Submitted to: Allelopathy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2012
Publication Date: 10/3/2012
Citation: Williams, R.D., Bartholomew, P.W. 2012. Radish (Raphanus sativus) seed size affects germination response to coumarin. Allelopathy Journal. 30(2):271-280.
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. Earlier we showed that when seeds of hairy vetch were separated in to classes of different sizes, the smaller seed were more sensitive to an allelochemical. However, the quality among the seed sizes differed and the smaller vetch seed had poorer germination than the medium and large seed. Here we repeated the study with radish seed which had excellent germination regardless of seed size. There was no significant difference in germination to 10-5 M coumarin with respect to seed size. However exposure to 10-3 M coumarin slightly delayed and significantly reduced seed germination and this effect was more noticeable in the smaller seed. The final germination at 72 h was 90%, 76%, and 63% for the large, medium and small radish seed. These results confirm our earlier report, as well as 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. 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. Earlier we demonstrated that when hairy vetch (Vicia villosa Roth.) seeds were separated as to seed size, the smaller seed were more sensitive to an allelochemical, and this sensitivity is selective with in a species with respect to large versus small seed. Here we confirm those observations with radish (Raphanus sativus L.) seed. Radish seed were carefully screened into three seed-size classes: large (>2.4 mm), medium (>2.0 but <2.4) and small (<2.0 mm) seeds. Water uptake during imbibition was greater in the small seed as compared to the large and medium seed, but there was no significant difference in the final germination among the three seed sizes. When coumarin (10-3 M) was added to the uptake solution the water uptake was slightly less in the treatment solution as compared to the control (water only) solution, and this difference in uptake was more noticeable in the smaller seeds. There was no difference in the germination rate or final germination percentage between coumarin at 10-5M and the water control in the large and medium seeds. However, there was a marked statistical difference the seed germination response to coumarin at 10-3M for all seed sizes. Seed germination was delayed and reduced by 10-3M coumarin, and the effect of the reduction in germination increased as seed size decreased. Final germination was 90%, 78% and 63% for the large, medium and small seeds, respectively. As suggested by the literature, small seed are more sensitive to allelochemicals and this sensitivity might have long term affects as to shifts in weed patterns in areas where cover-corps are used as part of the weed management system.