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Weeds cause considerable losses in horticultural and agricultural crops. Weeds are still predominantly controlled with herbicides. To reduce the use of chemicals, a better understanding of the biology of weeds is required. In this thesis the effect of environmental conditions on dormancy and germination of Chenopodium album L., Polygonum persicaria L., P. lapathifolium L. subsp. lapathifolium, Sisymbrium officinale (L.) Scop. and Spergula arvensis L. was investigated.It was shown that changes in dormancy of these species were regulated by temperature. Soil moisture and nitrate content did not affect these changes. The dormancy status of the seeds was visualized by the range of temperatures over which germination of exhumed seeds was possible. During relief of dormancy, seeds could germinate over a progressively wider range of temperatures. During induction of dormancy, this range became narrower.Germination of C.album, S. officinale and S.arvensis was stimulated by light, nitrate and desiccation. These factors all increased the width of the range of temperatures over which germination could proceed and therefore affected the expression of dormancy. That is, seeds seemed less dormant and they could germinate during a longer period of the year. Endogenous nitrate, that entered the seeds via the mother plant during seed development, only temporarily stimulated germination. After burial the effect disappeared because of equalization of the nitrate content. The effect of desiccation was stronger, the more seeds were desiccated.With descriptive models the changes in the range of germination temperatures of the investigated species and the effect of nitrate upon these changes were simulated for a period of three years as a function of soil temperature during burial. When the field temperature after exhumation and the germination-temperature range overlapped, germination was possible. Accordingly, temperature had a dual effect. Germination depended on the one hand on the actual field temperature after exhumation , on the other hand on the width of the germination-temperature range, which was determined by the dormancy status of the seeds and was regulated by soil temperature during burial . When nitrate was added during the test, the germinationtemperature range became wider and germination could occur during a longer period of the year.

In a growth experiment at phosphate levels varying between 0,0005 and 0.1 mM phosphate, relative growth rates and other growth parameters were determined in Carex species (C. rostrata Stokes, C. limosa L., C. lasiocarpa Ehrh., C. diandra Schrank, and C. acutiformis Ehrh., listed in order of increasing nutrient availability of their natural habitats). In all species, more efficient utilization of the phosphate was observed with decreasing phosphate levels, together with reduced fresh shoot ratio. In addition, each species shows characteristics which may help it to grow under low phosphate conditions: relatively high phosphate level in the plant (C. rostrata), a low fresh shoot ratio (C. limosa) and regulation of uptake and translocation of phosphate in such a way that relative growth rate during the first two weeks is maintained or even increased for a longer period (C. rostrata, C. limosa, and C. diandra). In the studied Carex species, the contribution of fresh shoot ratio to relative growth rate is much larger than that of net assimilation rate. C. acutiformis, a species of eutrophic areas, had the highest relative growth rate.

SUMMARY In eight experiments the competition between sun and shade plants was studied in different light environments. In higher light intensities the competitive ability of sun species was definitely greater. In lower light intensities, competitive ability did not differ basically between sun and shade species, but seemed mainly to depend on the weight of the plant at the start of the expiment. It is concluded that the competitive ability of sun and shade plants does indeed correlate positively with the light intensity of their natural habitats. The effect of the red/far-red ratio on competitive ability cannot be predicted from the experiments, but it is probable that when competing, sun species will be disadvantaged by a low red/far-red ratio.

Summary Weed species in arable ecosystems are adapted to the conditions produced by agricultural management. In order to give preventive and curative guidelines to weed management one needs to predict the composition of the weed vegetation after certain (soil) cultivation measures. Dormancy mechanisms of seeds in the seedbank and the environmental conditions in the furrow before and after cultivation are usually such that the vegetation composition is heterogeneous. Multivariate analyses enable us to describe and classify weed vegetation and to analyse its relationship to environmental factors. Resume Analyses multivariances en malherbologie Les especes d'adventices dans les ecosystemes cultives sont adaptees aux conditions creees par les conditions culturales. Afin de donner des conseils preventifs et curatifs de desherbage, on a besoin de prevoir la composition de la flore apres certaines facons culturales (/sol). Les mecanismes de dormance des semences du stock grainier et les conditions environnementales dans le sillon avant et apres le travail du sol sont tels que normalement on a des population vegetatives heterogenes. Les analyses multivariances nous permettent de decrire et classifier les populations d'adventices et d'analyser leur relation avec les facteurs de l'environnement. Zusammenfassung Multivariate Analyse in der Unkrautforschung Die Unkrautarten in anbaufahigen Oekosystemen sind den Bedingungen angepasst, die jeweils durch die Anbaumethoden hervorgerufen werden. Um fur ein Unkrautmanagement praventive und kurative Richtlinien zu geben, ist es notwendig, die Zusammensetzung der nach bestimmten (Boden) Bearbeitungsmassnahmen auftretenden Unkrautflora vorherzusagen. Die Vegetationszu-sammensetzung ist gewohnlich heterogen wegen der Dormanzmechanismen des Samenvorrats im Boden und der Umweltbedingungen in der Furche vor und nach Bestellung des Bodens. Eine multivariate Analyse ermoglicht uns die Unkrautvegetation zu beschreiben und zu klassi-fizieren als auch ihre Beziehung zu Umweltfaktoren zu analysieren.

