Replacement series analysis of the competitive interaction between a weed and a crop as influenced by a plant parasitic nematode

Les méthodes permettant d'évaluer les effets de concurrence sont appliquées à un système nématode-mauvaise herbe-culture. Il a été cherché à connaître les relations entre le nématode [#Anguina amsinckiae (Steiner & Scott, 1935) Thorne, 1961], sa plante hôte (#Amsinckia intermedia Fischer & Meyer) et le blé (#Triticum aestivum$ L.). Des expériences basées sur des séries de substitution sont conduites en serre en considérant des rapports (mauvaise herbe : culture) de 0:1, 3:1, 1:1, 1:3, et 1:0. Ces expériences menées avec et sans nématodes sont répétées deux fois. Les résultats montrent que le nématode réduit de 25 % le poids sec de la mauvaise herbe, mais n'a aucun effet significatif sur le poids sec de la culture. Les indices de concurrence montrent que la culture est le concurrent dominant aussi bien en présence qu'en absence du nématode. Les mesures relatives à la différenciation des niches laissent supposer que les deux espèces végétales ne partagent pas entièrement, en l'absence du nématode, les mêmes ressources limitées et que la concurrence vis-à-vis de ces ressources est accrue en présence du nématode. (Résumé d'auteur

Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), a potential biological control agent for the submerged aquatic weed, Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae)

The leaf-mining fly, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), was investigated in its native range in South Africa, to determine its potential as a biological control agent for Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae), an invasive submerged macrophyte that is weedy in many parts of the world. The fly was found throughout the indigenous range of the plant in South Africa. High larval abundance was recorded at field sites with nearly all L. major shoots sampled ontaining larvae, with densities of up to 10 larvae per shoot. Adults laid batches of up to 15 eggs, usually on the abaxial sides of L. major leaves. The larvae mined internally, leaving the epidermal tissues of the upper and lower leaves intact. The larvae underwent three instars which took an average of 24 days and pupated within the leaf tissue, from which the adults emerged. Impact studies in the laboratory showed that H. lagarosiphon larval feeding significantly restricted the formation of L. major side branches. Based on its biology and damage caused to the plant, Hydrellia lagarosiphon could be considered as a useful biological control candidate for L. major in countries where the plant is invasive

Eficiência de absorção e utilização de nitrogênio por plantas de arroz e de dois ecótipos de arroz vermelho Efficiency of nitrogen uptake and utilization by rice and two red rice ecotypes

A adubação nitrogenada proporciona grandes benefícios ao arroz irrigado. No entanto, pouco se conhece sobre os seus efeitos em lavouras infestadas com plantas daninhas, especialmente com arroz vermelho, que é a espécie que causa os maiores danos à orizicultura do Rio Grande do Sul. Através de dois ensaios conduzidos em casa de vegetação e em câmara de crescimento, compararam-se as eficiências de absorção e de utilização de nitrogênio pela cv. BR-IRGA 410 com as de dois ecótipos de arroz vermelho, nos estádios de início do perfilhomento e de início do desenvolvimento da panícula. Para esta avaliação, foram utilizados estudos de cinética de absorção de nitrogênio pelas plantas em solução nutritiva e a análise de concentração e de quantidade de nitrogênio presente nas plantas. No início do perfilhomento, estádio em que a competição por luz é limitada, os três genótipos apresentam eficiências similares na absorção de nitrogênio. Adubação nitrogenada no início do desenvolvimento da panícula beneficia mais os ecótipos de arroz vermelho do que o arroz, em função da sua maior eficiência na absorção e na utilização de N.<br>Nitrogen promotes large benefits to flooded rice. However, little is known about its effects on infested fields, especially with red rice, which is the species that causes the highest damage to flooded rice in Rio Grande do Sul, Brazil. Two experiments were conducted at greenhouse and growth chamber conditions, to compare the efficiencies of uptake and utilization of nitrogen by the cultivar BR-IRGA 410 with two red rice ecotypes, at the growth stages of initial tillering and beginning of panicle differentiation. It were used studies on kinetics of nitrogen uptake by plants in nutrient solution and the analysis of concentration and amount of nitrogen in plants. In the beginning of the tillering, stage with low competition by light, the three genotypes show similar efficiencies on nitrogen uptake. Nitrogen fertilization at beginning of panicle differentation is more advantageous to red rice than to rice, dueto its higher efficiency on nitrogen uptake and utilization