Mean germination rate (hours−1) of (a) Populus luziarum and (b) Populus primaveralepensis, evaluated by temperature (24, 28, and 32 °C), water potential (0, –0.07, –0.15, –0.3, and –0.6 MPa), and their interaction. Different letters indicate statistical differences between means, determined by Student–Newman–Keuls multiple comparisons (α = 0.05).