SUMMARY Bush bean seedlings were grown at 16 and 25°C and two light intensities. At 25°C the relative growth rate (RGR) was twice that of 16°C and plant development took half the time. This suggests a linear proportionality between relative growth rate and temperature in this range because the minimum temperature for growth and development is about 8°C for bush beans. This relation implies that the relative growth rate has a specific maximum which depends on temperature. Slow extension growth induced by low temperature leads to a high level of carbohydrate reserves under conditions of high light intensity. This explains why at 16°C the dry matter content of seedling leaves rises gradually whereas it remains practically constant at 25°C. This also explains why leaf area per unit leaf dry weight drops at 16°C (fig. 2) so that leaf area ratio (LAR) decreases after reaching a maximum (fig. 1). This, however, does not lead to a drop in relative growth rate which suggests that a compensation through a higher net assimilation rate (NAR) occurs. At 25°C the decline of RGR in low light intensity is relatively strong, probably because of a high respiration level. Light intensity and temperature have little influence on the specific weight of leaves (fig. 4). Furthermore leaf area per unit fresh leaf weight appears to be proportional to the inverse of leaf thickness (fig. 5). These findings suggest that inner leaf structure maintains a certain homogeneity independent of outward conditions. Cotyledons are exhausted by the young seedling at a constant rate which is higher in high temperatures and low light intensities.

Summary A two-species model was used to summarize results of four experiments with two component mixtures of barley (Hordeum vulgare L.), peas (Pisum sativum L.), Chenopodium album L. and Sinapis alba L. grown in pots in greenhouses. The results, covering a wide range of plant densities both in mixtures and pure stands, showed that the model was not always able to describe the relationship between the yield of a species and the densities of itself and its companion species. The deviations from the model were in all instances greatest for the least competitive species. The model that quantifies competition and niche differentiation appeared to be too general because competition between species was not independent of the wide plant density ranges used here. The results are discussed in relation to possible improvements of the model and an analogy between the principles of assessing crop weed competition and the assessment of the relative potency of herbicides in bioassay studies is put forward. Resume Estimation de la competition entre cultures el adventices Un modele a especes a ete utilise pour resumer les resultats des essais conduits avec des melanges a deux composantes d'orge (Hordeum vulgare L.) de pois (Pisum sativum L.) de Chenopodium album L. et de Sinapis alba L. cultives en pots sous serre. Les resultats, couvrant un large eventail de densites de plantes a la fois en melange ou seules, montrent que le modele ne permet pas toujours de decrire la relation entre le rendement d'une espece et les densites de celle-ci et des especes l'accompagnant. Les differences avec le modele etaient dans tous les cas plus importantes pour l'espece la moins competitive. Le modele qui quantifie la competition et la differentiation des sites apparait etre trop general car la competition interespeces n'est pas independante de l'eventail tres large des densites de plantes utilise ici. Les resultats sont discutes en relation avec des ameliorations possibles du modele et une analogie entre les principes d'une estimation de la competition culture/adventice et l'estimation du potentiel relatif des herbicides dans les essais biologiques est prevue. Zusammenfassung Bestimmung der Konkurrenz zwischen Kultur-pflanzen und Unkrautern Fur die Zusammenfassung der Ergebnisse von Konkurrenzversuchen mit jeweils zwei Partnern wurde ein 2-Arten-Modell verwendet; die Versuche mit Gerste (Hordeum vulgare L.), Erbse (Pisum sativum L.), Weisem Gansefus (Chenopodium album L.) und Weisem Senf (Sinapis alba L.) wurden im Gewachshaus in Topfen mit unterschiedlicher Pflanzendichte, sowohl in den Misch- als auch in den Reinbestanden, durchgefuhrt. Mit dem Modell lies sich die Beziehung zwischen dem Ertrag und der Dichte der einzelnen Arten und ihrer Konkurrenzpartner nicht immer beschreiben, wobei die grosten Abweichungen fur die konkurrenzschwacheren Arten auftraten. Mit dem Modell lassen sich die Konkurrenz- und Nischendifferenzierung beschreiben, doch erwies es sich als zu allgemein, weil die Konkurrenz von den hier gewahlten weiten Dichlebereichen nicht unabhangig war. Die Ergebnisse wurden in Hinblick auf eine Weiterentwicklung des Modells diskutiert, und es wurde auf eine Analogie zwischen den Prinzipien der Bestimmung der Kulturpflanzen/Unkraut-Kon-kurrenz und der Bestimmung der relativen Wirksamkeit von Herbiziden in Biotests hingewiesen.

SUMMARY In three experiments the effects of light intensity and nutrient supply (nitrate or phosphate) and their combined effects on the growth and morphogenesis of two shade-tolerant plant species and a non-tolerant species were studied. Nutrient supply was limited by placing the plants on a standard nutrient solution for a limited period each day and placing them on a nitrogen-free or phosphate-free solution for the rest of the day. The effects of light intensity and nitrate supply on growth and morphogenesis showed a marked interaction: low nitrate supply caused a much greater decrease in the relative growth rate under high light intensity, because of much larger changes in the dry matter ditribution; the net assimilation rate was only slightly affected by nitrate supply. The effects of light intensity and phosphate supply on the dry matter distribution and the net assimilation rate both showed interaction, but the effects on the relative growth rate were independent. Low phosphate supply caused greater changes in the dry matter distribution under high light intensity and a greater decrease in the net assimilation rate under low light intensity; the relative growth rate decreased to the same extent under both high and low light intensities. The experimental data were compared with the balanced quantitative model for root/shoot ratios proposed by Thornley (1972). The results were very satisfactory, but it was concluded that the model must be used in its exact form and that the use of approximations cannot be allowed.