ALBERT

Notes: For implementation of simple yield loss models into threshold-based weed management systems, a thorough validation is needed over a great diversity of sites. Yield losses by competition wsth Sinapis alba L. (white mustard) as a model weed, were studied in 12 experiments in sugar beet (Beta vulgaris L.) and in 11 experiments in spring wheat (Triticum aestivum L.). Most data sets were heller described by a model based on the relative leaf area of the weed than by a hyperbolic model based on weed density. This leaf area model accounted for (part of) the effect of different emerging times of the S. alba whereas the density model did not. A parameter that allows the maximum yield loss to be smaller than 100% was mostly not needed to describe the effects of weed competition. The parameter that denotes the competitiveness of the weed species with respect to the crop decreased the later the relative leaf area of the mustard was determined. This decrease could be estimated from the differences in relative growth rate of the leaf area of crop and S. alba. However, the accuracy of this estimation was poor. The parameter value of the leaf area model varied considerably between sites and years. The results strongly suggest that the predictive ability of the leaf area model needs to be improved before it can be applied in weed management systems. Such improvement would require additional information about effects of abiotic factors on plant development and morphology and the definition of a time window for predictions with an acceptable level of error.

Notes: Destructive measurements to collect input data for models that predict yield loss from relative leaf area of weeds can be laborious. Alternative methods were tested in seven field experiments with sugar beet or spring wheat. Weeds with different morphologies showed the same linear relationships between relative leaf area, meas ured destructively, and cover, assessed by means of a frame, until 3 or 4 weeks after crop emer gence. At later growth stages, differences in weed morphology resulted in different relationships. Visual estimates of weed cover corresponded only roughly with cover assessments with a frame. The possibility of estimating relative leaf area of weeds with a reflectance technique was tested, assuming that for early growth stages the leaf area index of weeds can be considered as additional to that of the crop. In spring wheat, relative leaf areas of Sinapis alba L., sown at different times and densities, correlated well with characteristics based on infra-red reflectance. In sugar beet, these relationships were not as distinct. Techniques d'estimation de la surface foliaire relative et de la couverture des mauvaises herbes dans les cultures, en vue de prédictions de pertes en rendement Des prélèvements destructifs peuvent alimenter en données les modèles qui prédisent les pertes de rendement à partir de la surface foliaire relative des mauvaises herbes, mais ils sont exigeants en temps. Des méthodes alternatives ont été testées lors de sept expériences au champ dans de la betterave ou du blé de printemps. Jusqu'à trois ou quatre semaines après la levée de la culture, des mauvaises herbes possédant différentes morphologies montraient les mêmes relations linéaires entre d'une part la surface foliaire relative mesurée de manière destructive et d'autre part la couverture mesurée grâce à une grille. Aux stades de croissance ultérieurs, du fait des différences morphologiques entre les mauvaises herbes, les relations étaient différentes. Les estimations visuelles de la couverture en mauvaises herbes ne correspondaient qu'approximativement aux mesures de couverture effectuées au moyen de la grille. La possibilité d'estimer la surface foliaire relative des mauvaises herbes avec une technique de réflectance a étéévaluée, en supposant que, aux stades de croissance initiaux, l'indice de surface foliaire des mauvaises herbes peut être considéré comme additif de celui de la culture. Dans le blé de printemps, les surfaces foliaires relatives de Sinapis alba L., semées à différentes époques et à différentes densités, étaient bien corrélées avec des caractéristiques basées sur la réflectance infra-rouge. Dans la betterave à sucre, ces relations n'étaient pas aussi claires. Bestimmung der relativen Blattfläche und des Deckungsgrads von Unkräutern zur Prognose von Ertragsverlusten Die destruktive Gewinnung von Daten der relativen Blattfläche von Unkräutern für Modelle zur Vorhersage von Ertragsverlusten sind arbeitsaufwendig. Alternative Methoden wurden in 7 Versuchen in Zuckerrübe und Sommerweizen untersucht. Morphologisch unterschiedliche Unkräuter zeigten bis zu 3 oder 4 Wochen nach dem Auflaufen der Kulturpflanzen dieselben linearen Beziehungen zwischen der destruktiv gemessenen relativen Blattfläche und dem mittels eines Rahmens bestimmten Deckungsgrad. In späteren Entwicklungsstadien führten die morphologischen Unterschiede zu verschiedenen Verhältnissen. Bonituren des Unkrautdeckungsgrads entsprachen nur grob den mit dem Rahmen ermittelten Deckungswerten. Die Bestimmung der relativen Blattfläche der Unkräuter durch Messung der Lichtreflexion wurde geprüft, wobei angenommen werden kann, daß sich in den frühen Entwicklungsstadien der Blattflächenindex der Unkräuter mit dem der Kulturpflanzen summiert. Im Sommerweizen war die relative Blattfläche von zu verschiedenen Zeiten und in unterschiedlicher Dichte gesätem Sinapis alba L. mit den Daten der Infrarotreflexion gut korreliert, auf den Zuckerrübenflächen nicht so genau.

