Modelling the Effects of Water Stress and Temperature on Seed Germination of Radish and Cantaloupe

Cantaloupe (Cucumis melo L.) and radish (Raphanus sativus L.) are considered as important vegetables with potential for national and international markets due to their sugars, vitamins and minerals. This study arranged, therefore, to simultaneously investigate the effect of temperature (T) and water potential (psi) on seed germination (SG) of these plants using two hydrothermal time (HTT) models and to determine cardinal Ts and base water potential (psi (b(50))) for both species. The results indicated that SG of both species was more affected by psi than T (p <= 0.001). At Ts below an optimum temperature (T-o) the psi (b(50)) was constant (-0.582 and -0.760 MPa for radish and cantaloupe, respectively) and then increased linearly by 0.0481 and 0.0446 MPa degrees C-1 as T increased above T-o (as thermoinhibition) until 0 MPa at the ceiling temperature (T-c), respectively. As the first report, however, we observed that the T at which psi (b(50)) begins to change was the same here (that is, T-d=T-o), when determined by either model for both species. This result suggests that the assumption in Rowse and Finch-Savage's model (T-d is often less and or very close to T-o) may be invalid in some cases. For both species, the base temperature (T-b) and T-o were not affected by psi and were constant while there was an exception only for T-c for which the value declined with decreasing psi s (more negative). In general, the estimated T-b, T-o and T-c were 9.64, 21.3 and 33.0 degrees C for radish and 11.8, 28.3 and 45.7 degrees C for cantaloupe in the control condition (psi =0 MPa), respectively. The HTT models used here and their parameters, each with strengths and weaknesses, can be used as a predictive tool in both cantaloupe and radish SG simulation models. However, at first, we need to select an appropriate HTT model based on SG behavior of plant species and then use the best model for quantifying the response of SG across Ts and psi s.