity of glyphosate was investigated in maize seedlings following application of the herbicide to either roots or shoots. One-leaf maize seedlings (Zea mays L.) were maintained in graduated cylinders (250 mL) containing nutrient solution. Half of the test plants were placed in cylinders (100 mL) containing different ¹⁴C-glyphosate concentrations; the remainder received foliar appliation of ¹⁴C-glyphosate. After 26 h, the roots and the treated leaves were washed with distilled water, and the plants placed again in cylinders (250 mL) containing fresh nutrient solution for 5 days. Plants were weighed, and split into root, seed, cotyledon, coleoptile, mesocotyl, first leaf and apex. The recovery of ¹⁴C-glyphosate was over 86%. For both application treatments, the shoot apex was the major sink of the mobilized glyphosate (47.9 ± 2.93% for root absorption and 45.8 ± 2.91% for foliar absorption). Expressed on a tissue fresh weight basis, approximately 0.26 μg a.e. g⁻¹ of glyphosate in the apex produced a 50% reduction of plant fresh weight (ED₅₀) when the herbicide was applied to the root. However, the ED₅₀ following foliar absorption was only 0.042 μg a.e. g⁻¹ in the apex, thus maize seedlings were much more sensitive to foliar application of the herbicide.     

An Improved Method for Isolating Violet Root Rot Fungus, Helicobasidium mompa, from Basidiocarps

Basidiocarps of the violet root rot fungus, Helicobasidium mompa, are less frequently used for isolation than are mycelial strands and sclerotia even though the basidiocarps are conspicuously produced at the trunk base of diseased plants. Basidiocarps are also more suitable for storage. This paper describes an improved method for obtaining pure cultures from basidiocarps using microcentrifuge tubes to facilitate the awkward steps of rinsing fungal materials under a dissecting microscope. Received 28 June 2000/ Accepted in revised form 4 August 2000     

Cropping systems for the management of phytonematodes

Damage caused by nematodes is one of the limiting factors in crop production. Traditional nematode management is based on the use of crop rotations, resistant cultivars, nematicides, or combinations of these methods. For a crop like peanut (Arachis hypogaea), cultivars resistant to root-knot nematodes are not available. There are soybean (Glycine max) cultivars resistant to some of the species of root-knot nematodes (Meloidogyne spp.); however, most fields have nematode infestations composed of mixtures of species. Research at Auburn has shown that tropical crops can be used effectively in rotation to manage nematode problems. Rotations with American jointvetch (Aeschynomene americana), castor (Ricinus communis), hairy indigo (Indigofera hirsuta), partridge pea (Cassia fasciculata), sesame (Sesamum indicum), and velvetbean (Mucuna deeringiana) have resulted in good nematode control and increased yields of peanut and soybean. Some crops (castor, sesame) are considered ‘active’ in that they produce compounds that are nematicidal, whereas others (e.g. corn, sorghum) are simply non-host, that is, ‘passive’.     

麦茎谷蛾生物学特性及防治

麦茎谷蛾在山西南部麦区普遍发生，丘陵旱塬地区为害较重。大发生时，孕穗～灌浆期小麦被害穗率达３０％以上。药剂防治试验表明，３月上旬卵态幼虫孵化时用４０％甲基异柳磷、２０％菊马乳油等进行一次性防治，均有较好的效果。     

小麦根腐镰刀菌鉴定及其生物学特性

１９９１－１９９４年对山东省１１县市的小麦根腐病标样进行病原分离培养及致病性测定，鉴定出６种镰刀菌，有黄色镰刀菌（Ｆｕｓａｒｉｕｍ ｃｕｌｍｏｒｕｍ（Ｓｍｉｔｈ Ｓａｃｃ）、禾谷镰刀苏（Ｆ ｇｒａｍｉｎｅａｒｕｍ Ｓｃｈｗ）、尖孢镰刀苏（Ｆ ｏｘｙｓｐｏｒｕｍ Ｓｃｈｌ）、串珠镰刀苏（Ｆ ｍｏｎｉｌｉｆｏｒｍｅ Ｓｈｅｌｄ）茄病镰刀苏（Ｆ ｓｏｌａｎｉ（Ｍａｒｔ） Ｓａｃｃ）及木贼镰刀菌（Ｆ ｅｑ