Early Growth and Development of Horseweed (<i>Conyza canadensis</i> (L.) Cronq.)

Abstract

Horseweed ( Conyza canadensis (L.) Cronq.) produces thousands of small elongated seeds which are botanically defined as achenes; yet, relative to the quantity of achenes produced, few seedlings survive to produce mature plants. The developmental progression from achene to 4 mm seedlings was documented, and seedling response to moisture deprivation was described. Radical protrusion through the pericarp occurred between 18 and 30 hours after onset of imbibition in water or when germinated on soil at or greater than field capacity. A ring of root hair initials formed immediately after radical emergence at the interface of what was to become the separation between the root and hypocotyl. By 48 hours post imbibition, radicals differentiated into a distinct root with root cap and a hypocotyl, and root hairs elongated. By 72 hours post imbibition, seedlings had emerged from the pericarp, and had: expanded photosynthetic cotyledons, a clearly defined hypocotyl, a ring of elongated root hairs exceeding 1 mm in length, and a root equal or longer than the hypocotyl. The epicotyl had not yet emerged, and the total seedling length was approximately 3 to 4 mm. Germination was delayed on soil at or below field capacity. More than 95% of two- and four-day - old seedlings that had been desiccated for more than 24 hours died after being rehydrated.