Markedly different patterns of imbibition in seeds of 48 Acacia species

Abstract

The 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’.

Germination and Early Growth of <em>Acacia nubica </em>Benth.<em> In vitro</em>

“…Improved seed germination using 30 g/L sucrose compared to low and high concentrations was also reported on another Acacia species, namely A. sieberiana [14], and on other tree species such as Prunus armeniaca [24], Boswellia serrata [25], and Drypetes roxburghii [26]. However, sucrose at 0-25 g/L revealed the same percentage germination of Olea europaea [23]. The pattern of germination was also affected by the sucrose level, as the lower and intermediate concentrations (10-30 g/L) revealed a peak of germination at 12 days, compared to 9 days at the higher concentrations (40 and 50 g/L).…”

“…This arrangement of emergence limits uniformity of germination, spreading the chances of germination inability over several seasons. After studying 48 Acacia species, Burrows et al [23] concluded that the lens part of the testa of pretreated seed delayed water from reaching the embryo, ensuring not all seeds in a population germinate after short rain events. In this process, some seeds of a single species in one treatment imbibe early, while others remain un-imbibed for a while before imbibing.…”

Pre-Germinative Treatments and Morphophysiological Traits in Enterolobium cyclocarpum and Piscidia piscipula (Fabaceae) from the Yucatan Peninsula, Mexico

“…and the percentage of imbibed seeds was also low (≤4%). In species of the genus Vachellia belonging to Fabaceae, Burrows [ 21 ] reported differential responses in the percentage of imbibed seeds for 48 Australian species. In terms of imbibition rate (fresh weight gain through time), Galíndez [ 22 ] documented similar low imbibition in two populations of Amburana cearensis (4.6% ± 3.13% and 6.6% ± 2.81% respectively), but high imbibition values in Myroxylon peruiferum (81.4% ± 4.01% and 95.2% ± 3% respectively).…”

Germination and Early Growth of Acacia nubica Benth. In vitro

“…This arrangement of emergence limits the uniformity of germination, spreading the chances of germination inability over several seasons. After studying 48 Acacia species, Burrows et al [24] concluded that the lens part of the testa of pretreated seed delayed water from reaching the embryo, ensuring not all seeds in a population germinate after short rain events. In this process, some seeds of a single species in one treatment imbibe early, while others remain un-imbibed for a while before imbibing.…”