Onion (Allium cepa L.) seed germination affected by temperature and water potential: Hydrothermal time model

Abstract

A useful implement for seed ecological studies is quantifying the germination behavior against variable environmental conditions. The influences of temperature and water potential on the germination response of onion seeds can be described by using the thermal time (TT), hydrotime (HT), and hydrothermal time (HTT) models. The germination behaviour of onion seeds (Allium cepa L.) was studied over a range of constant temperatures and water availability. The efficiency of hydrothermal time models was evaluated by the germination parameters for two onion cultivars (‘Sepidan’ and ‘Zargan’). The onion seeds were germinated in the laboratory over a water potential range from 0 to − 1 MPa at 5, 10, 15, 20, 25, 30, 35, and 40 °C. Final germination dropped with decreasing water potential, particularly at water potential less than − 0.2 MPa. The model analysis indicated that the dynamics of seed germination was generally well described by the TT model within water potential less than − 1 MPa (R2 = 0.69–0.97), HT (R2 = 0.94–0.99), and HTT models (R2 = 0.95–0.96). The Dent-like was found to be the best model to predict germination rate (RMSE = 0.00006, and 0.000133, R2 = 0.98, and 0.97 for ‘Sepidan’ and ‘Zargan’, respectively). The base, sub-optimum, supra-optimum and the ceiling temperatures for onion seed germination were estimated − 2.5, 19.3, 29.8, and 39.3 °C for ‘Sepidan’ and 0, 25.19, 27.37, and 42.39 °C for ‘Zargan’ at 0 MPa. Thermal time constants were 2650 and 3329.67 °C.h at − 0.8 MPa and 1602.83 and 1521.44 °C.h for ‘Sepidan’ and ‘Zargan’ seeds at 0 MPa. Hydrotime constants for ‘Sepidan’ and ‘Zargan’ seeds were 115.54 and 128.037 MPa.h at 5 °C and 20.8 and 26.93 MPa.h at 30 °C. Hydrothermal time constants for ‘Sepidan’ and ‘Zargan’ seeds were 1249.792 and 1122.78 MPa.°C.h.