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

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.

Keywords: Adenanthera pavonina (Fabaceae: Mimosoideae); artificial burial; summer temperatures; water gap.

Figure 1.

Mean percentage increase in seed mass is plotted against time for untreated, mechanically scarified and hot-water-treated mimetic seeds of Adenanthera pavonina sown on moist filter paper and stored in ambient laboratory conditions. Error bars represent the standard deviation of the mean.

Figure 2.

Mean percentage of Adenanthera pavonina seeds that broke dormancy when treated with hot water of different temperatures for 45 s. Different letters denote significant differences between treatments.

Figure 3.

Mean percentage of Adenanthera pavonina seeds that broke dormancy and germinated after being stored for either 1 or 2 months at summer temperatures of 15/60 °C and subsequently moved to autumn temperatures of either 15/45 °C or constant 30 °C. Different lower-case letters denote a significant difference between storage temperature after storing at 15/45 °C. Different upper-case letters denote a significant difference between 15/45 and 30 °C after 1- and 2-month storage period.

Figure 4.

Photomicrographs of longitudinal sections of Adenanthera pavonina seed coat in (A) non-hilar region and (B) hilar region. E, endosperm; H, hilum; LI, light line; Ml, mesophyll layer; Om, outer macrosclerids; Os, osteosclerids; PL, palisade layer. White arrows mark the thin palisade layer on the hilar surface.

Figure 5.

Scanning electron micrographs of (A) dormant and (B) non-dormant seeds showing the hilar side of the seeds. H, hilum; L, lens; M, micropyle. Scale: 700 µm.

Figure 6.

Percentage increase in mass of Adenanthera pavonina seeds dipped in hot water for 45 s and covered with petroleum jelly at either the lens or micropyle and hilum. Error bars represent the standard deviation of the mean.

Figure 7.

Maximum and minimum soil temperature in °C recorded at 3 and 7 cm below the soil surface at the burial site between August 2013 and August 2015. Seeds of Adenanthera pavonina buried at (A) 3 cm depth and (B) 7 cm depth were exhumed every 6 months from soil and assessed for germination at 20/25 °C. Germination of seeds buried in soil, germinated in the laboratory and germinated after dipping in boiling water are shown in percentages. Different lower-case letters indicate a significant difference in percentage of seeds germinated in the field and laboratory (combined).