Residues of legume crops used to increase soil fertility may also serve as sources of phytotoxins that can suppress the germination and early growth of weed and crop species. To test the hypothesis that weed and crop susceptibility to extracts of red clover shoots would be inversely proportional to seed mass, we (1) identified 18 weeds and 44 crops whose 100-seed masses ranged from 20 to 26,250 mg; (2) exposed their seeds in petri dishes and filter paper to a 2% aqueous extract of ‘Marathon’ red clover shoots or distilled water; and (3) measured germination percentage and radicle length of germinated seeds after incubation for 4 days. In a second experiment, we assessed germination and radicle growth of four crop and four weed species after exposure to 1% extracts of Marathon or ‘Cherokee’ red clover or distilled water. Germination inhibition by red clover extracts was greatest for lighter seeds and least for heavier seeds in Experiment 1 (P = 0.0005), but was unrelated to seed mass in Experiment 2. Radicle inhibition by red clover extracts was inversely proportional to seed mass in both Experiment 1 (P < 0.0001) and Experiment 2 (P = 0.0047), and, in Experiment 1, was greater for monocots than dicots (P = 0.0002). Our findings corroborate the general relationship between seed mass and stress tolerance observed by other investigators and indicate that small-seeded monocots are most likely to be susceptible to phytotoxins contained in red clover shoots.

Nomenclature: Red clover, Trifolium pratense L.