Determination of the water gap and the germination ecology of Adenanthera pavonina (Fabaceae, Mimosoideae); the adaptive role of physical dormancy in mimetic seeds.

Matthew Biddick,Ganesh K Jaganathan,Kirsty J Yule

doi:10.1093/aobpla/ply048

Matthew Biddick, Ganesh K Jaganathan + Show 1 more

Open Access

https://doi.org/10.1093/aobpla/ply048

Copy DOI

Abstract

Dormancy caused by impermeable seed coats, i.e. physical dormancy (PY), regulates the timing of seed germination in species of several genera belonging to 18 angiosperm families. Physical dormancy also occurs in some mimetic species whose seeds mimic brightly coloured, fleshy fruits or arilled seeds. However, the conditions that break dormancy, as well as the location of water gaps in mimetic seeds, remain unclear. Here, we investigated the adaptive role of impermeable coats in the mimetic seeds of Adenanthera pavonina (Fabaceae: Mimosoideae). Specifically, we explored: (i) the conditions that break PY; (ii) the location of the primary water gap that forms during dormancy break; and (iii) the effect of seasonal temperature regimes on seed germination. Seeds were subjected to hot-water treatment, rapid temperature fluctuations and storage at temperatures mimicking summer and autumn conditions. Seed coat anatomy and water-gap regions were characterized using scanning electron microscopy (SEM) and light microscopy. Seeds were artificially buried in the field at 3 and 7 cm depths and exhumed every 6 months for 2 years to monitor germination. Adenanthera pavonina had impermeable seed coats, and thus PY. Seeds treated with hot water and exposed to summer–autumn temperature regimes broke dormancy. Water entered only through the lens (Type-II simple) due to dislodgement of the palisade layer. Seeds buried at 3 cm depth had significantly higher germination than those buried at 7 cm depth, with germination primarily occurring in autumn. Seeds required high summer temperatures followed by moderate autumn temperatures to become permeable to water and germinate in the field during the wet season. We conclude that the impermeable seed coat of A. pavonina is an adaptation that synchronizes germination with the growing season.

Highlights

Dormancy caused by impermeable seed coats, i.e. physical dormancy (PY), regulates the timing of seedAoB PLANTS https://academic.oup.com/aobplaJaganathan et al – Adaptive role of dormancy in mimetic seeds palisade layer of lignified Malphian cells become impermeable to water and, occasionally, gases (Baskin et al 2000; Jaganathan 2016)
Imbibition Fresh, intact seeds were tested for an impermeable seed coat via their imbibition ability and compared to seeds treated with mechanical scarification or hot water
We report four key findings: (i) mature seeds have an impermeable seed coat that render them physically dormant; (ii) dormancy is effectively broken by hot-water treatment (>90 °C) and summer–autumn climatic conditions, both in the laboratory and in the field; (iii) imbibition occurs only at the lens, which is the primary water gap; (iv) germination in the soil is determined by seasonal temperature changes that occur during summer and autumn

Summary

Introduction

Dormancy caused by impermeable seed coats, i.e. physical dormancy (PY), regulates the timing of seedAoB PLANTS https://academic.oup.com/aobplaJaganathan et al – Adaptive role of dormancy in mimetic seeds palisade layer of lignified Malphian cells become impermeable to water and, occasionally, gases (Baskin et al 2000; Jaganathan 2016). ‘water gaps’, open, allowing imbibition of the embryo (Baskin and Baskin 2014). Environmental changes, such as seasonal temperature fluctuations, fire, or passing through the gut of an animal can break dormancy, causing germination to occur in the subsequent growing season when conditions are favourable (Baskin and Baskin 2014; Jaganathan 2015; Jaganathan et al 2016). Physical dormancy occurs in some mimetic species whose seeds mimic brightly coloured, fleshy fruits or arilled seeds (Peres and van Roosmalen 1996). The adaptive role of impermeable seed coats in mimetic species remains largely unknown

Methods

Results

Discussion

Conclusion