Seed germination is strongly dependent on temperature. Reduced germination rate as a consequence of seed aging might be due to changing the type of response function to temperature or changing the parameters that govern the function. The objectives of this research were: (1) to determine the effect of seed aging on the type of response function describing the relationship of germination rate to temperature, and (2) to evaluate how the parameter(s) of the response function are changed when the germination rate is reduced as a result of seed aging. Seeds (cv. ‘Zagros’) were kept at a high temperature (43°C) and high relative humidity (90-95%) to create different classes of seed aging. Seed aging had no effect on the type of response function or cardinal temperatures. A dent-like function adequately described the response of germination rate to temperature in all the aging treatments. Cardinal temperatures of 2.2°C for the base, 28.6°C for the lower optimum, 38.0°C for upper optimum and 45°C for ceiling temperatures were obtained. Inherent maximum rate of germination (Rmax) was the sole parameter that was affected by seed aging periods and characterized differences between seed aging periods with respect to germination rate at various temperatures.

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