Abstract
AbstractThe seeds of most Australian acacias have pronounced physical dormancy (PY). While fire and hot water (HW) treatments cause the lens to ‘pop’ almost instantaneously, for many Acacia species the increase in germination percentage can be gradual. If PY is broken instantly by HW treatment, why is germination often an extended process? Control and HW treatments were performed on seeds of 48 species of Acacia. Seeds were placed on a moist substrate and imbibition was assessed by frequently weighing individual seeds. In the two soft-seeded species all control seeds were fully imbibed within 6–24 h, while in hard-seeded species very few control seeds imbibed over several weeks. In 10 species over 50% of the HW-treated seeds imbibed within 30 h, but mostly the percentage of imbibed seeds gradually increased over several weeks. Some seeds in a replicate would imbibe early, while others would remain unimbibed for many days or weeks then, remarkably, become fully imbibed in less than 24 h. While HW treatment broke PY almost instantaneously, it appeared that in many Acacia species some other part of the testa slowed water from reaching the embryo. This process of having staggered imbibition may be a way of ensuring not all seeds in a population germinate after small rain events. Thus it appears the lens acts as a ‘fire gauge’ while some other part of the seed coat acts as a ‘rain gauge’.
Highlights
Seeds of almost all Australian Acacia species have physical dormancy (PY) due to a waterimpermeable epidermal layer of palisade cells in the testa (Burrows et al, 2009, 2018)
We studied seed imbibition at the individual seed level for a wide range of Australian Acacia species to investigate why a staggered germination can occur when PY has clearly been broken at the lens
Most imbibition of control seeds in the first 24 h was due to water entry through bruchid exit holes, often minute, in the seed coat, with physical damage such as cracks in the testa observed
Summary
Seeds of almost all Australian Acacia species have physical dormancy (PY) due to a waterimpermeable epidermal layer of palisade cells in the testa (Burrows et al, 2009, 2018). Acacia seeds can remain viable and with PY unbroken after many decades in the soil and after storage in seed stores (Cavanagh, 1980). Following various natural events (e.g. fire) or nursery treatments (e.g. hot or boiling water), a very small part of the palisade layer (the lens) ‘pops’ almost instantaneously, breaking PY and allowing the seed to imbibe and germinate when moisture becomes available. Rate of imbibition depends on water availability, seed structure and composition, water permeability of seed coat layers and temperature (Obroucheva, 1999). Imbibition is usually slower at lower temperatures, which is usually attributed to the increased viscosity of water at lower temperatures and to the effect of temperature on plant membranes (Booth and Bai, 1999). The effect of temperature on imbibition of Acacia seeds has rarely been studied
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