Estimation of Cardinal Temperatures and Thermal Time Requirement for Cyperus difformis Seed Germination

The aim of this study was to quantify the response of germination rate to temperature and to find cardinal temperatures and thermal time required for different germination percentiles in Cyperus difformis. We compared 4 non-linear regression models [dent-like، segmented and beta (4 and 5 parameter)] to describe the germination rate-temperature relationships of C. difformis over seven constant temperatures (ranging from 15 to 45 ˚C، with 5 ˚C intervals). Different statistical indices [Root Mean of Squares of Error (RMSE)، Akaike Information Criterion (AICc)] were used to compare models performances. The dent-like model was found to be the best model to predict germination rate (RMSE=0. 0009، AICc=-380. 8). The base، the lower optimum، the upper optimum and the maximum temperatures for the germination of C. difformis were estimated to be 14. 73، 34. 34، 38. 54 and 45. 01˚C، respectively. The thermal time required to reach 50 and 95% germination was 43. 50 and 65. 01 degree-days، respectively. The cardinal temperatures depended on the model used for their estimation. Overall، the dent-like model was better suited than the other models to estimate the cardinal temperatures for the germination of C. difformis.