Key features of the seed germination response to high temperatures

Abstract

The responses of seed germination rates (GRs) and final germination percentages to soil temperature follow trends similar to those for many plant processes. There is an optimum temperature for seed germination (Tₒ), which is defined as the soil temperature at which the highest germination percentage is achieved by a seed population in the shortest possible period of time (Mayer & Poljakoff-Mayber, 1975). At both sub-optimal (< Tₒ) and supra-optimal (> Tₒ) temperatures, the reductions in the GR and germination percentage scale with the amount by which the soil temperature is less than or greater than Tₒ, respectively, until threshold temperatures (base temperature Tb and ceiling temperature Tc) are reached at which germination is completely prevented (Fig. 1). These responses of seed germination to temperature and their underlying physiology have been thoroughly described in the literature and are very familiar to seed scientists. Specific aspects, such as thermoinhibition, when seeds fail to germinate at high soil temperatures, are of great importance for commercial crops and have been investigated for many crop species. However, there are two key features of germination commonly observed within seed germination studies that are nonetheless seldom discussed in the literature. First, Tₒ is often not a specific temperature, but rather a range of temperatures seen as a broad, curvilinear peak in the plot of GR vs temperature (GR–T). Second, the GR–T response varies between the seed percentiles in the population, where the order in which seeds germinate is specified by the seed percentile (G), such that G = 1 is the first percentile of seeds to germinate, G = 2 is the second percentile to germinate, and so on, up to G = 100, which represents complete germination of a seed population. Fast-germinating seeds (the lowest percentiles) often have higher values for Tₒ and Tc, compared with slower germinating seeds (the higher percentiles), which reach their Tₒ and Tc at lower temperatures. From an extensive literature review of GR–T responses, we identify the extent to which these two key features are shown by the data. We discuss two common mathematical models that have been used to describe the GR response to temperature, and which model more accurately describes the important features of GR–T. Lastly, we discuss how this more accurate model aligns with our physiological and ecological understanding of the seed germination process.