Germination ecology of Rorippa subumbellata (Tahoe yellow cress), an endangered, endemic species of Lake Tahoe

We studied the germination ecology of Rorippa subumbellata Rollins (Tahoe Yellow Cress, Brassicaceae), an endangered perennial plant endemic to the shores of Lake Tahoe (California and Nevada, USA). The effects of light, temperature, cold stratification, dry storage, and dispersal by hydrochory were tested in both greenhouse and growth chamber experiments. Seeds from four collection years (2001, 2002, 2003, and 2004) were either cold stratified for 14, 30, 60, or 90 days or not stratified and incubated in three alternating day/night temperature regimes (13/−1, 18/4, and 24/10°C) for thirty days. Hydrochory and light experiments were carried out in a cool greenhouse (mean temperature 21°C). Temperature was the most important factor for germination, and germination reached 60-80% when germinated at 24°C. Cold stratification at 5°C for more than a week decreased germination. Seeds required light to germinate. Seeds did survive flotation, but with decreased germination. Seeds dry stored for four years or less germinated nearly as readily as freshly harvested seeds, and germination in the older seed lots exhibited less sensitivity to lower temperatures.