Notes: Summary This paper tests the hypothesis that increased soil nitrogen supply reduces the growth of late-emerging weeds in wheat and potato by enhancing canopy leaf area development and thereby reducing the availability of light for weed growth. Two series of experiments were conducted: one in spring wheat (1997, 1999) with sown Stellaria media and one in potato (1998, 1999) with naturally emerged weeds, including S. media. For each crop, two cultivars were grown at three levels of nitrogen supply. In wheat, as in a monoculture of S. media, total dry weight and seed number of the weed increased with soil nitrogen supply, whereas in potato the opposite was found. Increased soil nitrogen supply increased the nitrogen uptake of S. media in wheat, despite the reduced light availability, indicating that S. media in wheat was limited by nitrogen. In potato, on the other hand, growth of S. media was limited by light availability, which decreased with increased soil nitrogen supply. We conclude that the differences in response of S. media in wheat and potato to additional nitrogen supply are attributable to the dual influence of soil nitrogen supply on light and nitrogen availability, which are mediated by the crops.

Notes: Summary Plasticity in life-history characteristics was investigated in three populations of Plantago major L. ssp. pleiosperma (Pilger), a self-compatible, wind pollinated species with a high self-fertilization rate. The populations studied were selected for their marked differences in biomass accumulation and habitat characteristics such as nutrient availability and interspecific interaction. Plants, raised from seeds collected at three sites, were grown in a greenhouse at three nutrient levels. In addition, a reciprocal transplant experiment was carried out. In both experiments variances in variables of growth and reproduction were largely due to environmental factors. Besides this overall result, population and population x environment interaction effects existed for most of the variables. Differences in plasticity between populations were further analysed. In the greenhouse experiment plants from a river-side population showed a high degree of plasticity in reproductive effort, whereas plants from two other populations, one series from a beach plain and the other from a salt meadow, showed a high degree of plasticity in shoot-root ratio. Plasticity in biomass allocation to either vegetative or generative parts is suggested to be an important response to selective forces related to either interspecific competition or temporal variability.

Notes: Summary An attempt was made to relate variation in life-history characteristics within a population of Plantago major ssp. pleiosperma to small-scale environmental variability. At a beach plain, embanked in 1966, a mosaic environment was distinguished with spatial variability in vegetation structure as well as in nutrient availability and water content of the soil. Differences between three subsites in comtemporary selection were demonstrated, e.g. in shoot morphology and allocation to reproductive tissue. The effects of nutrient supply and waterlogging on morphology and life history were studied on lines from the three subsites in a greenhouse. For most of the traits high levels of phenotypic plasticity were observed, covering almost entirely the observed phenotypic variability at the beach plain. In all treatments lines from the shrubs had, however, a higher leaf-area ratio as well as delayed flowering when compared to lines from more open subsites. In addition, in a reciprocal transplant experiment it was demonstrated that lines from the shrubs had larger shoots with e.g. broader leaves in the shady environment of the shrubs than other lines. From the experiments no indications were obtained that lines from any subsite were especially adapted to specific levels of nutrient supply or water content of the soil. With respect to these environmental factors P. major ssp. pleisoperma might occur and reproduce at all subsites by means of phenotypic plasticity, e.g. in plant form. However, it is suggested that spatial variability in vegetation structure caused a population subdivision in allocation patterns, leaf form and life history at the beach plain, over distances of about 15–25 m. This differentation occurred during primary succession over a period of twenty years.

Notes: Abstract During two consecutive years the effects of intercropping fresh market white cabbage with two species of clover on pest populations and yield were studied. White cabbage cv. Minicole was intercropped withTrifolium repens (white clover) andTrifolium subterraneum (subterranean clover) as compared to the monocrop. During the season observations were made on pest population developments, especially ofMamestra brassicae L. (cabbage moth),Brevicoryne brassicae L. (cabbage aphid),Delia brassicae L. (cabbage root fly), and evaluation of caterpillar feeding injury. At harvest the yield in quantity and quality was determined to be able to assess the gross financial result. Intercropping effects in terms of suppression of oviposition and larval populations of various pests were found. Although no pesticides were used and competition reduced the weight, the quality of the intercropped cabbages lead to a better financial result compared to the monocropped cabbage crop. The results are discussed in the perspective of the practical implications in the context of IPM.

Notes: Abstract As part of a research project on the variation in life-history characteristics within a population of Plantago major L. ssp. pleiosperma, seasonal and spatial variability in the availability of macronutrients (N, P, and K) were examined on a small scale in the 0–25 cm soil depth at a primary beach plain site, embanked since 1966. On the basis of distinct differences, among other things, in plant biomass, an a priori division into three different types of microhabitat occurring in a mosaic distribution pattern was made: an overall low-lying area (subsite 1) with slightly elevated patches of 0.5 to 1.5 m in diameter (subsite 2) and rather large patches, 20 to 40 m in diameter, of sea buckthorn shrubs, with small and relatively open spots (subsite 3) in the transitional zone from lower area into scrub. All three subsite types were studied within a total area of approximately 2000 m2. Three methods of analysis were applied: an inventory survey (sampling once at the start of the growing season), an analysis of the seasonal variation (sampling at approximately monthly intervals during the period April-November), and an assessment of nitrogen mineralization potentials in the laboratory (sampling once at the beginning of the growing season). All three procedures clearly demonstrated the occurrence of differences in the availability of nutrients over very short distances, i.e. a pronounced spatial variability among subsites. Particularly the availability of N and P appeared to have increased at the subsites 2 and 3, when compared to subsite 1. This small-scale differentiation in soil properties has occurred in an essentially homogeneous parent material (e.g. in texture and carbonate content) over a period of about 20 years. Besides a spatial variability, statistically significant temporal fluctuations were observed in the availability of N, P, and K. Relative fluctuations of mineral N (as indicated by the range/mean ratio) were especially large at the subsites 2 and 3.