Quantifying perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) seed germination responses to water potential and temperature with a hydrothermal time model

In New Zealand, a commonly sown pasture combination is perennial ryegrass with white clover. The hydrothermal time model (HTT) was used to quantify germination under different temperature (T) and water potential (WP) conditions for white clover 'Nomad' and perennial ryegrass 'Stellar AR1'. Seeds were germinated in vitro under a range of T x WP combinations, and HTT models were fitted to germination time course data. Rapid germination of white clover under optimum T and WP was reflected by a much lower value for its HTT constant compared with perennial ryegrass (10.8 vs 80.7 MPa degrees C d). However, the lower base water potential for perennial ryegrass (-1.29 vs -0.495 MPa) and a higher threshold temperature for thermoinhibition (21.8 vs 18.3 degrees C), meant that predicted perennial ryegrass germination was more resilient in warm dry conditions. HTT models can be used to assist decisions about the timing of perennial ryegrass and white clover sowing.