The effect of drought stress on inbreeding depression in four populations of the Mediterranean outcrossing plant Crepis sancta (Asteraceae)

The effect of physiological stress on the magnitude of inbreeding depressio n in plants has been the subject of few studies and is currently controvers ial because of contradictory results. We measured the inbreeding depression at three drought stress levels, precisely defined by a preliminary physiol ogical experiment. We also tested the hypothesis that more highly self-comp atible populations exhibit reduced inbreeding depression due to purging of deleterious mutations. The study was conducted on two populations of the an nual and allogamous plant Crepis sancta collected from the French Mediterra nean region and two other populations from marginal areas with various self -incompatibility levels. Drought stress did not increase inbreeding depress ion in terms of plant mortality but significantly increased the inbreeding depression for the date of first flowering, number of heads per plant and r elative growth rate. The most self-fertile marginal population showed an ab sence of inbreeding depression in most of the measured traits indicating th at purging could have taken place in this population. The three others popu lations showed relatively low and similar estimates of inbreeding depressio n (delta approximate to 0.35). The relatively low values obtained compared to the results found in allogamous plants suggests that the absence of comp etition for C. sancta in our experiment probably underestimated the effects of inbreeding in natural populations where competition occurs.