Modeling seed germination in Melisa officinalis L. in response to temperature and water potential

Seed germination is greatly influenced by both temperature (T) and water potential (psi) and these factors largely determine germination rate (GR) in the field. Quantitative information about T and psi effects on seed germination in lemon balm (Melisa officinalis L.) is scarce. The main objective of this study was to quantify seed germination responses of lemon balm to T and psi, and to determine cardinal temperatures in a laboratory experiment. A segmented model was used to describe the effects of psi (i.e., T) on GR and other germination parameters. The segmented model estimates were 7.2 A degrees C for base (T (b)), 28.9 A degrees C for optimum (T (o)), 40.1 A degrees C for ceiling temperature (T (c)) and 1.64 physiological days (f (o)) (equivalent to a GR(max) of 0.610 d(-1) and a thermal time of 35.6 A degrees C days) to reach 50 % maximum germination in the control (0 MPa) treatment (R (2) = 0.99, RMSE = 0.005 day(-1)). The inherent maximum rate of germination (days) was calculated by the [GR(max) = 1/f (o)] model. psi affected cardinal temperatures. From 0 to -0.76 MPa, when psi increased, T (b) was a constant 7.2 A degrees C to -0.38 MPa and increased linearly to 20.1 A degrees C as psi decreased. T (o) and f (o) increased linearly from 28.9 to 30 A degrees C, and from 1.64 to 5.4 day(-1), respectively as psi decreased. However, there was no signification difference in T (o) as psi decreased nor did T (c) decrease from 40.1 to 35 A degrees C as psi decreased. T (b), T (c) and GR(max) were the sole parameters affected by psi and could be used to characterize differences between psi treatments with respect to GR at various Ts. Therefore, the segmented model and its parameters can be used in lemon balm germination simulation models.