Effect of seed size and sub-zero imbibition-temperature on the thermal time model of winterfat ( Eurotia lanata (Pursh) Moq.)

Abstract

Thermal time has been widely used to quantify and model seed germination. However, several assumptions in the thermal time model remain untested. The purpose of this study was to test variations in parameters of the thermal time model among seed size classes and sub-zero imbibition-temperature treatments using two non-dormant seed collections of winterfat ( Eurotia lanata). Seeds originating from Utah, USA, were imbibed at sub-zero temperature for 0, 1, 3 and 5 days and then were germinated at 2, 5, 10, 15, 20, and 25 °C. Estimated base temperature ( T b) was lower in the large seed class (−4.48 °C) than the small seed class (−3.53 °C). Thermal time for 50% germination ( θ T(50)) and the standard deviation of thermal times ( σ θ T ) were not significantly different between seed collections and seed size classes. Seeds germinated faster after imbibition at sub-zero temperature, but the enhanced germination rate was mainly attributed to thermal time accumulation during imbibition rather than the often speculated physiological changes. A thermal germination rate index (TGRI) was proposed, which successfully accounted for the thermal time accumulation during imbibition. Sub-zero imbibition-temperature treatments did, however, alter T b, θ T(50) and σ θ T for both collections and seed size classes as evidenced by the higher model fit of the modified model than the original model. Base temperature in the thermal time model of winterfat was lower for larger seeds, which enables large seeds to accumulate more thermal time at a given temperature. This is particularly important for species such as winterfat because seeds of this species germinate early in the season.