Thermal requirements for seed germination of underutilized Lippia species

Seed germination requirements of three underutilized Lippia (Verbenaceae) species were investigated. Seeds of L. graveolens, L. javanica and L. multiflora were incubated at a range of constant temperatures (5-35 °C) in the light (12 h light/12 h darkness). The effect of GA3 and KNO3 on seed germination was also tested in the same range of germination temperatures. Germination responses to accumulated temperature were then characterized by a thermal time approach. Two different trends of germination response to temperature were detected among species, regardless of the applied treatment. Seeds of L. graveolens and L. javanica started to germinate at 15 °C, reached their maximum germination at 25 °C and then decreased at warmer temperatures, while those of L. multiflora reached their maximum at the highest tested temperature (35 °C). Base temperature for germination (Tb, °C) and the thermal constant for 50% germination (S, °Cd) of untreated seeds were ca. 2.5 and 7.5 °C and 76.92 and 84.3 °Cd, for L. graveolens and L. javanica, respectively. A Tb of ca. 20 °C could only be roughly estimated for L. multiflora, due to the low germination achieved at temperatures < 35 °C. GA3 and KNO3 had an overall inefficacy on enhancing seed germination of these species. Thermal thresholds of treated seeds could also be estimated for L. javanica, with a Tb of ca. 11 °C for both treatments and a S of 70.92 and 69.44 °Cd for GA3 and KNO3, respectively. Practical applications of the findings of this study may support the propagation of these underutilized Lippia species.