The effect of environmental conditions on the seasonal dormancy pattern and germination of weed seeds.

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 <strong>actual field temperature <u>after exhumation</u></strong> , 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 <strong>soil temperature <u>during burial</u></strong> . When nitrate was added during the test, the germinationtemperature range became wider and germination could occur during a longer period of the year