Early growth and development of Horseweed (Conyza canadensis (L.) Cronq.)

Authors: Molin, William; Parys, Katherine; Beck, Christine

Submitted to: American Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2020
Publication Date: 1/9/2020
Publication URL: https://handle.nal.usda.gov/10113/6949577
Citation: Molin, W.T., Parys, K.A., Beck, C.L. 2020. Early growth and development of Horseweed (Conyza canadensis (L.) Cronq.). American Journal of Plant Sciences. 11:40-50.

Interpretive Summary: Horseweed plants produce thousands of seed, but relative to the seed rain produced, few seedlings survive overwintering. Scientists in the USDA-ARS Crop Production Systems Research Unit, Stoneville, MS, conducted research to elucidate the early growth stage of horseweed seed and seedling response to dehydration. The results showed that when seeds were provided a suitable substrate, germination and seedling development proceeded quickly passing through defined stages of development, and the seedlings were extremely sensitive to drying as might be observed several days after a rain event. These results are important to farmers because they define a stage of development most susceptible to horseweed control by cultivation.

Technical Abstract: Horseweed (Conyza canadensis (L.) Cronq.) produces thousands of small, elongated seed, 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 below field capacity. More than 95% of two- and four-day old seedlings that had been desiccated for more than 24 hours failed to resume growth after being rehydrated and died.