Thermal time requirements for germination, emergence and seedling development of adventive legume and grass species

Abstract

Thermal time (Tt, °C days) was used to quantify germination, emergence and leaf appearance of temperate grassland species from natural and commercial seed populations. Germination requirements of six clovers (Trifolium arvensis, T. dubium, T. glomeratum, T. striatum, T. subterraneum, T. repens) and four grasses (Bromus erectus, Festuca rubra, F. ovina, Achnatherum calamagrostis) were determined from constant temperatures from 5 to 35°C. The base temperatures (Tb) were all less than 2.5°C, except for B. erectus (5.8°C). The four grasses had slower rates of germination and a wider tolerance of high temperatures than the clovers. All five annual clovers had a lower thermal time requirement for emergence and seedling development than the perennial T. repens. For four of the species used, the internationally recommended seed testing procedures that requires prechilling was found to be unnecessary and for two others the currently recommended optimum temperatures are higher than the optimum found. The thermal time approach has provided a robust summary of large datasets generated from controlled and field conditions. These easily transferable coefficients can be used to quantify seed and seedling responses to temperature for individual species and should form the basis for redefining seed testing regimes.