Does seed quality affect the thermal thresholds of seed germination? Case Study: Farm saved seeds of wheat (Triticum aestivum L.) Cultivar Chamran

The models based on thermal time concept are useful tools for describing germination in relation to the time and temperature. The aim of this study was to determine the effect of seed quality on the thermal germination thresholds of wheat seed lots. For this purpose, germination of 16 seed lots was evaluated using the thermal time models based on Normal and Log-Normal distributions according to two different approaches. In the first approach, the base temperature (Tb) was assumed constant for all seeds in the population and the variation of germination timing among seeds was attributed to the variation in thermal time (θT) required for germination of each given fraction. In the second approach, the θT required for germination of the whole population was assumed to be constant and the variation of germination timing among seeds was attributed to the variation of their Tb. Goodness of fit criteria showed that the prediction accuracy of thermal time models in the first approach was more than the second approach. According to the first approach, Log-Normal distribution in comparison with normal distribution gave better and more accurate fits to the germination time courses of wheat seed lots in response to temperature. Seed quality of wheat seed lots significantly affected their maximum germination, cold tolerance threshold, rate and uniformity of germination. Based on predictions of Log-Normal thermal time model, estimates of maximum germination, Tb, thermal time requirement for start of germination and Standard deviation of θT distribution ranged from 73 to 99%, 0.26 to 3.68 C, 250.32 to 590.04 C h and 3.74 to 6.49 C h depending on the seed lot, respectively.