Climatic control of seed longevity of Silene during the post-zygotic phase: do seeds from warm, dry climates possess higher maturity and desiccation tolerance than seeds from cold, wet climates?

Seed longevity derived from artificial ageing experiments has been shown to be related to the macroclimate at the sampling site and with individual seed traits. Nevertheless, the ecological interpretation of artificial seed longevity remains to be clarified. In this article, the ecological significance of seed longevity expressed by the p₅₀ index was explored using 16 closely related populations of the genus Silene L. Seeds were subjected to an artificial ageing treatment at 45 °C, 60% relative humidity and regularly tested for germination. The decline of viability to 50% (p₅₀ index) was calculated using probit analysis. Its relationship with known ecological predictors of seed longevity was assessed by multiple regression analysis. Values of p₅₀ ranged from 3.7 to 68.3 days. Seed lots which normally experience drought during the post-zygotic phase in a dry, warm environment, were long-lived. Consistently, precipitation during the driest months of the reproductive period was the strongest predictor of p₅₀ variability (partial regression, R² = 0.424). We could not find any relationship between ex situ seed longevity and soil seed bank formation or seed size. Our results indicate that ex situ seed longevity has an ecological significance related to long-term climatic differences at population site. Increased longevity presumably reflects resistance to desiccation stress attained by seeds through local adaptation to warm, dry climates. We cannot exclude that seed tolerance to ageing could also represent phenotypic plasticity mediated by a drier maturation environment. The vulnerability of seeds from wet, cool climates and the adaptive response of seed longevity to different environments may have implications for ex situ conservation in the face of climate warming.