Hairy vetch (Vicia villosa) seed size affects germination response to coumarin

The inhibition of seed germination by an allelochemical is generally greater in small seeds than in large seeds. This response may have significant impact on weed control by allelopathic cover crops, where the small-seeded weeds would be more effectively controlled than large-seeded species. The studies reporting these results used a large number of plant species that varied in seed size, which might have introduced differences in germination characteristics or various parameters required for growth and emergence. One approach to avoid factors associated with the use of a mixed seed size population is to separate a large population of seeds from a single species into various size classes. This allows the analysis of the effects of a given allelochemical directly as a function of seed size. Here, hairy vetch (Vicia villosa Roth.) seeds were separated into three classes based on their seed weight (large, 54 mg; medium, 40 mg; and small, 26 mg) and compared their germination response to coumarin at 10⁻³ and 10⁻⁵ M. There was no significant difference in germination to 10 ⁵ M coumarin with respect to seed size. However, germination was reduced and delayed in seeds exposed to 10⁻³ M coumarin, and this effect was more noticeable in the small seed class. Expressed as a percent of control, germination for large, medium and small seeds was 72, 76, and 55% respectively and germination of small seeds was also delayed by two days. These results confirm that small seeds are more sensitive to an allelochemical and this sensitivity is selective within a species with respect to small versus large seed. If expressed by a cover crop, this selection pressure could have significant impact on the weed community over time.