Allelopathy and Autotoxicity in Alfalfa: Characterization and Effects of Preceding Crops and Residue Incorporation

Alfalfa (Medicago sativa L.) is known to be both autotoxic and allelopathic. Greenhouse and laboratory experiments were conducted to determine if ‘WL-316’ alfalfa exhibits short-term autotoxicity and long-term autotoxicity and allelopathy. Long-term autotoxicity and allelopathy of alfalfa were verified at Urbana, IL, by comparing the germination and growth of alfalfa and sorghum [Sorghum bicolor (L.) Moench] on Flanagan silt loam (fine, montmorillonitic, mesic Aquic Argiudoll) previously cropped to alfalfa (alfalfa-soil) and sorghum (sorghum-soil). Short-term autotoxicity of alfalfa was investigated by studying the effect of incorporating its roots only and both roots and shoots on the germination and growth of alfalfa in alfalfasoil and sorghum-soil. The data were further supported by a laboratory bioassay of seedling exudate and shoot leachate of alfalfa and sorghum. Plant height and fresh weight per plant of alfalfa and fresh weight per plant of sorghum were lower on alfalfa-soil than on sorghum-soil. Germination percentages of both alfalfa and sorghum and plant height of sorghum were unaffected by the preceding crop. The two soils differed in nutrient content, but fertility was high and should not have been limiting to the growth of either crop. As a result, allelopathic/autotoxic compounds in alfalfa-soil were implicated in the growth inhibition of the two crops. Soil incorporation of fresh alfalfa roots only or both roots and shoots reduced alfalfa emergence, plant height, and dry weight per plant. Primary effects of water-soluble inhibitory compounds from alfalfa shoot appeared to be on germination and radicle elongation, the latter being apparently more sensitive than the former. Alfalfa allelopathy seems to be more severe than autotoxicity. A flow diagram describes different kinds of allelopathy and autotoxicity and various situations that verify the existence of a particular kind of allelopathy or autotoxicity Part of Project no. 375 of the Agric. Exp. Stn., College of Agic., Univ. of Illinois at Urbana-Champaign.