Population differentiation and natural selection for water-use efficiency in Impatiens capensis (Balsaminaceae)

In xeric environments, water-use efficiency (WUE) is likely to be a target of natural selection. Higher WUE is expected to confer a fitness advantage in drought conditions, but this prediction has rarely been tested. To examine the relationships between WUE and fitness, two common garden experiments were conducted. Inbred lines of Impatiens capensis derived from wet and relatively dry sites were planted into dry field conditions in an open and a woodland site. The lines from the dry population increased their instantaneous WUE in response to the dry conditions of the field to a greater degree than lines from the wet population. High instantaneous WUE also conferred a fitness advantage in the stressful conditions of the field. Thus, WUE may be an important trait for I. capensis populations in adapting to drier conditions.