A perennial invader's seed and rhizome differ in cold tolerance and apparent local adaptation
Extreme cold plays a key role in the range boundaries of plants. Winter survival is central to their persistence, but not all structures are equally susceptible to frost kill and, therefore, limiting to distributions. Furthermore, we expect intraspecific variation in cold tolerance both within and among tissue types. In a laboratory setting, we determined freezing tolerances of two overwintering propagule types - seeds and rhizomes - of the globally invasive Johnsongrass (Sorghum halepense), testing apparent emergence and electrolyte leakage as a proxy for cell death. We used 18 genotypes from agricultural and non-agricultural habitats spanning the climatic extremes occupied by Johnsongrass in the US. Single node rhizome fragments had an average LT90 of-5.1 degrees C with no significant variation based on home climate or ecotype. Seeds frozen at-85 degrees C suffered a decline in germinability to 10% from 25% at 22 degrees C. Population origin did not affect seed response to any temperature. However, non-agricultural seeds germinated more and faster than agricultural seeds from the coldest climates, with a reversed relationship among warmest origin seeds. Regardless of ecotype, seeds from the cold/dry and wet/warm sectors of Johnsongrass's range germinated more and faster. Drastic differences in cold tolerance between seeds and rhizome and evidence for seeds' local adaptation to land use and climate suggest that its spread is likely limited by winter rhizome survival, as well as adaptability of germination behavior to longer winters. These findings shed light on Johnsongrass' dispersal dynamics and help identify future avenues for mechanistically understanding its range limitation.