Dormancy-breaking and germination requirements in seeds of the summer annual Palafoxia callosa (Asteraceae) from central Texas

Seeds (achenes) of Palafoxia callosa from central Texas were dormant at maturity in late summer, and stratification was a pre-requisite for germination. With an increase in length of the stratification period at 5°C from 2 to 8 weeks, the minimum thermoperiod at which ≥50% of seeds germinated decreased from 30(12 h)/15(12 h)°C to 15/6°C. Seeds germinated to 94 and 93% in light (14-h daily photoperiod) at 15/6 and 20/10°C, respectively, after 12 weeks (2,016 h) at 5°C in light. Seeds stratified for 12 weeks in darkness germinated to 42 and 55% in darkness at 15/6 and 20/10°C, respectively. However, seeds kept continuously at 15/6°C in light for 14 weeks (total of 1,176 h at 6°C) germinated to 95%. After 12 weeks of dry storage under ambient laboratory conditions, the highest germination in light and in darkness was 19% (at 30/15°C) and 8% (at 20/10°C), respectively. An overlap between the minimum temperature at which seeds can germinate and habitat temperatures would occur in late January or early February in central Texas. Germination in winter ensures that seedlings are well-established before the onset of summer drought in the shallow, rocky limestone soils of its habitat.