Degree Type


Date of Award


Degree Name

Doctor of Philosophy




The competitive effects of maize plants on weed growth, seed production, and seed dormancy were investigated. Weed seed dormancy was studied using ultrafreezing and electron microscope techniques. The importance of weed seed dormancy in weed control was discussed;Plants of four weed species, Setaria luteschens (Weigel) Hubb., Setaria viridis (L.) Beauv., Polygonum pensylvanicum L., and Abutilon theophrasti Medic., were grown free from competition with maize. Height of Pennsylvania smartweed and velvetleaf plants increased for plants in competition with maize plants; other plant growth parameters decreased. Plant growth and seed production for green and yellow foxtail decreased for plants that were in competition with maize plants. Reduction in weed seed production was over 90% for each of the four weed species when grown with maize competition;Maize competition also affected the dormancy of Polygonum pensylvanicum L. weed seeds. Although all seeds were dormant when harvested, 4% of the seeds from plants with maize competition and 35% of the seeds from plants without maize competition germinated after 15 weeks of prechilling; germinations were 94% and 92% respectively after 30 weeks of prechilling. When viewed with a scanning electron electron microscope, seed coats of seeds from plants with maize competition appeared well-channeled; but, seed coats from plants without maize competition were compact. Dormancy of seeds from plants growing without maize competition may result from an impermeable seed coat;The embryo epidermis was investigated using a transmission electron microscope. More lipid bodies and intercellular junctions occurred in the embryo epidermis of seeds (not prechilled or imbibed) from plants with maize competition. Dormancy of seeds from maize competition may result from embryo immaturity;Weed seed dormancy was also investigated using ultrafreezing. Echinochloa crus-gali (L.) Beauv. seeds were ultrafrozen in liquid nitrogen (-196C) for 5 minutes and thawed for 1 hour. Germination increased from 60% for not ultrafrozen seeds to 99% for seeds ultrafrozen 10 times. Seed coats were viewed with scanning electron, transmission electron, and light microscopes. No cracks or structural changes were apparent in seeds ultrafrozen 10 times. Also, water imbibition was the same. Ultrafreezing probably did not increase seed coat permeability to water;A transmission electron microscope was used to investigate the secretory epithelium of unimbibed seed that had been ultrafrozen 0, 4 or 10 times. Lipid bodies were dissociated by ultrafreezing. Lipid bodies reformed after water imbibition; but, protein bodies were digested faster in the secretory epithelium of ultrafrozen seeds.



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James Lowell Jordan



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97 pages