Differential germination by dimorphic achenes of yellow starthistle (Centaurea solstitialisL.) under water stress

Yellow starthistle germination in an environmental chamber was 74, 38, 3, and 0% at osmotic potentials of 0, −0•5, −1•0, and −1•5 MPa, respectively. Salinity levels of 0, 4, 8, and 12 dSm−1had an inconsistent effect on germination. The presence of a pappus (p-achene) was associated with an 11 and 15% increase in germination over those achenes without a pappus (np-achene) at 0 and −1•5 MPa, respectively. Field germination of p-achenes was 29 and 33% greater than np-achenes at La Grande, OR and Walla Walla, WA, respectively. Total germination (field plus lab) was not influenced by time of burial (3 months) or 1- and 2-cm burial depths. Dimorphic achenes appear to be associated with success in exploiting highly variable soil moisture regimes in semi-arid environments.