• An analysis was made of seed traits along a north–south gradient spanning 19° 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.

Nine seedlots of the widely planted southern and central European native tree species Acer pseudoplatanus L. were collected along a north–south gradient spanning 21° of latitude in Europe. We investigated how the heat sum during seed development influences seed maturity as assessed by physical, physiological and biochemical traits. Using principal component analysis we found predictable and consistent patterns in all traits, which correlated with heat sum. For example, compared with fruits from their native range (Italy and France, heat sum >3000°C d), fruits from the coldest location (Scotland; heat sum of 1873°C d) were shorter (c. 30 v. 42 mm), germinated over a narrower temperature range (5–20 v. 5–35°C) and had smaller embryos (28 v. > 70 mg) with a higher water content (c. 63 v. 48%), less negative solute potentials (c. –2.4 v. –4.1 MPa) and were more desiccation sensitive (critical water potential of –20.2 v. –55.4 to –60.7 MPa). The observed level of desiccation-tolerance for the French and Italian seedlots is more consistent with the intermediate category than the previous classification of A. pseudoplatanus as recalcitrant. Our results demonstrate that a lower heat sum causes fruits from northern Europe to be dispersed before maximum potential seed quality is achieved.

Background and Aims Epigenetic regulation plays an important role in the management of plant growth, development and response to stress factors, and several reports have indicated that DNA methylation plays a critical role in seed development and viability. This study examines changes in 5-methylcytosine (m⁵C) levels in the DNA of seeds during ageing, a process that has important implications for plant conservation and agriculture. Methods Changes in the global level of m⁵C were measured in mature seeds of oak, Quercus robur. The extent of DNA methylation was measured using a protocol based on two-dimensional thin-layer chromatography. Viability of seeds was determined by germination and seedling emergence tests. Key Results An ageing-related decrease in total m⁵C during storage of recalcitrant seeds was highly and significantly correlated with a decrease in seed viability, as reflected by a reduction in germination (r = 0·8880) and seedling emergence (r = 0·8269). Conclusions The decrease in viability during ageing of Q. robur seeds is highly correlated with a global decline in the amount of m⁵C in genomic DNA, and it is possible that this may represent a typical response to ageing and senescence in recalcitrant seeds. Potential mechanisms that drive changes in genomic DNA methylation during ageing are discussed, together with their implications for seed viability.

The effects of storage and deep desiccation on structural changes of DNA in orthodox seeds are poorly characterized. In this study we analyzed the 5-methylcytosine (m⁵C) global content of DNA isolated from seeds of common pear (Pyrus communis L.) that had been subjected to extreme desiccation, and the seedlings derived from these seeds. Germination and seedling emergence tests were applied to determine seed viability after their desiccation. In parallel, analysis of the global content of m⁵C in dried seeds and DNA of seedlings obtained from such seeds was performed with a 2D TLC method. Desiccation of fresh seeds to 5.3% moisture content (mc) resulted in a slight reduction of DNA methylation, whereas severe desiccation down to 2–3% mc increased DNA methylation. Strong desiccation of seeds resulted in the subsequent generation of seedlings of shorter height. A 1-year period of seed storage induced a significant increase in the level of DNA methylation in seeds. It is possible that alterations in the m⁵C content of DNA in strongly desiccated pear seeds reflect a reaction of desiccation-tolerant (orthodox) seeds to severe desiccation. Epigenetic changes were observed not only in severely desiccated seeds but also in 3-month old seedlings obtained from these seeds. With regard to seed storage practices, epigenetic assessment could be used by gene banks for early detection of structural changes in the DNA of stored seeds.

We investigated the sensitivity of dormant Polish-provenance Fraxinus excelsior L. seeds to extreme desiccation and liquid nitrogen (LN, −196 °C). Germination and seedling emergence tests revealed that the critical water content (WC) of the desiccated seeds was 0.04–0.06 g H₂O g⁻¹ dry mass, g g⁻¹ (0.04 g g⁻¹ in the germination test and 0.06 g g⁻¹ in the seedling emergence test). The seeds that were within the safe WC range (0.06–0.24 g g⁻¹ in the germination test and 0.08–0.24 g g⁻¹ in the seedling emergence test) tolerated freezing in LN. A 2-year seed storage period in LN after desiccation to a safe WC did not decrease the germination after thawing as compared with a 2-year seed storage at −3 °C. When the seeds were stored in LN after dormancy breaking, the germination rate after storage was lower, because the seeds had to be desiccated to the lower level (~ 0.11 g g⁻¹) within the seeds’ safe WC range after stratification (before storage). Secondary dormancy was induced in seeds that were desiccated after stratification. The results of this study confirm the feasibility of long-term cryopreservation of European ash seeds in forest gene banks.