This study demonstrates that Anthemis chrysantha, a ‘Critically Endangered’ annual plant, produces two morphs of achenes: white and dark achenes, which differ in size, mass, anatomy and germination behaviour. Fresh white achenes germinated at all temperatures assayed from 10 to 25°C in both continuous darkness and 12-h photoperiod, ranging between 24% at 25°C in darkness and 89% at 12/20°C in light, whereas fresh dark achenes did not germinate under any temperature or light conditions. To identify differences in dormancy type between the two morphs, germination of dry-stored achenes, and achenes stratified at 5 or 25°C for 2 months were tested in both darkness and light at 5, 15 and 12/20°C for dry-stored and warm-stratified (25°C) achenes; and at 15, 25 and 12/20°C for cold-stratified (5°C) achenes. Of the white achenes, 90% germinated during the cold stratification period. In general, dry storage and warm stratification did not increase germination compared to fresh achenes. However, dark achenes did not germinate under any conditions. Dark achene dormancy was only broken by mechanical scarification or by excising the embryo (germination reached 71%). An anatomical study showed that the mesocarp of dark achenes had no intercellular spaces and was much thicker and stronger than that of white achenes, making the entry of water difficult, and also preventing germination by mechanical restriction. This study demonstrated that dormancy in the dark achenes is likely caused by the thickness of their pericarp, physically impeding germination and hampering imbibition of water.
