The germination behaviour of S. nigrum L. and S. physalifolium Rusby var. nitidibacatum (Bitter) Edmonds is compared, based on temperature requirements during imbibition. Three seed lots of S. nigrum had base temperatures (Tb) between 7.5°C and 10°C, showing a lower Tb when the period of freezing days, during which each population was collected, was reduced. S. physalifolium has a higher value for Tb at constant temperatures (21°C) that can be interpreted as a dormancy constraint. This constraint is released by alternating temperatures at amplitudes exceeding 5°C and with the high temperature above 21°C by apparently reducing Tb to 12.5°C.

This implies that for S. physaiifolium temperature has a dual effect on germination. It is the driving force for changes in dormancy, but germination also depends on the temperature. The role of temperature for S. nigrum is simpler: each population collected showed differences in the thermal time required for germination that could be related to the temperature regimen of the original environment.