EFFECT OF ALFALFA PLANT AND SOIL EXTRACTS ON GERMINATION AND GROWTH OF ALFALFA

Alfalfa (Medicago sativa L.) plants contain water-soluble substances t hat inhibit the germination and seedling growth of alfalfa. Determinin g where allelochemicals may be found in alfalfa in the greatest concen tration would aid in trying to isolate the compound or compounds respo nsible for autotoxicity. This study investigated the allelopathic effe cts of various alfalfa plant parts, and the soil in which alfalfa had been grown, on alfalfa germination and seedling growth. Aqueous extrac ts of alfalfa leaf, stem, flower, seed, and foot plant parts were made to determine their effects on germination and dry weights of hypocoty l, radicle and total length of 5-d-old alfalfa seedlings over a range of extract concentrations. Soil samples (Flanagan series: fine, montmo rillonitic, mesic Aquic Argiudolls) from around alfalfa plants at the vegetative and reproductive stages were compared with sterilized and n onsterilized soil formerly seeded with alfalfa, hairy vetch (Vicia vil losa Both), and winter rye (Secale cereale L.). Increasing the aqueous extracts concentrations of separated alfalfa plant parts significantl y inhibited alfalfa germination, seedling length and weight. Radicle l ength was more sensitive to extract source than seed germination or hy pocotyl length. Based on 5-d-old alfalfa radicle length growth, and av eraged across all extract concentrations, the degree of toxicity of di fferent alfalfa plant parts and soil from around alfalfa can be classi fied in order of decreasing inhibition as follows: leaf, seed, complet e plant mixture, soil, root, flower, and stem. Leaf extracts (12 g kg( -1)) caused a 48% decrease in water uptake by alfalfa seed. Soil in wh ich alfalfa had previously grown was the most inhibitory to alfalfa gr owth after 25 d of growth compared with soil where winter rye or hairy vetch had previously grown. Inhibitory effects were greater for soil collected around alfalfa grown at the reproductive than the vegetative growth stage. These data indicate that alfalfa autotoxicity may resul t from a release of one or more water-soluble compounds from alfalfa l eaf tissue.