The lens in focus – lens structure in seeds of 51 Australian Acacia species and its implications for imbibition and germination

Abstract

Acacia s. str. (Mimosoideae, Fabaceae) is the largest plant genus in Australia (~1000 species). Its seeds have physical dormancy from a hard, water-impermeable testa. Heat from fire (natural systems) and hot water (nursery production) can break this dormancy. It is often reported that these treatments ‘soften’ or ‘crack’ the seed coat, but in practice they only affect a minute part of the seed coat, the lens. We examined lens structure in a wide range of Acacia species to determine what diversity of testa and lens structure was present, if there were differing responses to a hot water dormancy breaking treatment and if there were structural differences between soft- and hard-seeded species. Seed morphology, testa and lens structure were examined before and after hot water treatment (~90°C for one minute), in 51 species of Australian Acacia from all seven sections, from all states and territories of Australia and from a wide range of environments. Five of the species had been noted to produce non-dormant seed (‘soft-seeded’ species). Average seed mass per species ranged from 3.1 to 257.9 mg (overall average 24.2 mg, median 13.8 mg). Almost all species had a relatively thick seed coat (average 132.2 µm) with well-developed palisade cells (average 41.5 µm long) and a lens which ‘popped’ in response to hot water treatment. For 44 species ranging in average seed mass from 3.1 to 43.9 mg (×14 range), the unpopped lens area only ranged ×3 (11480–36040 µm2). The lens was small (in 88% of species the average length of the unpopped lens was <300 µm) and the unpopped lens area was a minute proportion of seed surface area (average 0.10%). A. harpophylla (soft-seeded species) had a thin testa (37.3 µm) without obvious palisade cells and did not have a functional lens. In hard-seeded species the morphology of the popped lens varied widely, from a simple mound to complete detachment. A functional lens is not a universal feature in all genera of the Mimosoideae, including several species in a genus (Senegalia) previously included in Acacia s. lat. On the basis of the 51 investigated species a lens was present in all Australian acacias, although non-functional in two soft-seeded species. Although the lens was, on average, only ~1/1000th of the surface area of an Acacia seed and thus easily overlooked, it can have a profound influence on imbibition and germination. An assessment of lens structure, before and after heat treatment, can be of considerable use when interpreting the results of Acacia germination experiments.

Evolutionary history of fire‐stimulated resprouting, flowering, seed release and germination

“…is of particular interest. Apart from the challenge of documenting its fire‐related traits (Burrows et al, ), fossil evidence for its origin is in dispute: Brown, Murphy, & Ladiges () took it at 18 Mya on the basis of polyad pollen (a special feature of acacias) that is generally accepted, but Milne () interpreted similar pollen as Acacia ‐related 10 My earlier. Without accurately identified and dated fossils, molecular clock estimates are flawed, and the more fossils that are located, the increased likelihood that some will be older than those currently accepted as oldest.…”

“…Fire promotes the germination of soil‐stored seeds. The mechanisms vary greatly from ( i ) heat cracking the thick cuticle, opening the lens or dislodging the strophiole to allow imbibition to occur, ( ii ) chemicals in smoke, charate or ash that stimulate germination by breaking physiological dormancy, to ( iii ) wide diurnal temperature fluctuations that rupture parts of the seed coat and increase permeability to water (Brits, ; Cochrane et al, ; Flematti et al, ; Burrows, Alden, & Robinson, ). This ensures that germination is cued to post‐fire conditions when light, water and nutrient resources are optimal.…”

“…Here, they have invaded non‐fire‐prone arid areas (e.g. Mariosousa in the Sonoran Desert, ‘soft’‐seeded acacias in central Australia; Burrows et al, ) or rainforests (e.g. Koompassia malaccensis in Malaysia; Sasaki, ), frequently burnt (inter)tropical grasslands where seed storage is redundant [Lamont et al, ; e.g.…”

Markedly different patterns of imbibition in seeds of 48 Acacia species

“…For A. cambagei, only a control treatment was conducted as there was an insufficient number of seeds for a HW treatment. As in A. harpophylla, there was 100% imbibition and average 153% increase in mass after 23 h, with 67% germination after 71 h. Unlike the two species above, A. oswaldii had a hard seed coat, well-developed palisade cells and a functional lens (Burrows et al, 2018). Twenty-three per cent of the control seeds had imbibed after 25 h and additional control seeds imbibed over the next 8 days, with a final average imbibition of about 68% after 8 days.…”

“…Seed plus aril mass was based on the 60 seeds per species from the present study. The other four parameters could not be based on the seeds used in the present study (would have either destroyed seeds or disturbed the area around the hilum and lens) so data from Burrows et al (2018) was used as this study used the same seed batches. Rank correlation was also investigated of average HW imbibition t 50 with average annual rainfall for 29 species for which a specific collection locality and average annual rainfall were known.…”