Dormancy release as influenced by duration of outdoor winter chilling in Florence (Italy) was studied under different photoperiodic and temperature treatments in collected twigs of two European (Ulmus glabra Huds. and Ulmus minor Mill.) and four Asian (Ulmus pumila L., Ulmus parvifolia Jacq., Ulmus macrocarpa Hance and Ulmus villosa Brandis) elm clones. Photoperiod had no effect on dormancy release, and there was no evidence that photoperiod affected bud burst during quiescence in the studied elm clones. Thermal time (day degrees >0 degrees C) to bud burst decreased in all the clones with increasing outdoor chilling. Although all the clones exhibited a rather weak dormancy, they significantly differed from each other. Dormancy was released earlier in the Asian than in the European clones, and the clones could be ranked from the U. pumila clone (very weak and short dormancy) to the U. minor clone (relatively stronger and longer dormancy), the other clones being intermediate. In all the clones except U. minor, the observed decrement in thermal time to bud burst was efficiently explained as an inverse exponential function of the number of chill days < or =5 degrees C received outdoor in autumn and winter. Endodormancy, as measured by the single-node cuttings test, was weak and short in all the clones. The latter result suggests that correlative inhibitions were largely responsible for preventing bud burst during winter in these elm clones.