Germination response of Hylocereus setaceus (Salm-Dyck ex DC.) Ralf Bauer (Cactaceae) seeds to temperature and reduced water potentials

The germination response of Hylocereus setaceus seeds to isothermic incubation at different water potentials was analysed by using the thermal time and hydrotime models, aiming to describe some germination parameters of the population and to test the validity of the models to describe the response of the seeds to temperature and water potential. Hylocereus setaceus seeds germinated relatively well in a wide range of temperatures and the germination was rate limited from 11 to 20 degrees C interval and beyond 30 degrees C until 40 degrees C, in which the germination rate respectively shifts positively and negatively with temperature. The minimum or base temperature (T(b)) for the germination of H. setaceus was 7 degrees C, and the ceiling temperature varied nearly from 43.5 to 59 degrees C depending on the percent fraction, with median set on 49.8 degrees C. The number of degrees day necessary for 50% of the seeds to germinate in the infra-optimum temperature range was 39.3 degrees C day, whereas at the supra-optimum interval the value of theta = 77 was assumed to be constant throughout. Germination was sensitive to decreasing values of Psi in the medium, and both the germinability and the germination rate shift negatively with the reduction of Psi, but the rate of reduction changed with temperature. The values of base water potential (Psi(b)) shift to zero with increasing temperatures and such variation reflects in the relatively greater effect of low Psi on germination in supra optimum range of T. In general, the model described better the germination time courses at lower than at higher water potentials. The analysis also suggest that T(b) may not be independent of Psi and that Psi(b(g)) may change as a function of temperature at the infra-otimum temperature range.