Mechanisms underpinning the onset of seed coat impermeability and dormancy-break in Astragalus adsurgens

Ganesh K Jaganathan,Matthew Biddick,Yashu Yang,Kang Han,Yingying Han,Danping Song,Jiajin Li,Baolin Liu

doi:10.1038/s41598-019-46158-z

Ganesh K Jaganathan, Matthew Biddick + Show 6 more

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https://doi.org/10.1038/s41598-019-46158-z

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Abstract

Impermeable seed coats, i.e. physical dormancy (PY) influence the germination ecology of plants from 18 angiosperm families. Astragalus adsurgens (Fabaceae; Papilinoidaae) is a perennial plant widespread in temperate regions that is thought to produce both permeable and impermeable seeds. Why seeds vary in the permeability of their coat, in addition to the mechanisms by which impermeable seeds break dormancy, are not completely understood. However, seeds are often consumed by herbivores; a phenomenon that might facilitate the germination of impermeable seeds. Here, we tested whether: (1) moisture content plays a significant role in the onset of seed coat impermeability (and therefore PY) at similar ranges reported for species from tropical ecosystems; and (2) the presence of impermeable coats offer any benefits for seed survival when consumed by animals. We tested these hypotheses using A. adsurgens seeds collected from Inner Mongolia, China. Freshly collected seeds with a moisture content of 9.7% were permeable to water and therefore not physically dormant. However, seeds became impermeable when dried below a threshold of 6.5% moisture content. Treating impermeable seeds with hydrochloric acid effectively broke dormancy. Scanning electron microscope (SEM) revealed that HCl treated seeds had a narrow opening in the hilum and extra-hilar regions, through which water entered. Seeds with impermeable coats survived significantly better than permeable seeds when consumed by cows. Irrespective of coat permeability, most seeds were egested between 12 and 24 h. In seeds that maintained dormancy after gut passage, this was broken by additional acid scarification. Overall results suggest that: (1) seed coat impermeability is induced by reduced moisture content; (2) imbibition primarily occurs at the hilum and extra-hilar region; and (3) impermeable seeds may benefit from endozoochory.

Highlights

Astragalus adsurgens (Fabaceae: Papilionoideae) is a perennial plant inhabiting arid regions of China, as well as temperate North America, Siberia, Mongolia, and rarely in Japan[1,2]
Seeds that are shed at higher moisture content might be dried to levels that could onset impermeability when they persist in the soil, i.e. secondary dormancy, a feature where seeds are dispersed as non-dormant from mother plant and they become dormant after shedding It is crucial to understanding the mechanisms underpinning the onset of physical dormancy (PY) in temperate species
A. adsurgens seeds germinated at a wide-range of temperatures both in light and darkness (Fig. 3); consistent with seeds from the Qubei deserts of China[5]

Summary

Introduction

Astragalus adsurgens (Fabaceae: Papilionoideae) is a perennial plant inhabiting arid regions of China, as well as temperate North America, Siberia, Mongolia, and rarely in Japan[1,2]. It grows wild along the Yellow River valley in central and eastern China[3] and Tibetan grasslands[4]. Seeds that are shed at higher moisture content might be dried to levels that could onset impermeability when they persist in the soil, i.e. secondary dormancy, a feature where seeds are dispersed as non-dormant from mother plant and they become dormant after shedding It is crucial to understanding the mechanisms underpinning the onset of PY in temperate species. More detailed studies in some Fabaceae species have revealed that structures other than lens and hilar slit, e.g. pleurogram could act as a water-gap[39]

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