Alternating temperature combined with darkness resets base temperature for germination (Tb ) in photoblastic seeds of Lippia and Aloysia (Verbenaceae).

Thermal time models for seed germination assume a continuum of rate responses in the sub-optimal temperature range. Generally, the models describe germination performance in non-dormant seeds at constant temperatures, yet alternating temperature (AT) is a feature of many natural environments. We studied the possible interacting effects of AT on germination progress in photoblastic seeds of three aromatic-medicinal Verbenaceae species in the genera Lippia and Aloysia. For Lippia turbinata f. turbinata and L. turbinata f. magnifolia seed, germination only occurred in light conditions, while for L. integrifolia and Aloysia citriodora it was significantly higher in the light than in darkness. Although relative light germination (RLG) was not different between constant and AT in the sub-optimal range, AT raised the base temperature for germination progress (Tb ) from ca. 3-6 °C in constant temperature to 7-12 °C in AT. Among the species, thermal time for 50% seed germination [θT(50) ] was 55-100 °Cd at constant temperature. Although AT resulted in slight modifications to θT(50) , the germination rate at comparable average temperatures in the sub-optimal range was slower than under constant temperatures. For all species, the proportion of germinated seeds was similar for constant and AT. Our results suggest that an interaction between cool temperature and darkness during AT treatment limits the temperature range permissive for germination in these positively photoblastic seed, reflecting both close adaptation to the natural ecology and niche requirements of the species.