Seed dormancy alleviation by warm stratification progressively widens the germination window in Mediterranean climate Rutaceae

Abstract

Context In ecosystems where rainfall is episodic or highly seasonal, plant recruitment from a soil-stored seed bank occurs during periods of elevated soil moisture conducive to germination and seedling establishment. The release of seed dormancy in response to environmental conditions has significant consequences for the temperature window over which germination occurs, and as such the timing of germination can vary between years. Aims We aimed to understand in seeds of two species of Rutaceae, Diplolaena dampieri and Rhadinothamnus anceps, how dormancy loss and germination timing is influenced by warm stratification. Methods We tested the germination response to temperatures between 5 and 30°C following increasing durations (1–12 and 4–12 weeks) of warm stratification at 20, 25, or 30°C. Key results Warm stratification for 1–8 weeks at 30°C progressively alleviated seed dormancy in D. dampieri and R. anceps, increasing germination proportion from ~0.1 to ~0.5 in both species. Stratification duration was optimal at 30°C for between 4 and 8 weeks depending on species. Warm stratification was not affected by water stress down to −0.8 Mpa. Application of aerosol smoke did not significantly improve germination, and heat treatments had a negative effect on final germination proportion. Conclusions As dormancy was progressively alleviated, the range of temperatures that support germination increased for D. dampieri and decreased for R. anceps, allowing for confirmation of type 1 and type 2 non-deep physiological dormancy (PD), respectively. Implications Arising from this conclusion, we suggest that in Mediterranean climates, type 1 and 2 PD dictate risk-taking and risk-avoiding ecological strategies by shifting the thermal requirements for germination towards that characteristic of the early- or mid-germination season. Classification of non-deep PD may offer a structured approach to predict how temperature requirements shift during dormancy loss, which will provide insight into seed germination response to year-to-year variation in seasonal environmental conditions.