A Steep Cline in Flowering Time for Brassica rapa in Southern California: Population‐Level Variation in the Field and the Greenhouse

We examined clinal variation in flowering time in a series of Brassica rapa populations occupying a 4-kmlong urban greenbelt in southern California. Field studies on two populations showed that mean flowering date for populations of this winter annual differed by up to 3 wk during the 1998–1999 season, when an El Nin˜o event elevated precipitation well beyond normal levels. The degree of population difference was compressed in 1999–2000, when drought delayed onset of the growing season. In both years, the population occupying the drier site flowered earlier. Fitness functions, estimated separately for the two sites, indicated directional selection for early flowering in the dry site and a stabilizing component to selection at the wet site. Greenhouse experiments using other populations along the cline and conducted under different photoperiod conditions showed that when grown in a common environment, dry-site plants flower earlier than wet-site plants. This indicates a genetic basis for between-population variation. The degree of among-population variation in flowering phenology is compressed by long photoperiods.