Seed germination niche of the halophyte Suaeda maritima to combined salinity and temperature is characterised by a halothermal time model

Sensitivity to salinity during seed germination potentially serves as a niche specifier for halophytes. Quantification of germination niche with respect to salinity, and any interaction with temperature, is therefore important to understand, particularly regarding the impacts of sea level rise in response to climate change. Population-based germination models quantify the response to temperature and water potential. However, there is limited applicability to salinity. Here, we develop a new approach considering thermal time (for temperature), halo time (for salinity) and halothermal time (for combined temperature and salinity), to quantify the germination of Suaeda maritima, a model halophyte with a worldwide distribution. We find salt concentration has a profound effect on the estimated base temperature for germination, Tb, being c. −4 °C in 600 mM NaCl compared with c. 1 °C in water. The predicted maximum concentration of NaCl for 50% germination to occur (NaClmax50) is 1280 mM at 5 °C and decreases (lower concentration) as the temperature increases. Overall, germination becomes slower as the concentration of salt increases (i.e., longer halo time) or the temperature decreases (i.e., longer thermal time). Combining all parameters into a halothermal time model is most successful at 5 and 10 °C. However, Na+ accumulation in the seed which coincides with maintained seed moisture content, regardless of salt concentration, influences the model fit. The models reveal two different germination niches: at cold temperatures, germination is slow but temperature and salinity are not necessarily limiting; and at warmer temperatures, germination is fastest but constrained by NaClmax50.