Alfalfa (Medicago sativa L.) autotoxicity: Current status

Allelopathy plays a key role in both natural and managed ecosystems, especially agroecosystems. Autotoxicity in alfalfa (Medicago sativa), is specialized form of allelopathy, in which one or more chemical compounds from older plants affect their own seedlings. The chemical exudates released from alfalfa cause a “soil sickness” problem in agriculture and crop rotation has been suggested as the best solution. Autotoxic chemical(s) are moved from the plant, mainly the leaves, to the soil by transfer mechanisms and their subsequent dissipation occurs in the soil. There are two major practical considerations regarding alfalfa autotoxicity, (i) the needed delay of up to 2 years for replanting alfalfa after destroying an old alfalfa stand and (ii) inability to overseed directly to increase the plant density of thinned established alfalfa stands. An autotoxic zone of about 20 cm radius exists around the old plants. Characteristics of the autotoxic chemical and its effects are: (a) extractable from fresh alfalfa herbage and is not a product of microbial action, (b) more concentrated in alfalfa herbage than in roots, (c) water-soluble and moves in the soil, (d) delays or inhibits germination, (e) inhibits alfalfa root growth and causes swelling, curling and discolouration of the root, (f) roots lack root hairs and (g) reduces alfalfa root growth more than it reduces seed germination. Genetic solutions can address the problem by altering the plant production of autotoxic chemical(s), or increasing the tolerance of alfalfa seedlings to the chemical(s). Until the chemical(s) is identified, there will be a need for precise bioassays to develop methodologies to assess ecological effects and potentials for genetic solutions. Once the chemical(s) is known, more specific studies on ecology and variety improvement, perhaps using molecular techniques, should be feasible.