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

Authors: Williams, Robert; Bartholomew, Paul

Submitted to: Allelopathy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2010
Publication Date: 4/1/2011
Citation: Williams, R.D., Bartholomew, P.W. 2011. Hairy vetch (Vicia villosa) seed size affects germination response to coumarin. Allelopathy Journal. 27(2):237-244.

Interpretive Summary: Cover crops are used between the summer harvest of one crop and the spring replanting of another. Often these crops are plowed into the soil to serve as a green manure, providing both nitrogen and organic matter for the following crop. Many of the best cover crops have allelopathic properties that aid the crop in controlling weeds. Several researchers have found that seed of small seeded species are inhibited to a greater extent by extracts of cover crops, or by the pure chemical, than seed of larger seeded species. However, when seed of different sizes from different species are compared other factors that influence germination, for example differences in seed coats or optimum germination temperatures that may hide allelopathic effects. To avoid this problem we screened bulk seed of hairy vetch, a common legume forage, into three sizes that varied in 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 exposure to 10-3 M coumarin delayed and reduced seed germination and this effect was more noticeable in the small seed class. Expressed as a percent of control, germination for large, medium and small seed was 72%, 76%, and 55%, respectively, and germination of the small seed was delayed by two days. These results confirm those reported in the literature that small seeds do appear to have a greater sensitivity to an allelochemical, and that this sensitivity is selective within a species with respect to small versus large seed. If cover crops selectively control smaller seeded weeds, over time the weed population will shift to larger seeded weed species, which may mean that management practices will have to change to control the shift in the weed population.

Technical Abstract: 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 we separate hairy vetch (Vicia villosa Roth.) seed into three classes that varied in seed weight (large, 54 mg; medium, 40 mg; and small, 26 mg) and compare 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 exposure to 10-3 M coumarin delayed and reduced seed germination and this effect was more noticeable in the small seed class. Expressed as a percent of control, germination for large, medium and small seed was 72%, 76%, and 55%, respectively, and germination of the small seed was delayed by two days. These results confirm those reported in the literature on seeds from a number of species with different sized seeds, i.e., that small seeds do appear to have a greater sensitivity to an allelochemical, and that 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.