Determination of cardinal temperatures of lemon balm (Melissa officinalis L.) seeds in response to temperatures and water potentials

Germination is series of biochemical reactions that depends mainly on temperature and moisture. Therefore, seed germination rate of lemon balm (Mellissa officinalis L.) were evaluated in temperatures of 20, 23, 25,27, 30 and 32ºC and water potentials of 0, -0.2, -0.4, -0.6 and -0.8MPa. According to the results, germination temperature window was not very wide in a way that germination temperature range was 23-32ºC. Three non-linear regression models (Dent-like, Segmented and Beta) were used to evaluate cardinal temperatures. According to the indices including RMSE, the coefficient of determination (R2) and correlation coefficient (r), the best model was Dent-like which can better estimate germination rate in response to temperature and water potential. The base, the lower and the upper and the ceiling temperatures of lemon balm were 17.31, 30.26, 31.15 and 35ºC in the optimum conditions of water potential, respectively. Biological time for germination ranged from 41 to 137 h in different water potentials. Biological time decreased about 96 h by increasing 1 MPa in water potential. Estimated parameters and the relations can be used to predict time to germination or emergence in lemon balm under wide environmental conditions from normal to drought stress.