Developmental heat sum influences recalcitrant seed traits in Aesculus hippocastanum across Europe

An analysis was made of seed traits along a north-south gradient spanning 19degrees of latitude in Europe using Aesculus hippocastanum, which originates from Greece and the Balkans and has been introduced throughout Europe. Because summer temperatures decrease with increasing latitude in Europe, we tested the hypothesis that the heat sum during seed development influences physical, physiological and biochemical seed traits. Seeds from Greece (within the natural range) had a fresh mass five times higher than those from Scotland (most northerly seed lot), and a lower axis moisture content and solute potential. In addition, Greek seeds germinated at cooler temperatures, and were more desiccation tolerant. Principal component analysis (PCA) showed that the observed patterns in seed characteristics were consistent and predictable: a single PCA axis explained 86.6% of the variation in the dataset. This axis correlated strongly with the heat sum accumulated by the seeds during development. The results support the notion that seeds from further north were shed less developed as a consequence of cooler temperatures during development, and provide a quantitative explanation for intraspecies variability in recalcitrant seed traits for this and, possibly, other species.