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Title: Do soil microbes and abrasion by soil particles influence persistence and loss of physical dormancy in seeds of tropical pioneers?

Author
item ZALAMEA, P - Smithsonian Tropical Research
item SARMIENTO, C - Smithsonian Tropical Research
item ARNOLD, A - University Of Arizona
item Davis, Adam
item DALLING, J - University Of Illinois

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2014
Publication Date: 1/13/2015
Citation: Zalamea, P.C., Sarmiento, C., Arnold, A.E., Davis, A.S., Dalling, J.W. 2015. Do soil microbes and abrasion by soil particles influence persistence and loss of physical dormancy in seeds of tropical pioneers? Frontiers in Plant Science. DOI: 10.3389/fpls.2014.00799.

Interpretive Summary: Soil seedbanks (SSB) are important to the recruitment of early successional species, such as weeds and other pioneer plant species, as they protect populations of these plants from environments unsuitable for seed germination and seedling establishment. Understanding the mechanisms resulting in seed persistence in the SSB can contribute to improved management of weedy and invasive species. Seed persistence in SSB is often associated with physical dormancy, where seed or fruit coats are impermeable at the time of dispersal. Germination ecology literature often states, without empirical evidence, that dormancy-break experienced by these physically dormant seeds is the result of microbial action and/or abrasion by soil particles. We tested whether the seed dormancy-break in physically dormant seeds is controlled by microbial action and/or soil abrasion in four widely distributed tropical pioneer tree species. Seeds were buried at five sites in lowland tropical forest in Panama, and recovered at one, three, six and twelve months after burial. Germination, microbial infection, and seed coat permeability and thickness were measured. In contrast to expectation, the proportion of seeds infected by cultivable microbes decreased as a function of burial time for all species, while the proportion of permeable seeds decreased in three species. No change in seed coat thickness was observed for any of the four species. Our data refute the hypothesis that seed dormancy in physically dormant seeds is controlled by physical abrasion and/or microbial action. Instead, we highlight the existence of multiple dormancy types, where a fraction of each seed cohort is dispersed in a permeable state and germinate immediately regardless of microsite, while the impermeable seed fraction contributes to the SSB.

Technical Abstract: Germination from the soil seed bank (SSB) is an important determinant of species composition in tropical forest gaps, with seed persistence in the SSB allowing plants to recruit several decades after seed dispersal. The capacity to form a persistent SSB is often associated with physical dormancy, where seed or fruit coats are impermeable at the time of dispersal. Germination ecology literature often states, without empirical evidence, that dormancy-break experienced by these physically dormant seeds is the result of microbial action and/or abrasion by soil particles. We tested whether the seed dormancy-break in physically dormant seeds is controlled by microbial action and/or soil abrasion in four widely distributed tropical pioneer tree species (Apeiba aspera, Luehea seemannii, Ochroma pyramidale, and Cochlospermum vitifolium). Seeds were buried at five sites in lowland tropical forest in Panama, and recovered at one, three, six and twelve months after burial. Germination, microbial infection, and seed coat permeability and thickness were measured. In contrast to expectation, the proportion of seeds infected by cultivable microbes decreased as a function of burial time for all species, while the proportion of permeable seeds decreased in three species. No change in seed coat thickness was observed for any of the four species. In addition, under identical germination conditions we found that seeds stored in dark and dry conditions for two years in the laboratory showed a higher proportion of seed germination than fresh seeds. Our data refute the hypothesis that seed dormancy in physically dormant seeds is controlled by physical abrasion and/or microbial action. Instead, we highlight the existence of multiple dormancy phenotypes, where a fraction of each seed cohort is dispersed in a permeable state and germinate immediately regardless of microsite, while the impermeable seed fraction contributes to the SSB. Fluctuating temperatures associated with gap conditions are capable of directly breaking physical dormancy by loosening the seed's chalazal plug.