Quantifying seed germination response of melon (Cucumis melo L.) to temperature and water potential: Thermal time, hydrotime and hydrothermal time models

Abstract

Quantifying the germination behaviour of seeds against fluctuating environmental conditions can use as a useful tool for seed ecological studies. The response of seed germination rate to temperature (T) and water potential (Ψ) can be described using thermal time (TT), hydrotime (HT), and hydrothermal time (HTT) models. The germination behaviour of Persian melons (C. melo L.) were studied over a range of constant temperatures and water availability to assess the performance of hydro-thermal time models and to provide a data set of germination thresholds and parameters for the Persian melons. The seeds of melons were germinated in the laboratory over a water potential range from 0 to –1.25 MPa at 10, 15, 20, 25, 30, 35, 40 and 45 °C. The results showed that germination percentage was affected by water potential, temperature and their interactions. Germination increased with increasing temperature within the range of 15–25 °C, then decreased as temperature increased from 25 to 45 °C. Furthermore, final germination percentages were reduced with decreasing Ψ, although significant reduction was observed at Ψs less than –0.25 MPa. The model analysis showed that the dynamics of seed germination were generally well described by the TT model within Ψs range < –1 MPa (R2 = 83–96), HT (R2= 0.77–0.93) and HTT models (R2=0.89–0.90). Using the models analysis, the base, optimum and ceiling germination temperatures were estimated to be 9.4, 27.3 and 41.4 °C, respectively. Thermal time analysis showed that thermal time required for germination of Persian melon, Tb and Tc were affected by water availability. Furthermore, the estimated minimum water potential threshold for germination was –1.19 MPa using the HT model, but it shifted towards a higher values with temperatures above or below 25 °C. The amount of hydrothermal time required to germinate was 33 MPa °C days on the suboptimal temperatures range and increased by 36% with increasing temperature from the suboptimal temperatures to the supraoptimal temperatures. The HTT model showed that the Ψb(50) increased by 0.09 MPa with every degree increase in temperature above To. The interaction of Ψ and temperature affected the model parameters and finally the performance of germination models, then varying model parameters with changing environmental condition should be considered for predicting germination time in a complex growth environment.