Seed hydration-dehydration in an allelochemical (coumarin) alters germination and seedling growth

Generally, when individual allelochemical compounds are tested at concentrations commonly found in the soil, using seed germination or seedling growth bioassays, germination is only delayed and seedling growth is inhibited slightly. However, seeds in the soil may pass through several hydration-dehydration cycles prior to germination. We examined the effect of imbibition of an allelochemical on subsequent seed germination and seedling growth, by exposing radish (Raphanus sativus L.) seeds to coumarin during hydration-dehydration cycles. Seed germination after 24 h exposure to the allelochemical, was 92%, 85%, 51%, 0% in the control and coumarin at 10(-5), 10(-4) and 10(-3) M, respectively, and was 99%, 96%, 96% and 24%, respectively at 48 h after exposure. When seeds were exposed to 10(-5) and 10(-3) M coumarin, during imbibition for 2, 4 or 6 h, and transferred to water, Subsequent germination was delayed. The longer the exposure to 10(-3) M coumarin, the greater was delay in germination. Seeds exposed to coumarin (10(-5) or 10(-3) M) for 5 h and air dried for 14 h still showed effect of allelochemical pretreatment. Seeds imbibed in 10(-5) or 10(-3) M coumarin, or water for 5 h, air dried for 14 h and germinated in water, 10(-5) M or 10(-3) M coumarin, reduced the radicle lengths at 48 h. If the seeds were hydrated in coumarin and dehydrated several times, subsequent radicle elongation was also reduced. Seeds hydrated in 10(-3) M coumarin for 1 to 3 cycles and geminated in either 10(-5) M coumarin or water showed a decrease, while seeds hydrated in water for 1 to 3 cycles and germinated in 10(-3) M coumarin showed a slight increase in radicle length with increased number of cycles. It appears that seeds in the soil could absorb an allelochemical during hydration - dehydration that would inhibit or delay germination and/or affect seedling growth rate, which would provide neighboring seedlings opportunity for faster establishment and a competitive advantage.